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Advanced MS-DOS Programming
The following document is from the Microsoft Programmer’s Library 1.3 CD-ROM.
Advanced MS-DOS Programming
════════════════════════════════════════════════════════════════════════════
Advanced MS-DOS Programming
The Microsoft(R) Guide for Assembly Language and C Programmers
By Ray Duncan
════════════════════════════════════════════════════════════════════════════
PUBLISHED BY
Microsoft Press
A Division of Microsoft Corporation
16011 NE 36th Way, Box 97017, Redmond, Washington 98073-9717
Copyright (C) 1986, 1988 by Ray Duncan
Published 1986. Second edition 1988.
All rights reserved. No part of the contents of this book may be
reproduced or transmitted in any form or by any means without the written
permission of the publisher.
Library of Congress Cataloging in Publication Data
Duncan, Ray, 1952-
Advanced MS-DOS programming.
Rev. ed. of: Advanced MS-DOS. (C)1986.
Includes index.
1. MS-DOS (Computer operating system) 2. Assembler language
(Computer program language) 3. C (Computer program language)
I. Duncan, Ray, 1952- Advanced MS-DOS. II. Title.
QA76.76.063D858 1988 005.4'46 88-1251
ISBN 1-55615-157-8
Printed and bound in the United States of America.
1 2 3 4 5 6 7 8 9 FGFG 3 2 1 0 9 8
Distributed to the book trade in the United States by Harper & Row.
Distributed to the book trade in Canada by General Publishing Company,
Ltd.
Penguin Books Ltd., Harmondworth, Middlesex, England
Penguin Books Australia Ltd., Ringwood, Victoria, Australia
Penguin Books N.Z. Ltd., 182-190 Wairu Road, Auckland 10, New Zealand
British Cataloging in Publication Data available
IBM(R), PC/AT(R), and PS/2(R) are registered trademarks of International
Business Machines Corporation. CodeView(R), Microsoft(R), MS-DOS(R), and
XENIX(R) are registered trademarks and InPort TM is a trademark of
Microsoft Corporation.
──────────────────────────────────────────────────────────────────────────
Technical Editor: Mike Halvorson Production Editor: Mary Ann Jones
──────────────────────────────────────────────────────────────────────────
Dedication
For Carolyn
────────────────────────────────────────────────────────────────────────────
Contents
Road Map to Figures and Tables
Acknowledgments
Introduction
SECTION 1 PROGRAMMING FOR MS-DOS
Chapter 1 Genealogy of MS-DOS
Chapter 2 MS-DOS in Operation
Chapter 3 Structure of MS-DOS Application Programs
Chapter 4 MS-DOS Programming Tools
Chapter 5 Keyboard and Mouse Input
Chapter 6 Video Display
Chapter 7 Printer and Serial Port
Chapter 8 File Management
Chapter 9 Volumes and Directories
Chapter 10 Disk Internals
Chapter 11 Memory Management
Chapter 12 The EXEC Function
Chapter 13 Interrupt Handlers
Chapter 14 Installable Device Drivers
Chapter 15 Filters
Chapter 16 Compatibility and Portability
SECTION 2 MS-DOS FUNCTIONS REFERENCE
SECTION 3 IBM ROM BIOS AND MOUSE FUNCTIONS REFERENCE
SECTION 4 LOTUS/INTEL/MICROSOFT EMS FUNCTIONS REFERENCE
Index
────────────────────────────────────────────────────────────────────────────
Road Map to Figures and Tables
MS-DOS versions and release dates
MS-DOS memory map
Structure of program segment prefix (PSP)
Structure of .EXE load module
Register conditions at program entry
Segments, groups, and classes
Macro Assembler switches
C Compiler switches
Linker switches
MAKE switches
ANSI escape sequences
Video attributes
Structure of normal file control block (FCB)
Structure of extended file control block
MS-DOS error codes
Structure of boot sector
Structure of directory entry
Structure of fixed-disk master block
LIM EMS error codes
Intel 80x86 internal interrupts (faults)
Intel 80x86, MS-DOS, and ROM BIOS interrupts
Device-driver attribute word
Device-driver command codes
Structure of BIOS parameter block (BPB)
Media descriptor byte
────────────────────────────────────────────────────────────────────────────
Acknowledgments
My renewed thanks to the outstanding editors and production staff at
Microsoft Press, who make beautiful books happen, and to the talented
Microsoft developers, who create great programs to write books about.
Special thanks to Mike Halvorson, Jeff Hinsch, Mary Ann Jones, Claudette
Moore, Dori Shattuck, and Mark Zbikowski; if this book has anything unique
to offer, these people deserve most of the credit.
────────────────────────────────────────────────────────────────────────────
Introduction
Advanced MS-DOS Programming is written for the experienced C or
assembly-language programmer. It provides all the information you need to
write robust, high-performance applications under the MS-DOS operating
system. Because I believe that working, well-documented programs are
unbeatable learning tools, I have included detailed programming examples
throughout──including complete utility programs that you can adapt to your
own needs.
This book is both a tutorial and a reference and is divided into four
sections, so that you can find information more easily. Section 1
discusses MS-DOS capabilities and services by functional group in the
context of common programming issues, such as user input, control of the
display, memory management, and file handling. Special classes of
programs, such as interrupt handlers, device drivers, and filters, have
their own chapters.
Section 2 provides a complete reference guide to MS-DOS function calls,
organized so that you can see the calling sequence, results, and version
dependencies of each function at a glance. I have also included notes,
where relevant, about quirks and special uses of functions as well as
cross-references to related functions. An assembly-language example is
included for each entry in Section 2.
Sections 3 and 4 are references to IBM ROM BIOS, Microsoft Mouse driver,
and Lotus/Intel/Microsoft Expanded Memory Specification functions. The
entries in these two sections have the same form as in Section 2, except
that individual programming examples have been omitted.
The programs in this book were written with the marvelous Brief editor
from Solution Systems and assembled or compiled with Microsoft Macro
Assembler version 5.1 and Microsoft C Compiler version 5.1. They have been
tested under MS-DOS versions 2.1, 3.1, 3.3, and 4.0 on an 8088-based IBM
PC, an 80286-based IBM PC/AT, and an 80386-based IBM PS/2 Model 80. As far
as I am aware, they do not contain any software or hardware dependencies
that will prevent them from running properly on any IBM PC─compatible
machine running MS-DOS version 2.0 or later.
Changes from the First Edition
Readers who are familiar with the first edition will find many changes in
the second edition, but the general structure of the book remains the
same. Most of the material comparing MS-DOS to CP/M and UNIX/XENIX has
been removed; although these comparisons were helpful a few years ago,
MS-DOS has become its own universe and deserves to be considered on its
own terms.
The previously monolithic chapter on character devices has been broken
into three more manageable chapters focusing on the keyboard and mouse,
the display, and the serial port and printer. Hardware-dependent video
techniques have been de-emphasized; although this topic is more important
than ever, it has grown so complex that it requires a book of its own. A
new chapter discusses compatibility and portability of MS-DOS applications
and also contains a brief introduction to Microsoft OS/2, the new
multitasking, protected-mode operating system.
A road map to vital figures and tables has been added, following the Table
of Contents, to help you quickly locate the layouts of the program segment
prefix, file control block, and the like.
The reference sections at the back of the book have been extensively
updated and enlarged and are now complete through MS-DOS version 4.0, the
IBM PS/2 Model 80 ROM BIOS and the VGA video adapter, the Microsoft Mouse
driver version 6.0, and the Lotus/Intel/Microsoft Expanded Memory
Specification version 4.0.
In the two years since Advanced MS-DOS Programming was first published,
hundreds of readers have been kind enough to send me their comments, and I
have tried to incorporate many of their suggestions in this new edition.
As before, please feel free to contact me via MCI Mail (user name LMI),
CompuServe (user ID 72406,1577), or BIX (user name rduncan).
Ray Duncan Los Angeles, California September 1988
────────────────────────────────────────────────────────────────────────────
SECTION 1 PROGRAMMING FOR MS-DOS
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Chapter 1 Genealogy of MS-DOS
In only seven years, MS-DOS has evolved from a simple program loader into
a sophisticated, stable operating system for personal computers that are
based on the Intel 8086 family of microprocessors (Figure 1-1). MS-DOS
supports networking, graphical user interfaces, and storage devices of
every description; it serves as the platform for thousands of application
programs; and it has over 10 million licensed users──dwarfing the combined
user bases of all of its competitors.
The progenitor of MS-DOS was an operating system called 86-DOS, which was
written by Tim Paterson for Seattle Computer Products in mid-1980. At that
time, Digital Research's CP/M-80 was the operating system most commonly
used on microcomputers based on the Intel 8080 and Zilog Z-80
microprocessors, and a wide range of application software (word
processors, database managers, and so forth) was available for use with
CP/M-80.
To ease the process of porting 8-bit CP/M-80 applications into the new
16-bit environment, 86-DOS was originally designed to mimic CP/M-80 in
both available functions and style of operation. Consequently, the
structures of 86-DOS's file control blocks, program segment prefixes, and
executable files were nearly identical to those of CP/M-80. Existing
CP/M-80 programs could be converted mechanically (by processing their
source-code files through a special translator program) and, after
conversion, would run under 86-DOS either immediately or with very little
hand editing.
Because 86-DOS was marketed as a proprietary operating system for Seattle
Computer Products' line of S-100 bus, 8086-based microcomputers, it made
very little impact on the microcomputer world in general. Other vendors of
8086-based microcomputers were understandably reluctant to adopt a
competitor's operating system and continued to wait impatiently for the
release of Digital Research's CP/M-86.
In October 1980, IBM approached the major microcomputer-software houses in
search of an operating system for the new line of personal computers it
was designing. Microsoft had no operating system of its own to offer
(other than a stand-alone version of Microsoft BASIC) but paid a fee to
Seattle Computer Products for the right to sell Paterson's 86-DOS. (At
that time, Seattle Computer Products received a license to use and sell
Microsoft's languages and all 8086 versions of Microsoft's operating
system.) In July 1981, Microsoft purchased all rights to 86-DOS, made
substantial alterations to it, and renamed it MS-DOS. When the first IBM
PC was released in the fall of 1981, IBM offered MS-DOS (referred to as
PC-DOS 1.0) as its primary operating system.
IBM also selected Digital Research's CP/M-86 and Softech's P-system as
alternative operating systems for the PC. However, they were both very
slow to appear at IBM PC dealers and suffered the additional disadvantages
of higher prices and lack of available programming languages. IBM threw
its considerable weight behind PC-DOS by releasing all the IBM-logo PC
application software and development tools to run under it. Consequently,
most third-party software developers targeted their products for PC-DOS
from the start, and CP/M-86 and P-system never became significant factors
in the IBM PC─compatible market.
In spite of some superficial similarities to its ancestor CP/M-80, MS-DOS
version 1.0 contained a number of improvements over CP/M-80, including the
following:
■ An improved disk-directory structure that included information about a
file's attributes (such as whether it was a system or a hidden file),
its exact size in bytes, and the date that the file was created or last
modified
■ A superior disk-space allocation and management method, allowing
extremely fast sequential or random record access and program loading
■ An expanded set of operating-system services, including
hardware-independent function calls to set or read the date and time, a
filename parser, multiple-block record I/O, and variable record sizes
■ An AUTOEXEC.BAT batch file to perform a user-defined series of commands
when the system was started or reset
IBM was the only major computer manufacturer (sometimes referred to as
OEM, for original equipment manufacturer) to ship MS-DOS version 1.0 (as
PC-DOS 1.0) with its products. MS-DOS version 1.25 (equivalent to IBM
PC-DOS 1.1) was released in June 1982 to fix a number of bugs and also to
support double-sided disks and improved hardware independence in the DOS
kernel. This version was shipped by several vendors besides IBM, including
Texas Instruments, COMPAQ, and Columbia, who all entered the personal
computer market early. Due to rapid decreases in the prices of RAM and
fixed disks, MS-DOS version 1 is no longer in common use.
MS-DOS version 2.0 (equivalent to PC-DOS 2.0) was first released in March
1983. It was, in retrospect, a new operating system (though great care was
taken to maintain compatibility with MS-DOS version 1). It contained many
significant innovations and enhanced features, including those listed on
the following page.
■ Support for both larger-capacity floppy disks and hard disks
■ Many UNIX/XENIX-like features, including a hierarchical file structure,
file handles, I/O redirection, pipes, and filters
■ Background printing (print spooling)
■ Volume labels, plus additional file attributes
■ Installable device drivers
■ A user-customizable system-configuration file that controlled the
loading of additional device drivers, the number of system disk
buffers, and so forth
■ Maintenance of environment blocks that could be used to pass
information between programs
■ An optional ANSI display driver that allowed programs to position the
cursor and control display characteristics in a hardware-independent
manner
■ Support for the dynamic allocation, modification, and release of memory
by application programs
■ Support for customized user command interpreters (shells)
■ System tables to assist application software in modifying its currency,
time, and date formats (known as international support)
MS-DOS version 2.11 was subsequently released to improve international
support (table-driven currency symbols, date formats, decimal-point
symbols, currency separators, and so forth), to add support for 16-bit
Kanji characters throughout, and to fix a few minor bugs. Version 2.11
rapidly became the base version shipped for 8086/8088-based personal
computers by every major OEM, including Hewlett-Packard, Wang, Digital
Equipment Corporation, Texas Instruments, COMPAQ, and Tandy.
MS-DOS version 2.25, released in October 1985, was distributed in the Far
East but was never shipped by OEMs in the United States and Europe. In
this version, the international support for Japanese and Korean character
sets was extended even further, additional bugs were repaired, and many of
the system utilities were made compatible with MS-DOS version 3.0.
MS-DOS version 3.0 was introduced by IBM in August 1984 with the release
of the 80286-based PC/AT machines. It represented another major rewrite of
the entire operating system and included the important new features listed
on the following page.
■ Direct control of the print spooler by application software
■ Further expansion of international support for currency formats
■ Extended error reporting, including a code that suggests a recovery
strategy to the application program
■ Support for file and record locking and sharing
■ Support for larger fixed disks
MS-DOS version 3.1, which was released in November 1984, added support for
the sharing of files and printers across a network. Beginning with version
3.1, a new operating-system module called the redirector intercepts an
application program's requests for I/O and filters out the requests that
are directed to network devices, passing these requests to another machine
for processing.
Since version 3.1, the changes to MS-DOS have been evolutionary rather
than revolutionary. Version 3.2, which appeared in 1986, generalized the
definition of device drivers so that new media types (such as 3.5-inch
floppy disks) could be supported more easily. Version 3.3 was released in
1987, concurrently with the new IBM line of PS/2 personal computers, and
drastically expanded MS-DOS's multilanguage support for keyboard mappings,
printer character sets, and display fonts. Version 4.0, delivered in 1988,
was enhanced with a visual shell as well as support for very large file
systems.
While MS-DOS has been evolving, Microsoft has also put intense efforts
into the areas of user interfaces and multitasking operating systems.
Microsoft Windows, first shipped in 1985, provides a multitasking,
graphical user "desktop" for MS-DOS systems. Windows has won widespread
support among developers of complex graphics applications such as desktop
publishing and computer-aided design because it allows their programs to
take full advantage of whatever output devices are available without
introducing any hardware dependence.
Microsoft Operating System/2 (MS OS/2), released in 1987, represents a new
standard for application developers: a protected-mode, multitasking,
virtual-memory system specifically designed for applications requiring
high-performance graphics, networking, and interprocess communications.
Although MS OS/2 is a new product and is not a derivative of MS-DOS, its
user interface and file system are compatible with MS-DOS and Microsoft
Windows, and it offers the ability to run one real-mode (MS-DOS)
application alongside MS OS/2 protected-mode applications. This
compatibility allows users to move between the MS-DOS and OS/2
environments with a minimum of difficulty.
┌─────────────┐
│ MS-DOS 1.0 │ 1981: First operating system on IBM PC
│ PC-DOS 1.0 │
└──────┬──────┘
│
┌──────▼──────┐
│ MS-DOS 1.25 │ Double-sided disk support and bug fixes added:
│ PC-DOS 1.1 │ widely distributed by OEMs other than IBM
└──────┬──────┘
│
┌──────▼──────┐ 1983: Introduced with IBM PC/XT;
│ MS-DOS 2.0 │ support for UNIX/XENIX-like hierarchical
│ PC-DOS 2.0 │ file structure and hard disks added
└──────┬──────┘
├──────────────────────────────────────┐
┌──────▼──────┐ ┌──────▼──────┐
│ MS-DOS 2.01 │ 2.0 with international │ PC-DOS 2.1 │ Introduced with PCjr
└──────┬──────┘ support └─────────────┘ 2.0 with bug fixes
│
┌──────▼──────┐
│ MS-DOS 2.11 │ 2.01 with bug fixes
└──────┬──────┘
├──────────────────────────────────────┐
┌──────▼──────┐ 1984: Introduced with ┌──────▼──────┐ 1985: Far East OEMs;
│ MS-DOS 3.0 │ PC/AT; support for │ MS-DOS 2.25 │ support for extended
│ PC-DOS 3.0 │ 1.2 MB floppy disk, └─────────────┘ character sets
└──────┬──────┘ larger hard disk added
│
┌──────▼──────┐
│ MS-DOS 3.1 │ Support for Microsoft ┌─────────────┐ 1985: Graphical
│ PC-DOS 3.1 │ Networks added │ Windows │ user interface
└──────┬──────┘ │ 1.0 │ for MS-DOS
│ └──────┬──────┘
┌──────▼──────┐ │
│ MS-DOS 3.2 │ 1986: Support for 3.5- │
│ PC-DOS 3.2 │ inch disks added │
└──────┬──────┘ │
│ ┌──────▼──────┐ 1987: Compatibility
┌──────▼──────┐ 1987: Introduced with │ Windows │ with OS/2
│ MS-DOS 3.3 │ IBM PS/2; generalized │ 2.0 │ Presentation Manager
│ PC-DOS 3.3 │ code-page (font) └─────────────┘
└──────┬──────┘ support
│
┌──────▼──────┐ 1988: Support for
│ MS-DOS 4.0 │ logical volumes larger
│ PC-DOS 4.0 │ than 32 MB; visual shell
└─────────────┘
Figure 1-1. The evolution of MS-DOS.
What does the future hold for MS-DOS? Only the long-range planning teams
at Microsoft and IBM know for sure. But it seems safe to assume that
MS-DOS, with its relatively small memory requirements, adaptability to
diverse hardware configurations, and enormous base of users, will remain
important to programmers and software publishers for years to come.
────────────────────────────────────────────────────────────────────────────
Chapter 2 MS-DOS in Operation
It is unlikely that you will ever be called upon to configure the MS-DOS
software for a new model of computer. Still, an acquaintance with the
general structure of MS-DOS can often be very helpful in understanding the
behavior of the system as a whole. In this chapter, we will discuss how
MS-DOS is organized and how it is loaded into memory when the computer is
turned on.
The Structure of MS-DOS
MS-DOS is partitioned into several layers that serve to isolate the kernel
logic of the operating system, and the user's perception of the system,
from the hardware it is running on. These layers are
■ The BIOS (Basic Input/Output System)
■ The DOS kernel
■ The command processor (shell)
We'll discuss the functions of each of these layers separately.
The BIOS Module
The BIOS is specific to the individual computer system and is provided by
the manufacturer of the system. It contains the default resident
hardware-dependent drivers for the following devices:
■ Console display and keyboard (CON)
■ Line printer (PRN)
■ Auxiliary device (AUX)
■ Date and time (CLOCK$)
■ Boot disk device (block device)
The MS-DOS kernel communicates with these device drivers through I/O
request packets; the drivers then translate these requests into the proper
commands for the various hardware controllers. In many MS-DOS systems,
including the IBM PC, the most primitive parts of the hardware drivers are
located in read-only memory (ROM) so that they can be used by stand-alone
applications, diagnostics, and the system startup program.
The terms resident and installable are used to distinguish between the
drivers built into the BIOS and the drivers installed during system
initialization by DEVICE commands in the CONFIG.SYS file. (Installable
drivers will be discussed in more detail later in this chapter and in
Chapter 14.)
The BIOS is read into random-access memory (RAM) during system
initialization as part of a file named IO.SYS. (In PC-DOS, the file is
called IBMBIO.COM.) This file is marked with the special attributes hidden
and system.
The DOS Kernel
The DOS kernel implements MS-DOS as it is seen by application programs.
The kernel is a proprietary program supplied by Microsoft Corporation and
provides a collection of hardware-independent services called system
functions. These functions include the following:
■ File and record management
■ Memory management
■ Character-device input/output
■ Spawning of other programs
■ Access to the real-time clock
Programs can access system functions by loading registers with
function-specific parameters and then transferring to the operating system
by means of a software interrupt.
The DOS kernel is read into memory during system initialization from the
MSDOS.SYS file on the boot disk. (The file is called IBMDOS.COM in
PC-DOS.) This file is marked with the attributes hidden and system.
The Command Processor
The command processor, or shell, is the user's interface to the operating
system. It is responsible for parsing and carrying out user commands,
including the loading and execution of other programs from a disk or other
mass-storage device.
The default shell that is provided with MS-DOS is found in a file called
COMMAND.COM. Although COMMAND.COM prompts and responses constitute the
ordinary user's complete perception of MS-DOS, it is important to realize
that COMMAND.COM is not the operating system, but simply a special class
of program running under the control of MS-DOS.
COMMAND.COM can be replaced with a shell of the programmer's own design by
simply adding a SHELL directive to the system-configuration file
(CONFIG.SYS) on the system startup disk. The product COMMAND-PLUS from ESP
Systems is an example of such an alternative shell.
More about COMMAND.COM
The default MS-DOS shell, COMMAND.COM, is divided into three parts:
■ A resident portion
■ An initialization section
■ A transient module
The resident portion is loaded in lower memory, above the DOS kernel and
its buffers and tables. It contains the routines to process Ctrl-C and
Ctrl-Break, critical errors, and the termination (final exit) of other
transient programs. This part of COMMAND.COM issues error messages and is
responsible for the familiar prompt
Abort, Retry, Ignore?
The resident portion also contains the code required to reload the
transient portion of COMMAND.COM when necessary.
The initialization section of COMMAND.COM is loaded above the resident
portion when the system is started. It processes the AUTOEXEC.BAT batch
file (the user's list of commands to execute at system startup), if one is
present, and is then discarded.
The transient portion of COMMAND.COM is loaded at the high end of memory,
and its memory can also be used for other purposes by application
programs. The transient module issues the user prompt, reads the commands
from the keyboard or batch file, and causes them to be executed. When an
application program terminates, the resident portion of COMMAND.COM does a
checksum of the transient module to determine whether it has been
destroyed and fetches a fresh copy from the disk if necessary.
The user commands that are accepted by COMMAND.COM fall into three
categories:
■ Internal commands
■ External commands
■ Batch files
Internal commands, sometimes called intrinsic commands, are those carried
out by code embedded in COMMAND.COM itself. Commands in this category
include COPY, REN(AME), DIR(ECTORY), and DEL(ETE). The routines for the
internal commands are included in the transient part of COMMAND.COM.
External commands, sometimes called extrinsic commands or transient
programs, are the names of programs stored in disk files. Before these
programs can be executed, they must be loaded from the disk into the
transient program area (TPA) of memory. (See "How MS-DOS Is Loaded" in
this chapter.) Familiar examples of external commands are CHKDSK, BACKUP,
and RESTORE. As soon as an external command has completed its work, it is
discarded from memory; hence, it must be reloaded from disk each time it
is invoked.
Batch files are text files that contain lists of other intrinsic,
extrinsic, or batch commands. These files are processed by a special
interpreter that is built into the transient portion of COMMAND.COM. The
interpreter reads the batch file one line at a time and carries out each
of the specified operations in order.
In order to interpret a user's command, COMMAND.COM first looks to see if
the user typed the name of a built-in (intrinsic) command that it can
carry out directly. If not, it searches for an external command
(executable program file) or batch file by the same name. The search is
carried out first in the current directory of the current disk drive and
then in each of the directories specified in the most recent PATH command.
In each directory inspected, COMMAND.COM first tries to find a file with
the extension .COM, then .EXE, and finally .BAT. If the search fails for
all three file types in all of the possible locations, COMMAND.COM
displays the familiar message
Bad command or file name
If a .COM file or a .EXE file is found, COMMAND.COM uses the MS-DOS EXEC
function to load and execute it. The EXEC function builds a special data
structure called a program segment prefix (PSP) above the resident portion
of COMMAND.COM in the transient program area. The PSP contains various
linkages and pointers needed by the application program. Next, the EXEC
function loads the program itself, just above the PSP, and performs any
relocation that may be necessary. Finally, it sets up the registers
appropriately and transfers control to the entry point for the program.
(Both the PSP and the EXEC function will be discussed in more detail in
Chapters 3 and 12.) When the transient program has finished its job, it
calls a special MS-DOS termination function that releases the transient
program's memory and returns control to the program that caused the
transient program to be loaded (COMMAND.COM, in this case).
A transient program has nearly complete control of the system's resources
while it is executing. The only other tasks that are accomplished are
those performed by interrupt handlers (such as the keyboard input driver
and the real-time clock) and operations that the transient program
requests from the operating system. MS-DOS does not support sharing of the
central processor among several tasks executing concurrently, nor can it
wrest control away from a program when it crashes or executes for too
long. Such capabilities are the province of MS OS/2, which is a
protected-mode system with preemptive multitasking (time-slicing).
How MS-DOS Is Loaded
When the system is started or reset, program execution begins at address
0FFFF0H. This is a feature of the 8086/8088 family of microprocessors and
has nothing to do with MS-DOS. Systems based on these processors are
designed so that address 0FFFF0H lies within an area of ROM and contains a
jump machine instruction to transfer control to system test code and the
ROM bootstrap routine (Figure 2-1).
The ROM bootstrap routine reads the disk bootstrap routine from the first
sector of the system startup disk (the boot sector) into memory at some
arbitrary address and then transfers control to it (Figure 2-2). (The
boot sector also contains a table of information about the disk format.)
The disk bootstrap routine checks to see if the disk contains a copy of
MS-DOS. It does this by reading the first sector of the root directory and
determining whether the first two files are IO.SYS and MSDOS.SYS (or
IBMBIO.COM and IBMDOS.COM), in that order. If these files are not present,
the user is prompted to change disks and strike any key to try again.
┌───────────────────────────────────────────────┐
│ ROM bootstrap routine │
├───────────────────────────────────────────────┤
│ │
├───────────────────────────────────────────────┤ ◄ Top of RAM
│ │
│ │
└──────────────────────┐ │
┌────────────────────┐ └────────────────────────┘
│ └──────────────────────────┐
│ │
│ │
│ │
00400H ├───────────────────────────────────────────────┤
│ Interrupt vectors │
00000H └───────────────────────────────────────────────┘
Figure 2-1. A typical 8086/8088-based computer system immediately after
system startup or reset. Execution begins at location 0FFFF0H, which
contains a jump instruction that directs program control to the ROM
bootstrap routine.
┌───────────────────────────────────────────────┐
│ ROM bootstrap routine │
├───────────────────────────────────────────────┤
│ │
├───────────────────────────────────────────────┤ ◄ Top of RAM
│ │
├───────────────────────────────────────────────┤
│ Disk bootstrap routine │
├───────────────────────────────────────────────┤ ◄ Arbitrary
│ │ load location
│ │
└──────────────────────┐ │
┌────────────────────┐ └────────────────────────┘
│ └──────────────────────────┐
│ │
│ │
00400H ├───────────────────────────────────────────────┤
│ Interrupt vectors │
00000H └───────────────────────────────────────────────┘
Figure 2-2. The ROM bootstrap routine loads the disk bootstrap routine
into memory from the first sector of the system startup disk and then
transfers control to it.
If the two system files are found, the disk bootstrap reads them into
memory and transfers control to the initial entry point of IO.SYS (Figure
2-3). (In some implementations, the disk bootstrap reads only IO.SYS into
memory, and IO.SYS in turn loads the MSDOS.SYS file.)
The IO.SYS file that is loaded from the disk actually consists of two
separate modules. The first is the BIOS, which contains the linked set of
resident device drivers for the console, auxiliary port, printer, block,
and clock devices, plus some hardware-specific initialization code that is
run only at system startup. The second module, SYSINIT, is supplied by
Microsoft and linked into the IO.SYS file, along with the BIOS, by the
computer manufacturer.
SYSINIT is called by the manufacturer's BIOS initialization code. It
determines the amount of contiguous memory present in the system and then
relocates itself to high memory. Then it moves the DOS kernel, MSDOS.SYS,
from its original load location to its final memory location, overlaying
the original SYSINIT code and any other expendable initialization code
that was contained in the IO.SYS file (Figure 2-4).
Next, SYSINIT calls the initialization code in MSDOS.SYS. The DOS kernel
initializes its internal tables and work areas, sets up the interrupt
vectors 20H through 2FH, and traces through the linked list of resident
device drivers, calling the initialization function for each. (See Chapter
14.)
┌───────────────────────────────────────────────┐
│ ROM bootstrap routine │
├───────────────────────────────────────────────┤
│ │
├───────────────────────────────────────────────┤ ◄ Top of RAM
│ │
├───────────────────────────────────────────────┤
│ Disk bootstrap routine │
├───────────────────────────────────────────────┤
│ │
└──────────────────────┐ │
┌────────────────────┐ └────────────────────────┘
│ └──────────────────────────┐
│ │
├───────────────────────────────────────────────┤
│ DOS kernel (from MSDOS.SYS) │
├───────────────────────────────────────────────┤ ◄ In temporary
│ SYSINIT (from IO.SYS) │ location
├───────────────────────────────────────────────┤
│ BIOS (from IO.SYS) │
├───────────────────────────────────────────────┤
│ │
00400H ├───────────────────────────────────────────────┤
│ Interrupt vectors │
00000H └───────────────────────────────────────────────┘
Figure 2-3. The disk bootstrap reads the file IO.SYS into memory. This
file contains the MS-DOS BIOS (resident device drivers) and the SYSINIT
module. Either the disk bootstrap or the BIOS (depending upon the
manufacturer's implementation) then reads the DOS kernel into memory from
the MSDOS.SYS file.
These driver functions determine the equipment status, perform any
necessary hardware initialization, and set up the vectors for any external
hardware interrupts the drivers will service.
As part of the initialization sequence, the DOS kernel examines the
disk-parameter blocks returned by the resident block-device drivers,
determines the largest sector size that will be used in the system, builds
some drive-parameter blocks, and allocates a disk sector buffer. Control
then returns to SYSINIT.
When the DOS kernel has been initialized and all resident device drivers
are available, SYSINIT can call on the normal MS-DOS file services to open
the CONFIG.SYS file. This optional file can contain a variety of commands
that enable the user to customize the MS-DOS environment. For instance,
the user can specify additional hardware device drivers, the number of
disk buffers, the maximum number of files that can be open at one time,
and the filename of the command processor (shell).
If it is found, the entire CONFIG.SYS file is loaded into memory for
processing. All lowercase characters are converted to uppercase, and the
file is interpreted one line at a time to process the commands. Memory is
allocated for the disk buffer cache and the internal file control blocks
used by the handle file and record system functions. (See Chapter 8.) Any
device drivers indicated in the CONFIG.SYS file are sequentially loaded
into memory, initialized by calls to their init modules, and linked into
the device-driver list. The init function of each driver tells SYSINIT how
much memory to reserve for that driver.
┌───────────────────────────────────────────────┐
│ ROM bootstrap routine │
├───────────────────────────────────────────────┤
│ │
├───────────────────────────────────────────────┤ ◄ Top of RAM
│ SYSINIT module │
├───────────────────────────────────────────────┤
│ │
└──────────────────────┐ │
┌────────────────────┐ └────────────────────────┘
│ └──────────────────────────┐
│ │
├───────────────────────────────────────────────┤
│ Installable drivers │
├───────────────────────────────────────────────┤
│ File control blocks │
├───────────────────────────────────────────────┤
│ Disk buffer cache │
├───────────────────────────────────────────────┤
│ DOS kernel │
├───────────────────────────────────────────────┤ ◄ In final
│ BIOS │ location
├───────────────────────────────────────────────┤
│ │
├───────────────────────────────────────────────┤
00400H ├───────────────────────────────────────────────┤
│ Interrupt vectors │
00000H └───────────────────────────────────────────────┘
Figure 2-4. SYSINIT moves itself to high memory and relocates the DOS
kernel, MSDOS.SYS, downward to its final address. The MS-DOS disk buffer
cache and file control block areas are allocated, and then the installable
device drivers specified in the CONFIG.SYS file are loaded and linked into
the system.
After all installable device drivers have been loaded, SYSINIT closes all
file handles and reopens the console (CON), printer (PRN), and auxiliary
(AUX) devices as the standard input, standard output, standard error,
standard list, and standard auxiliary devices. This allows a
user-installed character-device driver to override the BIOS's resident
drivers for the standard devices.
Finally, SYSINIT calls the MS-DOS EXEC function to load the command
interpreter, or shell. (The default shell is COMMAND.COM, but another
shell can be substituted by means of the CONFIG.SYS file.) Once the shell
is loaded, it displays a prompt and waits for the user to enter a command.
MS-DOS is now ready for business, and the SYSINIT module is discarded
(Figure 2-5).
┌───────────────────────────────────────────────┐
│ ROM bootstrap routine │
├───────────────────────────────────────────────┤
│ │
├───────────────────────────────────────────────┤ ◄ Top of RAM
│ Transient part of COMMAND.COM │
├───────────────────────────────────────────────┤
└──────────────────────┐ │
┌────────────────────┐ └────────────────────────┘
│ └──────────────────────────┐
│ Transient program area │
├───────────────────────────────────────────────┤
│ Resident part of COMMAND.COM │
├───────────────────────────────────────────────┤
│ Installable drivers │
├───────────────────────────────────────────────┤
│ File control blocks │
├───────────────────────────────────────────────┤
│ Disk buffer cache │
├───────────────────────────────────────────────┤
│ DOS kernel │
├───────────────────────────────────────────────┤
│ BIOS │
├───────────────────────────────────────────────┤
│ │
00400H ├───────────────────────────────────────────────┤
│ Interrupt vectors │
00000H └───────────────────────────────────────────────┘
Figure 2-5. The final result of the MS-DOS startup process for a typical
system. The resident portion of COMMAND.COM lies in low memory, above the
DOS kernel. The transient portion containing the batch-file interpreter
and intrinsic commands is placed in high memory, where it can be overlaid
by extrinsic commands and application programs running in the transient
program area.
────────────────────────────────────────────────────────────────────────────
Chapter 3 Structure of MS-DOS Application Programs
Programs that run under MS-DOS come in two basic flavors: .COM programs,
which have a maximum size of approximately 64 KB, and .EXE programs, which
can be as large as available memory. In Intel 8086 parlance, .COM programs
fit the tiny model, in which all segment registers contain the same value;
that is, the code and data are mixed together. In contrast, .EXE programs
fit the small, medium, or large model, in which the segment registers
contain different values; that is, the code, data, and stack reside in
separate segments. .EXE programs can have multiple code and data segments,
which are respectively addressed by long calls and by manipulation of the
data segment (DS) register.
A .COM-type program resides on the disk as an absolute memory image, in a
file with the extension .COM. The file does not have a header or any other
internal identifying information. A .EXE program, on the other hand,
resides on the disk in a special type of file with a unique header, a
relocation map, a checksum, and other information that is (or can be) used
by MS-DOS.
Both .COM and .EXE programs are brought into memory for execution by the
same mechanism: the EXEC function, which constitutes the MS-DOS loader.
EXEC can be called with the filename of a program to be loaded by
COMMAND.COM (the normal MS-DOS command interpreter), by other shells or
user interfaces, or by another program that was previously loaded by EXEC.
If there is sufficient free memory in the transient program area, EXEC
allocates a block of memory to hold the new program, builds the program
segment prefix (PSP) at its base, and then reads the program into memory
immediately above the PSP. Finally, EXEC sets up the segment registers and
the stack and transfers control to the program.
When it is invoked, EXEC can be given the addresses of additional
information, such as a command tail, file control blocks, and an
environment block; if supplied, this information will be passed on to the
new program. (The exact procedure for using the EXEC function in your own
programs is discussed, with examples, in Chapter 12.)
.COM and .EXE programs are often referred to as transient programs. A
transient program "owns" the memory block it has been allocated and has
nearly total control of the system's resources while it is executing. When
the program terminates, either because it is aborted by the operating
system or because it has completed its work and systematically performed a
final exit back to MS-DOS, the memory block is then freed (hence the term
transient) and can be used by the next program in line to be loaded.
The Program Segment Prefix
A thorough understanding of the program segment prefix is vital to
successful programming under MS-DOS. It is a reserved area, 256 bytes
long, that is set up by MS-DOS at the base of the memory block allocated
to a transient program. The PSP contains some linkages to MS-DOS that can
be used by the transient program, some information MS-DOS saves for its
own purposes, and some information MS-DOS passes to the transient
program──to be used or not, as the program requires (Figure 3-1).
Offset
0000H ┌────────────────────────────────────────────────────────┐
│ Int 20H │
0002H ├────────────────────────────────────────────────────────┤
│ Segment, end of allocation block │
0004H ├────────────────────────────────────────────────────────┤
│ Reserved │
0005H ├────────────────────────────────────────────────────────┤
│ Long call to MS-DOS function dispatcher │
000AH ├────────────────────────────────────────────────────────┤
│ Previous contents of termination handler │
│ interrupt vector (Int 22H) │
000EH ├────────────────────────────────────────────────────────┤
│ Previous contents of Ctrl-C interrupt vector (Int 23H) │
0012H ├────────────────────────────────────────────────────────┤
│ Previous contents of critical-error handler │
│ interrupt vector (Int 24H) │
0016H ├────────────────────────────────────────────────────────┤
│ Reserved │
002CH ├────────────────────────────────────────────────────────┤
│ Segment address of environment block │
002EH ├────────────────────────────────────────────────────────┤
│ Reserved │
005CH ├────────────────────────────────────────────────────────┤
│ Default file control block #1 │
006CH ├────────────────────────────────────────────────────────┤
│ Default file control block #2 │
│ (overlaid if FCB #1 opened) │
008OH ├────────────────────────────────────────────────────────┤
└──────────────────────────┐ │
┌────────────────────────┐ └─────────────────────────────┘
│ └───────────────────────────────┐
│ Command tail and default disk transfer area (buffer) │
OOFFH └────────────────────────────────────────────────────────┘
Figure 3-1. The structure of the program segment prefix.
In the first versions of MS-DOS, the PSP was designed to be compatible
with a control area that was built beneath transient programs under
Digital Research's venerable CP/M operating system, so that programs could
be ported to MS-DOS without extensive logical changes. Although MS-DOS has
evolved considerably since those early days, the structure of the PSP is
still recognizably similar to its CP/M equivalent. For example, offset
0000H in the PSP contains a linkage to the MS-DOS process-termination
handler, which cleans up after the program has finished its job and
performs a final exit. Similarly, offset 0005H in the PSP contains a
linkage to the MS-DOS function dispatcher, which performs disk operations,
console input/output, and other such services at the request of the
transient program. Thus, calls to PSP:0000 and PSP:0005 have the same
effect as CALL 0000 and CALL 0005 under CP/M. (These linkages are not the
"approved" means of obtaining these services, however.)
The word at offset 0002H in the PSP contains the segment address of the
top of the transient program's allocated memory block. The program can use
this value to determine whether it should request more memory to do its
job or whether it has extra memory that it can release for use by other
processes.
Offsets 000AH through 0015H in the PSP contain the previous contents of
the interrupt vectors for the termination, Ctrl-C, and critical-error
handlers. If the transient program alters these vectors for its own
purposes, MS-DOS restores the original values saved in the PSP when the
program terminates.
The word at PSP offset 002CH holds the segment address of the environment
block, which contains a series of ASCIIZ strings (sequences of ASCII
characters terminated by a null, or zero, byte). The environment block is
inherited from the program that called the EXEC function to load the
currently executing program. It contains such information as the current
search path used by COMMAND.COM to find executable programs, the location
on the disk of COMMAND.COM itself, and the format of the user prompt used
by COMMAND.COM.
The command tail──the remainder of the command line that invoked the
transient program, after the program's name──is copied into the PSP
starting at offset 0081H. The length of the command tail, not including
the return character at its end, is placed in the byte at offset 0080H.
Redirection or piping parameters and their associated filenames do not
appear in the portion of the command line (the command tail) that is
passed to the transient program, because redirection is transparent to
applications.
To provide compatibility with CP/M, MS-DOS parses the first two parameters
in the command tail into two default file control blocks (FCBs) at
PSP:005CH and PSP:006CH, under the assumption that they may be filenames.
However, if the parameters are filenames that include a path
specification, only the drive code will be valid in these default FCBs,
because FCB-type file- and record-access functions do not support
hierarchical file structures. Although the default FCBs were an aid in
earlier years, when compatibility with CP/M was more of a concern, they
are essentially useless in modern MS-DOS application programs that must
provide full path support. (File control blocks are discussed in detail in
Chapter 8 and hierarchical file structures are discussed in Chapter 9.)
The 128-byte area from 0080H through 00FFH in the PSP also serves as the
default disk transfer area (DTA), which is set by MS-DOS before passing
control to the transient program. If the program does not explicitly
change the DTA, any file read or write operations requested with the FCB
group of function calls automatically use this area as a data buffer. This
is rarely useful and is another facet of MS-DOS's handling of the PSP that
is present only for compatibility with CP/M.
──────────────────────────────────────────────────────────────────────────
WARNING
Programs must not alter any part of the PSP below offset 005CH.
──────────────────────────────────────────────────────────────────────────
Introduction to .COM Programs
Programs of the .COM persuasion are stored in disk files that hold an
absolute image of the machine instructions to be executed. Because the
files contain no relocation information, they are more compact, and are
loaded for execution slightly faster, than equivalent .EXE files. Note
that MS-DOS does not attempt to ascertain whether a .COM file actually
contains executable code (there is no signature or checksum, as in the
case of a .EXE file); it simply brings any file with the .COM extension
into memory and jumps to it.
Because .COM programs are loaded immediately above the program segment
prefix and do not have a header that can specify another entry point, they
must always have an origin of 0100H, which is the length of the PSP.
Location 0100H must contain an executable instruction. The maximum length
of a .COM program is 65,536 bytes, minus the length of the PSP (256 bytes)
and a mandatory word of stack (2 bytes).
When control is transferred to the .COM program from MS-DOS, all of the
segment registers point to the PSP (Figure 3-2). The stack pointer
register contains 0FFFEH if memory allows; otherwise, it is set as high as
possible in memory minus 2 bytes. (MS-DOS pushes a zero word on the stack
before entry.)
SS:SP ┌────────────────────────────────────────────────────────┐
│ │
│ Stack grows downward from top of segment │
│ │ │
│ ▼ │
│ │
│ │ │
│ Program code and data │
│ │
CS:0100H ├────────────────────────────────────────────────────────┤
│ Program segment prefix │
CS:0000H └────────────────────────────────────────────────────────┘
DS:0000H
ES:0000H
SS:0000H
Figure 3-2. A memory image of a typical .COM-type program after loading.
The contents of the .COM file are brought into memory just above the
program segment prefix. Program, code, and data are mixed together in the
same segment, and all segment registers contain the same value.
Although the size of an executable .COM file can't exceed 64 KB, the
current versions of MS-DOS allocate all of the transient program area to
.COM programs when they are loaded. Because many such programs date from
the early days of MS-DOS and are not necessarily "well-behaved" in their
approach to memory management, the operating system simply makes the
worst-case assumption and gives .COM programs everything that is
available. If a .COM program wants to use the EXEC function to invoke
another process, it must first shrink down its memory allocation to the
minimum memory it needs in order to continue, taking care to protect its
stack. (This is discussed in more detail in Chapter 12.)
When a .COM program finishes executing, it can return control to MS-DOS by
several means. The preferred method is Int 21H Function 4CH, which allows
the program to pass a return code back to the program, shell, or batch
file that invoked it. However, if the program is running under MS-DOS
version 1, it must exit by means of Int 20H, Int 21H Function 0, or a
NEAR RETURN. (Because a word of zero was pushed onto the stack at entry, a
NEAR RETURN causes a transfer to PSP:0000, which contains an Int 20H
instruction.)
A .COM-type application can be linked together from many separate object
modules. All of the modules must use the same code-segment name and class
name, and the module with the entry point at offset 0100H within the
segment must be linked first. In addition, all of the procedures within a
.COM program should have the NEAR attribute, because all executable code
resides in one segment.
When linking a .COM program, the linker will display the message
Warning: no stack segment
This message can be ignored. The linker output is a .EXE file, which must
be converted into a .COM file with the MS-DOS EXE2BIN utility before
execution. You can then delete the .EXE file. (An example of this process
is provided in Chapter 4.)
An Example .COM Program
The HELLO.COM program listed in Figure 3-3 demonstrates the structure of
a simple assembly-language program that is destined to become a .COM file.
(You may find it helpful to compare this listing with the HELLO.EXE
program later in this chapter.) Because this program is so short and
simple, a relatively high proportion of the source code is actually
assembler directives that do not result in any executable code.
The NAME statement simply provides a module name for use during the
linkage process. This aids understanding of the map that the linker
produces. In MASM versions 5.0 and later, the module name is always the
same as the filename, and the NAME statement is ignored.
The PAGE command, when used with two operands, as in line 2, defines the
length and width of the page. These default respectively to 66 lines and
80 characters. If you use the PAGE command without any operands, a
formfeed is sent to the printer and a heading is printed. In larger
programs, use the PAGE command liberally to place each of your subroutines
on separate pages for easy reading.
The TITLE command, in line 3, specifies the text string (limited to 60
characters) that is to be printed at the upper left corner of each page.
The TITLE command is optional and cannot be used more than once in each
assembly-language source file.
──────────────────────────────────────────────────────────────────────────
1: name hello
2: page 55,132
3: title HELLO.COM--print hello on terminal
4:
5: ;
6: ; HELLO.COM: demonstrates various components
7: ; of a functional .COM-type assembly-
8: ; language program, and an MS-DOS
9: ; function call.
10: ;
11: ; Ray Duncan, May 1988
12: ;
13:
14: stdin equ 0 ; standard input handle
15: stdout equ 1 ; standard output handle
16: stderr equ 2 ; standard error handle
17:
18: cr equ 0dh ; ASCII carriage return
19: lf equ 0ah ; ASCII linefeed
20:
21:
22: _TEXT segment word public 'CODE'
23:
24: org 100h ; .COM files always have
25: ; an origin of 100h
26:
27: assume cs:_TEXT,ds:_TEXT,es:_TEXT,ss:_TEXT
28:
29: print proc near ; entry point from MS-DOS
30:
31: mov ah,40h ; function 40h = write
32: mov bx,stdout ; handle for standard output
33: mov cx,msg_len ; length of message
34: mov dx,offset msg ; address of message
35: int 21h ; transfer to MS-DOS
36:
37: mov ax,4c00h ; exit, return code = 0
38: int 21h ; transfer to MS-DOS
39:
40: print endp
41:
42:
43: msg db cr,lf ; message to display
44: db 'Hello World!',cr,lf
45:
46: msg_len equ $-msg ; length of message
47:
48:
49: _TEXT ends
50:
51: end print ; defines entry point
──────────────────────────────────────────────────────────────────────────
Figure 3-3. The HELLO.COM program listing.
Dropping down past a few comments and EQU statements, we come to a
declaration of a code segment that begins in line 22 with a SEGMENT
command and ends in line 49 with an ENDS command. The label in the
leftmost field of line 22 gives the code segment the name _TEXT. The
operand fields at the right end of the line give the segment the
attributes WORD, PUBLIC, and `CODE'. (You might find it helpful to read
the Microsoft Macro Assembler manual for detailed explanations of each
possible segment attribute.)
Because this program is going to be converted into a .COM file, all of its
executable code and data areas must lie within one code segment. The
program must also have its origin at offset 0100H (immediately above the
program segment prefix), which is taken care of by the ORG statement
in line 24.
Following the ORG instruction, we encounter an ASSUME statement on line
27. The concept of ASSUME often baffles new assembly-language programmers.
In a way, ASSUME doesn't "do" anything; it simply tells the assembler
which segment registers you are going to use to point to the various
segments of your program, so that the assembler can provide segment
overrides when they are necessary. It's important to notice that the
ASSUME statement doesn't take care of loading the segment registers with
the proper values; it merely notifies the assembler of your intent to do
that within the program. (Remember that, in the case of a .COM program,
MS-DOS initializes all the segment registers before entry to point to the
PSP.)
Within the code segment, we come to another type of block declaration that
begins with the PROC command on line 29 and closes with ENDP on line 40.
These two instructions declare the beginning and end of a procedure, a
block of executable code that performs a single distinct function. The
label in the leftmost field of the PROC statement (in this case, print)
gives the procedure a name. The operand field gives it an attribute. If
the procedure carries the NEAR attribute, only other code in the same
segment can call it, whereas if it carries the FAR attribute, code located
anywhere in the CPU's memory-addressing space can call it. In .COM
programs, all procedures carry the NEAR attribute.
For the purposes of this example program, I have kept the print procedure
ridiculously simple. It calls MS-DOS Int 21H Function 40H to send the
message Hello World! to the video screen, and calls Int 21H Function 4CH
to terminate the program.
The END statement in line 51 tells the assembler that it has reached the
end of the source file and also specifies the entry point for the program.
If the entry point is not a label located at offset 0100H, the .EXE file
resulting from the assembly and linkage of this source program cannot be
converted into a .COM file.
Introduction to .EXE Programs
We have just discussed a program that was written in such a way that it
could be assembled into a .COM file. Such a program is simple in
structure, so a programmer who needs to put together this kind of quick
utility can concentrate on the program logic and do a minimum amount of
worrying about control of the assembler. However, .COM-type programs have
some definite disadvantages, and so most serious assembly-language efforts
for MS-DOS are written to be converted into .EXE files.
Although .COM programs are effectively restricted to a total size of 64 KB
for machine code, data, and stack combined, .EXE programs can be
practically unlimited in size (up to the limit of the computer's available
memory). .EXE programs also place the code, data, and stack in separate
parts of the file. Although the normal MS-DOS program loader does not take
advantage of this feature of .EXE files, the ability to load different
parts of large programs into several separate memory fragments, as well as
the opportunity to designate a "pure" code portion of your program that
can be shared by several tasks, is very significant in multitasking
environments such as Microsoft Windows.
The MS-DOS loader always brings a .EXE program into memory immediately
above the program segment prefix, although the order of the code, data,
and stack segments may vary (Figure 3-4). The .EXE file has a header, or
block of control information, with a characteristic format (Figures 3-5
and 3-6). The size of this header varies according to the number of
program instructions that need to be relocated at load time, but it is
always a multiple of 512 bytes.
Before MS-DOS transfers control to the program, the initial values of the
code segment (CS) register and instruction pointer (IP) register are
calculated from the entry-point information in the .EXE file header and
the program's load address. This information derives from an END statement
in the source code for one of the program's modules. The data segment (DS)
and extra segment (ES) registers are made to point to the PSP so that the
program can access the environment-block pointer, command tail, and other
useful information contained there.
SS:SP ┌────────────────────────────────────────────────────────┐
│ │
│ Stack segment: │
│ stack grows downward from top of segment │
│ │ │
│ ▼ │
SS:0000H ├────────────────────────────────────────────────────────┤
│ Data segment │
├────────────────────────────────────────────────────────┤
│ Program code │
CS:0000H ├────────────────────────────────────────────────────────┤
│ Program segment prefix │
DS:0000H └────────────────────────────────────────────────────────┘
ES:0000H
Figure 3-4. A memory image of a typical .EXE-type program immediately
after loading. The contents of the .EXE file are relocated and brought
into memory above the program segment prefix. Code, data, and stack reside
in separate segments and need not be in the order shown here. The entry
point can be anywhere in the code segment and is specified by the END
statement in the main module of the program. When the program receives
control, the DS (data segment) and ES (extra segment) registers point to
the program segment prefix; the program usually saves this value and then
resets the DS and ES registers to point to its data area.
The initial contents of the stack segment (SS) and stack pointer (SP)
registers come from the header. This information derives from the
declaration of a segment with the attribute STACK somewhere in the
program's source code. The memory space allocated for the stack may be
initialized or uninitialized, depending on the stack-segment definition;
many programmers like to initialize the stack memory with a recognizable
data pattern so that they can inspect memory dumps and determine how much
stack space is actually used by the program.
When a .EXE program finishes processing, it should return control to
MS-DOS through Int 21H Function 4CH. Other methods are available, but
they offer no advantages and are considerably less convenient (because
they usually require the CS register to point to the PSP).
Byte
offset
0000H ┌────────────────────────────────────────────────────────┐
│ First of .EXE file signature (4DH) │
0001H ├────────────────────────────────────────────────────────┤
│ Second part of .EXE file signature (5AH) │
0002H ├────────────────────────────────────────────────────────┤
│ Length of file MOD 512 │
0004H ├────────────────────────────────────────────────────────┤
│ Size of file in 512-byte pages, including header │
0006H ├────────────────────────────────────────────────────────┤
│ Number of relocation-table items │
0008H ├────────────────────────────────────────────────────────┤
│ Size of header in paragraphs (16-byte units) │
000AH ├────────────────────────────────────────────────────────┤
│ Minimum number of paragraphs needed above program │
000CH ├────────────────────────────────────────────────────────┤
│ Maximum number of paragraphs desired above program │
000EH ├────────────────────────────────────────────────────────┤
│ Segment displacement of stack module │
0010H ├────────────────────────────────────────────────────────┤
│ Contents of SP register at entry │
0012H ├────────────────────────────────────────────────────────┤
│ Word checksum │
0014H ├────────────────────────────────────────────────────────┤
│ Contents of IP register at entry │
0016H ├────────────────────────────────────────────────────────┤
│ Segment displacement of code module │
0018H ├────────────────────────────────────────────────────────┤
│ Offset of first relocation item in file │
001AH ├────────────────────────────────────────────────────────┤
│ Overlay number (0 for resident part of program) │
001BH ├────────────────────────────────────────────────────────┤
│ Variable reserved space │
├────────────────────────────────────────────────────────┤
│ Relocation table │
├────────────────────────────────────────────────────────┤
│ Variable reserved space │
├────────────────────────────────────────────────────────┤
│ Program and data segments │
├────────────────────────────────────────────────────────┤
│ Stack segment │
└────────────────────────────────────────────────────────┘
Figure 3-5. The format of a .EXE load module.
The input to the linker for a .EXE-type program can be many separate
object modules. Each module can use a unique code-segment name, and the
procedures can carry either the NEAR or the FAR attribute, depending on
naming conventions and the size of the executable code. The programmer
must take care that the modules linked together contain only one segment
with the STACK attribute and only one entry point defined with an END
assembler directive. The output from the linker is a file with a .EXE
extension. This file can be executed immediately.
──────────────────────────────────────────────────────────────────────────
C>DUMP HELLO.EXE
0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 4D 5A 28 00 02 00 01 00 20 00 09 00 FF FF 03 00 MZ(..... .......
0010 80 00 20 05 00 00 00 00 1E 00 00 00 01 00 01 00 .. .............
0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
.
.
.
0200 B8 01 00 8E D8 B4 40 BB 01 00 B9 10 00 90 BA 08 ......@.........
0210 00 CD 21 B8 00 4C CD 21 0D 0A 48 65 6C 6C 6F 20 ..!..L.!..Hello
0220 57 6F 72 6C 64 21 0D 0A World!..
──────────────────────────────────────────────────────────────────────────
Figure 3-6. A hex dump of the HELLO.EXE program, demonstrating the
contents of a simple .EXE load module. Note the following interesting
values: the .EXE signature in bytes 0000H and 0001H, the number of
relocation-table items in bytes 0006H and 0007H, the minimum extra memory
allocation (MIN_ALLOC) in bytes 000AH and 000BH, the maximum extra memory
allocation (MAX_ALLOC) in bytes 000CH and 000DH, and the initial IP
(instruction pointer) register value in bytes 0014H and 0015H. See also
Figure 3-5.
An Example .EXE Program
The HELLO.EXE program in Figure 3-7 demonstrates the fundamental
structure of an assembly-language program that is destined to become a
.EXE file. At minimum, it should have a module name, a code segment, a
stack segment, and a primary procedure that receives control of the
computer from MS-DOS after the program is loaded. The HELLO.EXE program
also contains a data segment to provide a more complete example.
The NAME, TITLE, and PAGE directives were covered in the HELLO.COM example
program and are used in the same manner here, so we'll move to the first
new item of interest. After a few comments and EQU statements, we come to
a declaration of a code segment that begins on line 21 with a SEGMENT
command and ends on line 41 with an ENDS command. As in the HELLO.COM
example program, the label in the leftmost field of the line gives the
code segment the name _TEXT. The operand fields at the right end of the
line give the attributes WORD, PUBLIC, and `CODE'.
Following the code-segment instruction, we find an ASSUME statement on
line 23. Notice that, unlike the equivalent statement in the HELLO.COM
program, the ASSUME statement in this program specifies several different
segment names. Again, remember that this statement has no direct effect on
the contents of the segment registers but affects only the operation of
the assembler itself.
──────────────────────────────────────────────────────────────────────────
1: name hello
2: page 55,132
3: title HELLO.EXE--print Hello on terminal
4: ;
5: ; HELLO.EXE: demonstrates various components
6: ; of a functional .EXE-type assembly-
7: ; language program, use of segments,
8: ; and an MS-DOS function call.
9: ;
10: ; Ray Duncan, May 1988
11: ;
12:
13: stdin equ 0 ; standard input handle
14: stdout equ 1 ; standard output handle
15: stderr equ 2 ; standard error handle
16:
17: cr equ 0dh ; ASCII carriage return
18: lf equ 0ah ; ASCII linefeed
19:
20:
21: _TEXT segment word public 'CODE'
22:
23: assume cs:_TEXT,ds:_DATA,ss:STACK
24:
25: print proc far ; entry point from MS-DOS
26:
27: mov ax,_DATA ; make our data segment
28: mov ds,ax ; addressable...
29:
30: mov ah,40h ; function 40h = write
31: mov bx,stdout ; standard output handle
32: mov cx,msg_len ; length of message
33: mov dx,offset msg ; address of message
34: int 21h ; transfer to MS-DOS
35:
36: mov ax,4c00h ; exit, return code = 0
37: int 21h ; transfer to MS-DOS
38:
39: print endp
40:
41: _TEXT ends
42:
43:
44: _DATA segment word public 'DATA'
45:
46: msg db cr,lf ; message to display
47: db 'Hello World!',cr,lf
48:
49: msg_len equ $-msg ; length of message
50:
51: _DATA ends
52:
53:
54: STACK segment para stack `STACK'
55:
56: db 128 dup (?)
57:
58: STACK ends
59:
60: end print ; defines entry point
──────────────────────────────────────────────────────────────────────────
Figure 3-7. The HELLO.EXE program listing.
Within the code segment, the main print procedure is declared by the PROC
command on line 25 and closed with ENDP on line 39. Because the procedure
resides in a .EXE file, we have given it the FAR attribute as an example,
but the attribute is really irrelevant because the program is so small and
the procedure is not called by anything else in the same program.
The print procedure first initializes the DS register, as indicated in the
earlier ASSUME statement, loading it with a value that causes it to point
to the base of the data area. (MS-DOS automatically sets up the CS and SS
registers.) Next, the procedure uses MS-DOS Int 21H Function 40H to
display the message Hello World! on the screen, just as in the HELLO.COM
program. Finally, the procedure exits back to MS-DOS with an Int 21H
Function 4CH on lines 36 and 37, passing a return code of zero (which by
convention means a success).
Lines 44 through 51 declare a data segment named _DATA, which contains the
variables and constants the program will use. If the various modules of a
program contain multiple data segments with the same name, the linker will
collect them and place them in the same physical memory segment.
Lines 54 through 58 establish a stack segment; PUSH and POP instructions
will access this area of scratch memory. Before MS-DOS transfers control
to a .EXE program, it sets up the SS and SP registers according to the
declared size and location of the stack segment. Be sure to allow enough
room for the maximum stack depth that can occur at runtime, plus a safe
number of extra words for registers pushed onto the stack during an MS-DOS
service call. If the stack overflows, it may damage your other code and
data segments and cause your program to behave strangely or even to crash
altogether!
The END statement on line 60 winds up our brief HELLO.EXE program, telling
the assembler that it has reached the end of the source file and providing
the label of the program's point of entry from MS-DOS.
The differences between .COM and .EXE programs are summarized in Figure
3-8.
╓┌─┌──────────────────┌──────────────────────────┌───────────────────────────╖
.COM program .EXE program
──────────────────────────────────────────────────────────────────────────
Maximum size 65,536 bytes minus 256 No limit
bytes for PSP and 2 bytes
for stack
Entry point PSP:0100H Defined by END statement
AL at entry 00H if default FCB #1 has Same
valid drive, 0FFH if
invalid drive
.COM program .EXE program
──────────────────────────────────────────────────────────────────────────
AH at entry 00H if default FCB #2 has Same
valid drive, 0FFH if
invalid drive
CS at entry PSP Segment containing module
with entry point
IP at entry 0100H Offset of entry point within
its segment
DS at entry PSP PSP
ES at entry PSP PSP
SS at entry PSP Segment with STACK attribute
SP at entry 0FFFEH or top word in Size of segment defined with
available memory, STACK attribute
.COM program .EXE program
──────────────────────────────────────────────────────────────────────────
available memory, STACK attribute
whichever is lower
Stack at entry Zero word Initialized or uninitialized
Stack size 65,536 bytes minus 256 Defined in segment with
bytes for PSP and size of STACK attribute
executable code and data
Subroutine calls Usually NEAR NEAR or FAR
Exit method Int 21H Function 4CH Int 21H Function 4CH
preferred, NEAR RET if preferred
MS-DOS version 1
Size of file Exact size of program Size of program plus header
(multiple of 512 bytes)
──────────────────────────────────────────────────────────────────────────
.COM program .EXE program
──────────────────────────────────────────────────────────────────────────
Figure 3-8. Summary of the differences between .COM and .EXE programs,
including their entry conditions.
More About Assembly-Language Programs
Now that we've looked at working examples of .COM and .EXE
assembly-language programs, let's backtrack and discuss their elements a
little more formally. The following discussion is based on the Microsoft
Macro Assembler, hereafter referred to as MASM. If you are familiar with
MASM and are an experienced assembly-language programmer, you may want to
skip this section.
MASM programs can be thought of as having three structural levels:
■ The module level
■ The segment level
■ The procedure level
Modules are simply chunks of source code that can be independently
maintained and assembled. Segments are physical groupings of like items
(machine code or data) within a program and a corresponding segregation of
dissimilar items. Procedures are functional subdivisions of an executable
program──routines that carry out a particular task.
Program Modules
Under MS-DOS, the module-level structure consists of files containing the
source code for individual routines. Each source file is translated by the
assembler into a relocatable object module. An object module can reside
alone in an individual file or with many other object modules in an
object-module library of frequently used or related routines. The
Microsoft Object Linker (LINK) combines object-module files, often with
additional object modules extracted from libraries, into an executable
program file.
Using modules and object-module libraries reduces the size of your
application source files (and vastly increases your productivity), because
these files need not contain the source code for routines they have in
common with other programs. This technique also allows you to maintain the
routines more easily, because you need to alter only one copy of their
source code stored in one place, instead of many copies stored in
different applications. When you improve (or fix) one of these routines,
you can simply reassemble it, put its object module back into the library,
relink all of the programs that use the routine, and voilga: instant
upgrade.
Program Segments
The term segments refers to two discrete programming concepts: physical
segments and logical segments.
Physical segments are 64 KB blocks of memory. The Intel 8086/8088 and
80286 microprocessors have four segment registers, which are essentially
used as pointers to these blocks. (The 80386 has six segment registers,
which are a superset of those found on the 8086/8088 and 80286.) Each
segment register can point to the bottom of a different 64 KB area of
memory. Thus, a program can address any location in memory by appropriate
manipulation of the segment registers, but the maximum amount of memory
that it can address simultaneously is 256 KB.
As we discussed earlier in the chapter, .COM programs assume that all four
segment registers always point to the same place──the bottom of the
program. Thus, they are limited to a maximum size of 64 KB. .EXE programs,
on the other hand, can address many different physical segments and can
reset the segment registers to point to each segment as it is needed.
Consequently, the only practical limit on the size of a .EXE program is
the amount of available memory. The example programs throughout the
remainder of this book focus on .EXE programs.
Logical segments are the program components. A minimum of three logical
segments must be declared in any .EXE program: a code segment, a data
segment, and a stack segment. Programs with more than 64 KB of code or
data have more than one code or data segment. The routines or data that
are used most frequently are put into the primary code and data segments
for speed, and routines or data that are used less frequently are put into
secondary code and data segments.
Segments are declared with the SEGMENT and ENDS directives in the
following form:
name SEGMENT attributes
.
.
.
name ENDS
The attributes of a segment include its align type (BYTE, WORD, or PARA),
combine type (PUBLIC, PRIVATE, COMMON, or STACK), and class type. The
segment attributes are used by the linker when it is combining logical
segments to create the physical segments of an executable program. Most of
the time, you can get by just fine using a small selection of attributes
in a rather stereotypical way. However, if you want to use the full range
of attributes, you might want to read the detailed explanation in the MASM
manual.
Programs are classified into one memory model or another based on the
number of their code and data segments. The most commonly used memory
model for assembly-language programs is the small model, which has one
code and one data segment, but you can also use the medium, compact, and
large models (Figure 3-9). (Two additional models exist with which we
will not be concerning ourselves further: the tiny model, which consists
of intermixed code and data in a single segment── for example, a .COM file
under MS-DOS; and the huge model, which is supported by the Microsoft C
Optimizing Compiler and which allows use of data structures larger than 64
KB.)
Model Code segments Data segments
──────────────────────────────────────────────────────────────────────────
Small One One
Medium Multiple One
Compact One Multiple
Large Multiple Multiple
──────────────────────────────────────────────────────────────────────────
Figure 3-9. Memory models commonly used in assembly-language and C
programs.
For each memory model, Microsoft has established certain segment and class
names that are used by all its high-level-language compilers (Figure
3-10). Because segment names are arbitrary, you may as well adopt the
Microsoft conventions. Their use will make it easier for you to integrate
your assembly-language routines into programs written in languages such as
C, or to use routines from high-level-language libraries in your
assembly-language programs.
Another important Microsoft high-level-language convention is to use the
GROUP directive to name the near data segment (the segment the program
expects to address with offsets from the DS register) and the stack
segment as members of DGROUP (the automatic data group), a special name
recognized by the linker and also by the program loaders in Microsoft
Windows and Microsoft OS/2. The GROUP directive causes logical segments
with different names to be combined into a single physical segment so that
they can be addressed using the same segment base address. In C programs,
DGROUP also contains the local heap, which is used by the C runtime
library for dynamic allocation of small amounts of memory.
╓┌─┌───────────┌────────────┌───────────┌───────────┌────────────┌───────────╖
Memory Segment Align Combine Class Group
model name type type type
──────────────────────────────────────────────────────────────────────────
Memory Segment Align Combine Class Group
model name type type type
──────────────────────────────────────────────────────────────────────────
Small _TEXT WORD PUBLIC CODE
_DATA WORD PUBLIC DATA DGROUP
STACK PARA STACK STACK DGROUP
Medium module_TEXT WORD PUBLIC CODE
. WORD PUBLIC DATA DGROUP
.
.
_DATA
STACK PARA STACK STACK DGROUP
Compact _TEXT WORD PUBLIC CODE
data PARA PRIVATE FAR_DATA
. WORD PUBLIC DATA DGROUP
.
.
_DATA
STACK PARA STACK STACK DGROUP
Memory Segment Align Combine Class Group
model name type type type
──────────────────────────────────────────────────────────────────────────
STACK PARA STACK STACK DGROUP
Large module_TEXT WORD PUBLIC CODE
.
.
.
data PARA PRIVATE FAR_DATA
.
.
.
_DATA WORD PUBLIC DATA DGROUP
STACK PARA STACK STACK DGROUP
──────────────────────────────────────────────────────────────────────────
Figure 3-10. Segments, groups, and classes for the standard memory models
as used with assembly-language programs. The Microsoft C Optimizing
Compiler and other high-level-language compilers use a superset of these
segments and classes.
For pure assembly-language programs that will run under MS-DOS, you can
ignore DGROUP. However, if you plan to integrate assembly-language
routines and programs written in high-level languages, you'll want to
follow the Microsoft DGROUP convention. For example, if you are planning
to link routines from a C library into an assembly-language program, you
should include the line
DGROUP group _DATA,STACK
near the beginning of the program.
The final Microsoft convention of interest in creating .EXE programs is
segment order. The high-level compilers assume that code segments always
come first, followed by far data segments, followed by the near data
segment, with the stack and heap last. This order won't concern you much
until you begin integrating assembly-language code with routines from
high-level-language libraries, but it is easiest to learn to use the
convention right from the start.
Program Procedures
The procedure level of program structure is partly real and partly
conceptual. Procedures are basically just a fancy guise for subroutines.
Procedures within a program are declared with the PROC and ENDP directives
in the following form:
name PROC attribute
.
.
.
RET
name ENDP
The attribute carried by a PROC declaration, which is either NEAR or FAR,
tells the assembler what type of call you expect to use to enter the
procedure──that is, whether the procedure will be called from other
routines in the same segment or from routines in other segments. When the
assembler encounters a RET instruction within the procedure, it uses the
attribute information to generate the correct opcode for either a near
(intra-segment) or far (inter-segment) return.
Each program should have a main procedure that receives control from
MS-DOS. You specify the entry point for the program by including the name
of the main procedure in the END statement in one of the program's source
files. The main procedure's attribute (NEAR or FAR) is really not too
important, because the program returns control to MS-DOS with a function
call rather than a RET instruction. However, by convention, most
programmers assign the main procedure the FAR attribute anyway.
You should break the remainder of the program into procedures in an
orderly way, with each procedure performing a well-defined single
function, returning its results to its caller, and avoiding actions that
have global effects within the program. Ideally procedures invoke each
other only by CALL instructions, have only one entry point and one exit
point, and always exit by means of a RET instruction, never by jumping to
some other location within the program.
For ease of understanding and maintenance, a procedure should not exceed
one page (about 60 lines); if it is longer than a page, it is probably too
complex and you should delegate some of its function to one or more
subsidiary procedures. You should preface the source code for each
procedure with a detailed comment that states the procedure's calling
sequence, results returned, registers affected, and any data items
accessed or modified. The effort invested in making your procedures
compact, clean, flexible, and well-documented will be repaid many times
over when you reuse the procedures in other programs.
────────────────────────────────────────────────────────────────────────────
Chapter 4 MS-DOS Programming Tools
Preparing a new program to run under MS-DOS is an iterative process with
four basic steps:
■ Use of a text editor to create or modify an ASCII source-code file
■ Use of an assembler or high-level-language compiler (such as the
Microsoft Macro Assembler or the Microsoft C Optimizing Compiler) to
translate the source file into relocatable object code
■ Use of a linker to transform the relocatable object code into an
executable MS-DOS load module
■ Use of a debugger to methodically test and debug the program
Additional utilities the MS-DOS software developer may find necessary or
helpful include the following:
■ LIB, which creates and maintains object-module libraries
■ CREF, which generates a cross-reference listing
■ EXE2BIN, which converts .EXE files to .COM files
■ MAKE, which compares dates of files and carries out operations based on
the result of the comparison
This chapter gives an operational overview of the Microsoft programming
tools for MS-DOS, including the assembler, the C compiler, the linker, and
the librarian. In general, the information provided here also applies to
the IBM programming tools for MS-DOS, which are really the Microsoft
products with minor variations and different version numbers. Even if your
preferred programming language is not C or assembly language, you will
need at least a passing familiarity with these tools because all of the
examples in the IBM and Microsoft DOS reference manuals are written in one
of these languages.
The survey in this chapter, together with the example programs and
reference section elsewhere in the book, should provide the experienced
programmer with sufficient information to immediately begin writing useful
programs. Readers who do not have a background in C, assembly language, or
the Intel 80x86 microprocessor architecture should refer to the tutorial
and reference works listed at the end of this chapter.
File Types
The MS-DOS programming tools can create and process many different file
types. The following extensions are used by convention for these files:
╓┌─┌──────────┌──────────────────────────────────────────────────────────────╖
Extension File type
Extension File type
──────────────────────────────────────────────────────────────────────────
.ASM Assembly-language source file
.C C source file
.COM MS-DOS executable load module that does not require relocation
at runtime
.CRF Cross-reference information file produced by the assembler for
processing by CREF.EXE
.DEF Module-definition file describing a program's segment behavior
(MS OS/2 and Microsoft Windows programs only; not relevant to
normal MS-DOS applications)
.EXE MS-DOS executable load module that requires relocation at
runtime
.H C header file containing C source code for constants, macros,
and functions; merged into another C program with the #include
Extension File type
──────────────────────────────────────────────────────────────────────────
and functions; merged into another C program with the #include
directive
.INC Include file for assembly-language programs, typically
containing macros and/or equates for systemwide values such as
error codes
.LIB Object-module library file made up of one or more .OBJ files;
indexed and manipulated by LIB.EXE
.LST Program listing, produced by the assembler, that includes
memory locations, machine code, the original program text, and
error messages
.MAP Listing of symbols and their locations within a load module;
produced by the linker
.OBJ Relocatable-object-code file produced by an assembler or
compiler
Extension File type
──────────────────────────────────────────────────────────────────────────
compiler
.REF Cross-reference listing produced by CREF.EXE from the
information in a .CRF file
──────────────────────────────────────────────────────────────────────────
The Microsoft Macro Assembler
The Microsoft Macro Assembler (MASM) is distributed as the file MASM.EXE.
When beginning a program translation, MASM needs the following
information:
■ The name of the file containing the source program
■ The filename for the object program to be created
■ The destination of the program listing
■ The filename for the information that is later processed by the
cross-reference utility (CREF.EXE)
You can invoke MASM in two ways. If you enter the name of the assembler
alone, it prompts you for the names of each of the various input and
output files. The assembler supplies reasonable defaults for all the
responses except the source filename, as shown in the following example:
C>MASM <Enter>
Microsoft (R) Macro Assembler Version 5.10
Copyright (C) Microsoft Corp 1981, 1988. All rights reserved.
Source filename [.ASM]: HELLO <Enter>
Object filename [HELLO.OBJ]: <Enter>
Source listing [NUL.LST]: <Enter>
Cross-reference [NUL.CRF]: <Enter>
49006 Bytes symbol space free
0 Warning Errors
0 Severe Errors
C>
You can use a logical device name (such as PRN or COM1) at any of the MASM
prompts to send that output of the assembler to a character device rather
than a file. Note that the default for the listing and cross-reference
files is the NUL device──that is, no file is created. If you end any
response with a semicolon, MASM assumes that the remaining responses are
all to be the default.
A more efficient way to use MASM is to supply all parameters in the
command line, as follows:
MASM [options] source,[object],[listing],[crossref]
For example, the following command lines are equivalent to the preceding
interactive session:
C>MASM HELLO,,NUL,NUL <Enter>
or
C>MASM HELLO; <Enter>
These commands use the file HELLO.ASM as the source, generate the
object-code file HELLO.OBJ, and send the listing and cross-reference files
to the bit bucket.
MASM accepts several optional switches in the command line, to control
code generation and output files. Figure 4-1 lists the switches accepted
by MASM version 5.1. As shown in the following example, you can put
frequently used options in a MASM environment variable, where they will be
found automatically by the assembler:
C>SET MASM=/T /Zi <Enter>
The switches in the environment variable will be overridden by any that
you enter in the command line.
In other versions of the Microsoft Macro Assembler, additional or fewer
switches may be available. For exact instructions, see the manual for the
version of MASM that you are using.
╓┌─┌──────────┌──────────────────────────────────────────────────────────────╖
Switch Meaning
──────────────────────────────────────────────────────────────────────────
/A Arrange segments in alphabetic order.
/Bn Set size of source-file buffer (in KB).
/C Force creation of a cross-reference (.CRF) file.
/D Produce listing on both passes (to find phase errors).
/Dsymbol Define symbol as a null text string (symbol can be referenced
by conditional assembly directives in file).
/E Assemble for 80x87 numeric coprocessor emulator using IEEE
real-number format.
/Ipath Set search path for include files.
/L Force creation of a program-listing file.
/LA Force listing of all generated code.
/ML Preserve case sensitivity in all names (uppercase names
distinct from their lowercase equivalents).
/MX Preserve lowercase in external names only (names defined with
PUBLIC or EXTRN directives).
Switch Meaning
──────────────────────────────────────────────────────────────────────────
PUBLIC or EXTRN directives).
/MU Convert all lowercase names to uppercase.
/N Suppress generation of tables of macros, structures, records,
segments, groups, and symbols at the end of the listing.
/P Check for impure code in 80286/80386 protected mode.
/S Arrange segments in order of occurrence (default).
/T "Terse" mode; suppress all messages unless errors are
encountered during the assembly.
/V "Verbose" mode; report number of lines and symbols at end of
assembly.
/Wn Set error display (warning) level; n=0─2.
/X Force listing of false conditionals.
/Z Display source lines containing errors on the screen.
/Zd Include line-number information in .OBJ file.
/Zi Include line-number and symbol information in .OBJ file.
──────────────────────────────────────────────────────────────────────────
Figure 4-1. Microsoft Macro Assembler version 5.1 switches.
MASM allows you to override the default extensions on any file──a feature
that can be rather dangerous. For example, if in the preceding example you
had responded to the Object filename prompt with HELLO.ASM, the assembler
would have accepted the entry without comment and destroyed your source
file. This is not too likely to happen in the interactive command mode,
but you must be very careful with file extensions when MASM is used in a
batch file.
The Microsoft C Optimizing Compiler
The Microsoft C Optimizing Compiler consists of three executable files──
C1.EXE, C2.EXE, and C3.EXE──that implement the C preprocessor, language
translator, code generator, and code optimizer. An additional control
program, CL.EXE, executes the three compiler files in order, passing each
the necessary information about filenames and compilation options.
Before using the C compiler and the linker, you need to set up four
environment variables:
Variable Action
──────────────────────────────────────────────────────────────────────────
PATH=path Specifies the location of the three executable C
compiler files (C1, C2, and C3) if they are not
in the current directory; used by CL.EXE.
INCLUDE=path Specifies the location of #include files (default
extension .H) that are not found in the current
directory.
LIB=path Specifies the location(s) for object-code
libraries that are not found in the current
directory.
TMP=path Specifies the location for temporary working
files created by the C compiler and linker.
──────────────────────────────────────────────────────────────────────────
CL.EXE does not support an interactive mode or response files. You always
invoke it with a command line of the following form:
CL [options] file [file ...]
You may list any number of files──if a file has a .C extension, it will be
compiled into a relocatable-object-module (.OBJ) file. Ordinarily, if the
compiler encounters no errors, it automatically passes all resulting .OBJ
files and any additional .OBJ files specified in the command line to the
linker, along with the names of the appropriate runtime libraries.
The C compiler has many optional switches controlling its memory models,
output files, code generation, and code optimization. These are summarized
in Figure 4-2. The C compiler's arcane switch syntax is derived largely
from UNIX/XENIX, so don't expect it to make any sense.
╓┌─┌────────────────────────┌────────────────────────────────────────────────╖
Switch Meaning
──────────────────────────────────────────────────────────────────────────
/Ax Select memory model:
C = compact model
H = huge model
L = large model
M = medium model
Switch Meaning
──────────────────────────────────────────────────────────────────────────
M = medium model
S = small model (default)
/c Compile only; do not invoke linker.
/C Do not strip comments.
/D<name>[=text] Define macro.
/E Send preprocessor output to standard output.
/EP Send preprocessor output to standard output
without line numbers.
/F<n> Set stack size (in hexadecimal bytes).
/Fa [filename] Generate assembly listing.
/Fc [filename] Generate mixed source/object listing.
/Fe [filename] Force executable filename.
/Fl [filename] Generate object listing.
/Fm [filename] Generate map file.
/Fo [filename] Force object-module filename.
/FPx Select floating-point control:
a = calls with alternate math library
c = calls with emulator library
c87 = calls with 8087 library
Switch Meaning
──────────────────────────────────────────────────────────────────────────
c87 = calls with 8087 library
i = in-line with emulator (default)
i87 = in-line with 8087
/Fs [filename] Generate source listing.
/Gx Select code generation:
0 = 8086 instructions (default)
1 = 186 instructions
2 = 286 instructions
c = Pascal style function calls
s = no stack checking
t[n] = data size threshold
/H<n> Specify external name length.
/I<path> Specify additional #include path.
/J Specify default char type as unsigned.
/link [options] Pass switches and library names to linker.
/Ox Select optimization:
a = ignore aliasing
d = disable optimizations
i = enable intrinsic functions
Switch Meaning
──────────────────────────────────────────────────────────────────────────
i = enable intrinsic functions
l = enable loop optimizations
n = disable "unsafe" optimizations
p = enable precision optimizations
r = disable in-line return
s = optimize for space
/Ox t = optimize for speed (default)
w = ignore aliasing except across function
calls
x = enable maximum optimization (equivalent to
/Oailt /Gs)
/P Send preprocessor output to file.
/Sx Select source-listing control:
l<columns> = set line width
p<lines> = set page length
s<string> = set subtitle string
t<string> = set title string
/Tc<file> Compile file without .C extension.
/u Remove all predefined macros.
Switch Meaning
──────────────────────────────────────────────────────────────────────────
/u Remove all predefined macros.
/U<name> Remove specified predefined macro.
/V<string> Set version string.
/W<n> Set warning level (0─3).
/X Ignore "standard places" for include files.
/Zx Select miscellaneous compilation control:
a = disable extensions
c = make Pascal functions case-insensitive
d = include line-number information
e = enable extensions (default)
g = generate declarations
i = include symbolic debugging information
l = remove default library info
p<n> = pack structures on n-byte boundary
s = check syntax only
──────────────────────────────────────────────────────────────────────────
Figure 4-2. Microsoft C Optimizing Compiler version 5.1 switches.
The Microsoft Object Linker
The object module produced by MASM from a source file is in a form that
contains relocation information and may also contain unresolved references
to external locations or subroutines. It is written in a common format
that is also produced by the various high-level compilers (such as FORTRAN
and C) that run under MS-DOS. The computer cannot execute object modules
without further processing.
The Microsoft Object Linker (LINK), distributed as the file LINK.EXE,
accepts one or more of these object modules, resolves external references,
includes any necessary routines from designated libraries, performs any
necessary offset relocations, and writes a file that can be loaded and
executed by MS-DOS. The output of LINK is always in .EXE load-module
format. (See Chapter 3.)
As with MASM, you can give LINK its parameters interactively or by
entering all the required information in a single command line. If you
enter the name of the linker alone, the following type of dialog ensues:
C>LINK <Enter>
Microsoft (R) Overlay Linker Version 3.61
Copyright (C) Microsoft Corp 1983-1987. All rights reserved.
Object Modules [.OBJ]: HELLO <Enter>
Run File [HELLO.EXE]: <Enter>
List File [NUL.MAP]: HELLO <Enter>
Libraries [.LIB]: <Enter>
C>
If you are using LINK version 4.0 or later, the linker also asks for the
name of a module-definition (.DEF) file. Simply press the Enter key in
response to such a prompt. Module-definition files are used when building
Microsoft Windows or MS OS/2 "new .EXE" executable files but are not
relevant in normal MS-DOS applications.
The input file for this example was HELLO.OBJ; the output files were
HELLO.EXE (the executable program) and HELLO.MAP (the load map produced by
the linker after all references and addresses were resolved). Figure 4-3
shows the load map.
──────────────────────────────────────────────────────────────────────────
Start Stop Length Name Class
00000H 00017H 00018H _TEXT CODE
00018H 00027H 00010H _DATA DATA
00030H 000AFH 00080H STACK STACK
000B0H 000BBH 0000CH $$TYPES DEBTYP
000C0H 000D6H 00017H $$SYMBOLS DEBSYM
Address Publics by Name
Address Publics by Value
Program entry point at 0000:0000
──────────────────────────────────────────────────────────────────────────
Figure 4-3. Map produced by the Microsoft Object Linker (LINK) during the
generation of the HELLO.EXE program from Chapter 3. The program contains
one CODE, one DATA, and one STACK segment. The first instruction to be
executed lies in the first byte of the CODE segment. The $$TYPES and
$$SYMBOLS segments contain information for the CodeView debugger and are
not part of the program; these segments are ignored by the normal MS-DOS
loader.
You can obtain the same result more quickly by entering all parameters in
the command line, in the following form:
LINK options objectfile, [exefile], [mapfile], [libraries]
Thus, the command-line equivalent to the preceding interactive session is
C>LINK HELLO,HELLO,HELLO,, <Enter>
or
C>LINK HELLO,,HELLO; <Enter>
If you enter a semicolon as the last character in the command line, LINK
assumes the default values for all further parameters.
A third method of commanding LINK is with a response file. A response file
contains lines of text that correspond to the responses you would give the
linker interactively. You specify the name of the response file in the
command line with a leading @ character, as follows:
LINK @filename
You can also enter the name of a response file at any prompt. If the
response file is not complete, LINK will prompt you for the missing
information.
When entering linker commands, you can specify multiple object files with
the + operator or with spaces, as in the following example:
C>LINK HELLO+VMODE+DOSINT,MYPROG,,GRAPHICS; <Enter>
This command would link the files HELLO.OBJ, VMODE.OBJ, and DOSINT.OBJ,
searching the library file GRAPHICS.LIB to resolve any references to
symbols not defined in the specified object files, and would produce a
file named MYPROG.EXE. LINK uses the current drive and directory when they
are not explicitly included in a filename; it will not automatically use
the same drive and directory you specified for a previous file in the same
command line.
By using the + operator or space characters in the libraries field, you
can specify up to 32 library files to be searched. Each high-level-
language compiler provides default libraries that are searched
automatically during the linkage process if the linker can find them
(unless they are explicitly excluded with the /NOD switch). LINK looks for
libraries first in the current directory of the default disk drive, then
along any paths that were provided in the command line, and finally along
the path(s) specified by the LIB variable if it is present in the
environment.
LINK accepts several optional switches as part of the command line or at
the end of any interactive prompt. Figure 4-4 lists these switches. The
number of switches available and their actions vary among different
versions of LINK. See your Microsoft Object Linker instruction manual for
detailed information about your particular version.
╓┌─┌────────┌───────────────────────────┌────────────────────────────────────╖
Switch Full form Meaning
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
/A:n /ALIGNMENT:n Set segment sector alignment factor.
N must be a power of 2 (default =
512). Not related to logical-segment
alignment (BYTE, WORD, PARA, PAGE,
and so forth). Relevant to segmented
executable files (Microsoft Windows
and MS OS/2) only.
/B /BATCH Suppress linker prompt if a library
cannot be found in the current
directory or in the locations
specified by the LIB environment
variable.
/CO /CODEVIEW Include symbolic debugging
information in the .EXE file for use
by CodeView.
/CP /CPARMAXALLOC Set the field in the .EXE file header
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
/CP /CPARMAXALLOC Set the field in the .EXE file header
controlling the amount of memory
allocated to the program in addition
to the memory required for the
program's code, stack, and
initialized data.
/DO /DOSSEG Use standard Microsoft segment naming
and ordering conventions.
/DS /DSALLOCATE Load data at high end of the data
segment. Relevant to real-mode
programs only.
/E /EXEPACK Pack executable file by removing
sequences of repeated bytes and
optimizing relocation table.
/F /FARCALLTRANSLATION Optimize far calls to labels within
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
/F /FARCALLTRANSLATION Optimize far calls to labels within
the same physical segment for speed
by replacing them with near calls and
NOPs.
/HE /HELP Display information about available
options.
/HI /HIGH Load program as high in memory as
possible.
/I /INFORMATION Display information about progress of
linking, including pass numbers and
the names of object files being
linked.
/INC /INCREMENTAL Force production of .SYM and .ILK
files for subsequent use by ILINK
(incremental linker). May not be used
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
(incremental linker). May not be used
with /EXEPACK. Relevant to segmented
executable files (Microsoft Windows
and MS OS/2) only.
/LI /LINENUMBERS Write address of the first
instruction that corresponds to each
source-code line to the map file. Has
no effect if the compiler does not
include line-number information in
the object module. Force creation of
a map file.
/M[:n] /MAP[:n] Force creation of a .MAP file listing
all public symbols, sorted by name
and by location. The optional value n
is the maximum number of symbols that
can be sorted (default = 2048); when
n is supplied, the alphabetically
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
n is supplied, the alphabetically
sorted list is omitted.
/NOD /NODEFAULTLIBRARYSEARCH Skip search of any default compiler
libraries specified in the .OBJ file.
/NOE /NOEXTENDEDDICTSEARCH Ignore extended library dictionary
(if it is present). The extended
dictionary ordinarily provides the
linker with information about
inter-module dependencies, to speed
up linking.
/NOF /NOFARCALLTRANSLATION Disable optimization of far calls to
labels within the same segment.
/NOG /NOGROUPASSOCIATION Ignore group associations when
assigning addresses to data and code
items.
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
items.
/NOI /NOIGNORECASE Do not ignore case in names during
linking.
/NON /NONULLSDOSSEG Arrange segments as for /DOSSEG but
do not insert 16 null bytes at start
of _TEXT segment.
/NOP /NOPACKCODE Do not pack contiguous logical code
segments into a single physical
segment.
/O:n /OVERLAYINTERRUPT:n Use interrupt number n with the
overlay manager supplied with some
Microsoft high-level languages.
/PAC[:n] /PACKCODE[:n] Pack contiguous logical code segments
into a single physical code segment.
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
into a single physical code segment.
The optional value n is the maximum
size for each packed physical code
segment (default = 65,536 bytes).
Segments in different groups are not
packed.
/PADC:n /PADCODE:n Add n filler bytes to end of each
code module so that a larger module
can be inserted later with ILINK.
Relevant to segmented executable
files (Windows and MS OS/2) only.
/PADD:n /PADDATA:n Add n filler bytes to end of each
data module so that a larger module
can be inserted later with ILINK.
Relevant to segmented executable
files (Microsoft Windows and MS OS/2)
only.
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
only.
/PAU /PAUSE Pause during linking, allowing a
change of disks before .EXE file is
written.
/SE:n /SEGMENTS:n Set maximum number of segments in
linked program (default = 128).
/ST:n /STACK:n Set stack size of program in bytes;
ignore stack segment size
declarations within object modules
and definition file.
/W /WARNFIXUP Display warning messages for offsets
relative to a segment base that is
not the same as the group base.
Relevant to segmented executable
files (Microsoft Windows and MS OS/2)
Switch Full form Meaning
──────────────────────────────────────────────────────────────────────────
files (Microsoft Windows and MS OS/2)
only.
──────────────────────────────────────────────────────────────────────────
Figure 4-4. Switches accepted by the Microsoft Object Linker (LINK)
version 5.0. Earlier versions use a subset of these switches. Note that
any abbreviation for a switch is acceptable as long as it is sufficient to
specify the switch uniquely.
The EXE2BIN Utility
The EXE2BIN utility (EXE2BIN.EXE) transforms a .EXE file created by LINK
into an executable .COM file, if the program meets the following
prerequisites:
■ It cannot contain more than one declared segment and cannot
define a stack.
■ It must be less than 64 KB in length.
■ It must have an origin at 0100H.
■ The first location in the file must be specified as the entry point
in the source code's END directive.
Although .COM files are somewhat more compact than .EXE files, you should
avoid using them. Programs that use separate segments for code, data, and
stack are much easier to port to protected-mode environments such as MS
OS/2; in addition, .COM files do not support the symbolic debugging
information used by CodeView.
Another use for the EXE2BIN utility is to convert an installable device
driver──after it is assembled and linked into a .EXE file──into a
memory-image .BIN or .SYS file with an origin of zero. This conversion is
required in MS-DOS version 2, which cannot load device drivers as .EXE
files. The process of writing an installable device driver is discussed in
more detail in Chapter 14.
Unlike most of the other programming utilities, EXE2BIN does not have an
interactive mode. It always takes its source and destination filenames,
separated by spaces, from the MS-DOS command line, as follows:
EXE2BIN sourcefile [destinationfile]
If you do not supply the source-file extension, it defaults to .EXE; the
destination-file extension defaults to .BIN. If you do not specify a name
for the destination file, EXE2BIN gives it the same name as the source
file, with a .BIN extension.
For example, to convert the file HELLO.EXE into HELLO.COM, you would use
the following command line:
C>EXE2BIN HELLO.EXE HELLO.COM <Enter>
The EXE2BIN program also has other capabilities, such as pure binary
conversion with segment fixup for creating program images to be placed in
ROM; but because these features are rarely used during MS-DOS application
development, they will not be discussed here.
The CREF Utility
The CREF cross-reference utility CREF.EXE processes a .CRF file produced
by MASM, creating an ASCII text file with the default extension .REF. The
file contains a cross-reference listing of all symbols declared in the
program and the line numbers in which they are referenced. (See Figure
4-5.) Such a listing is very useful when debugging large
assembly-language programs with many interdependent procedures and
variables.
CREF may be supplied with its parameters interactively or in a single
command line. If you enter the utility name alone, CREF prompts you for
the input and output filenames, as shown in the following example:
C>CREF <Enter>
Microsoft (R) Cross-Reference Utility Version 5.10
Copyright (C) Microsoft Corp 1981-1985, 1987. All rights reserved.
Cross-reference [.CRF]: HELLO <Enter>
Listing [HELLO.REF]:
15 Symbols
C>
──────────────────────────────────────────────────────────────────────────
Microsoft Cross-Reference Version 5.10 Thu May 26 11:09:34 1988
HELLO.EXE --- print Hello on terminal
Symbol Cross-Reference (# definition, + modification)Cref-1
@CPU . . . . . . . . . . . . . . 1#
@VERSION . . . . . . . . . . . . 1#
CODE . . . . . . . . . . . . . . 21
CR . . . . . . . . . . . . . . . 17# 46 47
DATA . . . . . . . . . . . . . . 44
LF . . . . . . . . . . . . . . . 18# 46 47
MSG. . . . . . . . . . . . . . . 33 46#
MSG_LEN. . . . . . . . . . . . . 32 49#
PRINT. . . . . . . . . . . . . . 25# 39 60
STACK. . . . . . . . . . . . . . 23 54# 54 58
STDERR . . . . . . . . . . . . . 15#
STDIN. . . . . . . . . . . . . . 13#
STDOUT . . . . . . . . . . . . . 14# 31
_DATA. . . . . . . . . . . . . . 23 27 44# 51
_TEXT. . . . . . . . . . . . . . 21# 23 41
15 Symbols
──────────────────────────────────────────────────────────────────────────
Figure 4-5. Cross-reference listing HELLO.REF produced by the CREF
utility from the file HELLO.CRF, for the HELLO.EXE program example from
Chapter 3. The symbols declared in the program are listed on the left in
alphabetic order. To the right of each symbol is a list of all the lines
where that symbol is referenced. The number with a # sign after it denotes
the line where the symbol is declared. Numbers followed by a + sign
indicate that the symbol is modified at the specified line. The line
numbers given in the cross-reference listing correspond to the line
numbers generated by the assembler in the program-listing (.LST) file, not
to any physical line count in the original source file.
The parameters may also be entered in the command line in the following
form:
CREF CRF_file, listing_file
For example, the command-line equivalent to the preceding interactive
session is:
C>CREF HELLO,HELLO <Enter>
If CREF cannot find the specified .CRF file, it displays an error message.
Otherwise, it leaves the cross-reference listing in the specified file on
the disk. You can send the file to the printer with the COPY command, in
the following form:
COPY listing_file PRN:
You can also send the cross-reference listing directly to a character
device as it is generated by responding to the Listing prompt with the
name of the device.
The Microsoft Library Manager
Although the object modules that are produced by MASM or by high-level-
language compilers can be linked directly into executable load modules,
they can also be collected into special files called object-module
libraries. The modules in a library are indexed by name and by the public
symbols they contain, so that they can be extracted by the linker to
satisfy external references in a program.
The Microsoft Library Manager (LIB) is distributed as the file LIB.EXE.
LIB creates and maintains program libraries, adding, updating, and
deleting object files as necessary. LIB can also check a library file for
internal consistency or print a table of its contents (Figure 4-6).
LIB follows the command conventions of most other Microsoft programming
tools. You must supply it with the name of a library file to work on, one
or more operations to perform, the name of a listing file or device, and
(optionally) the name of the output library. If you do not specify a name
for the output library, LIB gives it the same name as the input library
and changes the extension of the input library to .BAK.
The LIB operations are simply the names of object files, with a prefix
character that specifies the action to be taken:
Prefix Meaning
──────────────────────────────────────────────────────────────────────────
- Delete an object module from the library.
* Extract a module and place it in a separate .OBJ file.
+ Add an object module or the entire contents of another library
to the library.
──────────────────────────────────────────────────────────────────────────
You can combine command prefixes. For example, -+ replaces a module, and
*- extracts a module into a new file and then deletes it from the library.
──────────────────────────────────────────────────────────────────────────
_abort............abort _abs..............abs
_access...........access _asctime..........asctime
_atof.............atof _atoi.............atoi
_atol.............atol _bdos.............bdos
_brk..............brk _brkctl...........brkctl
_bsearch..........bsearch _calloc...........calloc
_cgets............cgets _chdir............dir
_chmod............chmod _chsize...........chsize
.
.
.
_exit Offset: 00000010H Code and data size: 44H
__exit
_filbuf Offset: 00000160H Code and data size: BBH
__filbuf
_file Offset: 00000300H Code and data size: CAH
__iob __iob2 __lastiob
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 4-6. Extract from the table-of-contents listing produced by the
Microsoft Library Manager (LIB) for the Microsoft C library SLIBC.LIB. The
first part of the listing is an alphabetic list of all public names
declared in all of the modules in the library. Each name is associated
with the object module to which it belongs. The second part of the listing
is an alphabetic list of the object-module names in the library, each
followed by its offset within the library file and the actual size of the
module in bytes. The entry for each module is followed by a summary of the
public names that are declared within it.
When you invoke LIB with its name alone, it requests the other information
it needs interactively, as shown in the following example:
C>LIB <Enter>
Microsoft (R) Library Manager Version 3.08
Copyright (C) Microsoft Corp 1983-1987. All rights reserved.
Library name: SLIBC <Enter>
Operations: +VIDEO <Enter>
List file: SLIBC.LST <Enter>
Output library: SLIBC2 <Enter>
C>
In this example, LIB added the object module VIDEO.OBJ to the library
SLIBC.LIB, wrote a library table of contents into the file SLIBC.LST, and
named the resulting new library SLIBC2.LIB.
The Library Manager can also be run with a command line of the following
form:
LIB library [commands],[list],[newlibrary]
For example, the following command line is equivalent to the preceding
interactive session:
C>LIB SLIBC +VIDEO,SLIBC.LST,SLIBC2; <Enter>
As with the other Microsoft utilities, a semicolon at the end of the
command line causes LIB to use the default responses for any parameters
that are omitted.
Like LINK, LIB can also accept its commands from a response file. The
contents of the file are lines of text that correspond exactly to the
responses you would give LIB interactively. You specify the name of the
response file in the command line with a leading @ character, as follows:
LIB @filename
LIB has only three switches: /I (/IGNORECASE), /N (/NOIGNORECASE), and
/PAGESIZE:number. The /IGNORECASE switch is the default. The /NOIGNORECASE
switch causes LIB to regard as distinct any symbols that differ only in
the case of their component letters. You should place the /PAGESIZE
switch, which defines the size of a unit of allocation space for a given
library, immediately after the library filename. The library page size is
in bytes and must be a power of 2 between 16 and 32,768 (16, 32, 64, and
so forth); the default is 16 bytes. Because the index to a library is
always a fixed number of pages, setting a larger page size allows you to
store more object modules in that library; on the other hand, it will
result in more wasted space within the file.
The MAKE Utility
The MAKE utility (MAKE.EXE) compares dates of files and carries out
commands based on the result of that comparison. Because of this single,
rather basic capability, MAKE can be used to maintain complex programs
built from many modules. The dates of source, object, and executable files
are simply compared in a logical sequence; the assembler, compiler,
linker, and other programming tools are invoked as appropriate.
The MAKE utility processes a plain ASCII text file called, as you might
expect, a make file. You start the utility with a command-line entry in
the following form:
MAKE makefile [options]
By convention, a make file has the same name as the executable file that
is being maintained, but without an extension. The available MAKE switches
are listed in Figure 4-7.
A simple make file contains one or more dependency statements separated by
blank lines. Each dependency statement can be followed by a list of MS-DOS
commands, in the following form:
targetfile : sourcefile ...
command
command
.
.
.
If the date and time of any source file are later than those of the target
file, the accompanying list of commands is carried out. You may use
comment lines, which begin with a # character, freely in a make file. MAKE
can also process inference rules and macro definitions. For further
details on these advanced capabilities, see the Microsoft or IBM
documentation.
Switch Meaning
──────────────────────────────────────────────────────────────────────────
/D Display last modification date of each file as it is processed.
/I Ignore exit (return) codes returned by commands and programs
executed as a result of dependency statements.
/N Display commands that would be executed as a result of
dependency statements but do not execute those commands.
/S Do not display commands as they are executed.
/X Direct error messages from MAKE, or any program that MAKE runs,
<filename> to the specified file. If filename is a hyphen (-), direct
error messages to the standard output.
──────────────────────────────────────────────────────────────────────────
Figure 4-7. Switches for the MAKE utility.
A Complete Example
Let's put together everything we've learned about using the MS-DOS
programming tools so far. Figure 4-8 shows a sketch of the overall
process of building an executable program.
Assume that we have the source code for the HELLO.EXE program from Chapter
3 in the file HELLO.ASM. To assemble the source program into the
relocatable object module HELLO.OBJ with symbolic debugging information
included, also producing a program listing in the file HELLO.LST and a
cross-reference data file HELLO.CRF, we would enter
C>MASM /C /L /Zi /T HELLO; <Enter>
To convert the cross-reference raw-data file HELLO.CRF into a
cross-reference listing in the file HELLO.REF, we would enter
C>CREF HELLO,HELLO <Enter>
┌───────────────┐ ┌───────────────┐
│ MASM │ │ C or other │
│ source-code │ │ HLL source- │
│ file │ │ code file │
└───┬───────────┘ └───┬───────────┘
│ ┌─────────────────────┘ Compiler
┌───▼───────▼───┐
│ Relocatable │
│ object-module ├────┐
│ file (.OBJ) │ │
└───┬───────────┘ │
│ LIB │
┌───▼───────────┐ │ ┌───────────────┐
│ Object-module │ ▼ LINK │ Executable │
│ libraries ├─────────────► program │
│ (.LIB) │ │ (.EXE) │
└───────────────┘ │ └───┬───────────┘
│ │ EXE2BIN
┌───────────────┐ │ ┌───▼───────────┐
│ HLL │ │ │ Executable │
│ runtime ├──────┘ │ program │
│ libraries │ │ (.COM) │
└───────────────┘ └───────────────┘
Figure 4-8. Creation of an MS-DOS application program, from source code
to executable file.
To convert the relocatable object file HELLO.OBJ into the executable file
HELLO.EXE, creating a load map in the file HELLO.MAP and appending
symbolic debugging information to the executable file, we would enter
C>LINK /MAP /CODEVIEW HELLO; <Enter>
We could also automate the entire process just described by creating a
make file named HELLO (with no extension) and including the following
instructions:
hello.obj : hello.asm
masm /C /L /Zi /T hello;
cref hello,hello
hello.exe : hello.obj
link /MAP /CODEVIEW hello;
Then, when we have made some change to HELLO.ASM and want to rebuild the
executable HELLO.EXE file, we need only enter
C>MAKE HELLO <Enter>
Programming Resources and References
The literature on IBM PC─compatible personal computers, the Intel 80x86
microprocessor family, and assembly-language and C programming is vast.
The list below contains a selection of those books that I have found to be
useful and reliable. The list should not be construed as an endorsement by
Microsoft Corporation.
MASM Tutorials
Assembly Language Primer for the IBM PC and XT, by Robert Lafore. New
American Library, New York, NY, 1984. ISBN 0-452-25711-5.
8086/8088/80286 Assembly Language, by Leo Scanlon. Brady Books, Simon and
Schuster, New York, NY, 1988. ISBN 0-13-246919-7.
C Tutorials
Microsoft C Programming for the IBM, by Robert Lafore. Howard K. Sams &
Co., Indianapolis, IN, 1987. ISBN 0-672-22515-8.
Proficient C, by Augie Hansen. Microsoft Press, Redmond, WA, 1987. ISBN
1-55615-007-5.
Intel 80x86 Microprocessor References
iAPX 88 Book. Intel Corporation, Literature Department SV3-3, 3065 Bowers
Ave., Santa Clara, CA 95051. Order no. 210200.
iAPX 286 Programmer's Reference Manual. Intel Corporation, Literature
Department SV3-3, 3065 Bowers Ave., Santa Clara, CA 95051. Order no.
210498.
iAPX 386 Programmer's Reference Manual. Intel Corporation, Literature
Department SV3-3, 3065 Bowers Ave., Santa Clara, CA 95051. Order no.
230985.
PC, PC/AT, and PS/2 Architecture
The IBM Personal Computer from the Inside Out (Revised Edition), by Murray
Sargent and Richard L. Shoemaker. Addison-Wesley Publishing Company,
Reading, MA, 1986. ISBN 0-201-06918-0.
Programmer's Guide to PC & PS/2 Video Systems, by Richard Wilton.
Microsoft Press, Redmond, WA, 1987. ISBN 1-55615-103-9.
Personal Computer Technical Reference. IBM Corporation, IBM Technical
Directory, P. O. Box 2009, Racine, WI 53404. Part no. 6322507.
Personal Computer AT Technical Reference. IBM Corporation, IBM Technical
Directory, P. O. Box 2009, Racine, WI 53404. Part no. 6280070.
Options and Adapters Technical Reference. IBM Corporation, IBM Technical
Directory, P. O. Box 2009, Racine, WI 53404. Part no. 6322509.
Personal System/2 Model 30 Technical Reference. IBM Corporation, IBM
Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 68X2201.
Personal System/2 Model 50/60 Technical Reference. IBM Corporation, IBM
Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 68X2224.
Personal System/2 Model 80 Technical Reference. IBM Corporation, IBM
Technical Directory, P. O. Box 2009, Racine, WI 53404. Part no. 68X2256.
────────────────────────────────────────────────────────────────────────────
Chapter 5 Keyboard and Mouse Input
The fundamental means of user input under MS-DOS is the keyboard. This
follows naturally from the MS-DOS command-line interface, whose lineage
can be traced directly to minicomputer operating systems with Teletype
consoles. During the first few years of MS-DOS's existence, when
8088/8086-based machines were the norm, nearly every popular application
program used key-driven menus and text-mode displays.
However, as high-resolution graphics adapters (and 80286/80386-based
machines with enough power to drive them) have become less expensive,
programs that support windows and a graphical user interface have steadily
grown more popular. Such programs typically rely on a pointing device such
as a mouse, stylus, joystick, or light pen to let the user navigate in a
"point-and-shoot" manner, reducing keyboard entry to a minimum. As a
result, support for pointing devices has become an important consideration
for all software developers.
Keyboard Input Methods
Applications running under MS-DOS on IBM PC─compatible machines can use
several methods to obtain keyboard input:
■ MS-DOS handle-oriented functions
■ MS-DOS traditional character functions
■ IBM ROM BIOS keyboard-driver functions
These methods offer different degrees of flexibility, portability, and
hardware independence.
The handle, or stream-oriented, functions are philosophically derived from
UNIX/XENIX and were first introduced in MS-DOS version 2.0. A program uses
these functions by supplying a handle, or token, for the desired device,
plus the address and length of a buffer.
When a program begins executing, MS-DOS supplies it with predefined
handles for certain commonly used character devices, including the
keyboard:
Handle Device name Opened to
──────────────────────────────────────────────────────────────────────────
0 Standard input (stdin) CON
1 Standard output (stdout) CON
2 Standard error (stderr) CON
3 Standard auxiliary (stdaux) AUX
4 Standard printer (stdprn) PRN
──────────────────────────────────────────────────────────────────────────
These handles can be used for read and write operations without further
preliminaries. A program can also obtain a handle for a character device
by explicitly opening the device for input or output using its logical
name (as though it were a file). The handle functions support I/O
redirection, allowing a program to take its input from another device or
file instead of the keyboard, for example. Redirection is discussed in
detail in Chapter 15.
The traditional character-input functions are a superset of the character
I/O functions that were present in CP/M. Originally included in MS-DOS
simply to facilitate the porting of existing applications from CP/M, they
are still widely used. In MS-DOS versions 2.0 and later, most of the
traditional functions also support I/O redirection (although not as well
as the handle functions do).
Use of the IBM ROM BIOS keyboard functions presupposes that the program is
running on an IBM PC─compatible machine. The ROM BIOS keyboard driver
operates at a much more primitive level than the MS-DOS functions and
allows a program to circumvent I/O redirection or MS-DOS's special
handling of certain control characters. Programs that use the ROM BIOS
keyboard driver are inherently less portable than those that use the
MS-DOS functions and may interfere with the proper operation of other
programs; many of the popular terminate-and-stay-resident (TSR) utilities
fall into this category.
Keyboard Input with Handles
The principal MS-DOS function for keyboard input using handles is Int 21H
Function 3FH (Read File or Device). The parameters for this function are
a handle, the segment and offset of a buffer, and the length of the
buffer. (For a more detailed explanation of this function, see Section
II of this book, "MS-DOS Functions Reference.")
As an example, let's use the predefined standard input handle (0) and Int
21H Function 3FH to read a line from the keyboard:
──────────────────────────────────────────────────────────────────────────
buffer db 80 dup (?) ; keyboard input buffer
.
.
.
mov ah,3fh ; function 3fh = read file or device
mov bx,0 ; handle for standard input
mov cx,80 ; maximum bytes to read
mov dx,seg buffer ; DS:DX = buffer address
mov ds,dx
mov dx,offset buffer
int 21h ; transfer to MS-DOS
jc error ; jump if error detected
.
.
.
──────────────────────────────────────────────────────────────────────────
When control returns from Int 21H Function 3FH, the carry flag is clear if
the function was successful, and AX contains the number of characters
read. If there was an error, the carry flag is set and AX contains an
error code; however, this should never occur when reading the keyboard.
The standard input is redirectable, so the code just shown is not a
foolproof way of obtaining input from the keyboard. Depending upon whether
a redirection parameter was included in the command line by the user,
program input might be coming from the keyboard, a file, another character
device, or even the bit bucket (NUL device). To bypass redirection and be
absolutely certain where your input is coming from, you can ignore the
predefined standard input handle and open the console as though it were a
file, using the handle obtained from that open operation to perform your
keyboard input, as in the following example:
──────────────────────────────────────────────────────────────────────────
buffer db 80 dup (?) ; keyboard input buffer
fname db 'CON',0 ; keyboard device name
handle dw 0 ; keyboard device handle
.
.
.
mov ah,3dh ; function 3dh = open
mov al,0 ; mode = read
mov dx,seg fname ; DS:DX = device name
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if open failed
mov handle,ax ; save handle for CON
.
.
.
mov ah,3fh ; function 3fh = read file or device
mov bx,handle ; BX = handle for CON
mov cx,80 ; maximum bytes to read
mov dx,offset buffer ; DS:DX = buffer address
int 21h ; transfer to MS-DOS
jc error ; jump if error detected
.
.
.
──────────────────────────────────────────────────────────────────────────
When a programmer uses Int 21H Function 3FH to read from the keyboard, the
exact result depends on whether MS-DOS regards the handle to be in ASCII
mode or binary mode (sometimes known as cooked mode and raw mode). ASCII
mode is the default, although binary mode can be selected with Int 21H
Function 44H (IOCTL) when necessary.
In ASCII mode, MS-DOS initially places characters obtained from the
keyboard in a 128-byte internal buffer, and the user can edit the input
with the Backspace key and the special function keys. MS-DOS automatically
echoes the characters to the standard output, expanding tab characters to
spaces (although they are left as the ASCII code 09H in the buffer). The
Ctrl-C, Ctrl-S, and Ctrl-P key combinations receive special handling, and
the Enter key is translated to a carriage return─linefeed pair. When the
user presses Enter or Ctrl-Z, MS-DOS copies the requested number of
characters (or the actual number of characters entered, if less than the
number requested) out of the internal buffer into the calling program's
buffer.
In binary mode, MS-DOS never echoes input characters. It passes the
Ctrl-C, Ctrl-S, Ctrl-P, and Ctrl-Z key combinations and the Enter key
through to the application unchanged, and Int 21H Function 3FH does not
return control to the application until the exact number of characters
requested has been received.
Ctrl-C checking is discussed in more detail at the end of this chapter.
For now, simply note that the application programmer can substitute a
custom handler for the default MS-DOS Ctrl-C handler and thereby avoid
having the application program lose control of the machine when the user
enters a Ctrl-C or Ctrl-Break.
Keyboard Input with Traditional Calls
The MS-DOS traditional keyboard functions offer a variety of character and
line-oriented services with or without echo and Ctrl-C detection. These
functions are summarized on the following page.
Int 21H Function Action Ctrl-C checking
──────────────────────────────────────────────────────────────────────────
01H Keyboard input with echo Yes
06H Direct console I/O No
07H Keyboard input without echo No
08H Keyboard input without echo Yes
0AH Buffered keyboard input Yes
0BH Input-status check Yes
0CH Input-buffer reset and input Varies
──────────────────────────────────────────────────────────────────────────
In MS-DOS versions 2.0 and later, redirection of the standard input
affects all these functions. In other words, they act as though they were
special cases of an Int 21H Function 3FH call using the predefined
standard input handle (0).
The character-input functions (01H, 06H, 07H, and 08H) all return a
character in the AL register. For example, the following sequence waits
until a key is pressed and then returns it in AL:
──────────────────────────────────────────────────────────────────────────
mov ah,1 ; function 01h = read keyboard
int 21h ; transfer to MS-DOS
──────────────────────────────────────────────────────────────────────────
The character-input functions differ in whether the input is echoed to the
screen and whether they are sensitive to Ctrl-C interrupts. Although
MS-DOS provides no pure keyboard-status function that is immune to Ctrl-C,
a program can read keyboard status (somewhat circuitously) without
interference by using Int 21H Function 06H. Extended keys, such as the
IBM PC keyboard's special function keys, require two calls to a
character-input function.
As an alternative to single-character input, a program can use
buffered-line input (Int 21H Function 0AH) to obtain an entire line from
the keyboard in one operation. MS-DOS builds up buffered lines in an
internal buffer and does not pass them to the calling program until the
user presses the Enter key. While the line is being entered, all the usual
editing keys are active and are handled by the MS-DOS keyboard driver. You
use Int 21H Function 0AH as follows:
──────────────────────────────────────────────────────────────────────────
buff db 81 ; maximum length of input
db 0 ; actual length (from MS-DOS)
db 81 dup (0) ; receives keyboard input
.
.
.
mov ah,0ah ; function 0ah = read buffered line
mov dx,seg buff ; DS:DX = buffer address
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
.
.
.
──────────────────────────────────────────────────────────────────────────
Int 21H Function 0AH differs from Int 21H Function 3FH in several
important ways. First, the maximum length is passed in the first byte of
the buffer, rather than in the CX register. Second, the actual length is
returned in the second byte of the structure, rather than in the AX
register. Finally, when the user has entered one less than the specified
maximum number of characters, MS-DOS ignores all subsequent characters and
sounds a warning beep until the Enter key is pressed.
For detailed information about each of the traditional keyboard-input
functions, see Section II of this book, "MS-DOS Functions Reference."
Keyboard Input with ROM BIOS Functions
Programmers writing applications for IBM PC compatibles can bypass the
MS-DOS keyboard functions and choose from two hardware-dependent
techniques for keyboard input.
The first method is to call the ROM BIOS keyboard driver using Int 16H.
For example, the following sequence reads a single character from the
keyboard input buffer and returns it in the AL register:
──────────────────────────────────────────────────────────────────────────
mov ah,0 ; function 0=read keyboard
int 16h ; transfer to ROM BIOS
──────────────────────────────────────────────────────────────────────────
Int 16H Function 00H also returns the keyboard scan code in the AH
register, allowing the program to detect key codes that are not ordinarily
returned by MS-DOS. Other Int 16H services return the keyboard status
(that is, whether a character is waiting) or the keyboard shift state
(from the ROM BIOS data area 0000:0417H). For a more detailed explanation
of ROM BIOS keyboard functions, see Section III of this book, "IBM ROM
BIOS and Mouse Functions Reference."
You should consider carefully before building ROM BIOS dependence into an
application. Although this technique allows you to bypass any I/O
redirection that may be in effect, ways exist to do this without
introducing dependence on the ROM BIOS. And there are real disadvantages
to calling the ROM BIOS keyboard driver:
■ It always bypasses I/O redirection, which sometimes may not be
desirable.
■ It is dependent on IBM PC compatibility and does not work correctly,
unchanged, on some older machines such as the Hewlett-Packard
TouchScreen or the Wang Professional Computer.
■ It may introduce complicated interactions with TSR utilities.
The other and more hardware-dependent method of keyboard input on an IBM
PC is to write a new handler for ROM BIOS Int 09H and service the keyboard
controller's interrupts directly. This involves translation of scan codes
to ASCII characters and maintenance of the type-ahead buffer. In ordinary
PC applications, there is no reason to take over keyboard I/O at this
level; therefore, I will not discuss this method further here. If you are
curious about the techniques that would be required, the best reference is
the listing for the ROM BIOS Int 09H handler in the IBM PC or PC/AT
technical reference manual.
Ctrl-C and Ctrl-Break Handlers
In the discussion of keyboard input with the MS-DOS handle and traditional
functions, I made some passing references to the fact that Ctrl-C entries
can interfere with the expected behavior of those functions. Let's look at
this subject in more detail now.
During most character I/O operations, MS-DOS checks for a Ctrl-C (ASCII
code 03H) waiting at the keyboard and executes an Int 23H if one is
detected. If the system break flag is on, MS-DOS also checks for a Ctrl-C
entry during certain other operations (such as file reads and writes).
Ordinarily, the Int 23H vector points to a routine that simply terminates
the currently active process and returns control to the parent process──
usually the MS-DOS command interpreter.
In other words, if your program is executing and you enter a Ctrl-C,
accidentally or intentionally, MS-DOS simply aborts the program. Any files
the program has opened using file control blocks will not be closed
properly, any interrupt vectors it has altered may not be restored
correctly, and if it is performing any direct I/O operations (for example,
if it contains an interrupt driver for the serial port), all kinds of
unexpected events may occur.
Although you can use a number of partially effective methods to defeat
Ctrl-C checking, such as performing keyboard input with Int 21H Functions
06H and 07H, placing all character devices into binary mode, or turning
off the system break flag with Int 21H Function 33H, none of these is
completely foolproof. The simplest and most elegant way to defeat Ctrl-C
checking is simply to substitute your own Int 23H handler, which can take
some action appropriate to your program. When the program terminates,
MS-DOS automatically restores the previous contents of the Int 23H vector
from information saved in the program segment prefix. The following
example shows how to install your own Ctrl-C handler (which in this case
does nothing at all):
──────────────────────────────────────────────────────────────────────────
push ds ; save data segment
; set int 23h vector...
mov ax,2523h ; function 25h = set interrupt
; int 23h = vector for
; Ctrl-C handler
mov dx,seg handler ; DS:DX = handler address
mov ds,dx
mov dx,offset handler
int 21h ; transfer to MS-DOS
pop ds ; restore data segment
.
.
.
handler: ; a Ctrl-C handler
iret ; that does nothing
──────────────────────────────────────────────────────────────────────────
The first part of the code (which alters the contents of the Int 23H
vector) would be executed in the initialization part of the application.
The handler receives control whenever MS-DOS detects a Ctrl-C at the
keyboard. (Because this handler consists only of an interrupt return, the
Ctrl-C will remain in the keyboard input stream and will be passed to the
application when it requests a character from the keyboard, appearing on
the screen as ^C.)
When an Int 23H handler is called, MS-DOS is in a stable state. Thus, the
handler can call any MS-DOS function. It can also reset the segment
registers and the stack pointer and transfer control to some other point
in the application without ever returning control to MS-DOS with an IRET.
On IBM PC compatibles, an additional interrupt handler must be taken into
consideration. Whenever the ROM BIOS keyboard driver detects the key
combination Ctrl-Break, it calls a handler whose address is stored in the
vector for Int 1BH. The default ROM BIOS Int 1BH handler does nothing.
MS-DOS alters the Int 1BH vector to point to its own handler, which sets a
flag and returns; the net effect is to remap the Ctrl-Break into a Ctrl-C
that is forced ahead of any other characters waiting in the keyboard
buffer.
Taking over the Int 1BH vector in an application is somewhat tricky but
extremely useful. Because the keyboard is interrupt driven, a press of
Ctrl-Break lets the application regain control under almost any
circumstance──often, even if the program has crashed or is in an endless
loop.
You cannot, in general, use the same handler for Int 1BH that you use for
Int 23H. The Int 1BH handler is more limited in what it can do, because it
has been called as a result of a hardware interrupt and MS-DOS may have
been executing a critical section of code at the time the interrupt was
serviced. Thus, all registers except CS:IP are in an unknown state; they
may have to be saved and then modified before your interrupt handler can
execute. Similarly, the depth of the stack in use when the Int 1BH handler
is called is unknown, and if the handler is to perform stack-intensive
operations, it may have to save the stack segment and the stack pointer
and switch to a new stack that is known to have sufficient depth.
In normal application programs, you should probably avoid retaining
control in an Int 1BH handler, rather than performing an IRET. Because of
subtle differences among non-IBM ROM BIOSes, it is difficult to predict
the state of the keyboard controller and the 8259 Programmable Interrupt
Controller (PIC) when the Int 1BH handler begins executing. Also, MS-DOS
itself may not be in a stable state at the point of interrupt, a situation
that can manifest itself in unexpected critical errors during subsequent
I/O operations. Finally, MS-DOS versions 3.2 and later allocate a stack
from an internal pool for use by the Int 09H handler. If the Int 1BH
handler never returns, the Int 09H handler never returns either, and
repeated entries of Ctrl-Break will eventually exhaust the stack pool,
halting the system.
Because Int 1BH is a ROM BIOS interrupt and not an MS-DOS interrupt,
MS-DOS does not restore the previous contents of the Int 1BH vector when a
program exits. If your program modifies this vector, it must save the
original value and restore it before terminating. Otherwise, the vector
will be left pointing to some random area in the next program that runs,
and the next time the user presses Ctrl-Break a system crash is the best
you can hope for.
Ctrl-C and Ctrl-Break Handlers and High-Level Languages
Capturing the Ctrl-C and Ctrl-Break interrupts is straightforward when you
are programming in assembly language. The process is only slightly more
difficult with high-level languages, as long as you have enough
information about the language's calling conventions that you can link in
a small assembly-language routine as part of the program.
The BREAK.ASM listing in Figure 5-1 contains source code for a Ctrl-Break
handler that can be linked with small-model Microsoft C programs running
on an IBM PC compatible. The short C program in Figure 5-2 demonstrates
use of the handler. (This code should be readily portable to other C
compilers.)
──────────────────────────────────────────────────────────────────────────
page 55,132
title Ctrl-C & Ctrl-Break Handlers
name break
;
; Ctrl-C and Ctrl-Break handler for Microsoft C
; programs running on IBM PC compatibles
;
; by Ray Duncan
;
; Assemble with: C>MASM /Mx BREAK;
;
; This module allows C programs to retain control
; when the user enters a Ctrl-Break or Ctrl-C.
; It uses Microsoft C parameter-passing conventions
; and assumes the C small memory model.
;
; The procedure _capture is called to install
; a new handler for the Ctrl-C and Ctrl-Break
; interrupts (1bh and 23h). _capture is passed
; the address of a static variable, which will be
; set to true by the handler whenever a Ctrl-C
; or Ctrl-Break is detected. The C syntax is:
;
; static int flag;
; capture(&flag);
;
; The procedure _release is called by the C program
; to restore the original Ctrl-Break and Ctrl-C
; handler. The C syntax is:
; release();
;
; The procedure ctrlbrk is the actual interrupt
; handler. It receives control when a software
; int 1bh is executed by the ROM BIOS or int 23h
; is executed by MS-DOS. It simply sets the C
; program's variable to true (1) and returns.
;
args equ 4 ; stack offset of arguments,
; C small memory model
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII linefeed
_TEXT segment word public 'CODE'
assume cs:_TEXT
public _capture
_capture proc near ; take over Ctrl-Break
; and Ctrl-C interrupt vectors
push bp ; set up stack frame
mov bp,sp
push ds ; save registers
push di
push si
; save address of
; calling program's "flag"
mov ax,word ptr [bp+args]
mov word ptr cs:flag,ax
mov word ptr cs:flag+2,ds
; save address of original
mov ax,3523h ; int 23h handler
int 21h
mov word ptr cs:int23,bx
mov word ptr cs:int23+2,es
mov ax,351bh ; save address of original
int 21h ; int 1bh handler
mov word ptr cs:int1b,bx
mov word ptr cs:int1b+2,es
push cs ; set DS:DX = address
pop ds ; of new handler
mov dx,offset _TEXT:ctrlbrk
mov ax,02523h ; set int 23h vector
int 21h
mov ax,0251bh ; set int 1bh vector
int 21h
pop si ; restore registers
pop di
pop ds
pop bp ; discard stack frame
ret ; and return to caller
_capture endp
public _release
_release proc near ; restore original Ctrl-C
; and Ctrl-Break handlers
push bp ; save registers
push ds
push di
push si
lds dx,cs:int1b ; get address of previous
; int 1bh handler
mov ax,251bh ; set int 1bh vector
int 21h
lds dx,cs:int23 ; get address of previous
; int 23h handler
mov ax,2523h ; set int 23h vector
int 21h
pop si ; restore registers
pop di ; and return to caller
pop ds
pop bp
ret
release endp
ctrlbrk proc far ; Ctrl-C and Ctrl-Break
; interrupt handler
push bx ; save registers
push ds
lds bx,cs:flag ; get address of C program's
; "flag variable"
; and set the flag "true"
mov word ptr ds:[bx],1
pop ds ; restore registers
pop bx
iret ; return from handler
ctrlbrk endp
flag dd 0 ; far pointer to caller's
; Ctrl-Break or Ctrl-C flag
int23 dd 0 ; address of original
; Ctrl-C handler
int1b dd 0 ; address of original
; Ctrl-Break handler
_TEXT ends
end
──────────────────────────────────────────────────────────────────────────
Figure 5-1. BREAK.ASM: A Ctrl-C and Ctrl-Break interrupt handler that can
be linked with Microsoft C programs.
──────────────────────────────────────────────────────────────────────────
/*
TRYBREAK.C
Demo of BREAK.ASM Ctrl-Break and Ctrl-C
interrupt handler, by Ray Duncan
To create the executable file TRYBREAK.EXE, enter:
MASM /Mx BREAK;
CL TRYBREAK.C BREAK.OBJ
*/
#include <stdio.h>
main(int argc, char *argv[])
{
int hit = 0; /* flag for key press */
int c = 0; /* character from keyboard */
static int flag = 0; /* true if Ctrl-Break
or Ctrl-C detected */
puts("\n*** TRYBREAK.C running ***\n");
puts("Press Ctrl-C or Ctrl-Break to test handler,");
puts("Press the Esc key to exit TRYBREAK.\n");
capture(&flag); /* install new Ctrl-C and
Ctrl-Break handler and
pass address of flag */
puts("TRYBREAK has captured interrupt vectors.\n");
while(1)
{
hit = kbhit(); /* check for key press */
/* (MS-DOS sees Ctrl-C
when keyboard polled) */
if(flag != 0) /* if flag is true, an */
{ /* interrupt has occurred */
puts("\nControl-Break detected.\n");
flag = 0; /* reset interrupt flag */
}
if(hit != 0) /* if any key waiting */
{
c = getch(); /* read key, exit if Esc */
if( (c & 0x7f) == 0x1b) break;
putch(c); /* otherwise display it */
}
}
release(); /* restore original Ctrl-C
and Ctrl-Break handlers */
puts("\n\nTRYBREAK has released interrupt vectors.");
}
──────────────────────────────────────────────────────────────────────────
Figure 5-2. TRYBREAK.C: A simple Microsoft C program that demonstrates
use of the interrupt handler BREAK.ASM from Figure 5-1.
In the example handler, the procedure named capture is called with the
address of an integer variable within the C program. It saves the address
of the variable, points the Int 1BH and Int 23H vectors to its own
interrupt handler, and then returns.
When MS-DOS detects a Ctrl-C or Ctrl-Break, the interrupt handler sets the
integer variable within the C program to true (1) and returns. The C
program can then poll this variable at its leisure. Of course, to detect
more than one Ctrl-C, the program must reset the variable to zero again.
The procedure named release simply restores the Int 1BH and Int 23H
vectors to their original values, thereby disabling the interrupt handler.
Although it is not strictly necessary for release to do anything about Int
23H, this action does give the C program the option of restoring the
default handler for Int 23H without terminating.
Pointing Devices
Device drivers for pointing devices are supplied by the hardware
manufacturer and are loaded with a DEVICE statement in the CONFIG.SYS
file. Although the hardware characteristics of the available pointing
devices differ greatly, nearly all of their drivers present the same
software interface to application programs: the Int 33H protocol used by
the Microsoft Mouse driver. Version 6 of the Microsoft Mouse driver (which
was current as this was written) offers the following functions:
╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖
Function Meaning
──────────────────────────────────────────────────────────────────────────
00H Reset mouse and get status.
Function Meaning
──────────────────────────────────────────────────────────────────────────
00H Reset mouse and get status.
01H Show mouse pointer.
02H Hide mouse pointer.
03H Get button status and pointer position.
04H Set pointer position.
05H Get button-press information.
06H Get button-release information.
07H Set horizontal limits for pointer.
08H Set vertical limits for pointer.
09H Set graphics pointer type.
0AH Set text pointer type.
0BH Read mouse-motion counters.
0CH Install interrupt handler for mouse events.
0DH Turn on light pen emulation.
0EH Turn off light pen emulation.
0FH Set mickeys to pixel ratio.
10H Set pointer exclusion area.
13H Set double-speed threshold.
14H Swap mouse-event interrupt routines.
Function Meaning
──────────────────────────────────────────────────────────────────────────
14H Swap mouse-event interrupt routines.
15H Get buffer size for mouse-driver state.
16H Save mouse-driver state.
17H Restore mouse-driver state.
18H Install alternate handler for mouse events.
19H Get address of alternate handler.
1AH Set mouse sensitivity.
1BH Get mouse sensitivity.
1CH Set mouse interrupt rate.
1DH Select display page for pointer.
1EH Get display page for pointer.
1FH Disable mouse driver.
20H Enable mouse driver.
21H Reset mouse driver.
22H Set language for mouse-driver messages.
23H Get language number.
24H Get driver version, mouse type, and IRQ number.
──────────────────────────────────────────────────────────────────────────
Function Meaning
──────────────────────────────────────────────────────────────────────────
Although this list of mouse functions may appear intimidating, the average
application will only need a few of them.
A program first calls Int 33H Function 00H to initialize the mouse driver
for the current display mode and to check its status. At this point, the
mouse is "alive" and the application can obtain its state and position;
however, the pointer does not become visible until the process calls Int
33H Function 01H.
The program can then call Int 33H Functions 03H, 05H, and 06H to
monitor the mouse position and the status of the mouse buttons.
Alternatively, the program can register an interrupt handler for mouse
events, using Int 33H Function 0CH. This latter technique eliminates the
need to poll the mouse driver; the driver will notify the program by
calling the interrupt handler whenever the mouse is moved or a button is
pressed or released.
When the application is finished with the mouse, it can call Int 33H
Function 02H to hide the mouse pointer. If the program has registered an
interrupt handler for mouse events, it should disable further calls to the
handler by resetting the mouse driver again with Int 33H Function 00H.
For a complete description of the mouse-driver functions, see Section
III of this book, "IBM ROM BIOS and Mouse Functions Reference." Figure
5-3 shows a small demonstration program that polls the mouse continually,
to display its position and status.
──────────────────────────────────────────────────────────────────────────
/*
Simple Demo of Int 33H Mouse Driver
(C) 1988 Ray Duncan
Compile with: CL MOUDEMO.C
*/
#include <stdio.h>
#include <dos.h>
union REGS regs;
void cls(void); /* function prototypes */
void gotoxy(int, int);
main(int argc, char *argv[])
{
int x,y,buttons; /* some scratch variables */
/* for the mouse state */
regs.x.ax = 0; /* reset mouse driver */
int86(0x33, ®s, ®s); /* and check status */
if(regs.x.ax == 0) /* exit if no mouse */
{ printf("\nMouse not available\n");
exit(1);
}
cls(); /* clear the screen */
gotoxy(45,0); /* and show help info */
puts("Press Both Mouse Buttons To Exit");
regs.x.ax = 1; /* display mouse cursor */
int86(0x33, ®s, ®s);
do {
regs.x.ax = 3; /* get mouse position */
int86(0x33, ®s, ®s); /* and button status */
buttons = regs.x.bx & 3;
x = regs.x.cx;
y = regs.x.dx;
gotoxy(0,0); /* display mouse position */
printf("X = %3d Y = %3d", x, y);
} while(buttons != 3); /* exit if both buttons down */
regs.x.ax = 2; /* hide mouse cursor */
int86(0x33, ®s, ®s);
cls(); /* display message and exit */
gotoxy(0,0);
puts("Have a Mice Day!");
}
/*
Clear the screen
*/
void cls(void)
{
regs.x.ax = 0x0600; /* ROM BIOS video driver */
regs.h.bh = 7; /* int 10h function 06h */
regs.x.cx = 0; /* initializes a window */
regs.h.dh = 24;
regs.h.dl = 79;
int86(0x10, ®s, ®s);
}
/*
Position cursor to (x,y)
*/
void gotoxy(int x, int y)
{
regs.h.dl = x; /* ROM BIOS video driver */
regs.h.dh = y; /* int 10h function 02h */
regs.h.bh = 0; /* positions the cursor */
regs.h.ah = 2;
int86(0x10, ®s, ®s);
}
──────────────────────────────────────────────────────────────────────────
Figure 5-3. MOUDEMO.C: A simple Microsoft C program that polls the mouse
and continually displays the coordinates of the mouse pointer in the upper
left corner of the screen. The program uses the ROM BIOS video driver,
which is discussed in Chapter 6, to clear the screen and position the
text cursor.
────────────────────────────────────────────────────────────────────────────
Chapter 6 Video Display
The visual presentation of an application program is one of its most
important elements. Users frequently base their conclusions about a
program's performance and "polish" on the speed and attractiveness of its
displays. Therefore, a feel for the computer system's display facilities
and capabilities at all levels, from MS-DOS down to the bare hardware, is
important to you as a programmer.
Video Display Adapters
The video display adapters found in IBM PC─compatible computers have a
hybrid interface to the central processor. The overall display
characteristics, such as vertical and horizontal resolution, background
color, and palette, are controlled by values written to I/O ports whose
addresses are hardwired on the adapter, whereas the appearance of each
individual character or graphics pixel on the display is controlled by a
specific location within an area of memory called the regen buffer or
refresh buffer. Both the CPU and the video controller access this memory;
the software updates the display by simply writing character codes or bit
patterns directly into the regen buffer. (This is called memory-mapped
I/O.)
The following adapters are in common use as this book is being written:
■ Monochrome/Printer Display Adapter (MDA). Introduced with the original
IBM PC in 1981, this adapter supports 80-by-25 text display on a green
(monochrome) screen and has no graphics capabilities at all.
■ Color/Graphics Adapter (CGA). Also introduced by IBM in 1981, this
adapter supports 40-by-25 and 80-by-25 text modes and 320-by-200,
4-color or 640-by-200, 2-color graphics (all-points-addressable, or
APA) modes on composite or digital RGB monitors.
■ Enhanced Graphics Adapter (EGA). Introduced by IBM in 1985 and upwardly
compatible from the CGA, this adapter adds support for 640-by-350,
16-color graphics modes on digital RGB monitors. It also supports an
MDA-compatible text mode.
■ Multi-Color Graphics Array (MCGA). Introduced by IBM in 1987 with the
Personal System/2 (PS/2) models 25 and 30, this adapter is partially
compatible with the CGA and EGA and supports 640-by-480, 2-color or
320-by-200, 256-color graphics on analog RGB monitors.
■ Video Graphics Array (VGA). Introduced by IBM in 1987 with the PS/2
models 50, 60, and 80, this adapter is upwardly compatible from the EGA
and supports 640-by-480, 16-color or 320-by-200, 256-color graphics on
analog RGB monitors. It also supports an MDA-compatible text mode.
■ Hercules Graphics Card, Graphics CardPlus, and InColor Cards. These are
upwardly compatible from the MDA for text display but offer graphics
capabilities that are incompatible with all of the IBM adapters.
The locations of the regen buffers for the various IBM PC─compatible
adapters are shown in Figure 6-1.
┌───────────────────────────────────────────────────────┐
│ ROM BIOS │
FE000H ├───────────────────────────────────────────────────────┤
│ System ROM, Stand-alone BASIC, etc. │
F4000H ├───────────────────────────────────────────────────────┤
│ Reserved for BIOS extensions │
│ (hard-disk controller, etc.) │
C0000H ├───────────────────────────────────────────────────────┤
│ Reserved │
BC000H ├───────────────────────────────────────────────────────┤
│ 16 KB regen buffer for CGA, EGA, MCGA, and VGA │
│ in text modes and 200-line graphics modes │
B8000H ├───────────────────────────────────────────────────────┤
│ Reserved │
B1000H ├───────────────────────────────────────────────────────┤
│ 4 KB Monochrome Adapter regen buffer │
B0000H ├───────────────────────────────────────────────────────┤
│ Regen buffer area for EGA, MCGA, and VGA │
│ in 350-line or 480-line graphics modes │
A0000H ├───────────────────────────────────────────────────────┤
│ Transient part of COMMAND.COM │
├───────────────────────────────────────────────────────┤
│ Transient program area │
varies ├───────────────────────────────────────────────────────┤
│ MS-DOS and its buffers, │
│ tables, and device drivers │
00400H ├───────────────────────────────────────────────────────┤
│ Interrupt vectors │
00000H └───────────────────────────────────────────────────────┘
Figure 6-1. Memory diagram of an IBM PC─compatible personal computer,
showing the locations of the regen buffers for various adapters.
Support Considerations
MS-DOS offers several functions to transfer text to the display. Version 1
supported only Teletype-like output capabilities; version 2 added an
optional ANSI console driver to allow the programmer to clear the screen,
position the cursor, and select colors and attributes with standard escape
sequences embedded in the output. Programs that use only the MS-DOS
functions will operate properly on any computer system that runs MS-DOS,
regardless of the level of IBM hardware compatibility.
On IBM PC─compatible machines, the ROM BIOS contains a video driver that
programs can invoke directly, bypassing MS-DOS. The ROM BIOS functions
allow a program to write text or individual pixels to the screen or to
select display modes, video pages, palette, and foreground and background
colors. These functions are relatively efficient (compared with the MS-DOS
functions, at least), although the graphics support is primitive.
Unfortunately, the display functions of both MS-DOS and the ROM BIOS were
designed around the model of a cursor-addressable terminal and therefore
do not fully exploit the capabilities of the memory-mapped, high-bandwidth
display adapters used on IBM PC─compatible machines. As a result, nearly
every popular interactive application with full-screen displays or
graphics capability ignores both MS-DOS and the ROM BIOS and writes
directly to the video controller's registers and regen buffer.
Programs that control the hardware directly are sometimes called
"ill-behaved," because they are performing operations that are normally
reserved for operating-system device drivers. These programs are a severe
management problem in multitasking real-mode environments such as DesqView
and Microsoft Windows, and they are the main reason why such environments
are not used more widely. It could be argued, however, that the blame for
such problematic behavior lies not with the application programs but with
the failure of MS-DOS and the ROM BIOS──even six years after the first
appearance of the IBM PC──to provide display functions of adequate range
and power.
MS-DOS Display Functions
Under MS-DOS versions 2.0 and later, the preferred method for sending text
to the display is to use handle-based Int 21H Function 40H (Write File or
Device). When an application program receives control, MS-DOS has already
assigned it handles for the standard output (1) and standard error (2)
devices, and these handles can be used immediately. For example, the
sequence at the top of the following page writes the message hello to the
display using the standard output handle.
──────────────────────────────────────────────────────────────────────────
msg db 'hello' ; message to display
msg_len equ $-msg ; length of message
.
.
.
mov ah,40h ; function 40h = write file or device
mov bx,1 ; BX = standard output handle
mov cx,msg_len ; CX = message length
mov dx,seg msg ; DS:DX = address of message
mov ds,dx
mov dx,offset msg
int 21h ; transfer to MS-DOS
jc error ; jump if error detected
.
.
.
──────────────────────────────────────────────────────────────────────────
If there is no error, the function returns the carry flag cleared and the
number of characters actually transferred in register AX. Unless a Ctrl-Z
is embedded in the text or the standard output is redirected to a disk
file and the disk is full, this number should equal the number of
characters requested.
As in the case of keyboard input, the user's ability to specify
command-line redirection parameters that are invisible to the application
means that if you use the predefined standard output handle, you can't
always be sure where your output is going. However, to ensure that your
output actually goes to the display, you can use the predefined standard
error handle, which is always opened to the CON (logical console) device
and is not redirectable.
As an alternative to the standard output and standard error handles, you
can bypass any output redirection and open a separate channel to CON,
using the handle obtained from that open operation for character output.
For example, the following code opens the console display for output and
then writes the string hello to it:
──────────────────────────────────────────────────────────────────────────
fname db 'CON',0 ; name of CON device
handle dw 0 ; handle for CON device
msg db 'hello' ; message to display
msg_len equ $-msg ; length of message
.
.
.
mov ax,3d02h ; AH = function 3dh = open
; AL = mode = read/write
mov dx,seg fname ; DS:DX = device name
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if open failed
mov handle,ax ; save handle for CON
.
.
.
mov ah,40h ; function 40h = write
mov cx,msg_len ; CX = message length
mov dx,seg msg ; DS:DX = address of message
mov ds,dx
mov dx,offset msg
mov bx,handle ; BX = CON device handle
int 21h ; transfer to MS-DOS
jc error ; jump if error detected
.
.
.
──────────────────────────────────────────────────────────────────────────
As with the keyboard input functions, MS-DOS also supports traditional
display functions that are upwardly compatible from the corresponding CP/M
output calls:
■ Int 21H Function 02H sends the character in the DL register to the
standard output device. It is sensitive to Ctrl-C interrupts, and it
handles carriage returns, linefeeds, bell codes, and backspaces
appropriately.
■ Int 21H Function 06H transfers the character in the DL register to the
standard output device, but it is not sensitive to Ctrl-C interrupts.
You must take care when using this function, because it can also be
used for input and for status requests.
■ Int 21H Function 09H sends a string to the standard output device. The
string is terminated by the $ character.
With MS-DOS version 2 or later, these three traditional functions are
converted internally to handle-based writes to the standard output and
thus are susceptible to output redirection.
The sequence at the top of the following page sounds a warning beep by
sending an ASCII bell code (07H) to the display driver using the
traditional character-output call Int 21H Function 02H.
──────────────────────────────────────────────────────────────────────────
.
.
.
mov dl,7 ; 07h = ASCII bell code
mov ah,2 ; function 02h = display character
int 21h ; transfer to MS-DOS
.
.
.
──────────────────────────────────────────────────────────────────────────
The following sequence uses the traditional string-output call Int 21H
Function 09H to display a string:
──────────────────────────────────────────────────────────────────────────
msg db 'hello$'
.
.
.
mov dx,seg msg ; DS:DX = message address
mov ds,dx
mov dx,offset msg
mov ah,9 ; function 09h = write string
int 21h ; transfer to MS-DOS
.
.
.
──────────────────────────────────────────────────────────────────────────
Note that MS-DOS detects the $ character as a terminator and does not
display it on the screen.
Screen Control with MS-DOS Functions
With version 2.0 or later, if MS-DOS loads the optional device driver
ANSI.SYS in response to a DEVICE directive in the CONFIG.SYS file,
programs can clear the screen, control the cursor position, and select
foreground and background colors by embedding escape sequences in the text
output. Escape sequences are so called because they begin with an escape
character (1BH), which alerts the driver to intercept and interpret the
subsequent characters in the sequence. When the ANSI driver is not loaded,
MS-DOS simply passes the escape sequence to the display like any other
text, usually resulting in a chaotic screen.
The escape sequences that can be used with the ANSI driver for screen
control are a subset of those defined in the ANSI 3.64─1979 Standard.
These standard sequences are summarized in Figure 6-2. Note that case is
significant for the last character in an escape sequence and that numbers
must always be represented as ASCII digit strings, not as their binary
values. (A separate set of escape sequences supported by ANSI.SYS, but not
compatible with the ANSI standard, may be used for reprogramming and
remapping the keyboard.)
╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖
Escape sequence Meaning
──────────────────────────────────────────────────────────────────────────
Esc[2J Clear screen; place cursor in upper left corner (home
position).
Esc[K Clear from cursor to end of line.
Esc[row;colH Position cursor. (Row is the y coordinate in the range
1─25 and col is the x coordinate in the range 1─80 for
80-by-25 text display modes.) Escape sequences
terminated with the letter f instead of H have the same
effect.
Escape sequence Meaning
──────────────────────────────────────────────────────────────────────────
effect.
Esc[nA Move cursor up n rows.
Esc[nB Move cursor down n rows.
Esc[nC Move cursor right n columns.
Esc[nD Move cursor left n columns.
Esc[s Save current cursor position.
Esc[u Restore cursor to saved position.
Esc[6n Return current cursor position on the standard input
handle in the format Esc[row;colR.
Esc[nm Select character attributes:
0 = no special attributes
1 = high intensity
2 = low intensity
3 = italic
4 = underline
5 = blink
6 = rapid blink
7 = reverse video
8 = concealed text (no display)
Escape sequence Meaning
──────────────────────────────────────────────────────────────────────────
8 = concealed text (no display)
30 = foreground black
31 = foreground red
32 = foreground green
33 = foreground yellow
34 = foreground blue
35 = foreground magenta
36 = foreground cyan
37 = foreground white
40 = background black
41 = background red
42 = background green
43 = background yellow
44 = background blue
45 = background magenta
46 = background cyan
47 = background white
Esc[=nh Select display mode:
0 = 40-by-25, 16-color text (color burst off)
Escape sequence Meaning
──────────────────────────────────────────────────────────────────────────
0 = 40-by-25, 16-color text (color burst off)
1 = 40-by-25, 16-color text
2 = 80-by-25, 16-color text (color burst off)
3 = 80-by-25, 16-color text
4 = 320-by-200, 4-color graphics
5 = 320-by-200, 4-color graphics (color burst off)
6 = 620-by-200, 2-color graphics
14 = 640-by-200, 16-color graphics (EGA and VGA,
MS-DOS 4.0)
15 = 640-by-350, 2-color graphics (EGA and VGA,
MS-DOS 4.0)
16 = 640-by-350, 16-color graphics (EGA and VGA,
MS-DOS 4.0)
17 = 640-by-480, 2-color graphics (MCGA and VGA,
MS-DOS 4.0)
18 = 640-by-480, 16-color graphics (VGA, MS-DOS 4.0)
19 = 320-by-200, 256-color graphics (MCGA and VGA,
MS-DOS 4.0)
Escape sequences terminated with l instead of h have
Escape sequence Meaning
──────────────────────────────────────────────────────────────────────────
Escape sequences terminated with l instead of h have
the same effect.
Esc[=7h Enable line wrap.
Esc[=7l Disable line wrap.
──────────────────────────────────────────────────────────────────────────
Figure 6-2. The ANSI escape sequences supported by the MS-DOS ANSI.SYS
driver. Programs running under MS-DOS 2.0 or later may use these
functions, if ANSI.SYS is loaded, to control the appearance of the display
in a hardware-independent manner. The symbol Esc indicates an ASCII escape
code──a character with the value 1BH. Note that cursor positions in ANSI
escape sequences are one-based, unlike the cursor coordinates used by the
IBM ROM BIOS, which are zero-based. Numbers embedded in an escape sequence
must always be represented as a string of ASCII digits, not as their
binary values.
Binary Output Mode
Under MS-DOS version 2 or later, you can substantially increase display
speeds for well-behaved application programs without sacrificing hardware
independence by selecting binary (raw) mode for the standard output. In
binary mode, MS-DOS does not check between each character it transfers to
the output device for a Ctrl-C waiting at the keyboard, nor does it filter
the output string for certain characters such as Ctrl-Z.
Bit 5 in the device information word associated with a device handle
controls binary mode. Programs access the device information word by using
Subfunctions 00H and 01H of the MS-DOS IOCTL function (I/O Control, Int
21H Function 44H). For example, the sequence on the following page places
the standard output handle into binary mode.
──────────────────────────────────────────────────────────────────────────
; get device information...
mov bx,1 ; standard output handle
mov ax,4400h ; function 44h subfunction 00h
int 21h ; transfer to MS-DOS
mov dh,0 ; set upper byte of DX = 0
or dl,20h ; set binary mode bit in DL
; write device information...
; (BX still has handle)
mov ax,4401h ; function 44h subfunction 01h
int 21h ; transfer to MS-DOS
──────────────────────────────────────────────────────────────────────────
Note that if a program changes the mode of any of the standard handles, it
should restore those handles to ASCII (cooked) mode before it exits.
Otherwise, subsequent application programs may behave in unexpected ways.
For more detailed information on the IOCTL function, see Section II of
this book, "MS-DOS Functions Reference."
The ROM BIOS Display Functions
You can somewhat improve the display performance of programs that are
intended for use only on IBM PC─compatible machines by using the ROM BIOS
video driver instead of the MS-DOS output functions. Accessed by means of
Int 10H, the ROM BIOS driver supports the following functions for all of
the currently available IBM display adapters:
╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖
Function Action
──────────────────────────────────────────────────────────────────────────
Display mode control
00H Set display mode.
0FH Get display mode.
Cursor control
01H Set cursor size.
02H Set cursor position.
03H Get cursor position and size.
Writing to the display
09H Write character and attribute at cursor.
0AH Write character-only at cursor.
0EH Write character in teletype mode.
Reading from the display
08H Read character and attribute at cursor.
Function Action
──────────────────────────────────────────────────────────────────────────
Graphics support
0CH Write pixel.
0DH Read pixel.
Scroll or clear display
06H Scroll up or initialize window.
07H Scroll down or initialize window.
Miscellaneous
04H Read light pen.
05H Select display page.
0BH Select palette/set border color.
──────────────────────────────────────────────────────────────────────────
Additional ROM BIOS functions are available on the EGA, MCGA, VGA, and
PCjr to support the enhanced features of these adapters, such as
programmable palettes and character sets (fonts). Some of the functions
are valid only in certain display modes.
Each display mode is characterized by the number of colors it can display,
its vertical resolution, its horizontal resolution, and whether it
supports text or graphics memory mapping. The ROM BIOS identifies it with
a unique number. Section III of this book, "IBM ROM BIOS and Mouse
Functions Reference," documents all of the ROM BIOS Int 10H functions and
display modes.
As you can see from the preceding list, the ROM BIOS offers several
desirable capabilities that are not available from MS-DOS, including
initialization or scrolling of selected screen windows, modification of
the cursor shape, and reading back the character being displayed at an
arbitrary screen location. These functions can be used to isolate your
program from the hardware on any IBM PC─compatible adapter. However, the
ROM BIOS functions do not suffice for the needs of a high-performance,
interactive, full-screen program such as a word processor. They do not
support the rapid display of character strings at an arbitrary screen
position, and they do not implement graphics operations at the level
normally required by applications (for example, bit-block transfers and
rapid drawing of lines, circles, and filled polygons). And, of course,
they are of no use whatsoever in non-IBM display modes such as the
monochrome graphics mode of the Hercules Graphics Card.
Let's look at a simple example of a call to the ROM BIOS video driver. The
following sequence writes the string hello to the screen:
──────────────────────────────────────────────────────────────────────────
msg db 'hello'
msg_len equ $-msg
.
.
.
mov si,seg msg ; DS:SI = message address
mov ds,si
mov si,offset msg
mov cx,msg_len ; CX = message length
cld
next: lodsb ; get AL = next character
push si ; save message pointer
mov ah,0eh ; int 10h function 0eh = write
; character in teletype mode
mov bh,0 ; assume video page 0
mov bl,color ; (use in graphics modes only)
int 10h ; transfer to ROM BIOS
pop si ; restore message pointer
loop next ; loop until message done
.
.
.
──────────────────────────────────────────────────────────────────────────
(Note that the SI and DI registers are not necessarily preserved across a
call to a ROM BIOS video function.)
Memory-mapped Display Techniques
Display performance is best when an application program takes over
complete control of the video adapter and the refresh buffer. Because the
display is memory-mapped, the speed at which characters can be put on the
screen is limited only by the CPU's ability to copy bytes from one
location in memory to another. The trade-off for this performance is that
such programs are highly sensitive to hardware compatibility and do not
always function properly on "clones" or even on new models of IBM video
adapters.
Text Mode
Direct programming of the IBM PC─compatible video adapters in their text
display modes (sometimes also called alphanumeric display modes) is
straightforward. The character set is the same for all, and the cursor
home position──(x,y) = (0,0)──is defined to be the upper left corner of
the screen (Figure 6-3). The MDA uses 4 KB of memory starting at segment
B000H as a regen buffer, and the various adapters with both text and
graphics capabilities (CGA, EGA, MCGA, and VGA) use 16 KB of memory
starting at segment B800H. (See Figure 6-1.) In the latter case, the 16
KB is divided into "pages" that can be independently updated and
displayed.
(0,0)┌─────────────────────────────────┐(79,0)
│ │
│ │
│ │
│ │
│ │
│ │
│ │
(0,24)└─────────────────────────────────┘(79,24)
Figure 6-3. Cursor addressing for 80-by-25 text display modes (IBM ROM
BIOS modes 2, 3, and 7).
Each character-display position is allotted 2 bytes in the regen buffer.
The first byte (even address) contains the ASCII code of the character,
which is translated by a special hardware character generator into a
dot-matrix pattern for the screen. The second byte (odd address) is the
attribute byte. Several bit fields in this byte control such features as
blinking, intensity (highlighting), and reverse video, depending on the
adapter type and display mode (Figures 6-4 and 6-5). Figure 6-6 shows a
hex and ASCII dump of part of the video map for the MDA.
Display Background Foreground
──────────────────────────────────────────────────────────────────────────
No display (black) 000 000
No display (white)☼ 111 111
Underline 000 001
Normal video 000 111
Reverse video 111 000
──────────────────────────────────────────────────────────────────────────
Figure 6-4. Attribute byte for 80-by-25 monochrome text display mode on
the MDA, Hercules cards, EGA, and VGA (IBM ROM BIOS mode 7).
Value Color
──────────────────────────────────────────────────────────────────────────
0 Black
1 Blue
2 Green
3 Cyan
4 Red
5 Magenta
6 Brown
7 White
8 Gray
9 Light blue
10 Light green
11 Light cyan
12 Light red
13 Light magenta
14 Yellow
15 Intense white
──────────────────────────────────────────────────────────────────────────
Figure 6-5. Attribute byte for the 40-by-25 and 80-by-25 text display
modes on the CGA, EGA, MCGA, and VGA (IBM ROM BIOS modes 0─3). The table
of color values assumes default palette programming and that the B or I
bit controls intensity.
──────────────────────────────────────────────────────────────────────────
B000:0000 3e 07 73 07 65 07 6c 07 65 07 63 07 74 07 20 07
B000:0010 74 07 65 07 6d 07 70 07 20 07 20 07 20 07 20 07
B000:0020 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07
B000:0030 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07
B000:0040 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07
B000:0050 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07
B000:0060 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07
B000:0070 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07
B000:0080 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07
B000:0090 20 07 20 07 20 07 20 07 20 07 20 07 20 07 20 07
──────────────────────────────────────────────────────────────────────────
Figure 6-6. Example dump of the first 160 bytes of the MDA's regen
buffer. These bytes correspond to the first visible line on the screen.
Note that ASCII character codes are stored in even bytes and their
respective character attributes in odd bytes; all the characters in this
example line have the attribute normal video.
You can calculate the memory offset of any character on the display as the
line number (y coordinate) times 80 characters per line times 2 bytes per
character, plus the column number (x coordinate) times 2 bytes per
character, plus (for the text/graphics adapters) the page number times the
size of the page (4 KB per page in 80-by-25 modes; 2 KB per page in
40-by-25 modes). In short, the formula for the offset of the
character-attribute pair for a given screen position (x,y) in 80-by-25
text modes is
offset = ((y * 50H + x) * 2) + (page * 1000H)
In 40-by-25 text modes, the formula is
offset = ((y * 50H + x) * 2) + (page * 0800H)
Of course, the segment register being used to address the video buffer
must be set appropriately, depending on the type of display adapter.
As a simple example, assume that the character to be displayed is in the
AL register, the desired attribute byte for the character is in the AH
register, the x coordinate (column) is in the BX register, and the y
coordinate (row) is in the CX register. The following code stores the
character and attribute byte into the MDA's video refresh buffer at the
proper location:
──────────────────────────────────────────────────────────────────────────
push ax ; save char and attribute
mov ax,160
mul cx ; DX:AX = Y * 160
shl bx,1 ; multiply X by 2
add bx,ax ; BX = (Y*160) + (X*2)
mov ax,0b000h ; ES = segment of monochrome
mov es,ax ; adapter refresh buffer
pop ax ; restore char and attribute
mov es:[bx],ax ; write them to video buffer
──────────────────────────────────────────────────────────────────────────
More frequently, we wish to move entire strings into the refresh buffer,
starting at a given coordinate. In the next example, assume that the DS:SI
registers point to the source string, the ES:DI registers point to the
starting position in the video buffer (calculated as shown in the previous
example), the AH register contains the attribute byte to be assigned to
every character in the string, and the CX register contains the length of
the string. The following code moves the entire string into the refresh
buffer:
──────────────────────────────────────────────────────────────────────────
xfer: lodsb ; fetch next character
stosw ; store char + attribute
loop xfer ; until all chars moved
──────────────────────────────────────────────────────────────────────────
Of course, the video drivers written for actual application programs must
take into account many additional factors, such as checking for special
control codes (linefeeds, carriage returns, tabs), line wrap, and
scrolling.
Programs that write characters directly to the CGA regen buffer in text
modes must deal with an additional complicating factor──they must examine
the video controller's status port and access the refresh buffer only
during the horizontal retrace or vertical retrace intervals. (A retrace
interval is the period when the electron beam that illuminates the screen
phosphors is being repositioned to the start of a new scan line.)
Otherwise, the contention for memory between the CPU and the video
controller is manifest as unsightly "snow" on the display. (If you are
writing programs for any of the other IBM PC─compatible video adapters,
such as the MDA, EGA, MCGA, or VGA, you can ignore the retrace intervals;
snow is not a problem with these video controllers.)
A program can detect the occurrence of a retrace interval by monitoring
certain bits in the video controller's status register. For example,
assume that the offset for the desired character position has been
calculated as in the preceding example and placed in the BX register, the
segment for the CGA's refresh buffer is in the ES register, and an ASCII
character code to be displayed is in the CL register. The following code
waits for the beginning of a new horizontal retrace interval and then
writes the character into the buffer:
──────────────────────────────────────────────────────────────────────────
mov dx,03dah ; DX = video controller's
; status port address
cli ; disable interrupts
; if retrace is already
; in progress, wait for
; it to end...
wait1: in al,dx ; read status port
and al,1 ; check if retrace bit on
jnz wait1 ; yes, wait
; wait for new retrace
; interval to start...
wait2: in al,dx ; read status port
and al,1 ; retrace bit on yet?
jz wait2 ; jump if not yet on
mov es:[bx],cl ; write character to
; the regen buffer
sti ; enable interrupts again
──────────────────────────────────────────────────────────────────────────
The first wait loop "synchronizes" the code to the beginning of a
horizontal retrace interval. If only the second wait loop were used (that
is, if a character were written when a retrace interval was already in
progress), the write would occasionally begin so close to the end of a
horizontal retrace "window" that it would partially miss the retrace,
resulting in scattered snow at the left edge of the display. Notice that
the code also disables interrupts during accesses to the video buffer, so
that service of a hardware interrupt won't disrupt the synchronization
process.
Because of the retrace-interval constraints just outlined, the rate at
which you can update the CGA in text modes is severely limited when the
updating is done one character at a time. You can obtain better results by
calculating all the relevant addresses and setting up the appropriate
registers, disabling the video controller by writing to register 3D8H,
moving the entire string to the buffer with a REP MOVSW operation, and
then reenabling the video controller. If the string is of reasonable
length, the user won't even notice a flicker in the display. Of course,
this procedure introduces additional hardware dependence into your code
because it requires much greater knowledge of the 6845 controller.
Luckily, snow is not a problem in CGA graphics modes.
Graphics Mode
Graphics-mode memory-mapped programming for IBM PC─compatible adapters is
considerably more complicated than text-mode programming. Each bit or
group of bits in the regen buffer corresponds to an addressable point, or
pixel, on the screen. The mapping of bits to pixels differs for each of
the available graphics modes, with their differences in resolution and
number of supported colors. The newer adapters (EGA, MCGA, and VGA) also
use the concept of bit planes, where bits of a pixel are segregated into
multiple banks of memory mapped at the same address; you must manipulate
these bit planes by a combination of memory-mapped I/O and port
addressing.
IBM-video-systems graphics programming is a subject large enough for a
book of its own, but we can use the 640-by-200, 2-color graphics display
mode of the CGA (which is also supported by all subsequent IBM
text/graphics adapters) to illustrate a few of the techniques involved.
This mode is simple to deal with because each pixel is represented by a
single bit. The pixels are assigned (x,y) coordinates in the range (0,0)
through (639,199), where x is the horizontal displacement, y is the
vertical displacement, and the home position (0,0) is the upper left
corner of the display. (See Figure 6-7.)
(0,0)┌─────────────────────────────────┐(639,0)
│ │
│ │
│ │
│ │
│ │
│ │
│ │
(0,199)└─────────────────────────────────┘(639,199)
Figure 6-7. Point addressing for 640-by-200, 2-color graphics modes on
the CGA, EGA, MCGA, and VGA (IBM ROM BIOS mode 6).
Each successive group of 80 bytes (640 bits) represents one horizontal
scan line. Within each byte, the bits map one-for-one onto pixels, with
the most significant bit corresponding to the leftmost displayed pixel of
a set of eight pixels and the least significant bit corresponding to the
rightmost displayed pixel of the set. The memory map is set up so that all
the even y coordinates are scanned as a set and all the odd y coordinates
are scanned as a set; this mapping is referred to as the memory interlace.
To find the regen buffer offset for a particular (x,y) coordinate, you
would use the following formula:
offset = ((y AND 1) * 2000H) + (y/2 * 50H) + (x/8)
The assembly-language implementation of this formula is as follows:
──────────────────────────────────────────────────────────────────────────
; assume AX = Y, BX = X
shr bx,1 ; divide X by 8
shr bx,1
shr bx,1
push ax ; save copy of Y
shr ax,1 ; find (Y/2) * 50h
mov cx,50h ; with product in DX:AX
mul cx
add bx,ax ; add product to X/8
pop ax ; add (Y AND 1) * 2000h
and ax,1
jz label1
add bx,2000h
label1: ; now BX = offset into
; video buffer
──────────────────────────────────────────────────────────────────────────
After calculating the correct byte address, you can use the following
formula to calculate the bit position for a given pixel coordinate:
bit = 7 - (x MOD 8)
where bit 7 is the most significant bit and bit 0 is the least significant
bit. It is easiest to build an 8-byte table, or array of bit masks, and
use the operation X AND 7 to extract the appropriate entry from the table:
(X AND 7) Bit mask (X AND 7) Bit mask
──────────────────────────────────────────────────────────────────────────
0 80H 4 08H
1 40H 5 04H
2 20H 6 02H
3 10H 7 01H
──────────────────────────────────────────────────────────────────────────
The assembly-language implementation of this second calculation is as
follows:
──────────────────────────────────────────────────────────────────────────
table db 80h ; X AND 7 = offset 0
db 40h ; X AND 7 = offset 1
db 20h ; X AND 7 = offset 2
db 10h ; X AND 7 = offset 3
db 08h ; X AND 7 = offset 4
db 04h ; X AND 7 = offset 5
db 02h ; X AND 7 = offset 6
db 01h ; X AND 7 = offset 7
.
.
.
; assume BX = X coordinate
and bx,7 ; isolate 0─7 offset
mov al,[bx+table]
; now AL = mask from table
.
.
.
──────────────────────────────────────────────────────────────────────────
The program can then use the mask, together with the byte offset
previously calculated, to set or clear the appropriate bit in the video
controller's regen buffer.
────────────────────────────────────────────────────────────────────────────
Chapter 7 Printer and Serial Port
MS-DOS supports printers, plotters, modems, and other hard-copy output or
communication devices with device drivers for parallel ports and serial
ports. Parallel ports are so named because they transfer a byte──8 bits──
in parallel to the destination device over eight separate physical paths
(plus additional status and handshaking signals). The serial port, on the
other hand, communicates with the CPU with bytes but sends data to or
receives data from its destination device serially──a bit at a time──over
a single physical connection.
Parallel ports are typically used for high-speed output devices, such as
line printers, over relatively short distances (less than 50 feet). They
are rarely used for devices that require two-way communication with the
computer. Serial ports are used for lower-speed devices, such as modems
and terminals, that require two-way communication (although some printers
also have serial interfaces). A serial port can drive its device reliably
over much greater distances (up to 1000 feet) over as few as three wires──
transmit, receive, and ground.
The most commonly used type of serial interface follows a standard called
RS-232. This standard specifies a 25-wire interface with certain
electrical characteristics, the use of various handshaking signals, and a
standard DB-25 connector. Other serial-interface standards exist──for
example, the RS-422, which is capable of considerably higher speeds than
the RS-232── but these are rarely used in personal computers (except for
the Apple Macintosh) at this time.
MS-DOS has built-in device drivers for three parallel adapters, and for
two serial adapters on the PC or PC/AT and three serial adapters on the
PS/2. The logical names for these devices are LPT1, LPT2, LPT3, COM1,
COM2, and COM3. The standard printer (PRN) and standard auxiliary (AUX)
devices are normally aliased to LPT1 and COM1, but you can redirect PRN to
one of the serial ports with the MS-DOS MODE command.
As with keyboard and video display I/O, you can manage printer and
serial-port I/O at several levels that offer different degrees of
flexibility and hardware independence:
■ MS-DOS handle-oriented functions
■ MS-DOS traditional character functions
■ IBM ROM BIOS driver functions
In the case of the serial port, direct control of the hardware by
application programs is also common. I will discuss each of these I/O
methods briefly, with examples, in the following pages.
Printer Output
The preferred method of printer output is to use the handle write function
(Int 21H Function 40H) with the predefined standard printer handle (4).
For example, you could write the string hello to the printer as follows:
──────────────────────────────────────────────────────────────────────────
msg db 'hello' ; message for printer
msg_len equ $-msg ; length of message
.
.
.
mov ah,40h ; function 40h = write file or device
mov bx,4 ; BX = standard printer handle
mov cx,msg_len ; CX = length of string
mov dx,seg msg ; DS:DX = string address
mov ds,dx
mov dx,offset msg
int 21h ; transfer to MS-DOS
jc error ; jump if error
.
.
.
──────────────────────────────────────────────────────────────────────────
If there is no error, the function returns the carry flag cleared and the
number of characters actually transferred to the list device in register
AX. Under normal circumstances, this number should always be the same as
the length requested and the carry flag indicating an error should never
be set. However, the output will terminate early if your data contains an
end-of-file mark (Ctrl-Z).
You can write independently to several list devices (for example, LPT1,
LPT2) by issuing a specific open request (Int 21H Function 3DH) for each
device and using the handles returned to access the printers individually
with Int 21H Function 40H. You have already seen this general approach in
Chapters 5 and 6.
An alternative method of printer output is to use the traditional Int 21H
Function 05H, which transfers the character in the DL register to the
printer. (This function is sensitive to Ctrl-C interrupts.) For example,
the assembly-language code sequence at the top of the following page would
write the the string hello to the line printer.
──────────────────────────────────────────────────────────────────────────
msg db 'hello' ; message for printer
msg_len equ $-msg ; length of message
.
.
.
mov bx,seg msg ; DS:BX = string address
mov ds,bx
mov bx,offset msg
mov cx,msg_len ; CX = string length
next: mov dl,[bx] ; get next character
mov ah,5 ; function 05h = printer output
int 21h ; transfer to MS-DOS
inc bx ; bump string pointer
loop next ; loop until string done
.
.
.
──────────────────────────────────────────────────────────────────────────
Programs that run on IBM PC─compatible machines can obtain improved
printer throughput by bypassing MS-DOS and calling the ROM BIOS printer
driver directly by means of Int 17H. Section III of this book, "IBM ROM
BIOS and Mouse Functions Reference," documents the Int 17H functions in
detail. Use of the ROM BIOS functions also allows your program to test
whether the printer is off line or out of paper, a capability that MS-DOS
does not offer.
For example, the following sequence of instructions calls the ROM BIOS
printer driver to send the string hello to the line printer:
──────────────────────────────────────────────────────────────────────────
msg db 'hello' ; message for printer
msg_len equ $-msg ; length of message
.
.
.
mov bx,seg msg ; DS:BX = string address
mov ds,bx
mov bx,offset msg
mov cx,msg_len ; CX = string length
mov dx,0 ; DX = printer number
next: mov al,[bx] ; AL = character to print
mov ah,0 ; function 00h = printer output
int 17h ; transfer to ROM BIOS
inc bx ; bump string pointer
loop next ; loop until string done
.
.
.
──────────────────────────────────────────────────────────────────────────
Note that the printer numbers used by the ROM BIOS are zero-based, whereas
the printer numbers in MS-DOS logical-device names are one-based. For
example, ROM BIOS printer 0 corresponds to LPT1.
Finally, the most hardware-dependent technique of printer output is to
access the printer controller directly. Considering the functionality
already provided in MS-DOS and the IBM ROM BIOS, as well as the speeds of
the devices involved, I cannot see any justification for using direct
hardware control in this case. The disadvantage of introducing such
extreme hardware dependence for such a low-speed device would far outweigh
any small performance gains that might be obtained.
The Serial Port
MS-DOS support for serial ports (often referred to as the auxiliary device
in MS-DOS manuals) is weak compared with its keyboard, video-display, and
printer support. This is one area where the application programmer is
justified in making programs hardware dependent to extract adequate
performance.
Programs that restrict themselves to MS-DOS functions to ensure
portability can use the handle read and write functions (Int 21H Functions
3FH and 40H), with the predefined standard auxiliary handle (3) to
access the serial port. For example, the following code writes the string
hello to the serial port that is currently defined as the AUX device:
──────────────────────────────────────────────────────────────────────────
msg db 'hello' ; message for serial port
msg_len equ $-msg ; length of message
.
.
.
mov ah,40h ; function 40h = write file or device
mov bx,3 ; BX = standard aux handle
mov cx,msg_len ; CX = string length
mov dx,seg msg ; DS:DX = string address
mov ds,dx
mov dx,offset msg
int 21h ; transfer to MS-DOS
jc error ; jump if error
.
.
.
──────────────────────────────────────────────────────────────────────────
The standard auxiliary handle gives access to only the first serial port
(COM1). If you want to read or write COM2 and COM3 using the handle calls,
you must issue an open request (Int 21H Function 3DH) for the desired
serial port and use the handle returned by that function with Int 21H
Functions 3FH and 40H.
Some versions of MS-DOS have a bug in character-device handling that
manifests itself as follows: If you issue a read request with Int 21H
Function 3FH for the exact number of characters that are waiting in the
driver's buffer, the length returned in the AX register is the number of
characters transferred minus one. You can circumvent this problem by
always requesting more characters than you expect to receive or by placing
the device handle into binary mode using Int 21H Function 44H.
MS-DOS also supports two traditional functions for serial-port I/O. Int
21H Function 03H inputs a character from COM1 and returns it in the AL
register; Int 21H Function 04H transmits the character in the DL register
to COM1. Like the other traditional calls, these two are direct
descendants of the CP/M auxiliary-device functions.
For example, the following code sends the string hello to COM1 using the
traditional Int 21H Function 04H:
──────────────────────────────────────────────────────────────────────────
msg db 'hello' ; message for serial port
msg_len equ $-msg ; length of message
.
.
.
mov bx,seg msg ; DS:BX = string address
mov ds,bx
mov bx,offset msg
mov cx,msg_len ; CX = length of string
mov dl,[bx] ; get next character
mov ah,4 ; function 04h = aux output
int 21h ; transfer to MS-DOS
inc bx ; bump pointer to string
loop next ; loop until string done
.
.
.
──────────────────────────────────────────────────────────────────────────
MS-DOS translates the traditional auxiliary-device functions into calls on
the same device driver used by the handle calls. Therefore, it is
generally preferable to use the handle functions in the first place,
because they allow very long strings to be read or written in one
operation, they give access to serial ports other than COM1, and they are
symmetrical with the handle video-display, keyboard, printer, and file I/O
methods described elsewhere in this book.
Although the handle or traditional serial-port functions allow you to
write programs that are portable to any machine running MS-DOS, they have
a number of disadvantages:
■ The built-in MS-DOS serial-port driver is slow and is not interrupt
driven.
■ MS-DOS serial-port I/O is not buffered.
■ Determining the status of the auxiliary device requires a separate call
to the IOCTL function (Int 21H Function 44H)──if you request input and
no characters are ready, your program will simply hang.
■ MS-DOS offers no standardized function to configure the serial port
from within a program.
For programs that are going to run on the IBM PC or compatibles, a more
flexible technique for serial-port I/O is to call the IBM ROM BIOS
serial-port driver by means of Int 14H. You can use this driver to
initialize the serial port to a desired configuration and baud rate,
examine the status of the controller, and read or write characters.
Section III of this book, "IBM ROM BIOS and Mouse Functions Reference,"
documents the functions available from the ROM BIOS serial-port driver.
For example, the following sequence sends the character X to the first
serial port (COM1):
──────────────────────────────────────────────────────────────────────────
.
.
.
mov ah,1 ; function 01h = send character
mov al,'X' ; AL = character to transmit
mov dx,0 ; DX = serial-port number
int 14h ; transfer to ROM BIOS
and ah,80h ; did transmit fail?
jnz error ; jump if transmit error
.
.
.
──────────────────────────────────────────────────────────────────────────
As with the ROM BIOS printer driver, the serial-port numbers used by the
ROM BIOS are zero-based, whereas the serial-port numbers in MS-DOS
logical-device names are one-based. In this example, serial port 0
corresponds to COM1.
Unfortunately, like the MS-DOS auxiliary-device driver, the ROM BIOS
serial-port driver is not interrupt driven. Although it will support
higher transfer speeds than the MS-DOS functions, at rates greater than
2400 baud it may still lose characters. Consequently, most programmers
writing high-performance applications that use a serial port (such as
telecommunications programs) take complete control of the serial-port
controller and provide their own interrupt driver. The built-in functions
provided by MS-DOS, and by the ROM BIOS in the case of the IBM PC, are
simply not adequate.
Writing such programs requires a good understanding of the hardware. In
the case of the IBM PC, the chips to study are the INS8250 Asynchronous
Communications Controller and the Intel 8259A Programmable Interrupt
Controller. The IBM technical reference documentation for these chips is a
bit disorganized, but most of the necessary information is there if you
look for it.
The TALK Program
The simple terminal-emulator program TALK.ASM (Figure 7-1) is an example
of a useful program that performs screen, keyboard, and serial-port I/O.
This program recapitulates all of the topics discussed in Chapters 5
through 7. TALK uses the IBM PC's ROM BIOS video driver to put characters
on the screen, to clear the display, and to position the cursor; it uses
the MS-DOS character-input calls to read the keyboard; and it contains its
own interrupt driver for the serial-port controller.
──────────────────────────────────────────────────────────────────────────
name talk
page 55,132
.lfcond ; List false conditionals too
title TALK--Simple terminal emulator
;
; TALK.ASM--Simple IBM PC terminal emulator
;
; Copyright (c) 1988 Ray Duncan
;
; To assemble and link this program into TALK.EXE:
;
; C>MASM TALK;
; C>LINK TALK;
;
stdin equ 0 ; standard input handle
stdout equ 1 ; standard output handle
stderr equ 2 ; standard error handle
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII linefeed
bsp equ 08h ; ASCII backspace
escape equ 1bh ; ASCII escape code
dattr equ 07h ; display attribute to use
; while in emulation mode
bufsiz equ 4096 ; size of serial-port buffer
echo equ 0 ; 0 = full-duplex, -1 = half-duplex
equ -1
false equ 0
com1 equ true ; use COM1 if nonzero
com2 equ not com1 ; use COM2 if nonzero
pic_mask equ 21h ; 8259 interrupt mask port
pic_eoi equ 20h ; 8259 EOI port
if com1
com_data equ 03f8h ; port assignments for COM1
com_ier equ 03f9h
com_mcr equ 03fch
com_sts equ 03fdh
com_int equ 0ch ; COM1 interrupt number
int_mask equ 10h ; IRQ4 mask for 8259
endif
if com2
com_data equ 02f8h ; port assignments for COM2
com_ier equ 02f9h
com_mcr equ 02fch
com_sts equ 02fdh
com_int equ 0bh ; COM2 interrupt number
int_mask equ 08h ; IRQ3 mask for 8259
endif
_TEXT segment word public 'CODE'
assume cs:_TEXT,ds:_DATA,es:_DATA,ss:STACK
talk proc far ; entry point from MS-DOS
mov ax,_DATA ; make data segment addressable
mov ds,ax
mov es,ax
; initialize display for
; terminal emulator mode...
mov ah,15 ; get display width and
int 10h ; current display mode
dec ah ; save display width for use
mov columns,ah ; by the screen-clear routine
cmp al,7 ; enforce text display mode
je talk2 ; mode 7 ok, proceed
cmp al,3
jbe talk2 ; modes 0-3 ok, proceed
mov dx,offset msg1
mov cx,msg1_len
jmp talk6 ; print error message and exit
talk2: mov bh,dattr ; clear screen and home cursor
call cls
call asc_enb ; capture serial-port interrupt
; vector and enable interrupts
mov dx,offset msg2 ; display message
mov cx,msg2_len ; 'terminal emulator running'
mov bx,stdout ; BX = standard output handle
mov ah,40h ; function 40h = write file or device
int 21h ; transfer to MS-DOS
talk3: call pc_stat ; keyboard character waiting?
jz talk4 ; nothing waiting, jump
call pc_in ; read keyboard character
cmp al,0 ; is it a function key?
jne talk32 ; not function key, jump
call pc_in ; function key, discard 2nd
; character of sequence
jmp talk5 ; then terminate program
talk32: ; keyboard character received
if echo
push ax ; if half-duplex, echo
call pc_out ; character to PC display
pop ax
endif
call com_out ; write char to serial port
talk4: call com_stat ; serial-port character waiting?
jz talk3 ; nothing waiting, jump
call com_in ; read serial-port character
cmp al,20h ; is it control code?
jae talk45 ; jump if not
call ctrl_code ; control code, process it
jmp talk3 ; check keyboard again
talk45: ; noncontrol char received,
call pc_out ; write it to PC display
jmp talk4 ; see if any more waiting
talk5: ; function key detected,
; prepare to terminate...
mov bh,07h ; clear screen and home cursor
call cls
mov dx,offset msg3 ; display farewell message
mov cx,msg3_len
talk6: push dx ; save message address
push cx ; and message length
call asc_dsb ; disable serial-port interrupts
; and release interrupt vector
pop cx ; restore message length
pop dx ; and address
mov bx,stdout ; handle for standard output
mov ah,40h ; function 40h = write device
int 21h ; transfer to MS-DOS
mov ax,4c00h ; terminate program with
int 21h ; return code = 0
talk endp
com_stat proc near ; check asynch status; returns
; Z = false if character ready
; Z = true if nothing waiting
push ax
mov ax,asc_in ; compare ring buffer pointers
cmp ax,asc_out
pop ax
ret ; return to caller
stat endp
com_in proc near ; get character from serial-
; port buffer; returns
; new character in AL
push bx ; save register BX
com_in1: ; if no char waiting, wait
mov bx,asc_out ; until one is received
cmp bx,asc_in
je com_in1 ; jump, nothing waiting
mov al,[bx+asc_buf] ; character is ready,
; extract it from buffer
inc bx ; update buffer pointer
cmp bx,bufsiz
jne com_in2
xor bx,bx ; reset pointer if wrapped
com_in2:
mov asc_out,bx ; store updated pointer
pop bx ; restore register BX
ret ; and return to caller
com_in endp
com_out proc near ; write character in AL
; to serial port
push dx ; save register DX
push ax ; save character to send
mov dx,com_sts ; DX = status port address
com_out1: ; check if transmit buffer
in al,dx ; is empty (TBE bit = set)
and al,20h
jz com_out1 ; no, must wait
pop ax ; get character to send
mov dx,com_data ; DX = data port address
out dx,al ; transmit the character
pop dx ; restore register DX
ret ; and return to caller
com_out endp
pc_stat proc near ; read keyboard status; returns
; Z = false if character ready
; Z = true if nothing waiting
; register DX destroyed
mov al,in_flag ; if character already
or al,al ; waiting, return status
jnz pc_stat1
mov ah,6 ; otherwise call MS-DOS to
mov dl,0ffh ; determine keyboard status
int 21h
jz pc_stat1 ; jump if no key ready
mov in_char,al ; got key, save it for
mov in_flag,0ffh ; "pc_in" routine
pc_stat1: ; return to caller with
ret ; Z flag set appropriately
pc_stat endp
pc_in proc near ; read keyboard character,
; return it in AL
; DX may be destroyed
mov al,in_flag ; key already waiting?
or al,al
jnz pc_in1 ; yes, return it to caller
call pc_stat ; try to read a character
jmp pc_in
pc_in1: mov in_flag,0 ; clear char-waiting flag
mov al,in_char ; and return AL = character
ret
pc_in endp
pc_out proc near ; write character in AL
; to the PC's display
mov ah,0eh ; ROM BIOS function 0eh =
; "teletype output"
push bx ; save register BX
xor bx,bx ; assume page 0
int 10h ; transfer to ROM BIOS
pop bx ; restore register BX
ret ; and return to caller
pc_out endp
cls proc near ; clear display using
; char attribute in BH
; registers AX, CX,
; and DX destroyed
mov dl,columns ; set DL,DH = X,Y of
mov dh,24 ; lower right corner
mov cx,0 ; set CL,CH = X,Y of
; upper left corner
mov ax,600h ; ROM BIOS function 06h =
; "scroll or initialize
; window"
int 10h ; transfer to ROM BIOS
call home ; set cursor at (0,0)
ret ; and return to caller
cls endp
clreol proc near ; clear from cursor to end
; of line using attribute
; in BH, registers AX, CX,
; and DX destroyed
call getxy ; get current cursor position
mov cx,dx ; current position = "upper
; left corner" of window;
mov dl,columns ; "lower right corner" X is
; max columns, Y is same
; as upper left corner
mov ax,600h ; ROM BIOS function 06h =
; "scroll or initialize
; window"
int 10h ; transfer to ROM BIOS
ret ; return to caller
clreol endp
home proc near ; put cursor at home position
mov dx,0 ; set (X,Y) = (0,0)
call gotoxy ; position the cursor
ret ; return to caller
home endp
gotoxy proc near ; position the cursor
; call with DL = X, DH = Y
push bx ; save registers
push ax
mov bh,0 ; assume page 0
mov ah,2 ; ROM BIOS function 02h =
; set cursor position
int 10h ; transfer to ROM BIOS
pop ax ; restore registers
pop bx
ret ; and return to caller
gotoxy endp
getxy proc near ; get cursor position,
; returns DL = X, DH = Y
push ax ; save registers
push bx
push cx
mov ah,3 ; ROM BIOS function 03h =
; get cursor position
mov bh,0 ; assume page 0
int 10h ; transfer to ROM BIOS
pop cx ; restore registers
pop bx
pop ax
ret ; and return to caller
getxy endp
ctrl_code proc near ; process control code
; call with AL = char
cmp al,cr ; if carriage return
je ctrl8 ; just send it
cmp al,lf ; if linefeed
je ctrl8 ; just send it
cmp al,bsp ; if backspace
je ctrl8 ; just send it
cmp al,26 ; is it cls control code?
jne ctrl7 ; no, jump
mov bh,dattr ; cls control code, clear
call cls ; screen and home cursor
jmp ctrl9
ctrl7:
cmp al,escape ; is it Escape character?
jne ctrl9 ; no, throw it away
call esc_seq ; yes, emulate CRT terminal
jmp ctrl9
ctrl8: call pc_out ; send CR, LF, or backspace
; to the display
ctrl9: ret ; return to caller
ctrl_code endp
esc_seq proc near ; decode Televideo 950 escape
; sequence for screen control
call com_in ; get next character
cmp al,84 ; is it clear to end of line?
jne esc_seq1 ; no, jump
mov bh,dattr ; yes, clear to end of line
call clreol
jmp esc_seq2 ; then exit
esc_seq1:
cmp al,61 ; is it cursor positioning?
jne esc_seq2 ; no jump
call com_in ; yes, get Y parameter
sub al,33 ; and remove offset
mov dh,al
call com_in ; get X parameter
sub al,33 ; and remove offset
mov dl,al
call gotoxy ; position the cursor
esc_seq2: ; return to caller
ret
esc_seq endp
asc_enb proc near ; capture serial-port interrupt
; vector and enable interrupt
; save address of previous
; interrupt handler...
mov ax,3500h+com_int ; function 35h = get vector
int 21h ; transfer to MS-DOS
mov word ptr oldvec+2,es
mov word ptr oldvec,bx
; now install our handler...
push ds ; save our data segment
mov ax,cs ; set DS:DX = address
mov ds,ax ; of our interrupt handler
mov dx,offset asc_int
mov ax,2500h+com_int ; function 25h = set vector
int 21h ; transfer to MS-DOS
pop ds ; restore data segment
mov dx,com_mcr ; set modem-control register
mov al,0bh ; DTR and OUT2 bits
out dx,al
mov dx,com_ier ; set interrupt-enable
mov al,1 ; register on serial-
out dx,al ; port controller
in al,pic_mask ; read current 8259 mask
and al,not int_mask ; set mask for COM port
out pic_mask,al ; write new 8259 mask
ret ; back to caller
asc_enb endp
asc_dsb proc near ; disable interrupt and
; release interrupt vector
in al,pic_mask ; read current 8259 mask
or al,int_mask ; reset mask for COM port
out pic_mask,al ; write new 8259 mask
push ds ; save our data segment
lds dx,oldvec ; load address of
; previous interrupt handler
mov ax,2500h+com_int ; function 25h = set vector
int 21h ; transfer to MS-DOS
pop ds ; restore data segment
ret ; back to caller
asc_dsb endp
asc_int proc far ; interrupt service routine
; for serial port
sti ; turn interrupts back on
push ax ; save registers
push bx
push dx
push ds
mov ax,_DATA ; make our data segment
mov ds,ax ; addressable
cli ; clear interrupts for
; pointer manipulation
mov dx,com_data ; DX = data port address
in al,dx ; read this character
mov bx,asc_in ; get buffer pointer
mov [asc_buf+bx],al ; store this character
inc bx ; bump pointer
cmp bx,bufsiz ; time for wrap?
jne asc_int1 ; no, jump
xor bx,bx ; yes, reset pointer
asc_int1: ; store updated pointer
mov asc_in,bx
sti ; turn interrupts back on
mov al,20h ; send EOI to 8259
out pic_eoi,al
pop ds ; restore all registers
pop dx
pop bx
pop ax
iret ; return from interrupt
asc_int endp
_TEXT ends
_DATA segment word public 'DATA'
in_char db 0 ; PC keyboard input char
in_flag db 0 ; <>0 if char waiting
columns db 0 ; highest numbered column in
; current display mode (39 or 79)
msg1 db cr,lf
db 'Display must be text mode.'
db cr,lf
msg1_len equ $-msg1
msg2 db 'Terminal emulator running...'
db cr,lf
msg2_len equ $-msg2
msg3 db 'Exit from terminal emulator.'
db cr,lf
msg3_len equ $-msg3
oldvec dd 0 ; original contents of serial-
; port interrupt vector
asc_in dw 0 ; input pointer to ring buffer
asc_out dw 0 ; output pointer to ring buffer
asc_buf db bufsiz dup (?) ; communications buffer
_DATA ends
STACK segment para stack 'STACK'
db 128 dup (?)
STACK ends
end talk ; defines entry point
──────────────────────────────────────────────────────────────────────────
Figure 7-1. TALK.ASM: A simple terminal-emulator program for IBM
PC─compatible computers. This program demonstrates use of the MS-DOS and
ROM BIOS video and keyboard functions and direct control of the
serial-communications adapter.
The TALK program illustrates the methods that an application should use to
take over and service interrupts from the serial port without running
afoul of MS-DOS conventions.
The program begins with some equates and conditional assembly statements
that configure the program for half- or full-duplex and for the desired
serial port (COM1 or COM2). At entry from MS-DOS, the main routine of the
program──the procedure named talk──checks the status of the serial port,
initializes the display, and calls the asc_enb routine to take over the
serial-port interrupt vector and enable interrupts. The talk procedure
then enters a loop that reads the keyboard and sends the characters out
the serial port and then reads the serial port and puts the characters on
the display──in other words, it causes the PC to emulate a simple CRT
terminal.
The TALK program intercepts and handles control codes (carriage return,
linefeed, and so forth) appropriately. It detects escape sequences and
handles them as a subset of the Televideo 950 terminal capabilities. (You
can easily modify the program to emulate any other cursor-addressable
terminal.) When one of the PC's special function keys is pressed, the
program disables serial-port interrupts, releases the serial-port
interrupt vector, and exits back to MS-DOS.
There are several TALK program procedures that are worth your attention
because they can easily be incorporated into other programs. These are
listed in the table on the following page.
╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖
Procedure Action
──────────────────────────────────────────────────────────────────────────
asc_enb Takes over the serial-port interrupt vector and enables
interrupts by writing to the modem-control register of
the INS8250 and the interrupt-mask register of the
8259A.
asc_dsb Restores the original state of the serial-port
interrupt vector and disables interrupts by writing to
the interrupt-mask register of the 8259A.
asc_int Services serial-port interrupts, placing received
characters into a ring buffer.
com_stat Tests whether characters from the serial port are
waiting in the ring buffer.
com_in Removes characters from the interrupt handler's ring
buffer and increments the buffer pointers
appropriately.
Procedure Action
──────────────────────────────────────────────────────────────────────────
appropriately.
com_out Sends one character to the serial port.
cls Calls the ROM BIOS video driver to clear the screen.
clreol Calls the ROM BIOS video driver to clear from the
current cursor position to the end of the line.
home Places the cursor in the upper left corner of the
screen.
gotoxy Positions the cursor at the desired position on the
display.
getxy Obtains the current cursor position.
pc_out Sends one character to the PC's display.
Procedure Action
──────────────────────────────────────────────────────────────────────────
pc_stat Gets status for the PC's keyboard.
pc_in Returns a character from the PC's keyboard.
──────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Chapter 8 File Management
The dual heritage of MS-DOS──CP/M and UNIX/XENIX──is perhaps most clearly
demonstrated in its file-management services. In general, MS-DOS provides
at least two distinct operating-system calls for each major file or record
operation. This chapter breaks this overlapping battery of functions into
two groups and explains the usage, advantages, and disadvantages of each.
I will refer to the set of file and record functions that are compatible
with CP/M as FCB functions. These functions rely on a data structure
called a file control block (hence, FCB) to maintain certain bookkeeping
information about open files. This structure resides in the application
program's memory space. The FCB functions allow the programmer to create,
open, close, and delete files and to read or write records of any size at
any record position within such files. These functions do not support the
hierarchical (treelike) file structure that was first introduced in MS-DOS
version 2.0, so they can be used only to access files in the current
subdirectory for a given disk drive.
I will refer to the set of file and record functions that provide
compatibility with UNIX/XENIX as the handle functions. These functions
allow the programmer to open or create files by passing MS-DOS a
null-terminated string that describes the file's location in the
hierarchical file structure (the drive and path), the file's name, and its
extension. If the open or create operation is successful, MS-DOS returns a
16-bit token, or handle, that is saved by the application program and used
to specify the file in subsequent operations.
When you use the handle functions, the operating system maintains the data
structures that contain bookkeeping information about the file inside its
own memory space, and these structures are not accessible to the
application program. The handle functions fully support the hierarchical
file structure, allowing the programmer to create, open, close, and delete
files in any subdirectory on any disk drive and to read or write records
of any size at any byte offset within such files.
Although we are discussing the FCB functions first in this chapter for
historical reasons, new MS-DOS applications should always be written using
the more powerful handle functions. Use of the FCB functions in new
programs should be avoided, unless compatibility with MS-DOS version 1.0
is needed.
Using the FCB Functions
Understanding the structure of the file control block is the key to
success with the FCB family of file and record functions. An FCB is a
37-byte data structure allocated within the application program's memory
space; it is divided into many fields (Figure 8-1). Typically, the
program initializes an FCB with a drive code, a filename, and an extension
(conveniently accomplished with the parse-filename service, Int 21H
Function 29H) and then passes the address of the FCB to MS-DOS to open or
create the file. If the file is successfully opened or created, MS-DOS
fills in certain fields of the FCB with information from the file's entry
in the disk directory. This information includes the file's exact size in
bytes and the date and time the file was created or last updated. MS-DOS
also places certain other information within a reserved area of the FCB;
however, this area is used by the operating system for its own purposes
and varies among different versions of MS-DOS. Application programs should
never modify the reserved area.
For compatibility with CP/M, MS-DOS automatically sets the record-size
field of the FCB to 128 bytes. If the program does not want to use this
default record size, it must place the desired size (in bytes) into the
record-size field after the open or create operation. Subsequently, when
the program needs to read or write records from the file, it must pass the
address of the FCB to MS-DOS; MS-DOS, in turn, keeps the FCB updated with
information about the current position of the file pointer and the size of
the file. Data is always read to or written from the current disk transfer
area (DTA), whose address is set with Int 21H Function 1AH. If the
application program wants to perform random record access, it must set the
record number into the FCB before issuing each function call; when
sequential record access is being used, MS-DOS maintains the FCB and no
special intervention is needed from the application.
Byte offset
00H ┌───────────────────────────────────────────────────────┐
│ Drive identification │ Note 1
01H ├───────────────────────────────────────────────────────┤
│ Filename (8 characters) │ Note 2
09H ├───────────────────────────────────────────────────────┤
│ Extension (3 characters) │ Note 2
0CH ├───────────────────────────────────────────────────────┤
│ Current block number │ Note 9
0EH ├───────────────────────────────────────────────────────┤
│ Record size │ Note 10
10H ├───────────────────────────────────────────────────────┤
│ File size (4 bytes) │ Notes 3, 6
14H ├───────────────────────────────────────────────────────┤
│ Date created/updated │ Note 7
16H ├───────────────────────────────────────────────────────┤
│ Time created/updated │ Note 8
18H ├───────────────────────────────────────────────────────┤
│ Reserved │
20H ├───────────────────────────────────────────────────────┤
│ Current-record number │ Note 9
21H ├───────────────────────────────────────────────────────┤
│ Relative-record number (4 bytes) │ Note 5
└───────────────────────────────────────────────────────┘
Figure 8-1. Normal file control block. Total length is 37 bytes (25H
bytes). See notes on pages 133─34.
In general, MS-DOS functions that use FCBs accept the full address of the
FCB in the DS:DX register and pass back a return code in the AL register
(Figure 8-2). For file-management calls (open, close, create, and
delete), this return code is zero if the function was successful and 0FFH
(255) if the function failed. For the FCB-type record read and write
functions, the success code returned in the AL register is again zero, but
there are several failure codes. Under MS-DOS version 3.0 or later, more
detailed error reporting can be obtained by calling Int 21H Function 59H
(Get Extended Error Information) after a failed FCB function call.
When a program is loaded under MS-DOS, the operating system sets up two
FCBs in the program segment prefix, at offsets 005CH and 006CH. These are
often referred to as the default FCBs, and they are included to provide
upward compatibility from CP/M. MS-DOS parses the first two parameters in
the command line that invokes the program (excluding any redirection
directives) into the default FCBs, under the assumption that they may be
file specifications. The application must determine whether they really
are filenames or not. In addition, because the default FCBs overlap and
are not in a particularly convenient location (especially for .EXE
programs), they usually must be copied elsewhere in order to be used
safely. (See Chapter 3.)
──────────────────────────────────────────────────────────────────────────
; filename was previously
; parsed into "my_fcb"
mov dx,seg my_fcb ; DS:DX = address of
mov ds,dx ; file control block
mov dx,offset my_fcb
mov ah,0fh ; function 0fh = open
int 21h
or al,al ; was open successful?
jnz error ; no, jump to error routine
.
.
.
my_fcb db 37 dup (0) ; file control block
──────────────────────────────────────────────────────────────────────────
Figure 8-2. A typical FCB file operation. This sequence of code attempts
to open the file whose name was previously parsed into the FCB named
my_fcb.
Note that the structures of FCBs under CP/M and MS-DOS are not identical.
However, the differences lie chiefly in the reserved areas of the FCBs
(which should not be manipulated by application programs in any case), so
well-behaved CP/M applications should be relatively easy to port into
MS-DOS. It seems, however, that few such applications exist. Many of the
tricks that were played by clever CP/M programmers to increase performance
or circumvent the limitations of that operating system can cause severe
problems under MS-DOS, particularly in networking environments. At any
rate, much better performance can be achieved by thoroughly rewriting the
CP/M applications to take advantage of the superior capabilities of
MS-DOS.
You can use a special FCB variant called an extended file control block to
create or access files with special attributes (such as hidden or
read-only files), volume labels, and subdirectories. An extended FCB has a
7-byte header followed by the 37-byte structure of a normal FCB (Figure
8-3). The first byte contains 0FFH, which could never be a legal drive
code and thus indicates to MS-DOS that an extended FCB is being used. The
next 5 bytes are reserved and are unused in current versions of MS-DOS.
The seventh byte contains the attribute of the special file type that is
being accessed. (Attribute bytes are discussed in more detail in Chapter
9.) Any MS-DOS function that uses a normal FCB can also use an extended
FCB.
The FCB file- and record-management functions may be gathered into the
following broad classifications:
Byte
offset
00H ┌───────────────────────────────────────────────────────┐
│ 0FFH │ Note 11
01H ├───────────────────────────────────────────────────────┤
│ Reserved (5 bytes, must be zero) │
06H ├───────────────────────────────────────────────────────┤
│ Attribute byte │ Note 12
07H ├───────────────────────────────────────────────────────┤
│ Drive identification │ Note 1
08H ├───────────────────────────────────────────────────────┤
│ Filename (8 characters) │ Note 2
10H ├───────────────────────────────────────────────────────┤
│ Extension (3 characters) │ Note 2
13H ├───────────────────────────────────────────────────────┤
│ Current-block number │ Note 9
15H ├───────────────────────────────────────────────────────┤
│ Record size │ Note 10
17H ├───────────────────────────────────────────────────────┤
│ File size (4 bytes) │ Notes 3, 6
1BH ├───────────────────────────────────────────────────────┤
│ Date created/updated │ Note 7
1DH ├───────────────────────────────────────────────────────┤
│ Time created/updated │ Note 8
1FH ├───────────────────────────────────────────────────────┤
│ Reserved │
27H ├───────────────────────────────────────────────────────┤
│ Current-record number │ Note 9
28H ├───────────────────────────────────────────────────────┤
│ Relative-record number (4 bytes) │ Note 5
└───────────────────────────────────────────────────────┘
Figure 8-3. Extended file control block. Total length is 44 bytes (2CH
bytes). See notes on pages 133─34.
╓┌─┌────────────────────────┌────────────────────────────────────────────────╖
Function Action
──────────────────────────────────────────────────────────────────────────
Common FCB file operations
0FH Open file.
10H Close file.
16H Create file.
Common FCB record operations
14H Perform sequential read.
15H Perform sequential write.
Function Action
──────────────────────────────────────────────────────────────────────────
15H Perform sequential write.
21H Perform random read.
22H Perform random write.
27H Perform random block read.
28H Perform random block write.
Other vital FCB operations
1AH Set disk transfer address.
29H Parse filename.
Less commonly used FCB file operations
13H Delete file.
17H Rename file.
Less commonly used FCB record operations
23H Obtain file size.
24H Set relative-record number.
──────────────────────────────────────────────────────────────────────────
Function Action
──────────────────────────────────────────────────────────────────────────
Several of these functions have special properties. For example, Int 21H
Functions 27H (Random Block Read) and 28H (Random Block Write) allow
reading and writing of multiple records of any size and also update the
random-record field automatically (unlike Int 21H Functions 21H and
22H). Int 21H Function 28H can truncate a file to any desired size, and
Int 21H Function 17H used with an extended FCB can alter a volume label
or rename a subdirectory.
Section 2 of this book, "MS-DOS Functions Reference," gives detailed
specifications for each of the FCB file and record functions, along with
assembly-language examples. It is also instructive to compare the
preceding groups with the corresponding groups of handle-type functions
listed on pages 140─41.
──────────────────────────────────────────────────────────────────────────
Notes for Figures 8-1 and 8-3
1. The drive identification is a binary number: 00=default drive,
01=drive A:, 02=drive B:, and so on. If the application program
supplies the drive code as zero (default drive), MS-DOS fills in the
code for the actual current disk drive after a successful open or
create call.
2. File and extension names must be left justified and padded with
blanks.
3. The file size, date, time, and reserved fields should not be
modified by applications.
4. All word fields are stored with the least significant byte at the
lower address.
5. The relative-record field is treated as 4 bytes if the record size
is less than 64 bytes; otherwise, only the first 3 bytes of this
field are used.
6. The file-size field is in the same format as in the directory, with
the less significant word at the lower address.
7. The date field is mapped as in the directory. Viewed as a 16-bit
word (as it would appear in a register), the field is broken down as
follows:
F E D C B A 9 8 7 6 5 4 3 2 1 0
┌─────────────────────┬─────────────────────┬─────────────────────┐
│ Year │ Month │ Day │
└─────────────────────┴─────────────────────┴─────────────────────┘
Bits Contents
────────────────────────────────────────────────────────────────────────
00H─04H Day (1─31)
05H─08H Month (1─12)
09H─0FH Year, relative to 1980
────────────────────────────────────────────────────────────────────────
8. The time field is mapped as in the directory. Viewed as a 16-bit
word (as it would appear in a register), the field is broken down as
follows:
F E D C B A 9 8 7 6 5 4 3 2 1 0
┌───────────────────┬───────────────────────┬─────────────────────┐
│ Hours │ Minutes │ 2-second increments │
└───────────────────┴───────────────────────┴─────────────────────┘
Bits Contents
────────────────────────────────────────────────────────────────────────
00H─04H 2-second increments (0─29)
05H─0AH Minutes (0─59)
0BH─0FH Hours (0─23)
────────────────────────────────────────────────────────────────────────
9. The current-block and current-record numbers are used together on
sequential reads and writes. This simulates the behavior of CP/M.
10. The Int 21H open (0FH) and create (16H) functions set the
record-size field to 128 bytes, to provide compatibility with CP/M.
If you use another record size, you must fill it in after the open
or create operation.
11. An 0FFH (255) in the first byte of the structure signifies that it
is an extended file control block. You can use extended FCBs with
any of the functions that accept an ordinary FCB. (See also note
12.)
12. The attribute byte in an extended FCB allows access to files with
the special characteristics hidden, system, or read-only. You can
also use extended FCBs to read volume labels and the contents of
special subdirectory files.
──────────────────────────────────────────────────────────────────────────
FCB File-Access Skeleton
The following is a typical program sequence to access a file using the
FCB, or traditional, functions (Figure 8-4):
1. Zero out the prospective FCB.
2. Obtain the filename from the user, from the default FCBs, or from the
command tail in the PSP.
3. If the filename was not obtained from one of the default FCBs, parse
the filename into the new FCB using Int 21H Function 29H.
4. Open the file (Int 21H Function 0FH) or, if writing new data only,
create the file or truncate any existing file of the same name to zero
length (Int 21H Function 16H).
5. Set the record-size field in the FCB, unless you are using the default
record size. Recall that it is important to do this after a successful
open or create operation. (See Figure 8-5.)
6. Set the relative-record field in the FCB if you are performing random
record I/O.
7. Set the disk transfer area address using Int 21H Function 1AH, unless
the buffer address has not been changed since the last call to this
function. If the application never performs a set DTA, the DTA address
defaults to offset 0080H in the PSP.
8. Request the needed read- or write-record operation (Int 21H Function
14H─Sequential Read, 15H─Sequential Write, 21H─Random Read,
22H─Random Write, 27H─Random Block Read, 28H─Random Block Write).
9. If the program is not finished processing the file, go to step 6;
otherwise, close the file (Int 21H Function 10H). If the file was
used for reading only, you can skip the close operation under early
versions of MS-DOS. However, this shortcut can cause problems under
MS-DOS versions 3.0 and later, especially when the files are being
accessed across a network.
──────────────────────────────────────────────────────────────────────────
recsize equ 1024 ; file record size
.
.
.
mov ah,29h ; parse input filename
mov al,1 ; skip leading blanks
mov si,offset fname1 ; address of filename
mov di,offset fcb1 ; address of FCB
int 21h
or al,al ; jump if name
jnz name_err ; was bad
.
.
.
mov ah,29h ; parse output filename
mov al,1 ; skip leading blanks
mov si,offset fname2 ; address of filename
mov di,offset fcb2 ; address of FCB
int 21h
or al,al ; jump if name
jnz name_err ; was bad
.
.
.
mov ah,0fh ; open input file
mov dx,offset fcb1
int 21h
or al,al ; open successful?
jnz no_file ; no, jump
.
.
.
mov ah,16h ; create and open
mov dx,offset fcb2 ; output file
int 21h
or al,al ; create successful?
jnz disk_full ; no, jump
.
.
. ; set record sizes
mov word ptr fcb1+0eh,recsize
mov word ptr fcb2+0eh,recsize
.
.
.
mov ah,1ah ; set disk transfer
mov dx,offset buffer ; address for reads
int 21h ; and writes
.
next: . ; process next record
.
mov ah,14h ; sequential read from
mov dx,offset fcb1 ; input file
int 21h
cmp al,01 ; check for end of file
je file_end ; jump if end of file
cmp al,03
je file_end ; jump if end of file
or al,al ; other read fault?
jnz bad_read ; jump if bad read
.
.
.
mov ah,15h ; sequential write to
mov dx,offset fcb2 ; output file
int 21h
or al,al ; write successful?
jnz bad_write ; jump if write failed
.
.
.
jmp next ; process next record
.
file_end: . ; reached end of input
.
mov ah,10h ; close input file
mov dx,offset fcb1
int 21h
.
.
.
mov ah,10h ; close output file
mov dx,offset fcb2
int 21h
.
.
.
mov ax,4c00h ; exit with return
int 21h ; code of zero
.
.
.
fname1 db 'OLDFILE.DAT',0 ; name of input file
fname2 db 'NEWFILE.DAT',0 ; name of output file
fcb1 db 37 dup (0) ; FCB for input file
fcb2 db 37 dup (0) ; FCB for output file
buffer db recsize dup (?) ; buffer for file I/O
──────────────────────────────────────────────────────────────────────────
Figure 8-4. Skeleton of an assembly-language program that performs file
and record I/O using the FCB family of functions.
Byte Offset FCB before open FCB contents FCB after open
┌────────────────────┬────────────────────┬────────────────────┐
00H │ 00 │ Drive │ 03 │
├────────────────────┼────────────────────┼────────────────────┤
01H │ 4D │ │ 4D │
02H │ 59 │ │ 59 │
03H │ 46 │ │ 46 │
04H │ 49 │ Filename │ 49 │
05H │ 4C │ │ 4C │
06H │ 45 │ │ 45 │
07H │ 20 │ │ 20 │
08H │ 20 │ │ 20 │
├────────────────────┼────────────────────┼────────────────────┤
09H │ 44 │ │ 44 │
0AH │ 41 │ Extension │ 41 │
0BH │ 54 │ │ 54 │
├────────────────────┼────────────────────┼────────────────────┤
0CH │ 00 │ │ 00 │
0DH │ 00 │ Current block │ 00 │
├────────────────────┼────────────────────┼────────────────────┤
0EH │ 00 │ │ 80 │
0FH │ 00 │ Record size │ 00 │
├────────────────────┼────────────────────┼────────────────────┤
10H │ 00 │ │ 80 │
11H │ 00 │ │ 3D │
12H │ 00 │ File size │ 00 │
13H │ 00 │ │ 00 │
├────────────────────┼────────────────────┼────────────────────┤
14H │ 00 │ │ 43 │
15H │ 00 │ File date │ 0B │
├────────────────────┼────────────────────┼────────────────────┤
16H │ 00 │ │ A1 │
17H │ 00 │ File time │ 52 │
├────────────────────┼────────────────────┼────────────────────┤
18H │ 00 │ │ 03 │
19H │ 00 │ │ 02 │
1AH │ 00 │ │ 42 │
1BH │ 00 │ │ 73 │
1CH │ 00 │ Reserved │ 00 │
1DH │ 00 │ │ 01 │
1EH │ 00 │ │ 35 │
1FH │ 00 │ │ 0F │
├────────────────────┼────────────────────┼────────────────────┤
20H │ 00 │ Current record │ 00 │
├────────────────────┼────────────────────┼────────────────────┤
21H │ 00 │ │ 00 │
22H │ 00 │ Relative-record │ 00 │
23H │ 00 │ number │ 00 │
24H │ 00 │ │ 00 │
└────────────────────┴────────────────────┴────────────────────┘
Figure 8-5. A typical file control block before and after a successful
open call (Int 21H Function 0FH).
Points to Remember
Here is a summary of the pros and cons of using the FCB-related file and
record functions in your programs.
Advantages:
■ Under MS-DOS versions 1 and 2, the number of files that can be open
concurrently when using FCBs is unlimited. (This is not true under
MS-DOS versions 3.0 and later, especially if networking software is
running.)
■ File-access methods using FCBs are familiar to programmers with a CP/M
background, and well-behaved CP/M applications require little change in
logical flow to run under MS-DOS.
■ MS-DOS supplies the size, time, and date for a file to its FCB after
the file is opened. The calling program can inspect this information.
Disadvantages:
■ FCBs take up room in the application program's memory space.
■ FCBs offer no support for the hierarchical file structure (no access to
files outside the current directory).
■ FCBs provide no support for file locking/sharing or record locking in
networking environments.
■ In addition to the read or write call itself, file reads or writes
using FCBs require manipulation of the FCB to set record size and
record number, plus a previous call to a separate MS-DOS function to
set the DTA address.
■ Random record I/O using FCBs for a file containing variable-length
records is very clumsy and inconvenient.
■ You must use extended FCBs, which are incompatible with CP/M anyway, to
access or create files with special attributes such as hidden,
read-only, or system.
■ The FCB file functions have poor error reporting. This situation has
been improved somewhat in MS-DOS version 3 because a program can call
the added Int 21H Function 59H (Get Extended Error Information) after
a failed FCB function to obtain additional information.
■ Microsoft discourages use of FCBs. FCBs will make your program more
difficult to port to MS OS/2 later because MS OS/2 does not support
FCBs in protected mode at all.
Using the Handle Functions
The handle file- and record-management functions access files in a fashion
similar to that used under the UNIX/XENIX operating system. Files are
designated by an ASCIIZ string (an ASCII character string terminated by a
null, or zero, byte) that can contain a drive designator, path, filename,
and extension. For example, the file specification
C:\SYSTEM\COMMAND.COM
would appear in memory as the following sequence of bytes:
43 3A 5C 53 59 53 54 45 4D 5C 43 4F 4D 4D 41 4E 44 2E 43 4F 4D 00
When a program wishes to open or create a file, it passes the address of
the ASCIIZ string specifying the file to MS-DOS in the DS:DX registers
(Figure 8-6). If the operation is successful, MS-DOS returns a 16-bit
handle to the program in the AX register. The program must save this
handle for further reference.
──────────────────────────────────────────────────────────────────────────
mov ah,3dh ; function 3dh = open
mov al,2 ; mode 2 = read/write
mov dx,seg filename ; address of ASCIIZ
mov ds,dx ; file specification
mov dx,offset filename
int 21h ; request open from DOS
jc error ; jump if open failed
mov handle,ax ; save file handle
.
.
.
filename db 'C:\MYDIR\MYFILE.DAT',0 ; filename
handle dw 0 ; file handle
──────────────────────────────────────────────────────────────────────────
Figure 8-6. A typical handle file operation. This sequence of code
attempts to open the file designated in the ASCIIZ string whose address is
passed to MS-DOS in the DS:DX registers.
When the program requests subsequent operations on the file, it usually
places the handle in the BX register before the call to MS-DOS. All the
handle functions return with the CPU's carry flag cleared if the operation
was successful, or set if the operation failed; in the latter case, the AX
register contains a code describing the failure.
MS-DOS restricts the number of handles that can be active at any one
time──that is, the number of files and devices that can be open
concurrently when using the handle family of functions──in two different
ways:
■ The maximum number of concurrently open files in the system, for all
active processes combined, is specified by the entry
FILES=nn
in the CONFIG.SYS file. This entry determines the number of entries
to be allocated in the system file table; under MS-DOS version 3, the
default value is 8 and the maximum is 255. After MS-DOS is booted and
running, you cannot expand this table to increase the total number of
files that can be open. You must use an editor to modify the CONFIG.SYS
file and then restart the system.
■ The maximum number of concurrently open files for a single process is
20, assuming that sufficient entries are also available in the system
file table. When a program is loaded, MS-DOS preassigns 5 of its
potential 20 handles to the standard devices. Each time the process
issues an open or create call, MS-DOS assigns a handle from the
process's private allocation of 20, until all the handles are used up
or the system file table is full. In MS-DOS versions 3.3 and later, you
can expand the per-process limit of 20 handles with a call to Int 21H
Function 67H (Set Handle Count).
The handle file- and record-management calls may be gathered into the
following broad classifications for study:
╓┌─┌────────────────────────┌────────────────────────────────────────────────╖
Function Action
Function Action
──────────────────────────────────────────────────────────────────────────
Common handle file operations
3CH Create file (requires ASCIIZ string).
3DH Open file (requires ASCIIZ string).
3EH Close file.
Common handle record operations
42H Set file pointer (also used to find file size).
3FH Read file.
40H Write file.
Less commonly used handle operations
41H Delete file.
43H Get or modify file attributes.
44H IOCTL (I/O Control).
45H Duplicate handle.
46H Redirect handle.
56H Rename file.
57H Get or set file date and time.
5AH Create temporary file (versions 3.0 and later).
Function Action
──────────────────────────────────────────────────────────────────────────
5AH Create temporary file (versions 3.0 and later).
5BH Create file (fails if file already exists;
versions 3.0 and later).
5CH Lock or unlock file region (versions 3.0 and
later).
67H Set handle count (versions 3.3 and later).
68H Commit file (versions 3.3 and later).
6CH Extended open file (version 4).
──────────────────────────────────────────────────────────────────────────
Compare the groups of handle-type functions in the preceding table with
the groups of FCB functions outlined earlier, noting the degree of
functional overlap. Section 2 of this book, "MS-DOS Functions Reference,"
gives detailed specifications for each of the handle functions, along with
assembly-language examples.
Handle File-Access Skeleton
The following is a typical program sequence to access a file using the
handle family of functions (Figure 8-7):
1. Get the filename from the user by means of the buffered input service
(Int 21H Function 0AH) or from the command tail supplied by MS-DOS in
the PSP.
2. Put a zero at the end of the file specification in order to create an
ASCIIZ string.
3. Open the file using Int 21H Function 3DH and mode 2 (read/write
access), or create the file using Int 21H Function 3CH. (Be sure to
set the CX register to zero, so that you don't accidentally make a
file with special attributes.) Save the handle that is returned.
4. Set the file pointer using Int 21H Function 42H. You may set the
file-pointer position relative to one of three different locations:
the start of the file, the current pointer position, or the end of the
file. If you are performing sequential record I/O, you can usually
skip this step because MS-DOS will maintain the file pointer for you
automatically.
5. Read from the file (Int 21H Function 3FH) or write to the file (Int
21H Function 40H). Both of these functions require that the BX
register contain the file's handle, the CX register contain the length
of the record, and the DS:DX registers point to the data being
transferred. Both return the actual number of bytes transferred in the
AX register.
In a read operation, if the number of bytes read is less than the
number requested, the end of the file has been reached. In a write
operation, if the number of bytes written is less than the number
requested, the disk containing the file is full. Neither of these
conditions is returned as an error code; that is, the carry flag is
not set.
6. If the program is not finished processing the file, go to step 4;
otherwise, close the file (Int 21H Function 3EH). Any normal exit
from the program will also close all active handles.
──────────────────────────────────────────────────────────────────────────
recsize equ 1024 ; file record size
.
.
.
mov ah,3dh ; open input file
mov al,0 ; mode = read only
mov dx,offset fname1 ; name of input file
int 21h
jc no_file ; jump if no file
mov handle1,ax ; save token for file
.
.
.
mov ah,3ch ; create output file
mov cx,0 ; attribute = normal
mov dx,offset fname2 ; name of output file
int 21h
jc disk_full ; jump if create fails
mov handle2,ax ; save token for file
.
next: . ; process next record
.
mov ah,3fh ; sequential read from
mov bx,handle1 ; input file
mov cx,recsize
mov dx,offset buffer
int 21h
jc bad_read ; jump if read error
or ax,ax ; check bytes transferred
jz file_end ; jump if end of file
.
.
.
mov ah,40h ; sequential write to
mov bx,handle2 ; output file
mov cx,recsize
mov dx,offset buffer
int 21h
jc bad_write ; jump if write error
cmp ax,recsize ; whole record written?
jne disk_full ; jump if disk is full
.
.
.
jmp next ; process next record
.
file_end: . ; reached end of input
.
mov ah,3eh ; close input file
mov bx,handle1
int 21h
.
.
.
mov ah,3eh ; close output file
mov bx,handle2
int 21h
.
.
.
mov ax,4c00h ; exit with return
int 21h ; code of zero
.
.
.
fname1 db 'OLDFILE.DAT',0 ; name of input file
fname2 db 'NEWFILE.DAT',0 ; name of output file
handle1 dw 0 ; token for input file
handle2 dw 0 ; token for output file
buffer db recsize dup (?) ; buffer for file I/O
──────────────────────────────────────────────────────────────────────────
Figure 8-7. Skeleton of an assembly-language program that performs
sequential processing on an input file and writes the results to an output
file using the handle file and record functions. This code assumes that
the DS and ES registers have already been set to point to the segment
containing the buffers and filenames.
Points to Remember
Here is a summary of the pros and cons of using the handle file and record
operations in your program. Compare this list with the one given earlier
in the chapter for the FCB family of functions.
Advantages:
■ The handle calls provide direct support for I/O redirection and pipes
with the standard input and output devices in a manner functionally
similar to that used by UNIX/XENIX.
■ The handle functions provide direct support for directories (the
hierarchical file structure) and special file attributes.
■ The handle calls support file sharing/locking and record locking in
networking environments.
■ Using the handle functions, the programmer can open channels to
character devices and treat them as files.
■ The handle calls make the use of random record access extremely easy.
The current file pointer can be moved to any byte offset relative to
the start of the file, the end of the file, or the current pointer
position. Records of any length, up to an entire segment (65,535
bytes), can be read to any memory address in one operation.
■ The handle functions have relatively good error reporting in MS-DOS
version 2, and error reporting has been enhanced even further in MS-DOS
versions 3.0 and later.
■ Microsoft strongly encourages use of the handle family of functions in
order to provide upward compatibility with MS OS/2.
Disadvantages:
■ There is a limit per program of 20 concurrently open files and devices
using handles in MS-DOS versions 2.0 through 3.2.
■ Minor gaps still exist in the implementation of the handle functions.
For example, you must still use extended FCBs to change volume labels
and to access the contents of the special files that implement
directories.
MS-DOS Error Codes
When one of the handle file functions fails with the carry flag set, or
when a program calls Int 21H Function 59H (Get Extended Error
Information) following a failed FCB function or other system service, one
of the following error codes may be returned:
╓┌─┌────────────────────────┌────────────────────────────────────────────────╖
Value Meaning
──────────────────────────────────────────────────────────────────────────
MS-DOS version 2 error codes
01H Function number invalid
02H File not found
03H Path not found
04H Too many open files
05H Access denied
06H Handle invalid
07H Memory control blocks destroyed
08H Insufficient memory
09H Memory block address invalid
0AH (10) Environment invalid
0BH (11) Format invalid
0CH (12) Access code invalid
0DH (13) Data invalid
0EH (14) Unknown unit
Value Meaning
──────────────────────────────────────────────────────────────────────────
0EH (14) Unknown unit
0FH (15) Disk drive invalid
10H (16) Attempted to remove current directory
11H (17) Not same device
12H (18) No more files
Mappings to critical-error codes
13H (19) Write-protected disk
14H (20) Unknown unit
15H (21) Drive not ready
16H (22) Unknown command
17H (23) Data error (CRC)
18H (24) Bad request-structure length
19H (25) Seek error
1AH (26) Unknown media type
1BH (27) Sector not found
1CH (28) Printer out of paper
1DH (29) Write fault
1EH (30) Read fault
Value Meaning
──────────────────────────────────────────────────────────────────────────
1EH (30) Read fault
1FH (31) General failure
MS-DOS version 3 and later extended error codes
20H (32) Sharing violation
21H (33) File-lock violation
22H (34) Disk change invalid
23H (35) FCB unavailable
24H (36) Sharing buffer exceeded
25H─31H (37─49) Reserved
32H (50) Unsupported network request
33H (51) Remote machine not listening
34H (52) Duplicate name on network
35H (53) Network name not found
36H (54) Network busy
37H (55) Device no longer exists on network
38H (56) NetBIOS command limit exceeded
39H (57) Error in network adapter hardware
3AH (58) Incorrect response from network
Value Meaning
──────────────────────────────────────────────────────────────────────────
3AH (58) Incorrect response from network
3BH (59) Unexpected network error
3CH (60) Remote adapter incompatible
3DH (61) Print queue full
3EH (62) Not enough room for print file
3FH (63) Print file was deleted
40H (64) Network name deleted
41H (65) Network access denied
42H (66) Incorrect network device type
43H (67) Network name not found
44H (68) Network name limit exceeded
45H (69) NetBIOS session limit exceeded
46H (70) Temporary pause
47H (71) Network request not accepted
48H (72) Print or disk redirection paused
49H─4FH (73─79) Reserved
50H (80) File already exists
51H (81) Reserved
52H (82) Cannot make directory
Value Meaning
──────────────────────────────────────────────────────────────────────────
52H (82) Cannot make directory
53H (83) Fail on Int 24H (critical error)
54H (84) Too many redirections
55H (85) Duplicate redirection
56H (86) Invalid password
57H (87) Invalid parameter
58H (88) Net write fault
──────────────────────────────────────────────────────────────────────────
Under MS-DOS versions 3.0 and later, you can also use Int 21H Function
59H to obtain other information about the error, such as the error locus
and the recommended recovery action.
Critical-Error Handlers
In Chapter 5, we discussed how an application program can take over the
Ctrl-C handler vector (Int 23H) and replace the MS-DOS default handler, to
avoid losing control of the computer when the user enters a Ctrl-C or
Ctrl-Break at the keyboard. Similarly, MS-DOS provides a
critical-error-handler vector (Int 24H) that defines the routine to be
called when unrecoverable hardware faults occur. The default MS-DOS
critical-error handler is the routine that displays a message describing
the error type and the cue
Abort, Retry, Ignore?
This message appears after such actions as the following:
■ Attempting to open a file on a disk drive that doesn't contain a floppy
disk or whose door isn't closed
■ Trying to read a disk sector that contains a CRC error
■ Trying to print when the printer is off line
The unpleasant thing about MS-DOS's default critical-error handler is, of
course, that if the user enters an A for Abort, the application that is
currently executing is terminated abruptly and never has a chance to clean
up and make a graceful exit. Intermediate files may be left on the disk,
files that have been extended using FCBs are not properly closed so that
the directory is updated, interrupt vectors may be left pointing into the
transient program area, and so forth.
To write a truly bombproof MS-DOS application, you must take over the
critical-error-handler vector and point it to your own routine, so that
your program intercepts all catastrophic hardware errors and handles them
appropriately. You can use MS-DOS Int 21H Function 25H to alter the Int
24H vector in a well-behaved manner. When your application exits, MS-DOS
will automatically restore the previous contents of the Int 24H vector
from information saved in the program segment prefix.
MS-DOS calls the critical-error handler for two general classes of
errors── disk-related and non-disk-related──and passes different
information to the handler in the registers for each of these classes.
For disk-related errors, MS-DOS sets the registers as shown on the
following page. (Bits 3─5 of the AH register are relevant only in MS-DOS
versions 3.1 and later.)
╓┌─┌──────────────────┌─────────────────┌────────────────────────────────────╖
Register Bit(s) Significance
──────────────────────────────────────────────────────────────────────────
AH 7 0, to signify disk error
6 Reserved
5 0 = ignore response not allowed
1 = ignore response allowed
4 0 = retry response not allowed
1 = retry response allowed
3 0 = fail response not allowed
1 = fail response allowed
1─2 Area where disk error occurred
00 = MS-DOS area
01 = file allocation table
10 = root directory
11 = files area
0 0 = read operation
1 = write operation
AL 0─7 Drive code (0 = A, 1 = B, and so
forth)
DI 0─7 Driver error code
8─15 Not used
Register Bit(s) Significance
──────────────────────────────────────────────────────────────────────────
8─15 Not used
BP:SI Segment:offset of device-driver
header
──────────────────────────────────────────────────────────────────────────
For non-disk-related errors, the interrupt was generated either as the
result of a character-device error or because a corrupted memory image of
the file allocation table was detected. In this case, MS-DOS sets the
registers as follows:
Register Bit(s) Significance
──────────────────────────────────────────────────────────────────────────
AH 7 1, to signify a non-disk error
DI 0─7 Driver error code
8─15 Not used
BP:SI Segment:offset of device-driver
header
──────────────────────────────────────────────────────────────────────────
To determine whether the critical error was caused by a character device,
use the address in the BP:SI registers to examine the device attribute
word at offset 0004H in the presumed device-driver header. If bit 15 is
set, then the error was indeed caused by a character device, and the
program can inspect the name field of the driver's header to determine the
device.
At entry to a critical-error handler, MS-DOS has already disabled
interrupts and set up the stack as shown in Figure 8-8. A critical-error
handler cannot use any MS-DOS services except Int 21H Functions 01H
through 0CH (Traditional Character I/O), Int 21H Function 30H (Get MS-DOS
Version), and Int 21H Function 59H (Get Extended Error Information).
These functions use a special stack so that the context of the original
function (which generated the critical error) will not be lost.
┌───────┐─┐
│ Flags │ │
├───────┤ │ Flags and CS:IP pushed
│ CS │ ├─ on stack by original
├───────┤ │ Int 21H call
│ IP │ │
├───────┤═╡◄─SS:SP on entry to
│ ES │ │ Int 21H handler
├───────┤ │
│ DS │ │
├───────┤ │
│ BP │ │
├───────┤ │
│ DI │ │
├───────┤ ├─ Registers at point of
│ SI │ │ original Int 21H call
├───────┤ │
│ DX │ │
├───────┤ │
│ CX │ │
├───────┤ │
│ BX │ │
├───────┤ │
│ AX │ │
├───────┤═╡
│ Flags │ │
├───────┤ │
│ CS │ ├─ Return address for
├───────┤ │ Int 24H handler
│ IP │ │
└──────┘─┘
└───── SS:SP on entry to
Int 24H handler
Figure 8-8. The stack at entry to a critical-error handler.
The critical-error handler should return to MS-DOS by executing an IRET,
passing one of the following action codes in the AL register:
Code Meaning
──────────────────────────────────────────────────────────────────────────
0 Ignore the error (MS-DOS acts as though the original
function call had succeeded).
1 Retry the operation.
2 Terminate the process that encountered the error.
3 Fail the function (an error code is returned to the
requesting process). Versions 3.1 and later only.
──────────────────────────────────────────────────────────────────────────
The critical-error handler should preserve all other registers and must
not modify the device-driver header pointed to by BP:SI. A skeleton
example of a critical-error handler is shown in Figure 8-9.
──────────────────────────────────────────────────────────────────────────
; prompt message used by
; critical-error handler
prompt db cr,lf,'Critical Error Occurred: '
db 'Abort, Retry, Ignore, Fail? $'
keys db 'aArRiIfF' ; possible user response keys
keys_len equ $-keys ; (both cases of each allowed)
codes db 2,2,1,1,0,0,3,3 ; codes returned to MS-DOS kernel
; for corresponding response keys
;
; This code is executed during program's initialization
; to install the new critical-error handler.
;
.
.
.
push ds ; save our data segment
mov dx,seg int24 ; DS:DX = handler address
mov ds,dx
mov dx,offset int24
mov ax,2524h ; function 25h = set vector
int 21h ; transfer to MS-DOS
pop ds ; restore data segment
.
.
.
;
; This is the replacement critical-error handler. It
; prompts the user for Abort, Retry, Ignore, or Fail, and
; returns the appropriate code to the MS-DOS kernel.
;
int24 proc far ; entered from MS-DOS kernel
push bx ; save registers
push cx
push dx
push si
push di
push bp
push ds
push es
int24a: mov ax,seg prompt ; display prompt for user
mov ds,ax ; using function 9 (print string
mov es,ax ; terminated by $ character)
mov dx,offset prompt
mov ah,9
int 21h
mov ah,1 ; get user's response
int 21h ; function 1 = read one character
mov di,offset keys ; look up code for response key
mov cx,keys_len
cld
repne scasb
jnz int24a ; prompt again if bad response
; set AL = action code for MS-DOS
; according to key that was entered:
; 0 = ignore, 1 = retry, 2 = abort,
; 3 = fail
mov al,[di+keys_len-1]
pop es ; restore registers
pop ds
pop bp
pop di
pop si
pop dx
pop cx
pop bx
iret ; exit critical-error handler
int24 endp
──────────────────────────────────────────────────────────────────────────
Figure 8-9. A skeleton example of a replacement critical-error handler.
Example Programs: DUMP.ASM and DUMP.C
The programs DUMP.ASM (Figure 8-10) and DUMP.C (Figure 8-11) are
parallel examples of the use of the handle file and record functions. The
assembly-language version, in particular, illustrates features of a
well-behaved MS-DOS utility:
■ The program checks the version of MS-DOS to ensure that all the
functions it is going to use are really available.
■ The program parses the drive, path, and filename from the command tail
in the program segment prefix.
■ The program uses buffered I/O for speed.
■ The program sends error messages to the standard error device.
■ The program sends normal program output to the standard output device,
so that the dump output appears by default on the system console but
can be redirected to other character devices (such as the line printer)
or to a file.
The same features are incorporated into the C version of the program, but
some of them are taken care of behind the scenes by the C runtime library.
──────────────────────────────────────────────────────────────────────────
name dump
page 55,132
title DUMP--display file contents
;
; DUMP--Display contents of file in hex and ASCII
;
; Build: C>MASM DUMP;
; C>LINK DUMP;
;
; Usage: C>DUMP unit:\path\filename.exe [ >device ]
;
; Copyright (C) 1988 Ray Duncan
;
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII line feed
tab equ 09h ; ASCII tab code
blank equ 20h ; ASCII space code
cmd equ 80h ; buffer for command tail
blksize equ 16 ; input file record size
stdin equ 0 ; standard input handle
stdout equ 1 ; standard output handle
stderr equ 2 ; standard error handle
_TEXT segment word public 'CODE'
assume cs:_TEXT,ds:_DATA,es:_DATA,ss:STACK
dump proc far ; entry point from MS-DOS
push ds ; save DS:0000 for final
xor ax,ax ; return to MS-DOS, in case
push ax ; function 4ch can't be used
mov ax,_DATA ; make our data segment
mov ds,ax ; addressable via DS register
; check MS-DOS version
mov ax,3000h ; function 30h = get version
int 21h ; transfer to MS-DOS
cmp al,2 ; major version 2 or later?
jae dump1 ; yes, proceed
; if MS-DOS 1.x, display
; error message and exit
mov dx,offset msg3 ; DS:DX = message address
mov ah,9 ; function 9 = print string
int 21h ; transfer to MS-DOS
ret ; then exit the old way
dump1: ; check if filename present
mov bx,offset cmd ; ES:BX = command tail
call argc ; count command arguments
cmp ax,2 ; are there 2 arguments?
je dump2 ; yes, proceed
; missing filename, display
; error message and exit
mov dx,offset msg2 ; DS:DX = message address
mov cx,msg2_len ; CX = message length
jmp dump9 ; go display it
dump2: ; get address of filename
mov ax,1 ; AX = argument number
; ES:BX still = command tail
call argv ; returns ES:BX = address,
; and AX = length
mov di,offset fname ; copy filename to buffer
mov cx,ax ; CX = length
dump3: mov al,es:[bx] ; copy one byte
mov [di],al
inc bx ; bump string pointers
inc di
loop dump3 ; loop until string done
mov byte ptr [di],0 ; add terminal null byte
mov ax,ds ; make our data segment
mov es,ax ; addressable by ES too
; now open the file
mov ax,3d00h ; function 3dh = open file
; mode 0 = read only
mov dx,offset fname ; DS:DX = filename
int 21h ; transfer to MS-DOS
jnc dump4 ; jump, open successful
; open failed, display
; error message and exit
mov dx,offset msg1 ; DS:DX = message address
mov cx,msg1_len ; CX = message length
jmp dump9 ; go display it
dump4: mov fhandle,ax ; save file handle
dump5: ; read block of file data
mov bx,fhandle ; BX = file handle
mov cx,blksize ; CX = record length
mov dx,offset fbuff ; DS:DX = buffer
mov ah,3fh ; function 3fh = read
int 21h ; transfer to MS-DOS
mov flen,ax ; save actual length
cmp ax,0 ; end of file reached?
jne dump6 ; no, proceed
cmp word ptr fptr,0 ; was this the first read?
jne dump8 ; no, exit normally
; display empty file
; message and exit
mov dx,offset msg4 ; DS:DX = message address
mov cx,msg4_len ; CX = length
jmp dump9 ; go display it
dump6: ; display heading at
; each 128-byte boundary
test fptr,07fh ; time for a heading?
jnz dump7 ; no, proceed
; display a heading
mov dx,offset hdg ; DS:DX = heading address
mov cx,hdg_len ; CX = heading length
mov bx,stdout ; BX = standard output
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
dump7: call conv ; convert binary record
; to formatted ASCII
; display formatted output
mov dx,offset fout ; DX:DX = output address
mov cx,fout_len ; CX = output length
mov bx,stdout ; BX = standard output
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
jmp dump5 ; go get another record
dump8: ; close input file
mov bx,fhandle ; BX = file handle
mov ah,3eh ; function 3eh = close
int 21h ; transfer to MS-DOS
mov ax,4c00h ; function 4ch = terminate,
; return code = 0
int 21h ; transfer to MS-DOS
dump9: ; display message on
; standard error device
; DS:DX = message address
; CX = message length
mov bx,stderr ; standard error handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
mov ax,4c01h ; function 4ch = terminate,
; return code = 1
int 21h ; transfer to MS-DOS
dump endp
conv proc near ; convert block of data
; from input file
mov di,offset fout ; clear output format
mov cx,fout_len-2 ; area to blanks
mov al,blank
rep stosb
mov di,offset fout ; convert file offset
mov ax,fptr ; to ASCII for output
call w2a
mov bx,0 ; init buffer pointer
conv1: mov al,[fbuff+bx] ; fetch byte from buffer
mov di,offset foutb ; point to output area
; format ASCII part...
; store '.' as default
mov byte ptr [di+bx],'.'
cmp al,blank ; in range 20h-7eh?
jb conv2 ; jump, not alphanumeric
cmp al,7eh ; in range 20h-7eh?
ja conv2 ; jump, not alphanumeric
mov [di+bx],al ; store ASCII character
conv2: ; format hex part...
mov di,offset fouta ; point to output area
add di,bx ; base addr + (offset*3)
add di,bx
add di,bx
call b2a ; convert byte to hex
inc bx ; advance through record
cmp bx,flen ; entire record converted?
jne conv1 ; no, get another byte
; update file pointer
add word ptr fptr,blksize
ret
conv endp
w2a proc near ; convert word to hex ASCII
; call with AX = value
; DI = addr for string
; returns AX, DI, CX destroyed
push ax ; save copy of value
mov al,ah
call b2a ; convert upper byte
pop ax ; get back copy
call b2a ; convert lower byte
ret
w2a endp
b2a proc near ; convert byte to hex ASCII
; call with AL = binary value
; DI = addr for string
; returns AX, DI, CX modified
sub ah,ah ; clear upper byte
mov cl,16
div cl ; divide byte by 16
call ascii ; quotient becomes the first
stosb ; ASCII character
mov al,ah
call ascii ; remainder becomes the
stosb ; second ASCII character
ret
b2a endp
ascii proc near ; convert value 0-0fh in AL
; into "hex ASCII" character
add al,'0' ; offset to range 0-9
cmp al,'9' ; is it > 9?
jle ascii2 ; no, jump
add al,'A'-'9'-1 ; offset to range A-F,
ascii2: ret ; return AL = ASCII char
ascii endp
argc proc near ; count command-line arguments
; call with ES:BX = command line
; returns AX = argument count
push bx ; save original BX and CX
push cx ; for later
mov ax,1 ; force count >= 1
argc1: mov cx,-1 ; set flag = outside argument
argc2: inc bx ; point to next character
cmp byte ptr es:[bx],cr
je argc3 ; exit if carriage return
cmp byte ptr es:[bx],blank
je argc1 ; outside argument if ASCII blank
cmp byte ptr es:[bx],tab
je argc1 ; outside argument if ASCII tab
; otherwise not blank or tab,
jcxz argc2 ; jump if already inside argument
inc ax ; else found argument, count it
not cx ; set flag = inside argument
jmp argc2 ; and look at next character
argc3: pop cx ; restore original BX and CX
pop bx
ret ; return AX = argument count
argc endp
argv proc near ; get address & length of
; command line argument
; call with ES:BX = command line
; AX = argument #
; returns ES:BX = address
; AX = length
push cx ; save original CX and DI
push di
or ax,ax ; is it argument 0?
jz argv8 ; yes, jump to get program name
xor ah,ah ; initialize argument counter
argv1: mov cx,-1 ; set flag = outside argument
argv2: inc bx ; point to next character
cmp byte ptr es:[bx],cr
je argv7 ; exit if carriage return
cmp byte ptr es:[bx],blank
je argv1 ; outside argument if ASCII blank
cmp byte ptr es:[bx],tab
je argv1 ; outside argument if ASCII tab
; if not blank or tab...
jcxz argv2 ; jump if already inside argument
inc ah ; else count arguments found
cmp ah,al ; is this the one we're looking for?
je argv4 ; yes, go find its length
not cx ; no, set flag = inside argument
jmp argv2 ; and look at next character
argv4: ; found desired argument, now
; determine its length...
mov ax,bx ; save param starting address
argv5: inc bx ; point to next character
cmp byte ptr es:[bx],cr
je argv6 ; found end if carriage return
cmp byte ptr es:[bx],blank
je argv6 ; found end if ASCII blank
cmp byte ptr es:[bx],tab
jne argv5 ; found end if ASCII tab
argv6: xchg bx,ax ; set ES:BX = argument address
sub ax,bx ; and AX = argument length
jmp argvx ; return to caller
argv7: xor ax,ax ; set AX = 0, argument not found
jmp argvx ; return to caller
argv8: ; special handling for argv = 0
mov ax,3000h ; check if DOS 3.0 or later
int 21h ; (force AL = 0 in case DOS 1)
cmp al,3
jb argv7 ; DOS 1 or 2, return null param
mov es,es:[2ch] ; get environment segment from PSP
xor di,di ; find the program name by
xor al,al ; first skipping over all the
mov cx,-1 ; environment variables...
cld
argv9: repne scasb ; scan for double null (can't use
scasb ; SCASW since might be odd addr)
jne argv9 ; loop if it was a single null
add di,2 ; skip count word in environment
mov bx,di ; save program name address
mov cx,-1 ; now find its length...
repne scasb ; scan for another null byte
not cx ; convert CX to length
dec cx
mov ax,cx ; return length in AX
argvx: ; common exit point
pop di ; restore original CX and DI
pop cx
ret ; return to caller
argv endp
_TEXT ends
_DATA segment word public 'DATA'
fname db 64 dup (0) ; buffer for input filespec
fhandle dw 0 ; token from PCDOS for input file
flen dw 0 ; actual length read
fptr dw 0 ; relative address in file
fbuff db blksize dup (?) ; data from input file
fout db 'nnnn' ; formatted output area
db blank,blank
fouta db 16 dup ('nn',blank)
db blank
foutb db 16 dup (blank),cr,lf
fout_len equ $-fout
hdg db cr,lf ; heading for each 128 bytes
db 7 dup (blank) ; of formatted output
db '0 1 2 3 4 5 6 7 '
db '8 9 A B C D E F',cr,lf
hdg_len equ $-hdg
msg1 db cr,lf
db 'dump: file not found'
db cr,lf
msg1_len equ $-msg1
msg2 db cr,lf
db 'dump: missing file name'
db cr,lf
msg2_len equ $-msg2
msg3 db cr,lf
db 'dump: wrong MS-DOS version'
db cr,lf,'$'
msg4 db cr,lf
db 'dump: empty file'
db cr,lf
msg4_len equ $-msg4
_DATA ends
STACK segment para stack 'STACK'
db 64 dup (?)
STACK ends
end dump
──────────────────────────────────────────────────────────────────────────
Figure 8-10. The assembly-language version: DUMP.ASM.
──────────────────────────────────────────────────────────────────────────
/*
DUMP.C Displays the binary contents of a file in
hex and ASCII on the standard output device.
Compile: C>CL DUMP.C
Usage: C>DUMP unit:path\filename.ext
Copyright (C) 1988 Ray Duncan
*/
#include <stdio.h>
#include <io.h>
#include <fcntl.h>
#define REC_SIZE 16 /* input file record size */
main(int argc, char *argv[])
{
int fd; /* input file handle */
int status = 0; /* status from file read */
long fileptr = 0L; /* current file byte offset */
char filebuf[REC_SIZE]; /* data from file */
if(argc != 2) /* abort if missing filename */
{ fprintf(stderr,"\ndump: wrong number of parameters\n");
exit(1);
}
/* open file in binary mode,
abort if open fails */
if((fd = open(argv[1],O_RDONLY | O_BINARY) ) == -1)
{ fprintf(stderr, "\ndump: can't find file %s \n", argv[1]);
exit(1);
}
/* read and dump records
until end of file */
while((status = read(fd,filebuf,REC_SIZE) ) != 0)
{ dump_rec(filebuf, fileptr, status);
fileptr += REC_SIZE;
}
close(fd); /* close input file */
exit(0); /* return success code */
}
/*
Display record (16 bytes) in hex and ASCII on standard output
*/
dump_rec(char *filebuf, long fileptr, int length)
{
int i; /* index to current record */
if(fileptr % 128 == 0) /* display heading if needed */
printf("\n\n 0 1 2 3 4 5 6 7 8 9 A B C D E F")
printf("\n%04lX ",fileptr); /* display file offset */
/* display hex equivalent of
each byte from file */
for(i = 0; i < length; i++)
printf(" %02X", (unsigned char) filebuf[i]);
if(length != 16) /* spaces if partial record */
for (i=0; i<(16-length); i++) printf(" ");
/* display ASCII equivalent of
each byte from file */
printf(" ");
for(i = 0; i < length; i++)
{ if(filebuf[i] < 32 || filebuf[i] > 126) putchar('.');
else putchar(filebuf[i]);
}
}
──────────────────────────────────────────────────────────────────────────
Figure 8-11. The C version: DUMP.C.
The assembly-language version of the DUMP program contains a number of
subroutines that you may find useful in your own programming efforts.
These include the following:
Subroutine Action
──────────────────────────────────────────────────────────────────────────
argc Returns the number of command-line arguments.
argv Returns the address and length of a particular command-line
argument.
w2a Converts a binary word (16 bits) into hex ASCII for output.
b2a Converts a binary byte (8 bits) into hex ASCII for output.
ascii Converts 4 bits into a single hex ASCII character.
──────────────────────────────────────────────────────────────────────────
It is interesting to compare these two equivalent programs. The C program
contains only 77 lines, whereas the assembly-language program has 436
lines. Clearly, the C source code is less complex and easier to maintain.
On the other hand, if size and efficiency are important, the DUMP.EXE file
generated by the C compiler is 8563 bytes, whereas the assembly-language
DUMP.EXE file is only 1294 bytes and runs twice as fast as the C program.
────────────────────────────────────────────────────────────────────────────
Chapter 9 Volumes and Directories
Each file in an MS-DOS system is uniquely identified by its name and its
location. The location, in turn, has two components: the logical drive
that contains the file and the directory on that drive where the filename
can be found.
Logical drives are specified by a single letter followed by a colon (for
example, A:). The number of logical drives in a system is not necessarily
the same as the number of physical drives; for example, it is common for
large fixed-disk drives to be divided into two or more logical drives. The
key aspect of a logical drive is that it contains a self-sufficient file
system; that is, it contains one or more directories, zero or more
complete files, and all the information needed to locate the files and
directories and to determine which disk space is free and which is already
in use.
Directories are simply lists or catalogs. Each entry in a directory
consists of the name, size, starting location, attributes, and last
modification date and time of a file or another directory that the disk
contains. The detailed information about the location of every block of
data assigned to a file or directory is in a separate control area on the
disk called the file allocation table (FAT). (See Chapter 10 for a
detailed discussion of the internal format of directories and the FAT.)
Every disk potentially has two distinct kinds of directories: the root
directory and all other directories. The root directory is always present
and has a maximum number of entries, determined when the disk is
formatted; this number cannot be changed. The subdirectories of the root
directory, which may or may not be present on a given disk, can be nested
to any level and can grow to any size (Figure 9-1). This is the
hierarchical, or tree, directory structure referred to in earlier
chapters. Every directory has a name, except for the root directory, which
is designated by a single backslash (\) character.
MS-DOS keeps track of a "current drive" for the system and uses this drive
when a file specification does not include an explicit drive code.
Similarly, MS-DOS maintains a "current directory" for each logical drive.
You can select any particular directory on a drive by naming in order──
either from the root directory or relative to the current directory──the
directories that lead to its location in the tree structure. Such a list
of directories, separated by backslash delimiters, is called a path. When
a complete path from the root directory is prefixed by a logical drive
code and followed by a filename and extension, the resulting string is a
fully qualified filename and unambiguously specifies a file.
┌────────────┐
│ Drive │
│ identifier │
└─────┬──────┘
│
┌───────┴────────┐
│ Root directory │
│ (volume label) │
└─┬──┬──┬───┬──┬─┘
┌───────────────────┘ │ │ │ └───────────────────┐
│ ┌───────────┘ │ └───────────┐ │
┌────┴───┐ ┌────┴──────┐ ┌───┴────┐ ┌──────┴────┐ ┌───┴────┐
│ File A │ │ Directory │ │ File B │ │ Directory │ │ File C │
└────────┘ └─┬───────┬─┘ └────────┘ └─┬─────────┘ └─┬──────┘
│ │ │ │
│ │ │ │
┌─────┘ │ │ │
│ │ │ │
┌────┴──────┐ ┌──┴─────┐ ┌─────┴──┐ ┌───┴────┐
│ Directory │ │ File D │ │ File E │ │ File F │
└───────────┘ └────────┘ └────────┘ └────────┘
Figure 9-1. An MS-DOS file-system structure.
Drive and Directory Control
You can examine, select, create, and delete disk directories interactively
with the DIR, CHDIR (CD), MKDIR (MD), and RMDIR (RD) commands. You can
select a new current drive by entering the letter of the desired drive,
followed by a colon. MS-DOS provides the following Int 21H functions to
give application programs similar control over drives and directories:
Function Action
──────────────────────────────────────────────────────────────────────────
0EH Select current drive.
19H Get current drive.
39H Create directory.
3AH Remove directory.
3BH Select current directory.
47H Get current directory.
──────────────────────────────────────────────────────────────────────────
The two functions that deal with disk drives accept or return a binary
drive code──0 represents drive A, 1 represents drive B, and so on. This
differs from most other MS-DOS functions, which use 0 to indicate the
current drive, 1 for drive A, and so on.
The first three directory functions in the preceding list require an
ASCIIZ string that describes the path to the desired directory. As with
the handle-based file open and create functions, the address of the ASCIIZ
string is passed in the DS:DX registers. On return, the carry flag is
clear if the function succeeds or set if the function failed, with an
error code in the AX register. The directory functions can fail for a
variety of reasons, but the most common cause of an error is that some
element of the indicated path does not exist.
The last function in the preceding list, Int 21H Function 47H, allows you
to obtain an ASCIIZ path for the current directory on the specified or
default drive. MS-DOS supplies the path string without the drive
identifier or a leading backslash. Int 21H Function 47H is most commonly
used with Int 21H Function 19H to build fully qualified filenames. Such
filenames are desirable because they remain valid if the user changes the
current drive or directory.
Section 2 of this book, "MS-DOS Functions Reference," gives detailed
information on the drive and directory control functions.
Searching Directories
When you request an open operation on a file, you are implicitly
performing a search of a directory. MS-DOS examines each entry of the
directory to find a match for the filename you have given as an argument;
if the file is found, MS-DOS copies certain information from the directory
into a data structure that it can use to control subsequent read or write
operations to the file. Thus, if you wish to test for the existence of a
specific file, you need only perform an open operation and observe whether
it is successful. (If it is, you should, of course, perform a subsequent
close operation to avoid needless expenditure of handles.)
Sometimes you may need to perform more elaborate searches of a disk
directory. Perhaps you wish to find all the files with a certain
extension, a file with a particular attribute, or the names of the
subdirectories of a certain directory. Although the locations of a disk's
directories and the specifics of the entries that are found in them are of
necessity hardware dependent (for example, interpretation of the field
describing the starting location of a file depends upon the physical disk
format), MS-DOS does provide functions that will allow examination of a
disk directory in a hardware-independent fashion.
In order to search a disk directory successfully, you must understand two
types of MS-DOS search services. The first type is the "search for first"
function, which accepts a file specification──possibly including wildcard
characters──and looks for the first matching file in the directory of
interest. If it finds a match, the function fills a buffer owned by the
requesting program with information about the file; if it does not find a
match, it returns an error flag.
A program can call the second type of search service, called "search for
next," only after a successful "search for first." If the file
specification that was originally passed to "search for first" included
wildcard characters and at least one matching file was present, the
program can call "search for next" as many times as necessary to find all
additional matching files. Like "search for first," "search for next"
returns information about the matched files in a buffer designated by the
requesting program. When it can find no more matching files, "search for
next" returns an error flag.
As with nearly every other operation, MS-DOS provides two parallel sets of
directory-searching services:
Action FCB function Handle function
──────────────────────────────────────────────────────────────────────────
Search for first 11H 4EH
Search for next 12H 4FH
──────────────────────────────────────────────────────────────────────────
The FCB directory functions allow searches to match a filename and
extension, both possibly containing wildcard characters, within the
current directory for the specified or current drive. The handle directory
functions, on the other hand, allow a program to perform searches within
any directory on any drive, regardless of the current directory.
Searches that use normal FCBs find only normal files. Searches that use
extended FCBs, or the handle-type functions, can be qualified with file
attributes. The attribute bits relevant to searches are as follows:
Bit Significance
──────────────────────────────────────────────────────────────────────────
0 Read-only file
1 Hidden file
2 System file
3 Volume label
4 Directory
5 Archive needed (set when file modified)
──────────────────────────────────────────────────────────────────────────
The remaining bits of a search function's attribute parameter should be
zero. When any of the preceding attribute bits are set, the search
function returns all normal files plus any files with the specified
attributes, except in the case of the volume-label attribute bit, which
receives special treatment as described later in this chapter. Note that
by setting bit 4 you can include directories in a search, exactly as
though they were files.
Both the FCB and handle directory-searching functions require that the
disk transfer area address be set (with Int 21H Function 1AH), before the
call to "search for first," to point to a working buffer for use by
MS-DOS. The DTA address should not be changed between calls to "search for
first" and "search for next." When it finds a matching file, MS-DOS places
the information about the file in the buffer and then inspects the buffer
on the next "search for next" call, to determine where to resume the
search. The format of the data returned in the buffer is different for the
FCB and handle functions, so read the detailed descriptions in Section 2
of this book, "MS-DOS Functions Reference," before attempting to interpret
the buffer contents.
Figures 9-2 and 9-3 provide equivalent examples of searches for all
files in a given directory that have the .ASM extension, one example using
the FCB directory functions (Int 21H Functions 11H and 12H) and the
other using the handle functions (Int 21H Functions 4EH and 4FH). (Both
programs use the handle write function with the standard output handle to
display the matched filenames, to avoid introducing tangential differences
in the listings.)
──────────────────────────────────────────────────────────────────────────
start: ; set DTA address for buffer
; used by search functions
mov dx,seg buff ; DS:DX = buffer address
mov ds,dx
mov dx,offset buff
mov ah,1ah ; function 1ah = search for first
int 21h ; transfer to MS-DOS
; search for first match...
mov dx,offset fcb ; DS:DX = FCB address
mov ah,11h ; function 11h = search for first
int 21h ; transfer to MS-DOS
or al,al ; any matches at all?
jnz exit ; no, quit
disp: ; go to a new line...
mov dx,offset crlf ; DS:DX = CR-LF string
mov cx,2 ; CX = string length
mov bx,1 ; BX = standard output handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
; display matching file
mov dx,offset buff+1 ; DS:DX = filename
mov cx,11 ; CX = length
mov bx,1 ; BX = standard output handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
; search for next match...
mov dx,offset fcb ; DS:DX = FCB address
mov ah,12h ; function 12h = search for next
int 21h ; transfer to MS-DOS
or al,al ; any more matches?
jz disp ; yes, go show filename
exit: ; final exit point
mov ax,4c00h ; function 4ch = terminate,
; return code = 0
int 21h ; transfer to MS-DOS
.
.
.
crlf db 0dh,0ah ; ASCII carriage return-
; linefeed string
fcb db 0 ; drive = current
db 8 dup ('?') ; filename = wildcard
db 'ASM' ; extension = ASM
db 25 dup (0) ; remainder of FCB = zero
buff db 64 dup (0) ; receives search results
──────────────────────────────────────────────────────────────────────────
Figure 9-2. Example of an FCB-type directory search using Int 21H
Functions 11H and 12H. This routine displays the names of all files in
the current directory that have the .ASM extension.
──────────────────────────────────────────────────────────────────────────
start: ; set DTA address for buffer
; used by search functions
mov dx,seg buff ; DS:DX = buffer address
mov ds,dx
mov dx,offset buff
mov ah,1ah ; function 1ah = search for first
int 21h ; transfer to MS-DOS
; search for first match...
mov dx,offset fname ; DS:DX = wildcard filename
mov cx,0 ; CX = normal file attribute
mov ah,4eh ; function 4eh = search for first
int 21h ; transfer to MS-DOS
jc exit ; quit if no matches at all
disp: ; go to a new line...
mov dx,offset crlf ; DS:DX = CR-LF string
mov cx,2 ; CX = string length
mov bx,1 ; BX = standard output handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
; find length of filename...
mov cx,0 ; CX will be char count
; DS:SI = start of name
mov si,offset buff+30
disp1: lodsb ; get next character
or al,al ; is it null character?
jz disp2 ; yes, found end of string
inc cx ; else count characters
jmp disp1 ; and get another
disp2: ; display matching file...
; CX already contains length
; DS:DX = filename
mov dx,offset buff+30
mov bx,1 ; BX = standard output handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
; find next matching file...
mov ah,4fh ; function 4fh = search for next
int 21h ; transfer to MS-DOS
jnc disp ; jump if another match found
exit: ; final exit point
mov ax,4c00h ; function 4ch = terminate,
; return code = 0
int 21h ; transfer to MS-DOS
.
.
.
crlf db 0dh,0ah ; ASCII carriage return-
; linefeed string
fname db '*.ASM',0 ; ASCIIZ filename to
; be matched
buff db 64 dup (0) ; receives search results
──────────────────────────────────────────────────────────────────────────
Figure 9-3. Example of a handle-type directory search using Int 21H
Functions 4EH and 4FH. This routine also displays the names of all files
in the current directory that have a .ASM extension.
Moving Files
The rename file function that was added in MS-DOS version 2.0, Int 21H
Function 56H, has the little-advertised capability to move a file from
one directory to another. The function has two ASCIIZ parameters: the
"old" and "new" names for the file. If the old and new paths differ,
MS-DOS moves the file; if the filename or extension components differ,
MS-DOS renames the file. MS-DOS can carry out both of these actions in the
same function call.
Of course, the old and new directories must be on the same drive, because
the file's actual data is not moved at all; only the information that
describes the file is removed from one directory and placed in another
directory. Function 56H fails if the two ASCIIZ strings include different
logical-drive codes, if the file is read-only, or if a file with the same
name and location as the "new" filename already exists.
The FCB-based rename file service, Int 21H Function 17H, works only on
the current directory and cannot be used to move files.
Volume Labels
Support for volume labels was first added to MS-DOS in version 2.0. A
volume label is an optional name of from 1 to 11 characters that the user
assigns to a disk during a FORMAT operation. You can display a volume
label with the DIR, TREE, CHKDSK, or VOL command. Beginning with MS-DOS
version 3.0, you can use the LABEL command to add, display, or alter the
label after formatting. In MS-DOS version 4, the FORMAT program also
assigns a semi-random 32-bit binary ID to each disk it formats; you can
display this value, but you cannot change it.
The distinction between volumes and drives is important. A volume label is
associated with a specific storage medium. A drive identifier (such as A)
is associated with a physical device that a storage medium can be mounted
on. In the case of fixed-disk drives, the medium associated with a drive
identifier does not change (hence the name). In the case of floppy disks
or other removable media, the disk accessed with a given drive identifier
might have any volume label or none at all.
Hence, volume labels do not take the place of the logical-drive identifier
and cannot be used as part of a pathname to identify a file. In fact, in
MS-DOS version 2, the system does not use volume labels internally at all.
In MS-DOS versions 3.0 and later, a disk driver can use volume labels to
detect whether the user has replaced a disk while a file is open; this use
is optional, however, and is not implemented in all systems.
MS-DOS volume labels are implemented as a special type of entry in a
disk's root directory. The entry contains a time-and-date stamp and has an
attribute value of 8 (i.e., bit 3 set). Except for the attribute, a volume
label is identical to the directory entry for a file that was created but
never had any data written into it, and you can manipulate volume labels
with Int 21H functions much as you manipulate files. However, a volume
label receives special handling at several levels:
■ When you create a volume label after a disk is formatted, MS-DOS always
places it in the root directory, regardless of the current directory.
■ A disk can contain only one volume label; attempts to create additional
volume labels (even with different names) will fail.
■ MS-DOS always carries out searches for volume labels in the root
directory, regardless of the current directory, and does not also
return all normal files.
In MS-DOS version 2, support for volume labels is not completely
integrated into the handle file functions, and you must use extended FCBs
instead to manipulate volume labels. For example, the code in Figure 9-4
searches for the volume label in the root directory of the current drive.
You can also change volume labels with extended FCBs and the rename file
function (Int 21H Function 17H), but you should not attempt to remove an
existing volume label with Int 21H Function 13H under MS-DOS version 2,
because this operation can damage the disk's FAT in an unpredictable
manner.
In MS-DOS versions 3.0 and later, you can create a volume label in the
expected manner, using Int 21H Function 3CH and an attribute of 8, and
you can use the handle-type "search for first" function (4EH) to obtain
an existing volume label for a logical drive (Figure 9-5). However, you
still must use extended FCBs to change a volume label.
──────────────────────────────────────────────────────────────────────────
buff db 64 dup (?) ; receives search results
xfcb db 0ffh ; flag signifying extended FCB
db 5 dup (0) ; reserved
db 8 ; volume attribute byte
db 0 ; drive code (0 = current)
db 11 dup ('?') ; wildcard filename and extension
db 25 dup (0) ; remainder of FCB (not used)
.
.
.
; set DTA address for buffer
; used by search functions
mov dx,seg buff ; DS:DX = buffer address
mov ds,dx
mov dx,offset buff
mov ah,1ah ; function 1ah = set DTA
int 21h ; transfer to MS-DOS
; now search for label...
; DS:DX = extended FCB
mov dx,offset xfcb
mov ah,11h ; function 11h = search for first
int 21h ; transfer to MS-DOS
cmp al,0ffh ; search successful?
je no_label ; jump if no volume label
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 9-4. A volume-label search under MS-DOS version 2, using an
extended file control block. If the search is successful, the volume label
is returned in buff, formatted in the filename and extension fields of an
extended FCB.
──────────────────────────────────────────────────────────────────────────
buff db 64 dup (?) ; receives search results
wildcd db '*.*',0 ; wildcard ASCIIZ filename
.
.
.
; set DTA address for buffer
; used by search functions
mov dx,seg buff ; DS:DX = buffer address
mov ds,dx
mov dx,offset buff
mov ah,1ah ; function 1ah = set DTA
int 21h ; transfer to MS-DOS
; now search for label...
; DS:DX = ASCIIZ string
mov dx,offset wildcd
mov cx,8 ; CX = volume attribute
mov ah,4eh ; function 4eh = search for first
int 21h ; transfer to MS-DOS
jc no_label ; jump if no volume label
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 9-5. A volume-label search under MS-DOS version 3, using the
handle-type file functions. If the search is successful (carry flag
returned clear), the volume name is placed at location buff+1EH in the
form of an ASCIIZ string.
────────────────────────────────────────────────────────────────────────────
Chapter 10 Disk Internals
MS-DOS disks are organized according to a rather rigid scheme that is
easily understood and therefore easily manipulated. Although you will
probably never need to access the special control areas of a disk
directly, an understanding of their internal structure leads to a better
understanding of the behavior and performance of MS-DOS as a whole.
From the application programmer's viewpoint, MS-DOS presents disk devices
as logical volumes that are associated with a drive code (A, B, C, and so
on) and that have a volume name (optional), a root directory, and from
zero to many additional directories and files. MS-DOS shields the
programmer from the physical characteristics of the medium by providing a
battery of disk services through Int 21H. Using these services, the
programmer can create, open, read, write, close, and delete files in a
uniform way, regardless of the disk drive's size, speed, number of
read/write heads, number of tracks, and so forth.
Requests from an application program for file operations actually go
through two levels of translation before resulting in the physical
transfer of data between the disk device and random-access memory:
1. Beneath the surface, MS-DOS views each logical volume, whether it is
an entire physical unit such as a floppy disk or only a part of a
fixed disk, as a continuous sequence of logical sectors, starting at
sector 0. (A logical disk volume can also be implemented on other
types of storage. For example, RAM disks map a disk structure onto an
area of random-access memory.) MS-DOS translates an application
program's Int 21H file-management requests into requests for transfers
of logical sectors, using the information found in the volume's
directories and allocation tables. (For those rare situations where it
is appropriate, programs can also access logical sectors directly with
Int 25H and Int 26H.)
2. MS-DOS then passes the requests for logical sectors to the disk
device's driver, which maps them onto actual physical addresses (head,
track, and sector). Disk drivers are extremely hardware dependent and
are always written in assembly language for maximum speed. In most
versions of MS-DOS, a driver for IBM-compatible floppy- and fixed-disk
drives is built into the MS-DOS BIOS module (IO.SYS) and is always
loaded during system initialization; you can install additional
drivers for non-IBM-compatible disk devices by including the
appropriate DEVICE directives in the CONFIG.SYS file.
Each MS-DOS logical volume is divided into several fixed-size control
areas and a files area (Figure 10-1). The size of each control area
depends on several factors──the size of the volume and the version of
FORMAT used to initialize the volume, for example──but all of the
information needed to interpret the structure of a particular logical
volume can be found on the volume itself in the boot sector.
┌───────────────────────────────────────────────────────┐
│ Boot sector │
│ Reserved area │
├───────────────────────────────────────────────────────┤
│ File allocation table #1 │
├───────────────────────────────────────────────────────┤
│ Possible additional copies of FAT │
├───────────────────────────────────────────────────────┤
│ Root directory │
├───────────────────────────────────────────────────────┤
│ │
│ Files area │
│ │
└───────────────────────────────────────────────────────┘
Figure 10-1. Map of a typical MS-DOS logical volume. The boot sector
(logical sector 0) contains the OEM identification, BIOS parameter block
(BPB), and disk bootstrap. The remaining sectors are divided among an
optional reserved area, one or more copies of the file allocation table,
the root directory, and the files area.
The Boot Sector
Logical sector 0, known as the boot sector, contains all of the critical
information regarding the disk medium's characteristics (Figure 10-2).
The first byte in the sector is always an 80x86 jump instruction──either a
normal intrasegment JMP (opcode 0E9H) followed by a 16-bit displacement or
a "short" JMP (opcode 0EBH) followed by an 8-bit displacement and then by
an NOP (opcode 90H). If neither of these two JMP opcodes is present, the
disk has not been formatted or was not formatted for use with MS-DOS. (Of
course, the presence of the JMP opcode does not in itself ensure that the
disk has an MS-DOS format.)
Following the initial JMP instruction is an 8-byte field that is reserved
by Microsoft for OEM identification. The disk-formatting program, which is
specialized for each brand of computer, disk controller, and medium, fills
in this area with the name of the computer manufacturer and the
manufacturer's internal MS-DOS version number.
00H ┌───────────────────────────────────────────────┐
│ E9 XX XX or EB XX 90 │
03H ├───────────────────────────────────────────────┤
│ OEM name and version │
│ (8 bytes) │
OBH ├───────────────────────────────────────────────┤─┐
│ Bytes per sector (2 bytes) │ │
ODH ├───────────────────────────────────────────────┤ │
│ Sectors per allocation unit (1 byte) │ │
0EH ├───────────────────────────────────────────────┤ │
│ Reserved sectors, starting at 0 (2 bytes) │ │
10H ├───────────────────────────────────────────────┤ │
│ Number of FATs (1 byte) │ B
11H ├───────────────────────────────────────────────┤ P
│ Number of root-directory entries (2 bytes) │ B
13H ├───────────────────────────────────────────────┤ │
│ Total sectors in logical volume (2 bytes) │ │
15H ├───────────────────────────────────────────────┤ │ MS-DOS
│ Media descriptor byte │ │ version 2.0
16H ├───────────────────────────────────────────────┤ │
│ Number of sectors per FAT (2 bytes) │ │
18H ├───────────────────────────────────────────────┤═╡
│ Sectors per track (2 bytes) │ │
1AH ├───────────────────────────────────────────────┤ │
│ Number of heads (2 bytes) │ │ MS-DOS
1CH ├───────────────────────────────────────────────┤ │ version 3.0
│ Number of hidden sectors (4 bytes) │═╡
20H ├───────────────────────────────────────────────┤ │ MS-DOS
│ Total sectors in logical volume │ │ version 4.0
│ (MS-DOS 4.0 and volume size >32 MB) │ │
24H ├───────────────────────────────────────────────┤═╡
│ Physical drive number │ │
25H ├───────────────────────────────────────────────┤ │
│ Reserved │ │
26H ├───────────────────────────────────────────────┤ │
│ Extended boot signature record (29H) │ │ Additional
27H ├───────────────────────────────────────────────┤ │ MS-DOS 4.0
│ 32-bit binary volume ID │ │ information
2BH ├───────────────────────────────────────────────┤ │
│ Volume label (11 bytes) │ │
36H ├───────────────────────────────────────────────┤ │
│ Reserved (8 bytes) │ │
3EH ├───────────────────────────────────────────────┤─┘
│ Bootstrap │
└───────────────────────────────────────────────┘
Figure 10-2. Map of the boot sector of an MS-DOS disk. Note the JMP at
offset 0, the OEM identification field, the MS-DOS version 2 compatible
BIOS parameter block (bytes 0BH─17H), the three additional WORD fields for
MS-DOS version 3, the double-word number-of-sectors field and 32-bit
binary volume ID for MS-DOS version 4.0, and the bootstrap code.
The third major component of the boot sector is the BIOS parameter block
(BPB) in bytes 0BH through 17H. (Additional fields are present in MS-DOS
versions 3.0 and later.) This data structure describes the physical disk
characteristics and allows the device driver to calculate the proper
physical disk address for a given logical-sector number; it also contains
information that is used by MS-DOS and various system utilities to
calculate the address and size of each of the disk control areas (file
allocation tables and root directory).
The final element of the boot sector is the disk bootstrap routine. The
disk bootstrap is usually read into memory by the ROM bootstrap, which is
executed automatically when the computer is turned on. The ROM bootstrap
is usually just smart enough to home the head of the disk drive (move it
to track 0), read the first physical sector into RAM at a predetermined
location, and jump to it. The disk bootstrap is more sophisticated. It
calculates the physical disk address of the beginning of the files area,
reads the files containing the operating system into memory, and transfers
control to the BIOS module at location 0070:0000H. (See Chapter 2.)
Figures 10-3 and 10-4 show a partial hex dump and disassembly of a
PC-DOS 3.3 floppy-disk boot sector.
──────────────────────────────────────────────────────────────────────────
0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 EB 34 90 49 42 4D 20 20 33 2E 33 00 02 02 01 00 .4.IBM 3.3.....
0010 02 70 00 D0 02 FD 02 00 09 00 02 00 00 00 00 00 .p..............
0020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 12 ................
0030 00 00 00 00 01 00 FA 33 C0 8E D0 BC 00 7C 16 07 .......3.....|..
.
.
.
01C0 0D 0A 44 69 73 6B 20 42 6F 6F 74 20 66 61 69 6C ..Disk Boot fail
01D0 75 72 65 0D 0A 00 49 42 4D 42 49 4F 20 20 43 4F ure...IBMBIO CO
01E0 4D 49 42 4D 44 4F 53 20 20 43 4F 4D 00 00 00 00 MIBMDOS COM....
01F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA ..............U.
──────────────────────────────────────────────────────────────────────────
Figure 10-3. Partial hex dump of the boot sector (track 0, head 0, sector
1) of a PC-DOS version 3.3 floppy disk. This sector contains the OEM
identification, a copy of the BIOS parameter block describing the medium,
and the bootstrap routine that reads the BIOS into memory and transfers
control to it. See also Figures 10-2 and 10-4.
──────────────────────────────────────────────────────────────────────────
jmp $+54 ; jump to bootstrap
nop
db 'IBM 3.3' ; OEM identification
; BIOS parameter block
dw 512 ; bytes per sector
db 2 ; sectors per cluster
dw 1 ; reserved sectors
db 2 ; number of FATs
dw 112 ; root directory entries
dw 720 ; total sectors
db 0fdh ; media descriptor byte
dw 2 ; sectors per FAT
dw 9 ; sectors per track
dw 2 ; number of heads
dd 0 ; hidden sectors
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 10-4. Partial disassembly of the boot sector shown in Figure
10-3.
The Reserved Area
The boot sector is actually part of a reserved area that can span from one
to several sectors. The reserved-sectors word in the BPB, at offset 0EH in
the boot sector, describes the size of this area. Remember that the number
in the BPB field includes the boot sector itself, so if the value is 1 (as
it is on IBM PC floppy disks), the length of the reserved area is actually
0 sectors.
The File Allocation Table
When a file is created or extended, MS-DOS assigns it groups of disk
sectors from the files area in powers of 2. These are known as allocation
units or clusters. The number of sectors per cluster for a given medium is
defined in the BPB and can be found at offset 0DH in the disk's boot
sector. Below are some example cluster sizes:
Disk type Power of 2 Sectors/cluster
──────────────────────────────────────────────────────────────────────────
5.25" 180 KB floppy disk 0 1
5.25" 360 KB floppy disk 1 2
PC/AT fixed disk 2 4
PC/XT fixed disk 3 8
──────────────────────────────────────────────────────────────────────────
The file allocation table (FAT) is divided into fields that correspond
directly to the assignable clusters on the disk. These fields are 12 bits
in MS-DOS versions 1 and 2 and may be either 12 bits or 16 bits in
versions 3.0 and later, depending on the size of the medium (12 bits if
the disk contains fewer than 4087 clusters, 16 bits otherwise).
The first two fields in the FAT are always reserved. On IBM-compatible
media, the first 8 bits of the first reserved FAT entry contain a copy of
the media descriptor byte, which is also found in the BPB in the boot
sector. The second, third, and (if applicable) fourth bytes, which
constitute the remainder of the first two reserved FAT fields, always
contain 0FFH. The currently defined IBM-format media descriptor bytes are
as follows:
MS-DOS version
where first
Descriptor Medium supported
──────────────────────────────────────────────────────────────────────────
0F0H 3.5" floppy disk, 2-sided, 18-sector 3.3
0F8H Fixed disk 2.0
0F9H 5.25" floppy disk, 2-sided, 15-sector 3.0
3.5" floppy disk, 2-sided, 9-sector 3.2
0FCH 5.25" floppy disk, 1-sided, 9-sector 2.0
0FDH 5.25" floppy disk, 2-sided, 9-sector 2.0
8" floppy disk, 1-sided, single-density
0FEH 5.25" floppy disk, 1-sided, 8-sector 1.0
8" floppy disk, 1-sided, single-density
8" floppy disk, 2-sided, double-density
0FFH 5.25" floppy disk, 2-sided, 8-sector 1.1
──────────────────────────────────────────────────────────────────────────
The remainder of the FAT entries describe the use of their corresponding
disk clusters. The contents of the FAT fields are interpreted as follows:
Value Meaning
──────────────────────────────────────────────────────────────────────────
(0)000H Cluster available
(F)FF0─(F)FF6H Reserved cluster
(F)FF7H Bad cluster, if not part of chain
(F)FF8─(F)FFFH Last cluster of file
(X)XXX Next cluster in file
──────────────────────────────────────────────────────────────────────────
Each file's entry in a directory contains the number of the first cluster
assigned to that file, which is used as an entry point into the FAT. From
the entry point on, each FAT slot contains the cluster number of the next
cluster in the file, until a last-cluster mark is encountered.
At the computer manufacturer's option, MS-DOS can maintain two or more
identical copies of the FAT on each volume. MS-DOS updates all copies
simultaneously whenever files are extended or the directory is modified.
If access to a sector in a FAT fails due to a read error, MS-DOS tries the
other copies until a successful disk read is obtained or all copies are
exhausted. Thus, if one copy of the FAT becomes unreadable due to wear or
a software accident, the other copies may still make it possible to
salvage the files on the disk. As part of its procedure for checking the
integrity of a disk, the CHKDSK program compares the multiple copies
(usually two) of the FAT to make sure they are all readable and
consistent.
The Root Directory
Following the file allocation tables is an area known in MS-DOS versions
2.0 and later as the root directory. (Under MS-DOS version 1, it was the
only directory on the disk.) The root directory contains 32-byte entries
that describe files, other directories, and the optional volume label
(Figure 10-5). An entry beginning with the byte value E5H is available
for reuse; it represents a file or directory that has been erased. An
entry beginning with a null (zero) byte is the logical end-of-directory;
that entry and all subsequent entries have never been used.
00H ┌──────────────────────────────┐
│ Filename │ Note 1
08H ├──────────────────────────────┤
│ Extension │
0BH ├──────────────────────────────┤
│ File attribute │ Note 2
0CH ├──────────────────────────────┤
│ Reserved │
16H ├──────────────────────────────┤
│ Time created or last updated │ Note 3
18H ├──────────────────────────────┤
│ Date created or last updated │ Note 4
1AH ├──────────────────────────────┤
│ Starting cluster │
1CH ├──────────────────────────────┤
│ File size, 4 bytes │ Note 5
20H └──────────────────────────────┘
Figure 10-5. Format of a single entry in a disk directory. Total length
is 32 bytes (20H bytes).
──────────────────────────────────────────────────────────────────────────
Notes for Figure 10-5
1. The first byte of the filename field of a directory entry may
contain the following special information:
Value Meaning
────────────────────────────────────────────────────────────────────────
00H Directory entry has never been used; end of occupied
portion of directory.
05H First character of filename is actually E5H.
2EH Entry is an alias for the current or parent directory.
If the next byte is also 2EH, the cluster field
contains the cluster number of the parent directory
(zero if the parent directory is the root directory).
E5H File has been erased.
────────────────────────────────────────────────────────────────────────
2. The attribute byte of the directory entry is mapped as follows:
Bit Meaning
────────────────────────────────────────────────────────────────────────
0 Read-only; attempts to open file for write or to
delete file will fail.
1 Hidden file; excluded from normal searches.
2 System file; excluded from normal searches.
3 Volume label; can exist only in root directory.
4 Directory; excluded from normal searches.
5 Archive bit; set whenever file is modified.
6 Reserved.
7 Reserved.
────────────────────────────────────────────────────────────────────────
3. The time field is encoded as follows:
Bits Contents
────────────────────────────────────────────────────────────────────────
00H─04H Binary number of 2-second increments (0─29,
corresponding to 0─58 seconds)
05H─0AH Binary number of minutes (0─59)
0BH─0FH Binary number of hours (0─23)
────────────────────────────────────────────────────────────────────────
4. The date field is encoded as follows:
Bits Contents
────────────────────────────────────────────────────────────────────────
00H─04H Day of month (1─31)
05H─08H Month (1─12)
09H─0FH Year (relative to 1980)
────────────────────────────────────────────────────────────────────────
5. The file-size field is interpreted as a 4-byte integer, with the
low-order 2 bytes of the number stored first.
──────────────────────────────────────────────────────────────────────────
The root directory has a number of special properties. Its size and
position are fixed and are determined by the FORMAT program when a disk is
initialized. This information can be obtained from the boot sector's BPB.
If the disk is bootable, the first two entries in the root directory
always describe the files containing the MS-DOS BIOS and the MS-DOS
kernel. The disk bootstrap routine uses these entries to bring the
operating system into memory and start it up.
Figure 10-6 shows a partial hex dump of the first sector of the root
directory on a bootable PC-DOS 3.3 floppy disk.
──────────────────────────────────────────────────────────────────────────
0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 49 42 4D 42 49 4F 20 20 43 4F 4D 27 00 00 00 00 IBMBIO COM'....
0010 00 00 00 00 00 00 00 60 72 0E 02 00 54 56 00 00 .......'r...TV..
0020 49 42 4D 44 4F 53 20 20 43 4F 4D 27 00 00 00 00 IBMDOS COM'....
0030 00 00 00 00 00 00 00 60 71 0E 18 00 CF 75 00 00 .......'q....u..
0040 43 4F 4D 4D 41 4E 44 20 43 4F 4D 20 00 00 00 00 COMMAND COM ....
0050 00 00 00 00 00 00 00 60 71 0E 36 00 DB 62 00 00 .......'q.6..b..
0060 42 4F 4F 54 44 49 53 4B 20 20 20 28 00 00 00 00 BOOTDISK (....
0070 00 00 00 00 00 00 A1 00 21 00 00 00 00 00 00 00 ........!.......
0080 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0090 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 10-6. Partial hex dump of the first sector of the root directory
for a PC-DOS 3.3 disk containing the three system files and a volume
label.
The Files Area
The remainder of the volume after the root directory is known as the files
area. MS-DOS views the sectors in this area as a pool of clusters, each
containing one or more logical sectors, depending on the disk format. Each
cluster has a corresponding entry in the FAT that describes its current
use: available, reserved, assigned to a file, or unusable (because of
defects in the medium). Because the first two fields of the FAT are
reserved, the first cluster in the files area is assigned the number 2.
When a file is extended under versions 1 and 2, MS-DOS searches the FAT
from the beginning until it finds a free cluster (designated by a zero FAT
field); it then changes that FAT field to a last-cluster mark and updates
the previous last cluster of the file's chain to point to the new last
cluster. Under versions 3.0 and later, however, MS-DOS searches the FAT
from the most recently allocated cluster; this reduces file fragmentation
and improves overall access times.
Directories other than the root directory are simply a special type of
file. Their storage is allocated from the files area, and their contents
are 32-byte entries──in the same format as those used in the root
directory──that describe files or other directories. Directory entries
that describe other directories contain an attribute byte with bit 4 set,
zero in the file-length field, and the date and time that the directory
was created (Figure 10-7). The first cluster field points, of course, to
the first cluster in the files area that belongs to the directory. (The
directory's other clusters can be found only by tracing through the FAT.)
All directories except the root directory contain two special directory
entries with the names . and ... MS-DOS puts these entries in place when
it creates a directory, and they cannot be deleted. The . entry is an
alias for the current directory; its cluster field points to the cluster
in which it is found. The .. entry is an alias for the directory's parent
(the directory immediately above it in the tree structure); its cluster
field points to the first cluster of the parent directory. If the parent
is the root directory, the cluster field of the .. entry contains zero
(Figure 10-8).
──────────────────────────────────────────────────────────────────────────
.
.
.
0080 4D 59 44 49 52 20 20 20 20 20 20 10 00 00 00 00 MYDIR .....
0090 00 00 00 00 00 00 87 9A 9B 0A 2A 00 00 00 00 00 ..........*.....
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 10-7. Extract from the root directory of an MS-DOS disk, showing
the entry for a subdirectory named MYDIR. Bit 4 in the attribute byte is
set, the cluster field points to the first cluster of the subdirectory
file, the date and time stamps are valid, but the file length is zero.
──────────────────────────────────────────────────────────────────────────
0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 2E 20 20 20 20 20 20 20 20 20 20 10 00 00 00 00 . .....
0010 00 00 00 00 00 00 87 9A 9B 0A 2A 00 00 00 00 00 ..........*.....
0020 2E 2E 20 20 20 20 20 20 20 20 20 10 00 00 00 00 .. .....
0030 00 00 00 00 00 00 87 9A 9B 0A 00 00 00 00 00 00 ................
0040 4D 59 46 49 4C 45 20 20 44 41 54 20 00 00 00 00 MYFILE DAT ....
0050 00 00 00 00 00 00 98 9A 9B 0A 2B 00 15 00 00 00 ..........+.....
0060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
0070 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 10-8. Hex dump of the first block of the directory MYDIR. Note the
. and .. entries. This directory contains exactly one file, MYFILE.DAT.
Interpreting the File Allocation Table
Now that we understand how the disk is structured, let's see how we can
use this knowledge to find a FAT position from a cluster number.
If the FAT has 12-bit entries, use the following procedure:
1. Use the directory entry to find the starting cluster of the file in
question.
2. Multiply the cluster number by 1.5.
3. Use the integral part of the product as the offset into the FAT and
move the word at that offset into a register. Remember that a FAT
position can span a physical disk-sector boundary.
4. If the product is a whole number, AND the register with 0FFFH.
5. Otherwise, "logical shift" the register right 4 bits.
6. If the result is a value from 0FF8H through 0FFFH, the file has no
more clusters. Otherwise, the result is the number of the next cluster
in the file.
On disks with at least 4087 clusters formatted under MS-DOS version 3.0 or
later, the FAT entries use 16 bits, and the extraction of a cluster number
from the table is much simpler:
1. Use the directory entry to find the starting cluster of the file in
question.
2. Multiply the cluster number by 2.
3. Use the product as the offset into the FAT and move the word at that
offset into a register.
4. If the result is a value from 0FFF8H through 0FFFFH, the file has no
more clusters. Otherwise, the result is the number of the next cluster
in the file.
To convert cluster numbers to logical sectors, subtract 2, multiply the
result by the number of sectors per cluster, and add the logical-sector
number of the beginning of the data area (this can be calculated from the
information in the BPB).
As an example, let's work out the disk location of the file IBMBIO.COM,
which is the first entry in the directory shown in Figure 10-6. First, we
need some information from the BPB, which is in the boot sector of the
medium. (See Figures 10-3 and 10-4.) The BPB tells us that there are
■ 512 bytes per sector
■ 2 sectors per cluster
■ 2 sectors per FAT
■ 2 FATs
■ 112 entries in the root directory
From the BPB information, we can calculate the starting logical-sector
number of each of the disk's control areas and the files area by
constructing a table, as follows:
Length Sector
Area (sectors) numbers
──────────────────────────────────────────────────────────────────────────
Boot sector 1 00H
2 FATs * 2 sectors/FAT 4 01H─04H
112 directory entries 7 05H─0BH
*32 bytes/entry
/512 bytes/sector
Total sectors occupied by bootstrap, FATs, and 12
root directory
──────────────────────────────────────────────────────────────────────────
Therefore, the first sector of the files area is 12 (0CH).
The word at offset 01AH in the directory entry for IBMBIO.COM gives us the
starting cluster number for that file: cluster 2. To find the
logical-sector number of the first block in the file, we can follow the
procedure given earlier:
1. Cluster number - 2 = 2 - 2 = 0.
2. Multiply by sectors per cluster = 0 * 2 = 0.
3. Add logical-sector number of start of the files area = 0 + 0CH = 0CH.
So the calculated sector number of the beginning of the file IBMBIO.COM is
0CH, which is exactly what we expect knowing that the FORMAT program
always places the system files in contiguous sectors at the beginning of
the data area.
Now let's trace IBMBIO.COM's chain through the file allocation table
(Figures 10-9 and 10-10). This will be a little tedious, but a detailed
understanding of the process is crucial. In an actual program, we would
first read the boot sector using Int 25H, then calculate the address of
the FAT from the contents of the BPB, and finally read the FAT into
memory, again using Int 25H.
From IBMBIO.COM's directory entry, we already know that the first cluster
in the file is cluster 2. To examine that cluster's entry in the FAT, we
multiply the cluster number by 1.5, which gives 0003H as the FAT offset,
and fetch the word at that offset (which contains 4003H). Because the
product of the cluster and 1.5 is a whole number, we AND the word from the
FAT with 0FFFH, yielding the number 3, which is the number of the second
cluster assigned to the file.
──────────────────────────────────────────────────────────────────────────
0 1 2 3 4 5 6 7 8 9 A B C D E F
0000 FD FF FF 03 40 00 05 60 00 07 80 00 09 A0 00 0B ....@..'........
0010 C0 00 0D E0 00 0F 00 01 11 20 01 13 40 01 15 60 ......... ..@..'
0020 01 17 F0 FF 19 A0 01 1B C0 01 1D E0 01 1F 00 02 ................
0030 21 20 02 23 40 02 25 60 02 27 80 02 29 A0 02 2B ! .#@.%'.'..)..+
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 10-9. Hex dump of the first block of the file allocation table
(track 0, head 0, sector 2) for the PC-DOS 3.3 disk whose root directory
is shown in Figure 10-6. Notice that the first byte of the FAT contains
the media descriptor byte for a 5.25-inch, 2-sided, 9-sector floppy disk.
──────────────────────────────────────────────────────────────────────────
getfat proc near ; extracts the FAT field
; for a given cluster
; call AX = cluster #
; DS:BX = addr of FAT
; returns AX = FAT field
; other registers unchanged
push bx ; save affected registers
push cx
mov cx,ax
shl ax,1 ; cluster * 2
add ax,cx ; cluster * 3
test ax,1
pushf ; save remainder in Z flag
shr ax,1 ; cluster * 1.5
add bx,ax
mov ax,[bx]
popf ; was cluster * 1.5 whole number?
jnz getfat1 ; no, jump
and ax,0fffh ; yes, isolate bottom 12 bits
jmp getfat2
getfat1: mov cx,4 ; shift word right 4 bits
shr ax,cx
getfat2: pop cx ; restore registers and exit
pop bx
ret
getfat endp
──────────────────────────────────────────────────────────────────────────
Figure 10-10. Assembly-language procedure to access the file allocation
table (assumes 12-bit FAT fields). Given a cluster number, the procedure
returns the contents of that cluster's FAT entry in the AX register. This
simple example ignores the fact that FAT entries can span sector
boundaries.
To examine cluster 3's entry in the FAT, we multiply 3 by 1.5, which gives
4.5, and fetch the word at offset 0004H (which contains 0040H). Because
the product of 3 and 1.5 is not a whole number, we shift the word right
4 bits, yielding the number 4, which is the number of the third cluster
assigned to IBMBIO.COM.
In this manner, we can follow the chain through the FAT until we come to a
cluster (number 23, in this case) whose FAT entry contains the value
0FFFH, which is an end-of-file marker in FATs with 12-bit entries.
We have now established that the file IBMBIO.COM contains clusters 2
through 23 (02H─17H), from which we can calculate that logical sectors 0CH
through 38H are assigned to the file. Of course, the last cluster may be
only partially filled with actual data; the portion of the last cluster
used is the remainder of the file's size in bytes (found in the directory
entry) divided by the bytes per cluster.
Fixed-Disk Partitions
Fixed disks have another layer of organization beyond the logical volume
structure already discussed: partitions. The FDISK utility divides a fixed
disk into one or more partitions consisting of an integral number of
cylinders. Each partition can contain an independent file system and, for
that matter, its own copy of an operating system.
The first physical sector on a fixed disk (track 0, head 0, sector 1)
contains the master boot record, which is laid out as follows:
Bytes Contents
──────────────────────────────────────────────────────────────────────────
000─1BDH Reserved
1BE─1CDH Partition #1 descriptor
1CE─1DDH Partition #2 descriptor
1DE─1EDH Partition #3 descriptor
1EE─1FDH Partition #4 descriptor
1FE─1FFH Signature word (AA55H)
──────────────────────────────────────────────────────────────────────────
The partition descriptors in the master boot record define the size,
location, and type of each partition, as follows:
Byte(s) Contents
──────────────────────────────────────────────────────────────────────────
00H Active flag (0 = not bootable, 80H = bootable)
01H Starting head
02H─03H Starting cylinder/sector
04H Partition type
00H not used
01H FAT file system, 12-bit FAT entries
04H FAT file system, 16-bit FAT entries
05H extended partition
06H "huge partition" (MS-DOS versions 4.0 and later)
05H Ending head
06H─07H Ending cylinder/sector
08H─0BH Starting sector for partition, relative to beginning of
disk
0CH─0FH Partition length in sectorsThe active flag, which
indicates that the partition is bootable, can be set on
only one partition at a time.
──────────────────────────────────────────────────────────────────────────
MS-DOS treats partition types 1, 4, and 6 as normal logical volumes and
assigns them their own drive identifiers during the system boot process.
Partition type 5 can contain multiple logical volumes and has a special
extended boot record that describes each volume. The FORMAT utility
initializes MS-DOS fixed-disk partitions, creating the file system within
the partition (boot record, file allocation table, root directory, and
files area) and optionally placing a bootable copy of the operating system
in the file system.
Figure 10-11 contains a partial hex dump of a master block from a fixed
disk formatted under PC-DOS version 3.3. This dump illustrates the
partition descriptors for a normal partition with a 16-bit FAT and an
extended partition.
──────────────────────────────────────────────────────────────────────────
0000 .
.
.
0180 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
01A0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
01B0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 80 01
01C0 01 00 04 04 D1 02 11 00 00 00 EE FF 00 00 00 00
01D0 C1 04 05 04 D1 FD 54 00 01 00 02 53 00 00 00 00
01E0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
01F0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 55 AA
──────────────────────────────────────────────────────────────────────────
Figure 10-11. A partial hex dump of a master block from a fixed disk
formatted under PC-DOS version 3.3. This disk contains two partitions. The
first partition has a 16-bit FAT and is marked "active" to indicate that
it contains a bootable copy of PC-DOS. The second partition is an
"extended" partition. The third and fourth partition entries are not used
in this example.
────────────────────────────────────────────────────────────────────────────
Chapter 11 Memory Management
Current versions of MS-DOS can manage as much as 1 megabyte of contiguous
random-access memory. On IBM PCs and compatibles, the memory occupied by
MS-DOS and other programs starts at address 0000H and may reach as high as
address 09FFFFH; this 640 KB area of RAM is sometimes referred to as
conventional memory. Memory above this address is reserved for ROM
hardware drivers, video refresh buffers, and the like. Computers that are
not IBM compatible may use other memory layouts.
The RAM area under the control of MS-DOS is divided into two major
sections:
■ The operating-system area
■ The transient-program area
The operating-system area starts at address 0000H──that is, it occupies
the lowest portion of RAM. It holds the interrupt vector table, the
operating system proper and its tables and buffers, any additional
installable drivers specified in the CONFIG.SYS file, and the resident
part of the COMMAND.COM command interpreter. The amount of memory occupied
by the operating-system area varies with the version of MS-DOS used, the
number of disk buffers, the size of installed device drivers, and so
forth.
The transient-program area (TPA), sometimes called the memory arena, is
the remainder of memory above the operating-system area. The memory arena
is dynamically allocated in blocks called arena entries. Each arena entry
has a special control structure called an arena header, and all of the
arena headers are chained together. Three MS-DOS Int 21H functions allow
programs to allocate, resize, and release blocks of memory from the TPA:
Function Action
──────────────────────────────────────────────────────────────────────────
48H Allocate memory block.
49H Release memory block.
4AH Resize memory block.
──────────────────────────────────────────────────────────────────────────
MS-DOS itself uses these functions when loading a program from disk at the
request of COMMAND.COM or another program. The EXEC function, which is the
MS-DOS program loader, calls Int 21H Function 48H to allocate a memory
block for the loaded program's environment and another for the program
itself and its program segment prefix. It then reads the program from the
disk into the assigned memory area. When the program terminates, MS-DOS
calls Int 21H Function 49H to release all memory owned by the program.
Transient programs can also employ the MS-DOS memory-management functions
to dynamically manage the memory available in the TPA. Proper use of these
functions is one of the most important criteria of whether a program is
well behaved under MS-DOS. Well-behaved programs are most likely to be
portable to future versions of the operating system and least likely to
cause interference with other processes under multitasking user interfaces
such as Microsoft Windows.
Using the Memory-Allocation Functions
The memory-allocation functions have two common uses:
■ To shrink a program's initial memory allocation so that there is enough
room to load and execute another program under its control.
■ To dynamically allocate additional memory required by the program and
to release the same memory when it is no longer needed.
Shrinking the Initial Memory Allocation
Although many MS-DOS application programs simply assume they own all
memory, this assumption is a relic of MS-DOS version 1 (and CP/M), which
could support only one active process at a time. Well-behaved MS-DOS
programs take pains to modify only memory that they actually own and to
release any memory that they don't need.
Unfortunately, under current versions of MS-DOS, the amount of memory that
a program will own is not easily predicted in advance. It turns out that
the amount of memory allocated to a program when it is first loaded
depends upon two factors:
■ The type of file the program is loaded from
■ The amount of memory available in the TPA
MS-DOS always allocates all of the largest available memory block in the
TPA to programs loaded from .COM (memory-image) files. Because .COM
programs contain no file header that can pass segment and memory-use
information to MS-DOS, MS-DOS simply assumes the worst case and gives such
a program everything. MS-DOS will load the program as long as there is an
available memory block as large as the size of the file plus 256 bytes for
the PSP and 2 bytes for the stack. The .COM program, when it receives
control, must determine whether enough memory is available to carry out
its functions.
MS-DOS uses more complicated rules to allocate memory to programs loaded
from .EXE files. First, of course, a memory block large enough to hold the
declared code, data, and stack segments must be available in the TPA. In
addition, the linker sets two fields in a .EXE file's header to inform
MS-DOS about the program's memory requirements. The first field,
MIN_ALLOC, defines the minimum number of paragraphs required by the
program, in addition to those for the code, data, and stack segments. The
second, MAX_ALLOC, defines the maximum number of paragraphs of additional
memory the program would use if they were available.
When loading a .EXE file, MS-DOS first attempts to allocate the number of
paragraphs in MAX_ALLOC plus the number of paragraphs required by the
program itself. If that much memory is not available, MS-DOS assigns all
of the largest available block to the program, provided that this is at
least the amount specified by MIN_ALLOC plus the size of the program
image. If that condition is not satisfied, the program cannot be executed.
After a .COM or .EXE program is loaded and running, it can use Int 21H
Function 4AH (Resize Memory Block) to release all the memory it does not
immediately need. This is conveniently done right after the program
receives control from MS-DOS, by calling the resize function with the
segment of the program's PSP in the ES register and the number of
paragraphs that the program requires to run in the BX register (Figure
11-1).
──────────────────────────────────────────────────────────────────────────
.
.
.
org 100h
main proc near ; entry point from MS-DOS
; DS, ES = PSP address
mov sp,offset stk ; COM program must move
; stack to safe area
; release extra memory...
mov ah,4ah ; function 4Ah =
; resize memory block
; BX = paragraphs to keep
mov bx,(offset stk - offset main + 10FH) / 16
int 21h ; transfer to MS-DOS
jc error ; jump if resize failed
.
.
.
main endp
.
.
.
dw 64 dup (?) ; new stack area
stk equ $ ; new base of stack
end main ; defines entry point
──────────────────────────────────────────────────────────────────────────
Figure 11-1. An example of a .COM program releasing excess memory after
it receives control from MS-DOS. Int 21H Function 4AH is called with ES
pointing to the program's PSP and BX containing the number of paragraphs
that the program needs to execute. In this case, the new size for the
program's memory block is calculated as the program image size plus the
size of the PSP (256 bytes), rounded up to the next paragraph. .EXE
programs use similar code.
Dynamic Allocation of Additional Memory
When a well-behaved program needs additional memory space──for an I/O
buffer or an array of intermediate results, for example──it can call Int
21H Function 48H (Allocate Memory Block) with the desired number of
paragraphs. If a sufficiently large block of unallocated memory is
available, MS-DOS returns the segment address of the base of the assigned
area and clears the carry flag (0), indicating that the function was
successful.
If no unallocated block of sufficient size is available, MS-DOS sets the
carry flag (1), returns an error code in the AX register, and returns the
size (in paragraphs) of the largest block available in the BX register
(Figure 11-2). In this case, no memory has yet been allocated. The
program can use the value returned in the BX register to determine whether
it can continue in a "degraded" fashion, with less memory. If it can, it
must call Int 21H Function 48H again to allocate the smaller memory
block.
When the MS-DOS memory manager is searching the chain of arena headers to
satisfy a memory-allocation request, it can use one of the following
strategies:
■ First fit: Use the arena entry at the lowest address that is large
enough to satisfy the request.
■ Best fit: Use the smallest arena entry that will satisfy the request,
regardless of its location.
■ Last fit: Use the arena entry at the highest address that is large
enough to satisfy the request.
──────────────────────────────────────────────────────────────────────────
.
.
.
mov ah,48h ; function 48h = allocate mem bl
mov bx,0800h ; 800h paragraphs = 32 KB
int 21h ; transfer to MS-DOS
jc error ; jump if allocation failed
mov buff_seg,ax ; save segment of allocated bloc
.
.
.
mov es,buff_seg ; ES:DI = address of block
xor di,di
mov cx,08000h ; store 32,768 bytes
mov al,0ffh ; fill buffer with -1s
cld
rep stosb ; now perform fast fill
.
.
.
mov cx,08000h ; length to write, bytes
mov bx,handle ; handle for prev opened file
push ds ; save our data segment
mov ds,buff_seg ; let DS:DX = buffer address
mov dx,0
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
pop ds ; restore our data segment
jc error ; jump if write failed
.
.
.
mov es,buff_seg ; ES = seg of prev allocated blo
mov ah,49h ; function 49h = release mem blo
int 21h ; transfer to MS-DOS
jc error ; jump if release failed
.
error: .
.
handle dw 0 ; file handle
buff_seg dw 0 ; segment of allocated block
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 11-2. Example of dynamic memory allocation. The program requests a
32 KB memory block from MS-DOS, fills it with -1s, writes it to disk, and
then releases it.
If the arena entry selected is larger than the size requested, MS-DOS
divides it into two parts: one block of the size requested, which is
assigned to the program that called Int 21H Function 48H, and an unowned
block containing the remaining memory.
The default MS-DOS allocation strategy is first fit. However, under MS-DOS
versions 3.0 and later, an application program can change the strategy
with Int 21H Function 58H.
When a program is through with an allocated memory block, it should use
Int 21H Function 49H to release the block. If it does not, MS-DOS will
automatically release all memory allocations for the program when it
terminates.
Arena Headers
Microsoft has not officially documented the internal structure of arena
headers for the outside world at present. This is probably to deter
programmers from trying to manipulate their memory allocations directly
instead of through the MS-DOS functions provided for that purpose.
Arena headers have identical structures in MS-DOS versions 2 and 3. They
are 16 bytes (one paragraph) and are located immediately before the memory
area that they control (Figure 11-3). An arena header contains the
following information:
■ A byte signifying whether the header is a member or the last entry in
the entire chain of such headers
■ A word indicating whether the area it controls is available or whether
it already belongs to a program (if the latter, the word points to the
program's PSP)
■ A word indicating the size (in paragraphs) of the controlled memory
area (arena entry)
MS-DOS inspects the chain of arena headers whenever the program requests a
memory-block allocation, modification, or release function, or when a
program is EXEC'd or terminated. If any of the blocks appear to be
corrupted or if the chain is broken, MS-DOS displays the dreaded message
Memory allocation error
and halts the system.
In the example illustrated in Figure 11-3, COMMAND.COM originally loaded
PROGRAM1.COM into the TPA and, because it was a .COM file, COMMAND.COM
allocated it all of the TPA, controlled by arena header #1. PROGRAM1.COM
then used Int 21H Function 4AH (Resize Memory Block) to shrink its memory
allocation to the amount it actually needed to run and loaded and executed
PROGRAM2.EXE with the EXEC function (Int 21H Function 4BH). The EXEC
function obtained a suitable amount of memory, controlled by arena header
#2, and loaded PROGRAM2.EXE into it. PROGRAM2.EXE, in turn, needed some
additional memory to store some intermediate results, so it called Int 21H
Function 48H (Allocate Memory Block) to obtain the area controlled by
arena header #3. The highest arena header (#4) controls all of the
remaining TPA that has not been allocated to any program.
┌─────────────────────────────────────────────────┐◄ Top of RAM
│ Unowned RAM controlled by header #4 │ controlled by MS-DOS
├─────────────────────────────────────────────────┤
│ Arena header #4 │
├─────────────────────────────────────────────────┤
│ Memory area controlled by header #3; additional │
│ storage dynamically allocated by PROGRAM2.EXE │
├─────────────────────────────────────────────────┤
│ Arena header #3 │
├─────────────────────────────────────────────────┤
│ Memory area controlled by header #2, │
│ containing PROGRAM2.EXE │
├─────────────────────────────────────────────────┤
│ Arena header #2 │
├─────────────────────────────────────────────────┤
│ Memory area controlled by header #1, │
│ containing PROGRAM1.COM │
├─────────────────────────────────────────────────┤
│ Arena header #1 │
└─────────────────────────────────────────────────┘◄ Bottom of transient-
program area
Figure 11-3. An example diagram of MS-DOS arena headers and the
transient-program area. The environment blocks and their associated
headers have been omitted from this figure to increase its clarity.
Lotus/Intel/Microsoft Expanded Memory
When the IBM Personal Computer and MS-DOS were first released, the 640 KB
limit that IBM placed on the amount of RAM that could be directly managed
by MS-DOS seemed almost unimaginably huge. But as MS-DOS has grown in both
size and capabilities and the popular applications have become more
powerful, that 640 KB has begun to seem a bit crowded. Although personal
computers based on the 80286 and 80386 have the potential to manage up to
16 megabytes of RAM under operating systems such as MS OS/2 and XENIX,
this is little comfort to the millions of users of 8086/8088-based
computers and MS-DOS.
At the spring COMDEX in 1985, Lotus Development Corporation and Intel
Corporation jointly announced the Expanded Memory Specification 3.0 (EMS),
which was designed to head off rapid obsolescence of the older PCs because
of limited memory. Shortly afterward, Microsoft announced that it would
support the EMS and would enhance Microsoft Windows to use the memory made
available by EMS hardware and software. EMS versions 3.2 and 4.0, released
in fall 1985 and summer 1987, expanded support for multitasking operating
systems.
The LIM EMS (as it is usually known) has been an enormous success. EMS
memory boards are available from scores of manufacturers, and "EMS-aware"
software──especially spreadsheets, disk caches, and terminate-and-stay-
resident utilities──has become the rule rather than the exception.
What Is Expanded Memory?
The Lotus/Intel/Microsoft Expanded Memory Specification is a functional
definition of a bank-switched memory-expansion subsystem. It consists of
hardware expansion modules and a resident driver program specific to those
modules. In EMS versions 3.0 and 3.2, the expanded memory is made
available to application software as 16 KB pages mapped into a contiguous
64 KB area called the page frame, somewhere above the main memory area
used by MS-DOS/PC-DOS (0─640 KB). The exact location of the page frame is
user configurable, so it need not conflict with other hardware options. In
EMS version 4.0, the pages may be mapped anywhere in memory and can have
sizes other than 16 KB.
The EMS provides a uniform means for applications to access as much as 8
megabytes of memory (32 megabytes in EMS 4.0). The supporting software,
which is called the Expanded Memory Manager (EMM), provides a
hardware-independent interface between application software and the
expanded memory board(s). The EMM is supplied in the form of an
installable device driver that you link into the MS-DOS/PC-DOS system by
adding a line to the CONFIG.SYS file on the system boot disk.
Internally, the Expanded Memory Manager consists of two major portions,
which may be referred to as the driver and the manager. The driver portion
mimics some of the actions of a genuine installable device driver, in that
it includes initialization and output status functions and a valid device
header. The second, and major, portion of the EMM is the true interface
between application software and the expanded-memory hardware. Several
classes of services are provided:
■ Verification of functionality of hardware and software modules
■ Allocation of expanded-memory pages
■ Mapping of logical pages into the physical page frame
■ Deallocation of expanded-memory pages
■ Support for multitasking operating systems
Application programs communicate with the EMM directly, by means of
software Int 67H. MS-DOS versions 3.3 and earlier take no part in (and in
fact are completely oblivious to) any expanded-memory manipulations that
may occur. MS-DOS version 4.0 and Microsoft Windows, on the other hand,
are "EMS-aware" and can use the EMS memory when it is available.
Expanded memory should not be confused with extended memory. Extended
memory is the term used by IBM to refer to the memory at physical
addresses above 1 megabyte that can be accessed by an 80286 or 80386 CPU
in protected mode. Current versions of MS-DOS run the 80286 and 80386 in
real mode (8086-emulation mode), and extended memory is therefore not
directly accessible.
Checking for Expanded Memory
An application program can use either of two methods to test for the
existence of the Expanded Memory Manager:
■ Issue an open request (Int 21H Function 3DH) using the guaranteed
device name of the EMM driver: EMMXXXX0. If the open function succeeds,
either the driver is present or a file with the same name
coincidentally exists on the default disk drive. To rule out the
latter, the application can use IOCTL (Int 21H Function 44H)
subfunctions 00H and 07H to ensure that EMM is present. In either case,
the application should then use Int 21H Function 3EH to close the
handle that was obtained from the open function, so that the handle can
be reused for another file or device.
■ Use the address that is found in the Int 67H vector to inspect the
device header of the presumed EMM. Interrupt handlers and device
drivers must use this method. If the EMM is present, the name field at
offset 0AH of the device header contains the string EMMXXXX0. This
approach is nearly foolproof and avoids the relatively high overhead of
an MS-DOS open function. However, it is somewhat less well behaved
because it involves inspection of memory that does not belong to the
application.
These two methods of testing for the existence of the Expanded Memory
Manager are illustrated in Figures 11-4 and 11-5.
──────────────────────────────────────────────────────────────────────────
.
.
.
; attempt to "open" EMM...
mov dx,seg emm_name ; DS:DX = address of name
mov ds,dx ; of Expanded Memory Manager
mov dx,offset emm_name
mov ax,3d00h ; function 3dh, mode = 00h
; = open, read only
int 21h ; transfer to MS-DOS
jc error ; jump if open failed
; open succeeded, be sure
; it was not a file...
mov bx,ax ; BX = handle from open
mov ax,4400h ; function 44h subfunction 00h
; = IOCTL get device information
int 21h ; transfer to MS-DOS
jc error ; jump if IOCTL call failed
and dx,80h ; bit 7 = 1 if character device
jz error ; jump if it was a file
; EMM is present, be sure
; it is available...
; (BX still contains handle)
mov ax,4407h ; function 44h subfunction 07h
; = IOCTL get output status
int 21h ; transfer to MS-DOS
jc error ; jump if IOCTL call failed
or al,al ; test device status
jz error ; if AL = 0 EMM is not available
; now close handle ...
; (BX still contains handle)
mov ah,3eh ; function 3eh = close
int 21h ; transfer to MS-DOS
jc error ; jump if close failed
.
.
.
emm_name db 'EMMXXXX0',0 ; guaranteed device name for
; Expanded Memory Manager
──────────────────────────────────────────────────────────────────────────
Figure 11-4. Testing for the Expanded Memory Manager by means of the
MS-DOS open and IOCTL functions.
──────────────────────────────────────────────────────────────────────────
emm_int equ 67h ; Expanded Memory Manager
; software interrupt
.
.
.
; first fetch contents of
; EMM interrupt vector...
mov al,emm_int ; AL = EMM int number
mov ah,35h ; function 35h = get vector
int 21h ; transfer to MS-DOS
; now ES:BX = handler address
; assume ES:0000 points
; to base of the EMM...
mov di,10 ; ES:DI = address of name
; field in device header
; DS:SI = EMM driver name
mov si,seg emm_name
mov ds,si
mov si,offset emm_name
mov cx,8 ; length of name field
cld
repz cmpsb ; compare names...
jnz error ; jump if driver absent
.
.
.
emm_name db 'EMMXXXX0' ; guaranteed device name for
; Expanded Memory Manager
──────────────────────────────────────────────────────────────────────────
Figure 11-5. Testing for the Expanded Memory Manager by inspection of the
name field in the driver's device header.
Using Expanded Memory
After establishing that the memory-manager software is present, the
application program communicates with it directly by means of the "user
interrupt" 67H, bypassing MS-DOS/PC-DOS. The calling sequence for the EMM
is as follows:
──────────────────────────────────────────────────────────────────────────
mov ah,function ; AH determines service type
. ; load other registers with
. ; values specific to the
. ; requested service
int 67h
──────────────────────────────────────────────────────────────────────────
In general, AH contains the EMM function number, AL holds the subfunction
number (if any), BX holds a number of pages (if applicable), and DX
contains an EMM handle. Registers DS:SI and ES:DI are used to pass the
addresses of arrays or buffers. Section 4 of this book,
"Lotus/Intel/Microsoft EMS Functions Reference," details each of the
expanded memory functions.
Upon return from an EMM function, the AH register contains zero if the
function was successful; otherwise, it contains an error code with the
most significant bit set (Figures 11-6 and 11-7). Other values are
typically returned in the AL and BX registers or in a user-specified
buffer.
╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖
Error code Meaning
──────────────────────────────────────────────────────────────────────────
00H Function successful.
80H Internal error in Expanded Memory Manager software
(could be caused by corrupted memory image of driver).
81H Malfunction in expanded-memory hardware.
82H Memory manager busy.
83H Invalid handle.
84H Function requested by application not defined.
85H No more handles available.
86H Error in save or restore of mapping context.
87H Allocation request specified more logical pages than
physically available in system; no pages allocated.
Error code Meaning
──────────────────────────────────────────────────────────────────────────
physically available in system; no pages allocated.
88H Allocation request specified more logical pages than
currently available in system (request does not exceed
physical pages that exist, but some are already
allocated to other handles); no pages allocated.
Zero pages; cannot be allocated.
8AH Logical page requested to be mapped located outside
range of logical pages assigned to handle.
8BH Illegal physical page number in mapping request (not in
range
0─3).
8CH Page-mapping hardware-state save area full.
Error code Meaning
──────────────────────────────────────────────────────────────────────────
8DH Save of mapping context failed; save area already
contains context associated with requested handle.
8EH Restore of mapping context failed; save area does not
contain context for requested handle.
8FH Subfunction parameter not defined.
──────────────────────────────────────────────────────────────────────────
Figure 11-6. Expanded Memory Manager error codes common to EMS versions
3.0, 3.2, and 4.0. After a call to EMM, the AH register contains zero if
the function was successful or an error code in the range 80H through 8FH
if the function failed.
╓┌─┌──────────────────┌──────────────────────────────────────────────────────╖
Error code Meaning
──────────────────────────────────────────────────────────────────────────
Error code Meaning
──────────────────────────────────────────────────────────────────────────
90H Attribute type not defined.
91H Feature not supported.
92H Source and destination memory regions have same handle
and overlap; requested move was performed, but part of
source region was overwritten.
93H Specified length for source or destination memory
region is longer than actual allocated length.
94H Conventional-memory region and expanded-memory region
overlap.
95H Specified offset is outside logical page.
96H Region length exceeds 1 MB.
97H Source and destination memory regions have same handle
Error code Meaning
──────────────────────────────────────────────────────────────────────────
97H Source and destination memory regions have same handle
and overlap; exchange cannot be performed.
98H Memory source and destination types undefined.
99H This error code currently unused.
9AH Alternate map or DMA register sets supported, but the
alternate register set specified is not supported.
9BH Alternate map or DMA register sets supported, but all
alternate register sets currently allocated.
9CH Alternate map or DMA register sets not supported, and
specified alternate register set not zero.
9DH Alternate map or DMA register sets supported, but
alternate register set specified is either not defined
or not allocated.
Error code Meaning
──────────────────────────────────────────────────────────────────────────
or not allocated.
Dedicated DMA channels not supported.
9FH Dedicated DMA channels supported, but specified DMA
channel not supported.
A0H No handle found for specified name.
A1H Handle with this name already exists.
A2H Memory address wrap; sum of the source or destination
region base address and length exceeds 1 MB.
A3H Invalid pointer passed to function, or contents of
source array corrupted.
A4H Access to function denied by operating system.
──────────────────────────────────────────────────────────────────────────
Error code Meaning
──────────────────────────────────────────────────────────────────────────
──────────────────────────────────────────────────────────────────────────
Figure 11-7. Expanded Memory Manager error codes unique to EMS version
4.0. Most of these errors are related to the EMS functions for use by
operating systems and would not normally be encountered by application
programs.
An application program that uses expanded memory should regard that memory
as a system resource, like a file or a device, and employ only the
documented EMM services to allocate, access, and release expanded-memory
pages. Such a program can use the following general strategy:
1. Establish the presence of the Expanded Memory Manager by one of the
two methods demonstrated in Figures 11-4 and 11-5.
2. After the driver is known to be present, check its operational status
with EMS Function 40H.
3. Check the version number of EMM with EMS Function 46H, to ensure that
all services the application will request are available.
4. Obtain the segment of the page frame used by EMM with EMS Function
41H.
5. Allocate the desired number of expanded-memory pages with EMS Function
43H. If the allocation is successful, EMM returns a handle that the
application can use to refer to the expanded-memory pages that it
owns. This step is exactly analogous to opening a file and using the
handle obtained from the open function for read/write operations on
the file.
6. If the requested number of pages are not available, the application
can query EMM for the actual number of pages available (EMS Function
42H) and determine whether it can continue.
7. After the application has successfully allocated the needed number of
expanded-memory pages, it uses EMS Function 44H to map logical pages
in and out of the physical page frame in order to store and retrieve
data in expanded memory.
8. When the program finishes using its expanded-memory pages, it must
release them by calling EMS Function 45H. Otherwise, the pages will
be lost to use by other programs until the system is restarted.
Figure 11-8 shows a skeleton program that illustrates this general
approach.
An interrupt handler or device driver that uses EMS follows the same
general procedure outlined in steps 1 through 8, with a few minor
variations. It may need to acquire an EMS handle and allocate pages before
the operating system is fully functional; in particular, you cannot assume
that the MS-DOS Open File or Device, IOCTL, and Get Interrupt Vector
functions are available. Thus, such a handler or driver must use a
modified version of the "get interrupt vector" technique (Figure 11-5) to
test for the existence of EMM, fetching the contents of the Int 67H vector
directly.
A device driver or interrupt handler typically owns its expanded-memory
pages permanently (until the system is restarted) and never deallocates
them. Such a program must also take care to save and restore EMM's
page-mapping context (EMS Functions 47H and 48H) whenever it accesses
expanded memory, so that use of EMS by a foreground program will not
be disturbed.
The EMM relies on the good behavior of application software to avoid the
corruption of expanded memory. If several applications that use expanded
memory are running under a multitasking manager such as Microsoft Windows
and one or more of them does not abide strictly by EMM conventions, the
data of some or all of the applications may be destroyed.
──────────────────────────────────────────────────────────────────────────
.
.
.
mov ah,40h ; test EMM status
int 67h
or ah,ah
jnz error ; jump if bad status from EMM
mov ah,46h ; check EMM version
int 67h
or ah,ah
jnz error ; jump if couldn't get version
cmp al,030h ; make sure at least ver 3.0
jb error ; jump if wrong EMM version
mov ah,41h ; get page frame segment
int 67h
or ah,ah
jnz error ; jump if failed to get frame
mov page_frame,bx ; save segment of page frame
mov ah,42h ; get number of available pages
int 67h
or ah,ah
jnz error ; jump if get pages error
mov total_pages,dx ; save total EMM pages
mov avail_pages,bx ; save available EMM pages
or bx,bx
jz error ; abort if no pages available
mov ah,43h ; try to allocate EMM pages
mov bx,needed_pages
int 67h ; if allocation is successful
or ah,ah
jnz error ; jump if allocation failed
mov emm_handle,dx ; save handle for allocated pages
.
. ; now we are ready for other
. ; processing using EMM pages
.
; map in EMS memory page...
mov bx,log_page ; BX <- EMS logical page number
mov al,phys_page ; AL <- EMS physical page (0-3)
mov dx,emm_handle ; EMM handle for our pages
mov ah,44h ; function 44h = map EMS page
int 67h
or ah,ah
jnz error ; jump if mapping error
.
.
. ; program ready to terminate,
; give up allocated EMM pages...
mov dx,emm_handle ; handle for our pages
mov ah,45h ; EMS function 45h = release pages
int 67h
or ah,ah
jnz error ; jump if release failed
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 11-8. A program illustrating the general strategy for using
expanded memory.
Extended Memory
Extended memory is RAM storage at addresses above 1 megabyte (100000H)
that can be accessed by an 80286 or 80386 processor running in protected
mode. IBM PC/AT─ and PS/2─compatible machines can (theoretically) have as
much as 15 MB of extended memory installed, in addition to the usual 1 MB
of conventional memory.
Protected-mode operating systems such as Microsoft XENIX or MS OS/2 can
use extended memory for execution of programs. MS-DOS, on the other hand,
runs in real mode on an 80286 or 80386, and programs running under its
control cannot ordinarily execute from extended memory or even address
that memory for storage of data. However, the ROM BIOS contains two
routines that allow real-mode programs restricted access to extended
memory:
ROM BIOS function Action
──────────────────────────────────────────────────────────────────────────
Int 15H Function 87H Move extended-memory block.
Int 15H Function 88H Get extended-memory size.
──────────────────────────────────────────────────────────────────────────
These routines can be used by electronic disks (RAMdisks) and by other
programs that want to use extended memory for fast storage and retrieval
of information that would otherwise have to be written to a slower
physical disk drive. Section 3 of this book, "IBM ROM BIOS and Mouse
Functions Reference," documents both of these functions.
You should use these ROM BIOS routines with caution. Data stored in
extended memory is, of course, volatile; it is lost if the machine is
turned off. The transfer of data to or from extended memory involves a
switch from real mode to protected mode and back, which is a relatively
slow process on 80286-based machines; in some cases it is only marginally
faster than actually reading the data from a fixed disk. In addition,
programs that use the ROM BIOS extended-memory functions are not
compatible with the MS-DOS compatibility mode of MS OS/2.
Finally, a major deficit in these ROM BIOS functions is that they do not
make any attempt to arbitrate between two or more programs or drivers that
are using extended memory for temporary storage. For example, if an
application program and an installed RAMdisk driver attempt to put data in
the same area of extended memory, no error will be returned to either
program, but the data of one or both may be destroyed.
Figure 11-9 shows an example of the code necessary to transfer data to
and from extended memory.
──────────────────────────────────────────────────────────────────────────
bmdt db 30h dup (0) ; block move descriptor table
buff1 db 80h dup ('?') ; source buffer
buff2 db 80h dup (0) ; destination buffer
.
.
.
; copy 'buff1' to extended-
; memory address 100000h
mov dx,10h ; DX:AX = destination
mov ax,0 ; extended-memory address
mov bx,seg buff1 ; DS:BX = source conventional-
mov ds,bx ; memory address
mov bx,offset buff1
mov cx,80h ; CX = bytes to move
mov si,seg bmdt ; ES:SI = block move
mov es,si ; descriptor table
mov si,offset bmdt
call putblk ; request transfer
; fill buff2 from extended-
; memory address 100000h
mov dx,10h ; DX:AX = source extended-
mov ax,0 ; memory address
mov bx,seg buff2 ; DS:BX = destination
mov ds,bx ; conventional-memory address
mov bx,offset buff2
mov cx,80h ; CX = bytes to move
mov si,seg bmdt ; ES:SI = block move
mov es,si ; descriptor table
mov si,offset bmdt
call getblk ; request transfer
.
.
.
getblk proc near ; transfer block from extended
; memory to real memory
; call with
; DX:AX = source linear 32-bit
; extended-memory address
; DS:BX = segment and offset
; destination address
; CX = length in bytes
; ES:SI = block move descriptor
; table
; returns
; AH = 0 if transfer OK
mov es:[si+10h],cx ; store length into descriptors
mov es:[si+18h],cx
; store access rights bytes
mov byte ptr es:[si+15h],93h
mov byte ptr es:[si+1dh],93h
mov es:[si+12h],ax ; source extended-memory address
mov es:[si+14h],dl
; convert destination segment
; and offset to linear address
mov ax,ds ; segment * 16
mov dx,16
mul dx
add ax,bx ; + offset -> linear address
adc dx,0
mov es:[si+1ah],ax ; store destination address
mov es:[si+1ch],dl
shr cx,1 ; convert length to words
mov ah,87h ; int 15h function 87h = block move
int 15h ; transfer to ROM BIOS
ret ; back to caller
getblk endp
putblk proc near ; transfer block from real
; memory to extended memory
; call with
; DX:AX = dest linear 32-bit
; extended-memory address
; DS:BX = segment and offset
; source address
; CX = length in bytes
; ES:SI = block move descriptor
; table
; returns
; AH = 0 if transfer OK
mov es:[si+10h],cx ; store length into descriptors
mov es:[si+18h],cx
; store access rights bytes
mov byte ptr es:[si+15h],93h
mov byte ptr es:[si+1dh],93h
mov es:[si+1ah],ax ; store destination extended-
mov es:[si+1ch],dl ; memory address
; convert source segment and
; offset to linear address
mov ax,ds ; segment * 16
mov dx,16
mul dx
add ax,bx ; + offset -> linear address
adc dx,0
mov es:[si+12h],ax ; store source address
mov es:[si+14h],dl
shr cx,1 ; convert length to words
mov ah,87h ; int 15h function 87h = block move
int 15h ; transfer to ROM BIOS
ret ; back to caller
putblk endp
──────────────────────────────────────────────────────────────────────────
Figure 11-9. Moving blocks of data between conventional memory and
extended memory, using the ROM BIOS extended-memory functions. For
additional information on the format of the block move descriptor table,
see the entry for Int 15H Function 87H in Section 3 of this book, "IBM
ROM BIOS and Mouse Functions Reference." Note that you must specify the
extended-memory address as a 32-bit linear address rather than as a
segment and offset.
────────────────────────────────────────────────────────────────────────────
Chapter 12 The EXEC Function
The MS-DOS EXEC function (Int 21H Function 4BH) allows a program (called
the parent) to load any other program (called the child) from a storage
device, execute it, and then regain control when the child program is
finished.
A parent program can pass information to the child in a command line, in
default file control blocks, and by means of a set of strings called the
environment block (discussed later in this chapter). All files or devices
that the parent opened using the handle file-management functions are
duplicated in the newly created child task; that is, the child inherits
all the active handles of the parent task. Any file operations on those
handles by the child, such as seeks or file I/O, also affect the file
pointers associated with the parent's handles.
MS-DOS suspends execution of the parent program until the child program
terminates. When the child program finishes its work, it can pass an exit
code back to the parent, indicating whether it encountered any errors. It
can also, in turn, load other programs, and so on through many levels of
control, until the system runs out of memory.
The MS-DOS command interpreter, COMMAND.COM, uses the EXEC function to run
its external commands and other application programs. Many popular
commercial programs, such as database managers and word processors, use
EXEC to run other programs (spelling checkers, for example) or to load a
second copy of COMMAND.COM, thereby allowing the user to list directories
or copy and rename files without closing all the application files and
stopping the main work in progress. EXEC can also be used to load program
overlay segments, although this use is uncommon.
Making Memory Available
In order for a parent program to use the EXEC function to load a child
program, sufficient unallocated memory must be available in the transient
program area.
When the parent itself was loaded, MS-DOS allocated it a variable amount
of memory, depending upon its original file type──.COM or .EXE──and any
other information that was available to the loader. (See Chapter 11 for
further details.) Because the operating system has no foolproof way of
predicting how much memory any given program will require, it generally
allocates far more memory to a program than is really necessary.
Therefore, a prospective parent program's first action should be to use
Int 21H Function 4AH (Resize Memory Block) to release any excess memory
allocation of its own to MS-DOS. In this case, the program should call Int
21H Function 4AH with the ES register pointing to the program segment
prefix of the program releasing memory and the BX register containing the
number of paragraphs of memory to retain for that program. (See Figure
11-1 for an example.)
──────────────────────────────────────────────────────────────────────────
WARNING
A .COM program must move its stack to a safe area if it is reducing its
memory allocation to less than 64 KB.
──────────────────────────────────────────────────────────────────────────
Requesting the EXEC Function
To load and execute a child program, the parent must execute an Int 21H
with the registers set up as follows:
AH = 4BH
AL = 00H (subfunction to load child program)
DS:DX = segment:offset of pathname for child program
ES:BX = segment:offset of parameter block
The parameter block, in turn, contains addresses of other information
needed by the EXEC function.
The Program Name
The name of the program to be run, which the calling program provides to
the EXEC function, must be an unambiguous file specification (no wildcard
characters) and must include an explicit .COM or .EXE extension. If the
path and disk drive are not supplied in the program name, MS-DOS uses the
current directory and default disk drive. (The sequential search for .COM,
.EXE, and .BAT files in all the locations listed in the PATH variable is
not a function of EXEC, but rather of the internal logic of COMMAND.COM.)
You cannot EXEC a batch file directly; instead, you must EXEC a copy of
COMMAND.COM and pass the name of the batch file in the command tail, along
with the /C switch.
The Parameter Block
The parameter block contains the addresses of four data objects:
■ The environment block
■ The command tail
■ Two default file control blocks
The space reserved in the parameter block for the address of the
environment block is only 2 bytes and holds a segment address. The
remaining three addresses are all double-word addresses; that is, they are
4 bytes, with the offset in the first 2 bytes and the segment address in
the last 2 bytes.
The Environment Block
Each program that the EXEC function loads inherits a data structure called
an environment block from its parent. The pointer to the segment of the
block is at offset 002CH in the PSP. The environment block holds certain
information used by the system's command interpreter (usually COMMAND.COM)
and may also hold information to be used by transient programs. It has no
effect on the operation of the operating system proper.
If the environment-block pointer in the EXEC parameter block contains
zero, the child program acquires a copy of the parent program's
environment block. Alternatively, the parent program can provide a segment
pointer to a different or expanded environment. The maximum size of the
environment block is 32 KB, so very large chunks of information can be
passed between programs by this mechanism.
The environment block for any given program is static, implying that if
more than one generation of child programs is resident in RAM, each one
will have a distinct and separate copy of the environment block.
Furthermore, the environment block for a program that terminates and stays
resident is not updated by subsequent PATH and SET commands.
You will find more details about the environment block later in this
chapter.
The Command Tail
MS-DOS copies the command tail into the child program's PSP at offset
0080H, as described in Chapter 3. The information takes the form of a
count byte, followed by a string of ASCII characters, terminated by a
carriage return; the carriage return is not included in the count.
The command tail can include filenames, switches, or other parameters.
From the child program's point of view, the command tail should provide
the same information that would be present if the program had been run by
a direct user command at the MS-DOS prompt. EXEC ignores any
I/O-redirection parameters placed in the command tail; the parent program
must provide for redirection of the standard devices before the EXEC
call is made.
The Default File Control Blocks
MS-DOS copies the two default file control blocks pointed to by the EXEC
parameter block into the child program's PSP at offsets 005CH and 006CH.
To emulate the function of COMMAND.COM from the child program's point of
view, the parent program should use Int 21H Function 29H (the system
parse-filename service) to parse the first two parameters of the command
tail into the default file control blocks before invoking the EXEC
function.
File control blocks are not much use under MS-DOS versions 2 and 3,
because they do not support the hierarchical file structure, but some
application programs do inspect them as a quick way to get at the first
two switches or other parameters in the command tail. Chapter 8 discusses
file control blocks in more detail.
Returning from the EXEC Function
In MS-DOS version 2, the EXEC function destroys the contents of all
registers except the code segment (CS) and instruction pointer (IP).
Therefore, before making the EXEC call, the parent program must push the
contents of any other registers that are important onto the stack and then
save the stack segment (SS) and stack pointer (SP) registers in variables.
Upon return from a successful EXEC call (that is, the child program has
finished executing), the parent program should reload SS and SP from the
variables where they were saved and then pop the other saved registers off
the stack. In MS-DOS versions 3.0 and later, the stack and other registers
are preserved across the EXEC call in the usual fashion.
Finally, the parent can use Int 21H Function 4DH to obtain the
termination type and return code of the child program.
The EXEC function will fail under the following conditions:
■ Not enough unallocated memory is available to load and execute the
requested program file.
■ The requested program can't be found on the disk.
■ The transient portion of COMMAND.COM in highest RAM (which contains the
actual loader) has been destroyed and not enough free memory is
available to reload it (PC-DOS version 2 only).
Figure 12-1 summarizes the calling convention for function 4BH. Figure
12-2 shows a skeleton of a typical EXEC call. This particular example
uses the EXEC function to load and run the MS-DOS utility CHKDSK.COM. The
SHELL.ASM program listing later in this chapter (Figure 12-5) presents a
more complete example that includes the use of Int 21H Function 4AH to
free unneeded memory.
──────────────────────────────────────────────────────────────────────────
Called with:
AH = 4BH
AL = function type
00 = load and execute program
03 = load overlay
ES:BX = segment:offset of parameter block
DS:DX = segment:offset of program specification
Returns:
If call succeeded
Carry flag clear. In MS-DOS version 2, all registers except for CS:IP may
be destroyed. In MS-DOS versions 3.0 and later, registers are preserved in
the usual fashion.
If call failed
Carry flag set and AX = error code.
Parameter block format:
If AL = 0 (load and execute program)
Bytes 0─1 = segment pointer, environment block
Bytes 2─3 = offset of command-line tail
Bytes 4─5 = segment of command-line tail
Bytes 6─7 = offset of first file control block to be copied
into new PSP + 5CH
Bytes 8─9 = segment of first file control block
Bytes 10─11 = offset of second file control block to be copied
into new PSP + 6CH
Bytes 12─13 = segment of second file control block
If AL = 3 (load overlay)
Bytes 0─1 = segment address where file will be loaded
Bytes 2─3 = relocation factor to apply to loaded image
──────────────────────────────────────────────────────────────────────────
Figure 12-1. Calling convention for the EXEC function (Int 21H Function
4BH).
──────────────────────────────────────────────────────────────────────────
cr egu 0dh ; ASCII carriage return
.
.
.
mov stkseg,ss ; save stack pointer
mov stkptr,sp
mov dx,offset pname ; DS:DX = program name
mov bx,offset pars ; ES:BX = param block
mov ax,4b00h ; function 4bh, subfunction 00h
int 21h ; transfer to MS-DOS
mov ax,_DATA ; make our data segment
mov ds,ax ; addressable again
mov es,ax
cli ; (for bug in some 8088s)
mov ss,stkseg ; restore stack pointer
mov sp,stkptr
sti ; (for bug in some 8088s)
jc error ; jump if EXEC failed
.
.
.
stkseg dw 0 ; original SS contents
stkptr dw 0 ; original SP contents
pname db '\CHKDSK.COM',0 ; pathname of child program
pars dw envir ; environment segment
dd cmdline ; command line for child
dd fcb1 ; file control block #1
dd fcb2 ; file control block #2
cmdline db 4,' *.*',cr ; command line for child
fcb1 db 0 ; file control block #1
db 11 dup ('?')
db 25 dup (0)
fcb2 db 0 ; file control block #2
db 11 dup (' ')
db 25 dup (0)
envir segment para 'ENVIR' ; environment segment
db 'PATH=',0 ; empty search path
; location of COMMAND.COM
db 'COMSPEC=A:\COMMAND.COM',0
db 0 ; end of environment
envir ends
──────────────────────────────────────────────────────────────────────────
Figure 12-2. A brief example of the use of the MS-DOS EXEC call, with all
necessary variables and command blocks. Note the protection of the
registers for MS-DOS version 2 and the masking of interrupts during
loading of SS:SP to circumvent a bug in some early 8088 CPUs.
More About the Environment Block
The environment block is always paragraph aligned (starts at an address
that is a multiple of 16 bytes) and contains a series of ASCIIZ strings.
Each of the strings takes the following form:
NAME=PARAMETER
An additional zero byte (Figure 12-3) indicates the end of the entire set
of strings. Under MS-DOS version 3, the block of environment strings and
the extra zero byte are followed by a word count and the complete drive,
path, filename, and extension used by EXEC to load the program.
──────────────────────────────────────────────────────────────────────────
0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDEF
0000 43 4F 4D 53 50 45 43 3D 43 3A 5C 43 4F 4D 4D 41 COMSPEC=C:\COMMA
0010 4E 44 2E 43 4F 4D 00 50 52 4F 4D 50 54 3D 24 70 NDcom.PROMPT=$p
0020 24 5F 24 64 20 20 20 24 74 24 68 24 68 24 68 24 $_$d $t$h$h$h$
0030 68 24 68 24 68 20 24 71 24 71 24 67 00 50 41 54 h$h$h $q$q$g.PAT
0040 48 3D 43 3A 5C 53 59 53 54 45 4D 3B 43 3A 5C 41 H=C:\SYSTEM;C:\A
0050 53 4D 3B 43 3A 5C 57 53 3B 43 3A 5C 45 54 48 45 SM;C:\WS;C:\ETHE
0060 52 4E 45 54 3B 43 3A 5C 46 4F 52 54 48 5C 50 43 RNET;C:\FORTH\PC
0070 33 31 3B 00 00 01 00 43 3A 5C 46 4F 52 54 48 5C 31;....C:\FORTH\
0080 50 43 33 31 5C 46 4F 52 54 48 2E 43 4F 4D 00 20 PC31\FORTH.COM.
──────────────────────────────────────────────────────────────────────────
Figure 12-3. Dump of a typical environment block under MS-DOS version 3.
This particular example contains the default COMSPEC parameter and two
relatively complex PATH and PROMPT control strings that were set up by
entries in the user's AUTOEXEC file. Note the path and file specification
of the executing program following the double zeros at offset 0073H that
denote the end of the environment block.
Under normal conditions, the environment block inherited by a program will
contain at least three strings:
COMSPEC=variable
PATH=variable
PROMPT=variable
MS-DOS places these three strings into the environment block at system
initialization, during the interpretation of SHELL, PATH, and PROMPT
directives in the CONFIG.SYS and AUTOEXEC.BAT files. The strings tell the
MS-DOS command interpreter, COMMAND.COM, the location of its executable
file (to enable it to reload the transient portion), where to search for
executable external commands or program files, and the format of the user
prompt.
You can add other strings to the environment block, either interactively
or in batch files, with the SET command. Transient programs can use these
strings for informational purposes. For example, the Microsoft C Compiler
looks in the environment block for INCLUDE, LIB, and TMP strings to tell
it where to find its #include files and library files and where to build
its temporary working files.
Example Programs: SHELL.C and SHELL.ASM
As a practical example of use of the MS-DOS EXEC function, I have included
a small command interpreter called SHELL, with equivalent Microsoft C
(Figure 12-4) and Microsoft Macro Assembler (Figure 12-5) source code.
The source code for the assembly-language version is considerably more
complex than the code for the C version, but the names and functionality
of the various procedures are quite parallel.
──────────────────────────────────────────────────────────────────────────
/*
SHELL.C Simple extendable command interpreter
for MS-DOS versions 2.0 and later
Copyright 1988 Ray Duncan
Compile: C>CL SHELL.C
Usage: C>SHELL
*/
#include <stdio.h>
#include <process.h>
#include <stdlib.h>
#include <signal.h>
/* macro to return number of
elements in a structure */
#define dim(x) (sizeof(x) / sizeof(x[0]))
unsigned intrinsic(char *); /* function prototypes */
void extrinsic(char *);
void get_cmd(char *);
void get_comspec(char *);
void break_handler(void);
void cls_cmd(void);
void dos_cmd(void);
void exit_cmd(void);
struct cmd_table { /* intrinsic commands table */
char *cmd_name;
int (*cmd_fxn)();
} commands[] =
{ "CLS", cls_cmd,
"DOS", dos_cmd,
"EXIT", exit_cmd, };
static char com_spec[64]; /* COMMAND.COM filespec */
main(int argc, char *argv[])
{
char inp_buf[80]; /* keyboard input buffer */
get_comspec(com_spec); /* get COMMAND.COM filespec */
/* register new handler
for Ctrl-C interrupts */
if(signal(SIGINT, break_handler) == (int(*)()) -1)
{
fputs("Can't capture Control-C Interrupt", stderr);
exit(1);
}
while(1) /* main interpreter loop */
{
get_cmd(inp_buf); /* get a command */
if (! intrinsic(inp_buf) ) /* if it's intrinsic,
run its subroutine */
extrinsic(inp_buf); /* else pass to COMMAND.COM */
}
}
/*
Try to match user's command with intrinsic command
table. If a match is found, run the associated routine
and return true; else return false.
*/
unsigned intrinsic(char *input_string)
{
int i, j; /* some scratch variables */
/* scan off leading blanks */
while(*input_string == '\x20') input_string++ ;
/* search command table */
for(i=0; i < dim(commands); i++)
{
j = strcmp(commands[i].cmd_name, input_string);
if(j == 0) /* if match, run routine */
{
(*commands[i].cmd_fxn)();
return(1); /* and return true */
}
}
return(0); /* no match, return false */
}
/*
Process an extrinsic command by passing it
to an EXEC'd copy of COMMAND.COM.
*/
void extrinsic(char *input_string)
{
int status;
status = system(input_string); /* call EXEC function */
if(status) /* if failed, display
error message */
fputs("\nEXEC of COMMAND.COM failed\n", stderr);
}
/*
Issue prompt, get user's command from standard input,
fold it to uppercase.
*/
void get_cmd(char *buffer)
{
printf("\nsh: "); /* display prompt */
gets(buffer); /* get keyboard entry */
strupr(buffer); /* fold to uppercase */
}
/*
Get the full path and file specification for COMMAND.COM
from the COMSPEC variable in the environment.
*/
void get_comspec(char *buffer)
{
strcpy(buffer, getenv("COMSPEC"));
if(buffer[0] == NULL)
{
fputs("\nNo COMSPEC in environment\n", stderr);
exit(1);
}
}
/*
This Ctrl-C handler keeps SHELL from losing control.
It just reissues the prompt and returns.
*/
void break_handler(void)
{
signal(SIGINT, break_handler); /* reset handler */
printf("\nsh: "); /* display prompt */
}
/*
These are the subroutines for the intrinsic commands.
*/
void cls_cmd(void) /* CLS command */
{
printf("\033[2J"); /* ANSI escape sequence */
} /* to clear screen */
void dos_cmd(void) /* DOS command */
{
int status;
/* run COMMAND.COM */
status = spawnlp(P_WAIT, com_spec, com_spec, NULL);
if (status)
fputs("\nEXEC of COMMAND.COM failed\n",stderr);
}
void exit_cmd(void) /* EXIT command */
{
exit(0); /* terminate SHELL */
}
──────────────────────────────────────────────────────────────────────────
Figure 12-4. SHELL.C: A table-driven command interpreter written in
Microsoft C.
──────────────────────────────────────────────────────────────────────────
name shell
page 55,132
title SHELL.ASM--simple MS-DOS shell
;
; SHELL.ASM Simple extendable command interpreter
; for MS-DOS versions 2.0 and later
;
; Copyright 1988 by Ray Duncan
;
; Build: C>MASM SHELL;
; C>LINK SHELL;
;
; Usage: C>SHELL;
;
stdin equ 0 ; standard input handle
stdout equ 1 ; standard output handle
stderr equ 2 ; standard error handle
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII linefeed
blank equ 20h ; ASCII blank code
escape equ 01bh ; ASCII escape code
_TEXT segment word public 'CODE'
assume cs:_TEXT,ds:_DATA,ss:STACK
shell proc far ; at entry DS = ES = PSP
mov ax,_DATA ; make our data segment
mov ds,ax ; addressable
mov ax,es:[002ch] ; get environment segment
mov env_seg,ax ; from PSP and save it
; release unneeded memory...
; ES already = PSP segment
mov bx,100h ; BX = paragraphs needed
mov ah,4ah ; function 4ah = resize block
int 21h ; transfer to MS-DOS
jnc shell1 ; jump if resize OK
mov dx,offset msg1 ; resize failed, display
mov cx,msg1_length ; error message and exit
jmp shell4
shell1: call get_comspec ; get COMMAND.COM filespec
jnc shell2 ; jump if it was found
mov dx,offset msg3 ; COMSPEC not found in
mov cx,msg3_length ; environment, display error
jmp shell4 ; message and exit
shell2: mov dx,offset shell3 ; set Ctrl-C vector (int 23h)
mov ax,cs ; for this program's handler
mov ds,ax ; DS:DX = handler address
mov ax,2523h ; function 25h = set vector
int 21h ; transfer to MS-DOS
mov ax,_DATA ; make our data segment
mov ds,ax ; addressable again
mov es,ax
shell3: ; main interpreter loop
call get_cmd ; get a command from user
call intrinsic ; check if intrinsic function
jnc shell3 ; yes, it was processed
call extrinsic ; no, pass it to COMMAND.COM
jmp shell3 ; then get another command
shell4: ; come here if error detected
; DS:DX = message address
; CX = message length
mov bx,stderr ; BX = standard error handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
mov ax,4c01h ; function 4ch = terminate with
; return code = 1
int 21h ; transfer to MS-DOS
shell endp
intrinsic proc near ; decode user entry against
; the table "COMMANDS"
; if match, run the routine,
; and return carry = false
; if no match, carry = true
; return carry = true
mov si,offset commands ; DS:SI = command table
intr1: cmp byte ptr [si],0 ; end of table?
je intr7 ; jump, end of table found
mov di,offset inp_buf ; no, let DI = addr of user input
intr2: cmp byte ptr [di],blank ; scan off any leading blanks
jne intr3
inc di ; found blank, go past it
jmp intr2
intr3: mov al,[si] ; next character from table
or al,al ; end of string?
jz intr4 ; jump, entire string matched
cmp al,[di] ; compare to input character
jnz intr6 ; jump, found mismatch
inc si ; advance string pointers
inc di
jmp intr3
intr4: cmp byte ptr [di],cr ; be sure user's entry
je intr5 ; is the same length...
cmp byte ptr [di],blank ; next character in entry
jne intr6 ; must be blank or return
intr5: call word ptr [si+1] ; run the command routine
clc ; return carry flag = false
ret ; as success flag
intr6: lodsb ; look for end of this
or al,al ; command string (null byte)
jnz intr6 ; not end yet, loop
add si,2 ; skip over routine address
jmp intr1 ; try to match next command
intr7: stc ; command not matched, exit
ret ; with carry = true
intrinsic endp
extrinsic proc near ; process extrinsic command
; by passing it to
; COMMAND.COM with a
; " /C " command tail
mov al,cr ; find length of command
mov cx,cmd_tail_length ; by scanning for carriage
mov di,offset cmd_tail+1 ; return
cld
repnz scasb
mov ax,di ; calculate command-tail
sub ax,offset cmd_tail+2 ; length without carriage
mov cmd_tail,al ; return, and store it
; set command-tail address
mov word ptr par_cmd,offset cmd_tail
call exec ; and run COMMAND.COM
ret
extrinsic endp
get_cmd proc near ; prompt user, get command
; display the shell prompt
mov dx,offset prompt ; DS:DX = message address
mov cx,prompt_length ; CX = message length
mov bx,stdout ; BX = standard output handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
; get entry from user
mov dx,offset inp_buf ; DS:DX = input buffer
mov cx,inp_buf_length ; CX = max length to read
mov bx,stdin ; BX = standard input handle
mov ah,3fh ; function 3fh = read
int 21h ; transfer to MS-DOS
mov si,offset inp_buf ; fold lowercase characters
mov cx,inp_buf_length ; in entry to uppercase
gcmd1: cmp byte ptr [si],'a' ; check if 'a-z'
jb gcmd2 ; jump, not in range
cmp byte ptr [si],'z' ; check if 'a-z'
ja gcmd2 ; jump, not in range
sub byte ptr [si],'a'-'A' ; convert to uppercase
gcmd2: inc si ; advance through entry
loop gcmd1
ret ; back to caller
get_cmd endp
get_comspec proc near ; get location of COMMAND.COM
; from environment "COMSPEC="
; returns carry = false
; if COMSPEC found
; returns carry = true
; if no COMSPEC
mov si,offset com_var ; DS:SI = string to match...
call get_env ; search environment block
jc gcsp2 ; jump if COMSPEC not found
; ES:DI points past "="
mov si,offset com_spec ; DS:SI = local buffer
gcsp1: mov al,es:[di] ; copy COMSPEC variable
mov [si],al ; to local buffer
inc si
inc di
or al,al ; null char? (turns off carry)
jnz gcsp1 ; no, get next character
gcsp2: ret ; back to caller
get_comspec endp
get_env proc near ; search environment
; call DS:SI = "NAME="
; uses contents of "ENV_SEG"
; returns carry = false and ES:DI
; pointing to parameter if found,
; returns carry = true if no match
mov es,env_seg ; get environment segment
xor di,di ; initialize env offset
genv1: mov bx,si ; initialize pointer to name
cmp byte ptr es:[di],0 ; end of environment?
jne genv2 ; jump, end not found
stc ; no match, return carry set
ret
genv2: mov al,[bx] ; get character from name
or al,al ; end of name? (turns off carry)
jz genv3 ; yes, name matched
cmp al,es:[di] ; compare to environment
jne genv4 ; jump if match failed
inc bx ; advance environment
inc di ; and name pointers
jmp genv2
genv3: ; match found, carry = clear,
ret ; ES:DI = variable
genv4: xor al,al ; scan forward in environment
mov cx,-1 ; for zero byte
cld
repnz scasb
jmp genv1 ; go compare next string
get_env endp
exec proc near ; call MS-DOS EXEC function
; to run COMMAND.COM
mov stkseg,ss ; save stack pointer
mov stkptr,sp
; now run COMMAND.COM
mov dx,offset com_spec ; DS:DX = filename
mov bx,offset par_blk ; ES:BX = parameter block
mov ax,4b00h ; function 4bh = EXEC
; subfunction 0 =
; load and execute
int 21h ; transfer to MS-DOS
mov ax,_DATA ; make data segment
mov ds,ax ; addressable again
mov es,ax
cli ; (for bug in some 8088s)
mov ss,stkseg ; restore stack pointer
mov sp,stkptr
sti ; (for bug in some 8088s)
jnc exec1 ; jump if no errors
; display error message
mov dx,offset msg2 ; DS:DX = message address
mov cx,msg2_length ; CX = message length
mov bx,stderr ; BX = standard error handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
exec1: ret ; back to caller
exec endp
cls_cmd proc near ; intrinsic CLS command
mov dx,offset cls_str ; send the ANSI escape
mov cx,cls_str_length ; sequence to clear
mov bx,stdout ; the screen
mov ah,40h
int 21h
ret
cls_cmd endp
dos_cmd proc near ; intrinsic DOS command
; set null command tail
mov word ptr par_cmd,offset nultail
call exec ; and run COMMAND.COM
ret
dos_cmd endp
exit_cmd proc near ; intrinsic EXIT command
mov ax,4c00h ; call MS-DOS terminate
int 21h ; function with
; return code of zero
exit_cmd endp
_TEXT ends
STACK segment para stack 'STACK' ; declare stack segment
dw 64 dup (?)
STACK ends
_DATA segment word public 'DATA'
commands equ $ ; "intrinsic" commands table
; each entry is ASCIIZ string
; followed by the offset
; of the procedure to be
; executed for that command
db 'CLS',0
dw cls_cmd
db 'DOS',0
dw dos_cmd
db 'EXIT',0
dw exit_cmd
db 0 ; end of table
com_var db 'COMSPEC=',0 ; environment variable
; COMMAND.COM filespec
com_spec db 80 dup (0) ; from environment COMSPEC=
nultail db 0,cr ; null command tail for
; invoking COMMAND.COM
; as another shell
cmd_tail db 0,' /C ' ; command tail for invoking
; COMMAND.COM as a transient
inp_buf db 80 dup (0) ; command line from standard input
inp_buf_length equ $-inp_buf
cmd_tail_length equ $-cmd_tail-1
prompt db cr,lf,'sh: ' ; SHELL's user prompt
prompt_length equ $-prompt
env_seg dw 0 ; segment of environment block
msg1 db cr,lf
db 'Unable to release memory.'
db cr,lf
msg1_length equ $-msg1
msg2 db cr,lf
db 'EXEC of COMMAND.COM failed.'
db cr,lf
msg2_length equ $-msg2
msg3 db cr,lf
db 'No COMSPEC variable in environment.'
db cr,lf
msg3_length equ $-msg3
cls_str db escape,'[2J' ; ANSI escape sequence
cls_str_length equ $-cls_str ; to clear the screen
; EXEC parameter block
par_blk dw 0 ; environment segment
par_cmd dd cmd_tail ; command line
dd fcb1 ; file control block #1
dd fcb2 ; file control block #2
fcb1 db 0 ; file control block #1
db 11 dup (' ')
db 25 dup (0)
fcb2 db 0 ; file control block #2
db 11 dup (' ')
db 25 dup (0)
stkseg dw 0 ; original SS contents
stkptr dw 0 ; original SP contents
_DATA ends
end shell
──────────────────────────────────────────────────────────────────────────
Figure 12-5. SHELL.ASM: A simple table-driven command interpreter written
in Microsoft Macro Assembler.
The SHELL program is table driven and can easily be extended to provide a
powerful customized user interface for almost any application. When SHELL
takes control of the system, it displays the prompt
sh:
and waits for input from the user. After the user types a line terminated
by a carriage return, SHELL tries to match the first token in the line
against its table of internal (intrinsic) commands. If it finds a match,
it calls the appropriate subroutine. If it does not find a match, it calls
the MS-DOS EXEC function and passes the user's input to COMMAND.COM with
the /C switch, essentially using COMMAND.COM as a transient command
processor under its own control.
As supplied in these listings, SHELL "knows" exactly three internal
commands:
Command Action
──────────────────────────────────────────────────────────────────────────
CLS Uses the ANSI standard control sequence to clear the
display screen and home the cursor.
DOS Runs a copy of COMMAND.COM.
EXIT Exits SHELL, returning control of the system to the
next lower command interpreter.
──────────────────────────────────────────────────────────────────────────
You can quickly add new intrinsic commands to either the C version or the
assembly-language version of SHELL. Simply code a procedure with the
appropriate action and insert the name of that procedure, along with the
text string that defines the command, into the table COMMANDS. In
addition, you can easily prevent SHELL from passing certain "dangerous"
commands (such as MKDIR or ERASE) to COMMAND.COM simply by putting the
names of the commands to be screened out into the intrinsic command table
with the address of a subroutine that prints an error message.
To summarize, the basic flow of both versions of the SHELL program is
as follows:
1. The program calls MS-DOS Int 21H Function 4AH (Resize Memory Block)
to shrink its memory allocation, so that the maximum possible space
will be available for COMMAND.COM if it is run as an overlay. (This is
explicit in the assembly-language version only. To keep the example
code simple, the number of paragraphs to be reserved is coded as a
generous literal value, rather than being figured out at runtime from
the size and location of the various program segments.)
2. The program searches the environment for the COMSPEC variable, which
defines the location of an executable copy of COMMAND.COM. If it can't
find the COMSPEC variable, it prints an error message and exits.
3. The program puts the address of its own handler in the Ctrl-C vector
(Int 23H) so that it won't lose control if the user enters a Ctrl-C
or a Ctrl-Break.
4. The program issues a prompt to the standard output device.
5. The program reads a buffered line from the standard input device to
get the user's command.
6. The program matches the first blank-delimited token in the line
against its table of intrinsic commands. If it finds a match, it
executes the associated procedure.
7. If the program does not find a match in the table of intrinsic
commands, it synthesizes a command-line tail by appending the user's
input to the /C switch and then EXECs a copy of COMMAND.COM, passing
the address of the synthesized command tail in the EXEC parameter
block.
8. The program repeats steps 4 through 7 until the user enters the
command EXIT, which is one of the intrinsic commands, and which causes
SHELL to terminate execution.
In its present form, SHELL allows COMMAND.COM to inherit a full copy of
the current environment. However, in some applications it may be helpful,
or safer, to pass a modified copy of the environment block so that the
secondary copy of COMMAND.COM will not have access to certain information.
Using EXEC to Load Overlays
Loading overlays with the EXEC function is much less complex than using
EXEC to run another program. The overlay can be constructed as either a
memory image (.COM) or relocatable (.EXE) file and need not be the same
type as the program that loads it. The main program, called the root
segment, must carry out the following steps to load and execute an
overlay:
1. Make a memory block available to receive the overlay. The program that
calls EXEC must own the memory block for the overlay.
2. Set up the overlay parameter block to be passed to the EXEC function.
This block contains the segment address of the block that will receive
the overlay, plus a segment relocation value to be applied to the
contents of the overlay file (if it is a .EXE file). These are
normally the same value.
3. Call the MS-DOS EXEC function to load the overlay by issuing an Int
21H with the registers set up as follows:
AH = 4BH
AL = 03H (EXEC subfunction to load overlay)
DS:DX = segment:offset of overlay file pathname
ES:BX = segment:offset of overlay parameter block
Upon return from the EXEC function, the carry flag is clear if the
overlay was found and loaded. The carry flag is set if the file could
not be found or if some other error occurred.
4. Execute the code within the overlay by transferring to it with a far
call. The overlay should be designed so that either the entry point or
a pointer to the entry point is at the beginning of the module after
it is loaded. This technique allows you to maintain the root and
overlay modules separately, because the root module does not contain
any "magical" knowledge of addresses within the overlay segment.
To prevent users from inadvertently running an overlay directly from the
command line, you should assign overlay files an extension other than .COM
or .EXE. It is most convenient to relate overlays to their root segment by
assigning them the same filename but a different extension, such as .OVL
or .OV1, .OV2, and so on.
Figure 12-6 shows the use of EXEC to load and execute an overlay.
──────────────────────────────────────────────────────────────────────────
.
.
.
; allocate memory for overlay
mov bx,1000h ; get 64 KB (4096 paragraphs)
mov ah,48h ; function 48h = allocate block
int 21h ; transfer to MS-DOS
jc error ; jump if allocation failed
mov pars,ax ; set load address for overlay
mov pars+2,ax ; set relocation segment for overlay
; set segment of entry point
mov word ptr entry+2,ax
mov stkseg,ss ; save root's stack pointer
mov stkptr,sp
mov ax,ds ; set ES = DS
mov es,ax
mov dx,offset oname ; DS:DX = overlay pathname
mov bx,offset pars ; ES:BX = parameter block
mov ax,4b03h ; function 4bh, subfunction 03h
int 21h ; transfer to MS-DOS
mov ax,_DATA ; make our data segment
mov ds,ax ; addressable again
mov es,ax
cli ; (for bug in some early 8088s)
mov ss,stkseg ; restore stack pointer
mov sp,stkptr
sti ; (for bug in some early 8088s)
jc error ; jump if EXEC failed
; otherwise EXEC succeeded...
push ds ; save our data segment
call dword ptr entry ; now call the overlay
pop ds ; restore our data segment
.
.
.
oname db 'OVERLAY.OVL',0 ; pathname of overlay file
pars dw 0 ; load address (segment) for file
dw 0 ; relocation (segment) for file
entry dd 0 ; entry point for overlay
stkseg dw 0 ; save SS register
stkptr dw 0 ; save SP register
──────────────────────────────────────────────────────────────────────────
Figure 12-6. A code skeleton for loading and executing an overlay with
the EXEC function. The overlay file may be in either .COM or .EXE format.
────────────────────────────────────────────────────────────────────────────
Chapter 13 Interrupt Handlers
Interrupts are signals that cause the computer's central processing unit
to suspend what it is doing and transfer to a program called an interrupt
handler. Special hardware mechanisms that are designed for maximum speed
force the transfer. The interrupt handler determines the cause of the
interrupt, takes the appropriate action, and then returns control to the
original process that was suspended.
Interrupts are typically caused by events external to the central
processor that require immediate attention, such as the following:
■ Completion of an I/O operation
■ Detection of a hardware failure
■ "Catastrophes" (power failures, for example)
In order to service interrupts more efficiently, most modern processors
support multiple interrupt types, or levels. Each type usually has a
reserved location in memory, called an interrupt vector, that specifies
where the interrupt-handler program for that interrupt type is located.
This design speeds processing of an interrupt because the computer can
transfer control directly to the appropriate routine; it does not need a
central routine that wastes precious machine cycles determining the cause
of the interrupt. The concept of interrupt types also allows interrupts to
be prioritized, so that if several interrupts occur simultaneously, the
most important one can be processed first.
CPUs that support interrupts must also have the capability to block
interrupts while they are executing critical sections of code. Sometimes
the CPU can block interrupt levels selectively, but more frequently the
effect is global. While an interrupt is being serviced, the CPU masks all
other interrupts of the same or lower priority until the active handler
has completed its execution; similarly, it can preempt the execution of a
handler if a different interrupt with higher priority requires service.
Some CPUs can even draw a distinction between selectively masking
interrupts (they are recognized, but their processing is deferred) and
simply disabling them (the interrupt is thrown away).
The creation of interrupt handlers has traditionally been considered one
of the most arcane of programming tasks, suitable only for the elite cadre
of system hackers. In reality, writing an interrupt handler is, in itself,
straightforward. Although the exact procedure must, of course, be
customized for the characteristics of the particular CPU and operating
system, the guidelines on the following page are applicable to almost any
computer system.
A program preparing to handle interrupts must do the following:
1. Disable interrupts, if they were previously enabled, to prevent them
from occurring while interrupt vectors are being modified.
2. Initialize the vector for the interrupt of interest to point to the
program's interrupt handler.
3. Ensure that, if interrupts were previously disabled, all other vectors
point to some valid handler routine.
4. Enable interrupts again.
The interrupt handler itself must follow a simple but rigid sequence of
steps:
1. Save the system context (registers, flags, and anything else that the
handler will modify and that wasn't saved automatically by the CPU).
2. Block any interrupts that might cause interference if they were
allowed to occur during this handler's processing. (This is often done
automatically by the computer hardware.)
3. Enable any interrupts that should still be allowed to occur during
this handler's processing.
4. Determine the cause of the interrupt.
5. Take the appropriate action for the interrupt: receive and store data
from the serial port, set a flag to indicate the completion of a
disk-sector transfer, and so forth.
6. Restore the system context.
7. Reenable any interrupt levels that were blocked during this handler's
execution.
8. Resume execution of the interrupted process.
As in writing any other program, the key to success in writing an
interrupt handler is to program defensively and cover all the bases. The
main reason interrupt handlers have acquired such a mystical reputation is
that they are so difficult to debug when they contain obscure errors.
Because interrupts can occur asynchronously──that is, because they can be
caused by external events without regard to the state of the currently
executing process──bugs in interrupt handlers can cause the system as a
whole to behave quite unpredictably.
Interrupts and the Intel 80x86 Family
The Intel 80x86 family of microprocessors supports 256 levels of
prioritized interrupts, which can be triggered by three types of events:
■ Internal hardware interrupts
■ External hardware interrupts
■ Software interrupts
Internal Hardware Interrupts
Internal hardware interrupts, sometimes called faults, are generated by
certain events encountered during program execution, such as an attempt to
divide by zero. The assignment of such events to certain interrupt numbers
is wired into the processor and is not modifiable (Figure 13-1).
╓┌─┌──────────┌─────────┌────────────────────┌──────────┌─────────┌──────────╖
Interrupt Vector Interrupt 8086/88 80286 80386
level address trigger
──────────────────────────────────────────────────────────────────────────
00H 00H─03H Divide-by-zero x x x
01H 04H─07H Single step x x x
02H 08H─0BH Nonmaskable x x x
Interrupt Vector Interrupt 8086/88 80286 80386
level address trigger
──────────────────────────────────────────────────────────────────────────
02H 08H─0BH Nonmaskable x x x
interrupt (NMI)
03H 0CH─0FH Breakpoint x x x
04H 10H─13H Overflow x x x
05H 14H─17H BOUND exceeded x x
06H 18H─1BH Invalid opcode x x
07H 1CH─1FH Processor extension x x
not available
08H 20H─23H Double fault x x
09H 24H─27H Segment overrun x x
0AH 28H─2BH Invalid task-state x x
segment
0BH 2CH─2FH Segment not present x x
0CH 30H─33H Stack segment x x
overrun
0DH 34H─37H General protection x x
fault
0EH 38H─3BH Page fault x
Interrupt Vector Interrupt 8086/88 80286 80386
level address trigger
──────────────────────────────────────────────────────────────────────────
0EH 38H─3BH Page fault x
0FH 3CH─3FH Reserved
10H 40H─43H Numeric coprocessor x x
error
11H─1FH 44H─7FH Reserved
──────────────────────────────────────────────────────────────────────────
Figure 13-1. Internal interrupts (faults) on the Intel 8086/88, 80286,
and 80386 microprocessors.
External Hardware Interrupts
External hardware interrupts are triggered by peripheral device
controllers or by coprocessors such as the 8087/80287. These can be tied
to either the CPU's nonmaskable-interrupt (NMI) pin or its
maskable-interrupt (INTR) pin. The NMI line is usually reserved for
interrupts caused by such catastrophic events as a memory parity error or
a power failure.
Instead of being wired directly to the CPU, the interrupts from external
devices can be channeled through a device called the Intel 8259A
Programmable Interrupt Controller (PIC). The CPU controls the PIC through
a set of I/O ports, and the PIC, in turn, signals the CPU through the INTR
pin. The PIC allows the interrupts from specific devices to be enabled and
disabled, and their priorities to be adjusted, under program control.
A single PIC can handle only eight levels of interrupts. However, PICs can
be cascaded together in a treelike structure to handle as many levels as
desired. For example, 80286- and 80386-based machines with a
PC/AT-compatible architecture use two PICs wired together to obtain 16
individually configurable levels of interrupts.
INTR interrupts can be globally enabled and disabled with the CPU's STI
and CLI instructions. As you would expect, these instructions have no
effect on interrupts received on the CPU's NMI pin.
The manufacturer of the computer system and/or the manufacturer of the
peripheral device assigns external devices to specific 8259A PIC interrupt
levels. These assignments are realized as physical electrical connections
and cannot be modified by software.
Software Interrupts
Any program can trigger software interrupts synchronously simply by
executing an INT instruction. MS-DOS uses Interrupts 20H through 3FH to
communicate with its modules and with application programs. (For instance,
the MS-DOS function dispatcher is reached by executing an Int 21H.) The
IBM PC ROM BIOS and application software use other interrupts, with either
higher or lower numbers, for various purposes (Figure 13-2). These
assignments are simply conventions and are not wired into the hardware in
any way.
╓┌─┌──────────────────┌────────────────────────────────────┌─────────────────╖
Interrupt Usage Machine
──────────────────────────────────────────────────────────────────────────
00H Divide-by-zero PC, AT, PS/2
01H Single step PC, AT, PS/2
02H NMI PC, AT, PS/2
03H Breakpoint PC, AT, PS/2
Interrupt Usage Machine
──────────────────────────────────────────────────────────────────────────
03H Breakpoint PC, AT, PS/2
04H Overflow PC, AT, PS/2
05H ROM BIOS PrintScreen PC, AT, PS/2
BOUND exceeded AT, PS/2
06H Reserved PC
Invalid opcode AT, PS/2
07H Reserved PC
80287/80387 not present AT, PS/2
08H IRQ0 timer tick PC, AT, PS/2
Double fault AT, PS/2
09H IRQ1 keyboard PC, AT, PS/2
80287/80387 segment overrun AT, PS/2
0AH IRQ2 reserved PC
IRQ2 cascade from slave 8259A PIC AT, PS/2
Invalid task-state segment (TSS) AT, PS/2
0BH IRQ3 serial communications (COM2) PC, AT, PS/2
Segment not present AT, PS/2
0CH IRQ4 serial communications (COM1) PC, AT, PS/2
Stack segment overflow AT, PS/2
Interrupt Usage Machine
──────────────────────────────────────────────────────────────────────────
Stack segment overflow AT, PS/2
0DH IRQ5 fixed disk PC
IRQ5 parallel printer (LPT2) AT
Reserved PS/2
General protection fault AT, PS/2
0EH IRQ6 floppy disk PC, AT, PS/2
Page fault AT, PS/2
0FH IRQ7 parallel printer (LPT1) PC, AT, PS/2
10H ROM BIOS video driver PC, AT, PS/2
Numeric coprocessor fault AT, PS/2
11H ROM BIOS equipment check PC, AT, PS/2
12H ROM BIOS conventional-memory size PC, AT, PS/2
13H ROM BIOS disk driver PC, AT, PS/2
14H ROM BIOS communications driver PC, AT, PS/2
15H ROM BIOS cassette driver PC
ROM BIOS I/O system extensions AT, PS/2
16H ROM BIOS keyboard driver PC, AT, PS/2
17H ROM BIOS printer driver PC, AT, PS/2
18H ROM BASIC PC, AT, PS/2
Interrupt Usage Machine
──────────────────────────────────────────────────────────────────────────
18H ROM BASIC PC, AT, PS/2
19H ROM BIOS bootstrap PC, AT, PS/2
1AH ROM BIOS time of day AT, PS/2
1BH ROM BIOS Ctrl-Break PC, AT, PS/2
1CH ROM BIOS timer tick PC, AT, PS/2
1DH ROM BIOS video parameter table PC, AT, PS/2
1EH ROM BIOS floppy-disk parameters PC, AT, PS/2
1FH ROM BIOS font (characters 80H─FFH) PC, AT, PS/2
20H MS-DOS terminate process
21H MS-DOS function dispatcher
22H MS-DOS terminate address
23H MS-DOS Ctrl-C handler address
24H MS-DOS critical-error handler
address
25H MS-DOS absolute disk read
26H MS-DOS absolute disk write
27H MS-DOS terminate and stay resident
28H MS-DOS idle interrupt
29H MS-DOS reserved
Interrupt Usage Machine
──────────────────────────────────────────────────────────────────────────
29H MS-DOS reserved
2AH MS-DOS network redirector
2BH─2EH MS-DOS reserved
2FH MS-DOS multiplex interrupt
30H─3FH MS-DOS reserved
40H ROM BIOS floppy-disk driver (if PC, AT, PS/2
fixed disk installed)
41H ROM BIOS fixed-disk parameters PC
ROM BIOS fixed-disk parameters AT, PS/2
(drive 0)
42H ROM BIOS default video driver (if PC, AT, PS/2
EGA installed)
43H EGA, MCGA, VGA character table PC, AT, PS/2
44H ROM BIOS font (characters 00H─7FH) PCjr
46H ROM BIOS fixed-disk parameters AT, PS/2
(drive 1)
4AH ROM BIOS alarm handler AT, PS/2
5AH Cluster adapter PC, AT
5BH Used by cluster program PC, AT
Interrupt Usage Machine
──────────────────────────────────────────────────────────────────────────
5BH Used by cluster program PC, AT
60H─66H User interrupts PC, AT, PS/2
67H LIM EMS driver PC, AT, PS/2
68H─6FH Unassigned
70H IRQ8 CMOS real-time clock AT, PS/2
71H IRQ9 software diverted to IRQ2 AT, PS/2
72H IRQ10 reserved AT, PS/2
73H IRQ11 reserved AT, PS/2
74H IRQ12 reserved AT
IRQ12 mouse PS/2
75H IRQ13 numeric coprocessor AT, PS/2
76H IRQ14 fixed-disk controller AT, PS/2
77H IRQ15 reserved AT, PS/2
78H─7FH Unassigned
80H─F0H BASIC PC, AT, PS/2
F1H─FFH Not used PC, AT, PS/2
──────────────────────────────────────────────────────────────────────────
Figure 13-2. Interrupts with special significance on the IBM PC, PC/AT,
and PS/2 and compatible computers. Note that the IBM ROM BIOS uses several
interrupts in the range 00H─1FH, even though they were reserved by Intel
for CPU faults. IRQ numbers refer to Intel 8259A PIC priority levels.
The Interrupt-Vector Table
The bottom 1024 bytes of system memory are called the interrupt-vector
table. Each 4-byte position in the table corresponds to an interrupt type
(0 through 0FFH) and contains the segment and offset of the interrupt
handler for that level. Interrupts 0 through 1FH (the lowest levels) are
used for internal hardware interrupts; MS-DOS uses Interrupts 20H through
3FH; all the other interrupts are available for use by either external
hardware devices or system drivers and application software.
When an 8259A PIC or other device interrupts the CPU by means of the INTR
pin, it must also place the interrupt type as an 8-bit number (0 through
0FFH) on the system bus, where the CPU can find it. The CPU then
multiplies this number by 4 to find the memory address of the interrupt
vector to be used.
Servicing an Interrupt
When the CPU senses an interrupt, it pushes the program status word (which
defines the various CPU flags), the code segment (CS) register, and the
instruction pointer (IP) onto the machine stack and disables the interrupt
system. It then uses the 8-bit number that was jammed onto the system bus
by the interrupting device to fetch the address of the handler from the
vector table and resumes execution at that address.
Usually the handler immediately reenables the interrupt system (to allow
higher-priority interrupts to occur), saves any registers it is going to
use, and then processes the interrupt as quickly as possible. Some
external devices also require a special acknowledgment signal so that they
will know the interrupt has been recognized.
If the interrupt was funneled through an 8259A PIC, the handler must send
a special code called end of interrupt (EOI) to the PIC through its
control port to tell it when interrupt processing is completed. (The EOI
has no effect on the CPU itself.) Finally, the handler executes the
special IRET (INTERRUPT RETURN) instruction that restores the original
state of the CPU flags, the CS register, and the instruction pointer
(Figure 13-3).
Whether an interrupt was triggered by an external device or forced by
software execution of an INT instruction, there is no discernible
difference in the system state at the time the interrupt handler receives
control. This fact is convenient when you are writing and testing external
interrupt handlers because you can debug them to a large extent simply by
invoking them with software drivers.
──────────────────────────────────────────────────────────────────────────
pic_ctl equ 20h ; control port for 8259A
; interrupt controller
.
.
.
sti ; turn interrupts back on,
push ax ; save registers
push bx
push cx
push dx
push si
push di
push bp
push ds
push es
mov ax,cs ; make local data addressable
mov ds,ax
. ; do some stuff appropriate
. ; for this interrupt here
.
mov al,20h ; send EOI to 8259A PIC
mov dx,pic_ctl
out dx,al
pop es ; restore registers
pop ds
pop bp
pop di
pop si
pop dx
pop cx
pop bx
pop ax
iret ; resume previous processing
──────────────────────────────────────────────────────────────────────────
Figure 13-3. Typical handler for hardware interrupts on the 80x86 family
of microprocessors. In real life, the interrupt handler would need to save
and restore only the registers that it actually modified. Also, if the
handler made extensive use of the machine stack, it would need to save and
restore the SS and SP registers of the interrupted process and use its own
local stack.
Interrupt Handlers and MS-DOS
The introduction of an interrupt handler into your program brings with it
considerable hardware dependence. It goes without saying (but I am saying
it again here anyway) that you should avoid such hardware dependence in
MS-DOS applications whenever possible, to ensure that your programs will
be portable to any machine running current versions of MS-DOS and that
they will run properly under future versions of the operating system.
Valid reasons do exist, however, for writing your own interrupt handler
for use under MS-DOS:
■ To supersede the MS-DOS default handler for an internal hardware
interrupt (such as divide-by-zero, BOUND exceeded, and so forth).
■ To supersede the MS-DOS default handler for a defined system exception,
such as the critical-error handler or Ctrl-C handler.
■ To chain your own interrupt handler onto the default system handler for
a hardware device, so that both the system's actions and your own will
occur on an interrupt. (A typical example of this is the "clock-tick"
interrupt.)
■ To service interrupts not supported by the default MS-DOS device
drivers (such as the serial communications port, which can be used at
much higher speeds with interrupts than with polling).
■ To provide a path of communication between a program that terminates
and stays resident and other application software.
MS-DOS provides the following facilities to enable you to install
well-behaved interrupt handlers in a manner that does not interfere with
operating-system functions or other interrupt handlers:
Function Action
──────────────────────────────────────────────────────────────────────────
Int 21H Function 25H Set interrupt vector.
Int 21H Function 35H Get interrupt vector.
Int 21H Function 31H Terminate and stay resident.
──────────────────────────────────────────────────────────────────────────
These functions allow you to examine or modify the contents of the system
interrupt-vector table and to reserve memory for the use of a handler
without running afoul of other processes in the system or causing memory
use conflicts. Section 2 of this book, "MS-DOS Functions Reference,"
describes each of these functions in detail, with programming examples.
Handlers for external hardware interrupts under MS-DOS must operate under
some fairly severe restrictions:
■ Because the current versions of MS-DOS are not reentrant, a hardware
interrupt handler should never call the MS-DOS functions during the
actual interrupt processing.
■ The handler must reenable interrupts as soon as it gets control, to
avoid crippling other devices or destroying the accuracy of the system
clock.
■ A program should access the 8259A PIC with great care. The program
should not access the PIC unless that program is known to be the only
process in the system concerned with that particular interrupt level.
And it is vital that the handler issue an end-of-interrupt code to the
8259A PIC before performing the IRET; otherwise, the processing of
further interrupts for that priority level or lower priority levels
will be blocked.
Restrictions on handlers that replace the MS-DOS default handlers for
internal hardware interrupts or system exceptions (such as Ctrl-C or
critical errors) are not quite so stringent, but you must still program
the handlers with extreme care to avoid destroying system tables or
leaving the operating system in an unstable state.
The following are a few rules to keep in mind when you are writing an
interrupt driver:
■ Use Int 21H Function 25H (Set Interrupt Vector) to modify the
interrupt vector; do not write directly to the interrupt-vector table.
■ If your program is not the only process in the system that uses this
interrupt level, chain back to the previous handler after performing
your own processing on an interrupt.
■ If your program is not going to stay resident, fetch and save the
current contents of the interrupt vector before modifying it and then
restore the original contents when your program exits.
■ If your program is going to stay resident, use one of the terminate-
and-stay-resident functions (preferably Int 21H Function 31H) to
reserve the proper amount of memory for your handler.
■ If you are going to process hardware interrupts, keep the time that
interrupts are disabled and the total length of the service routine to
an absolute minimum. Remember that even after interrupts are reenabled
with an STI instruction, interrupts of the same or lower priority
remain blocked if the interrupt was received through the 8259A PIC.
ZERODIV, an Example Interrupt Handler
The listing ZERODIV.ASM (Figure 13-4) illustrates some of the principles
and guidelines on the previous pages. It is an interrupt handler for the
divide-by-zero internal interrupt (type 0). ZERODIV is loaded as a .COM
file (usually by a command in the system's AUTOEXEC file) but makes itself
permanently resident in memory as long as the system is running.
The ZERODIV program has two major portions: the initialization portion and
the interrupt handler.
The initialization procedure (called init in the program listing) is
executed only once, when the ZERODIV program is executed from the MS-DOS
level. The init procedure takes over the type 0 interrupt vector, prints a
sign-on message, then performs a terminate-and-stay-resident exit to
MS-DOS. This special exit reserves the memory occupied by the ZERODIV
program, so that it is not overwritten by subsequent application programs.
The interrupt handler (called zdiv in the program listing) receives
control when a divide-by-zero interrupt occurs. The handler preserves all
registers and then prints a message to the user asking whether to continue
or to abort the program. We can use the MS-DOS console I/O functions
within this particular interrupt handler because we can safely presume
that the application was in control when the interrupt occurred; thus,
there should be no chance of accidentally making overlapping calls upon
the operating system.
If the user enters a C to continue, the handler simply restores all the
registers and performs an IRET (INTERRUPT RETURN) to return control to the
application. (Of course, the results of the divide operation will be
useless.) If the user enters Q to quit, the handler exits to MS-DOS. Int
21H Function 4CH is particularly convenient in this case because it
allows the program to pass a return code and at the same time is the only
termination function that does not rely on the contents of any of the
segment registers.
For an example of an interrupt handler for external (communications port)
interrupts, see the TALK terminal-emulator program in Chapter 7. You may
also want to look again at the discussions of Ctrl-C and critical-error
exception handlers in Chapters 5 and 8.
──────────────────────────────────────────────────────────────────────────
name zdivide
page 55,132
title ZERODIV--Divide-by-zero handler
;
; ZERODIV.ASM--Terminate-and-stay-resident handler
; for divide-by-zero interrupts
;
; Copyright 1988 Ray Duncan
;
; Build: C>MASM ZERODIV;
; C>LINK ZERODIV;
; C>EXE2BIN ZERODIV.EXE ZERODIV.COM
; C>DEL ZERODIV.EXE
;
; Usage: C>ZERODIV
;
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII linefeed
beep equ 07h ; ASCII bell code
backsp equ 08h ; ASCII backspace code
_TEXT segment word public 'CODE'
org 100H
assume cs:_TEXT,ds:_TEXT,es:_TEXT,ss:_TEXT
init proc near ; entry point at load time
; capture vector for
; interrupt zero...
mov dx,offset zdiv ; DS:DX = handler address
mov ax,2500h ; function 25h = set vector
; interrupt type = 0
int 21h ; transfer to MS-DOS
; print sign-on message
mov dx,offset msg1 ; DS:DX = message address
mov ah,9 ; function 09h = display string
int 21h ; transfer to MS-DOS
; DX = paragraphs to reserve
mov dx,((offset pgm_len+15)/16)+10h
mov ax,3100h ; function 31h = terminate and
; stay resident
int 21h ; transfer to MS-DOS
init endp
zdiv proc far ; this is the divide-by-
; zero interrupt handler
sti ; enable interrupts
push ax ; save registers
push bx
push cx
push dx
push si
push di
push bp
push ds
push es
mov ax,cs ; make data addressable
mov ds,ax
; display message
; "Continue or Quit?"
mov dx,offset msg2 ; DS:DX = message address
mov ah,9 ; function 09h = display string
int 21h ; transfer to MS-DOS
zdiv1: mov ah,1 ; function 01h = read keyboard
int 21h ; transfer to MS-DOS
or al,20h ; fold char to lowercase
cmp al,'c' ; is it C or Q?
je zdiv3 ; jump, it's a C
cmp al,'q'
je zdiv2 ; jump, it's a Q
; illegal entry, send beep
; and erase the character
mov dx,offset msg3 ; DS:DX = message address
mov ah,9 ; function 09h = display string
int 21h ; transfer to MS-DOS
jmp zdiv1 ; try again
zdiv2: ; user chose "Quit"
mov ax,4cffh ; terminate current program
int 21h ; with return code = 255
zdiv3: ; user chose "Continue"
; send CR-LF pair
mov dx,offset msg4 ; DS:DX = message address
mov ah,9 ; function 09h = print string
int 21h ; transfer to MS-DOS
; what CPU type is this?
xor ax,ax ; to find out, we'll put
push ax ; zero in the CPU flags
popf ; and see what happens
pushf
pop ax
and ax,0f000h ; 8086/8088 forces
cmp ax,0f000h ; bits 12-15 true
je zdiv5 ; jump if 8086/8088
; otherwise we must adjust
; return address to bypass
; the divide instruction...
mov bp,sp ; make stack addressable
lds bx,[bp+18] ; get address of the
; faulting instruction
mov bl,[bx+1] ; get addressing byte
and bx,0c7h ; isolate mod & r/m fields
cmp bl,6 ; mod 0, r/m 6 = direct
jne zdiv4 ; not direct, jump
add word ptr [bp+18],4
jmp zdiv5
zdiv4: mov cl,6 ; otherwise isolate mod
shr bx,cl ; field and get instruction
mov bl,cs:[bx+itab] ; size from table
add [bp+18],bx
zdiv5: pop es ; restore registers
pop ds
pop bp
pop di
pop si
pop dx
pop cx
pop bx
pop ax
iret ; return from interrupt
zdiv endp
msg1 db cr,lf ; load-time sign-on message
db 'Divide by Zero Interrupt '
db 'Handler installed.'
db cr,lf,'$'
msg2 db cr,lf,lf ; interrupt-time message
db 'Divide by Zero detected: '
db cr,lf,'Continue or Quit (C/Q) ? '
db '$'
msg3 db beep ; used if bad entry
db backsp,' ',backsp,'$'
msg4 db cr,lf,'$' ; carriage return-linefeed
; instruction size table
itab db 2 ; mod = 0
db 3 ; mod = 1
db 4 ; mod = 2
db 2 ; mod = 3
pgm_len equ $-init ; program length
_TEXT ends
end init
──────────────────────────────────────────────────────────────────────────
Figure 13-4. A simple example of an interrrupt handler for use within the
MS-DOS environment. ZERODIV makes itself permanently resident in memory
and handles the CPU's internal divide-by-zero interrupt.
────────────────────────────────────────────────────────────────────────────
Chapter 14 Installable Device Drivers
Device drivers are the modules of an operating system that control the
hardware. They isolate the operating-system kernel from the specific
characteristics and idiosyncrasies of the peripheral devices interfaced to
the central processor. Thus, the driver's relationship to the kernel is
analogous to the operating system's relationship to application programs.
The installable device drivers that were introduced in MS-DOS version 2
give the user great flexibility. They allow the user to customize and
configure the computer for a wide range of peripheral devices, with a
minimum of troublesome interactions and without having to "patch" the
operating system. Even the most inexperienced user can install a new
device into a system by plugging in a card, copying a driver file to the
boot disk, and editing the system configuration file.
For those inclined to do their own programming, the MS-DOS installable
device drivers are interfaced to the hardware-independent kernel through a
simple and clearly defined scheme of function codes and data structures.
Given adequate information about the hardware, any competent assembly-
language programmer can expect to successfully interface even the most
bizarre device to MS-DOS without altering the operating system in the
slightest and without acquiring any special or proprietary knowledge about
its innards.
In retrospect, installable device drivers have proven to be one of the key
usability features of MS-DOS. I feel that they have been largely
responsible for the rapid proliferation and competitive pricing of
high-speed mass-storage devices for MS-DOS machines, and for the growing
confidence of the average user toward "tampering with" (upgrading) his or
her machine.
MS-DOS Device-Driver Types
Drivers written for MS-DOS fall into two distinct classes:
■ Block-device drivers
■ Character-device drivers
A driver's class determines what functions it must support, how it is
viewed by MS-DOS, and how it makes the associated physical device appear
to behave when an application program makes a request for I/O.
Character-Device Drivers
Character-device drivers control peripheral devices that perform input and
output one character (or byte) at a time, such as a terminal or printer. A
single character-device driver ordinarily supports a single hardware unit.
Each character device has a one-to-eight-character logical name, and an
application program can use this name to open the device for input or
output, as though it were a file. The logical name is strictly a means of
identification for MS-DOS and has no physical equivalent on the device.
MS-DOS's built-in character-device drivers for the console, serial port,
and printer are unique in that an application program can access them in
three different ways:
■ It can open them by name (CON, AUX, PRN, etc.) for input and output,
like any other character device.
■ It can use the special-purpose MS-DOS function calls (Int 21H Functions
01-0CH).
■ It can use the default handles (standard input, standard output,
standard error, standard auxiliary, and standard printer), which do not
need to be opened to be used.
The number of additional character-device drivers that can be installed is
limited only by available memory and by the requirement that each driver
have a unique logical name. If more than one driver uses the same logical
name, the last driver to be loaded will supersede any others and will
receive all I/O requests addressed to that logical name. This fact can
occasionally be turned to advantage; for example, it allows the user to
replace the system's default CON driver, which does not support cursor
positioning or character attributes, with the more powerful ANSI.SYS
driver.
ASCII vs Binary Mode
MS-DOS regards a handle associated with a character device to be in either
ASCII (cooked) mode or binary (raw) mode. The mode affects MS-DOS's
buffering of data for read and write requests. The driver itself is not
aware of the mode, and the mode does not affect its operation. An
application can select the mode of a handle with the IOCTL function (Int
21H Function 44H).
During ASCII-mode input, MS-DOS requests characters one at a time from the
driver and places them into its own internal buffer, echoing each to the
screen (if the input device is the keyboard) and checking each character
for a Ctrl-C (03H). When the number of characters requested by the
application program has been received, when a Ctrl-Z is detected, or when
the Enter key is pressed (in the case of the keyboard), MS-DOS terminates
the input and copies the data from its internal buffer into the requesting
program's buffer. Similarly, during ASCII-mode output, MS-DOS passes the
characters to the device driver one at a time and checks for a Ctrl-C
pending at the keyboard between each character. When a Ctrl-C is detected,
MS-DOS aborts the input or output operation and transfers to the routine
whose address is stored in the Int 23H vector.
In binary mode, MS-DOS reads or writes the exact number of bytes requested
by the application program, without regard to any control characters such
as Enter or Ctrl-C. MS-DOS passes the entire request through to the driver
in a single operation, instead of breaking it into single-character reads
or writes, and transfers the characters directly to or from the requesting
program's buffer.
Block-Device drivers
Block-device drivers usually control random-access mass-storage devices
such as floppy-disk drives and fixed disks, although they can also be used
to control non-random-access devices such as magnetic-tape drives. Block
devices transfer data in chunks, rather than one byte at a time. The size
of the blocks may be either fixed (disk drives) or variable (tape drives).
A block driver can support more than one hardware unit, map a single
physical unit onto two or more logical units, or both. Block devices do
not have file-like logical names, as character devices do. Instead, MS-DOS
assigns drive designators to the block-device units or logical drives in
an alphabetic sequence: A, B, and so forth. Each logical drive contains a
file system: boot block, file allocation table, root directory, and so
forth. (See Chapter 10.)
A block-device driver's position in the chain of all drivers determines
the first letter assigned to that driver. The number of logical drive
units that the driver supports determines the total number of letters
assigned to it.
Block-device drivers always read or write exactly the number of sectors
requested (barring hardware or addressing errors) and never filter or
otherwise manipulate the contents of the blocks being transferred.
Structure of an MS-DOS Device Driver
A device driver consists of three major parts (Figure 14-1):
■ A device header
■ A strategy (strat) routine
■ An interrupt (intr) routine
We'll discuss each of these in more detail as we work through this
chapter.
┌────────────────────────┬─────────────────────────┐
│ │ Initialization │
│ ├─────────────────────────┤
│ │ Media check │
│ ├─────────────────────────┤
│ │ Build BPB │
│ ├─────────────────────────┤
│ │ IOCTL read and write │
│ ├─────────────────────────┤
│ │ Status │
│ ├─────────────────────────┤
│ │ Read │
│ ├─────────────────────────┤
│ │ Write, write/verify │
│ ├─────────────────────────┤
│ Interrupt routine │ Output until busy │
│ ├─────────────────────────┤
│ │ Flush buffers │
│ ├─────────────────────────┤
│ │ Device open │
│ ├─────────────────────────┤
│ │ Device close │
│ ├─────────────────────────┤
│ │ Check whether removable │
│ ├─────────────────────────┤
│ │ Generic IOCTL │
│ ├─────────────────────────┤
│ │ Get/Set logical device │
│ └─────────────────────────┤
├──────────────────────────────────────────────────┤
│ Strategy routine │
├──────────────────────────────────────────────────┤
│ Device-driver header │
└──────────────────────────────────────────────────┘
Figure 14-1. General structure of an MS-DOS installable device driver.
The Device Header
The device header (Figure 14-2) lies at the beginning of the driver. It
contains a link to the next driver in the chain, a set of attribute flags
for the device (Figure 14-3), offsets to the executable strategy and
interrupt routines for the device, and the logical-device name (if it is a
character device such as PRN or COM1) or the number of logical units (if
it is a block device).
Byte offset
00H ┌──────────────────────────────────────────────┐
│ Link to next driver, offset │
02H ├──────────────────────────────────────────────┤
│ Link to next driver, segment │
04H ├──────────────────────────────────────────────┤
│ Device attribute word │
06H ├──────────────────────────────────────────────┤
│ Strategy entry point, offset │
08H ├──────────────────────────────────────────────┤
│ Interrupt entry point, offset │
0AH ├──────────────────────────────────────────────┤
│ Logical name (8 bytes) if character device │
│ Number of units (1 byte) if block device, │
│ followed by 7 bytes of reserved space │
└──────────────────────────────────────────────┘
Figure 14-2. Device-driver header. The offsets to the strat and intr
routines are offsets from the same segment used to point to the device
header.
╓┌─┌──────────────┌──────────────────────────────────────────────────────────╖
Bit Significance
──────────────────────────────────────────────────────────────────────────
15 1 if character device, 0 if block device
14 1 if IOCTL read and write supported
13 for block devices:
1 if BIOS parameter block in boot sector should be used to
determine media characteristics, 0 if media ID byte should
be used
for character devices:
1 if output until busy supported
12 Reserved (should be 0)
11 1 if open/close/removable media supported (MS-DOS 3.0 and
later)
Bit Significance
──────────────────────────────────────────────────────────────────────────
later)
7─10 Reserved (should be 0)
6 1 if generic IOCTL and get/set logical drive supported
(MS-DOS 3.2 and later)
5 Reserved (should be 0)
4 1 if CON driver and Int 29H fast-output function supported
3 1 if current CLOCK$ device
2 1 if current NUL device
1 for block devices:
1 if driver supports 32-bit sector addressing (MS-DOS 4.0)
for character devices:
1 if standard output device (stdout)
0 1 if current standard input device (stdin)
──────────────────────────────────────────────────────────────────────────
Figure 14-3. Device attribute word in device header. In block-device
drivers, only bits 6, 11, and 13─15 (and bit 1 in MS-DOS version 4.0) have
significance; the remainder should always be zero.
The Strategy Routine
MS-DOS calls the strategy routine (strat) for the device when the driver
is first loaded and installed, and again whenever an application program
issues an I/O request for the device. MS-DOS passes the strategy routine a
double-word pointer to a data structure called a request header. This
structure contains information about the type of operation to be
performed. In current versions of MS-DOS, the strategy routine never
actually performs any I/O operation but simply saves the pointer to the
request header. The strat routine must not make any Int 21H function
calls.
The first 13 bytes of the request header are the same for all
device-driver functions and are therefore referred to as the static
portion of the header. The number and contents of the subsequent bytes
vary according to the type of function being requested (Figure 14-4).
Both MS-DOS and the driver read and write information in the request
header.
The request header's most important component is a command code, or
function number, passed in its third byte to select a driver subfunction
such as read, write, or status. Other information passed to the driver in
the header includes unit numbers, transfer addresses, and sector or byte
counts.
──────────────────────────────────────────────────────────────────────────
;
; MS-DOS request header structure definition
;
Request struc ; request header template structure
Rlength db ? ; 0 length of request header
Unit db ? ; 1 unit number for this request
Command db ? ; 2 request header's command code
Status dw ? ; 3 driver's return status word
Reserve db 8 dup (?) ; 5 reserved area
Media db ? ; 13 media descriptor byte
Address dd ? ; 14 memory address for transfer
Count dw ? ; 18 byte/sector count value
Sector dw ? ; 20 starting sector value
Request ends ; end of request header template
──────────────────────────────────────────────────────────────────────────
Figure 14-4. Format of request header. Only the first 13 bytes are common
to all driver functions; the number and definition of the subsequent bytes
vary, depending upon the function type. The structure shown here is the
one used by the read and write subfunctions of the driver.
The Interrupt Routine
The last and most complex part of a device driver is the interrupt routine
(intr), which MS-DOS calls immediately after it calls the strategy
routine. The interrupt routine implements the device driver proper; it
performs (or calls other resident routines to perform) the actual input or
output operations, based on the information passed in the request header.
The strat routine may not make any Int 21H function calls, except for a
restricted set during driver initialization.
When an I/O function is completed, the interrupt routine uses the status
field in the request header to inform the DOS kernel about the outcome of
the requested I/O operation. It can use other fields in the request header
to pass back such useful information as counts of the actual sectors or
bytes transferred.
The interrupt routine usually consists of the following elements:
■ A collection of subroutines to implement the various function types
that may be requested by MS-DOS (sometimes called the command-code
routines)
■ A centralized entry point that saves all affected registers, extracts
the desired function code from the request header, and branches to the
appropriate command-code routine (typically accomplished with a jump
table)
■ A centralized exit point that stores status and error codes into the
request header (Figures 14-5 and 14-6) and restores the previous
contents of the affected registers
The command-code routines that implement the various functions supported
by an installable device driver are discussed in detail in the following
pages.
Bit(s) Significance
──────────────────────────────────────────────────────────────────────────
15 Error
12─14 Reserved
9 Busy
8 Done
0─7 Error code if bit 15 = 1
──────────────────────────────────────────────────────────────────────────
Figure 14-5. Values for the return status word of the request header.
Code Meaning
──────────────────────────────────────────────────────────────────────────
0 Write-protect violation
1 Unknown unit
2 Drive not ready
3 Unknown command
4 Data error (CRC)
5 Bad request-structure length
6 Seek error
7 Unknown medium
8 Sector not found
9 Printer out of paper
0AH Write fault
0BH Read fault
0CH General failure
0D─0EH Reserved
0FH Invalid disk change (MS-DOS versions 3.0 and later)
──────────────────────────────────────────────────────────────────────────
Figure 14-6. Driver error codes returned in bits 0 through 7 of the
return status word of the request header.
Although its name suggests otherwise, the interrupt routine is never
entered asynchronously (on an I/O completion interrupt, for example).
Thus, the division of function between strategy and interrupt routines is
completely artificial in the current versions of MS-DOS.
The Command-Code Routines
A total of 20 command codes are defined for MS-DOS device drivers. The
command codes (which are not consecutive), the names of the associated
driver-interrupt routines, and the MS-DOS versions in which they are first
supported are as follows:
╓┌─┌───────────┌────────────────────────┌───────────┌────────────┌───────────╖
Command Function Character Block MS-DOS
code driver driver version
──────────────────────────────────────────────────────────────────────────
0 Init (Initialization) X X 2.0
1 Media Check X 2.0
2 Build BPB X 2.0
3 IOCTL Read X X 2.0
4 Read X X 2.0
5 Nondestructive Read X 2.0
6 Input Status X 2.0
7 Flush Input Buffers X 2.0
8 Write X X 2.0
9 Write with Verify X 2.0
10 Output Status X 2.0
11 Flush Output Buffers X 2.0
Command Function Character Block MS-DOS
code driver driver version
──────────────────────────────────────────────────────────────────────────
11 Flush Output Buffers X 2.0
12 IOCTL Write X X 2.0
13 Device Open X X 3.0
14 Device Close X X 3.0
15 Removable Media X 3.0
16 Output Until Busy X 3.0
19 Generic IOCTL X X 3.2
23 Get Logical Device X 3.2
24 Set Logical Device X 3.2
──────────────────────────────────────────────────────────────────────────
As you can see from the preceding table, a driver's interrupt section must
support functions 0 through 12 under all versions of MS-DOS. Drivers
tailored for MS-DOS 3.0 and 3.1 can optionally support an additional four
functions, and MS-DOS drivers for versions 3.2 and later can support three
more (for a total of 20). MS-DOS inspects the bits in the attribute word
of the device-driver header to determine which of the optional functions a
driver supports, if any.
Some of the functions are relevant only for character-device drivers and
some only for block-device drivers; a few have meaning to both types. In
any case, both driver types should have an executable routine present for
each function, even if it does nothing except set the done flag in the
status word of the request header.
In the command-code descriptions that follow, RH refers to the request
header whose address was passed to the strategy routine in ES:BX, BYTE is
an 8-bit parameter, WORD is a 16-bit parameter, and DWORD is a far pointer
(a 16-bit offset followed by a 16-bit segment).
Function 00H (0): Driver Initialization
MS-DOS requests the driver's initialization function (init) only once,
when the driver is first loaded. This function performs any necessary
device hardware initialization, setup of interrupt vectors, and so forth.
The initialization routine must return the address of the position where
free memory begins after the driver code (the break address), so that
MS-DOS knows where it can build certain control structures and then load
the next installable driver. If this is a block-device driver, init must
also return the number of units and the address of a BPB pointer array.
MS-DOS uses the number of units returned by a block driver in the request
header to assign drive identifiers. For example, if the current maximum
drive is D and the driver being initialized supports four units, MS-DOS
will assign it the drive letters E, F, G, and H. Although the
device-driver header also has a field for number of units, MS-DOS does not
inspect it.
The BPB pointer array is an array of word offsets to BIOS parameter blocks
(Figure 14-7). Each unit defined by the driver must have one entry in the
array, although the entries can all point to the same BPB to conserve
memory. During the operating-system boot sequence, MS-DOS scans all the
BPBs defined by all the units in all the block-device drivers to determine
the largest sector size that exists on any device in the system and uses
this information to set its cache buffer size.
The operating-system services that the initialization code can invoke at
load time are very limited only Int 21H Functions 01H through 0CH and
30H. These are just adequate to check the MS-DOS version number and
display a driver-identification or error message.
Many programmers position the initialization code at the end of the driver
and return that address as the location of the first free memory, so that
MS-DOS will reclaim the memory occupied by the initialization routine
after the routine is finished with its work. If the initialization routine
finds that the device is missing or defective and wants to abort the
installation of the driver completely so that it does not occupy any
memory, it should return number of units as zero and set the free memory
address to CS:0000H. (A character-device driver that wants to abort its
installation should clear bit 15 of the attribute word in the driver
header and then set the units field and free memory address as though it
were a block-device driver.)
Byte(s) Contents
──────────────────────────────────────────────────────────────────────────
00─01H Bytes per sector
02H Sectors per allocation unit (power of 2)
03H─04H Number of reserved sectors (starting at sector 0)
05H Number of file allocation tables
06H─07H Maximum number of root-directory entries
08H─09H Total number of sectors in medium
0AH Media descriptor byte
0BH─0CH Number of sectors occupied by a single FAT
0DH─0EH Sectors per track (versions 3.0 and later)
0FH─10H Number of heads (versions 3.0 and later)
11H─12H Number of hidden sectors (versions 3.0 and later)
13H─14H High-order word of number of hidden sectors
(version 4.0)
15H─18H If bytes 8─9 are zero, total number of sectors in
medium (version 4.0)
19H─1EH Reserved, should be zero (version 4.0)
──────────────────────────────────────────────────────────────────────────
Figure 14-7. Structure of a BIOS parameter block (BPB). Every formatted
disk contains a copy of its BPB in the boot sector. (See Chapter 10.)
The initialization function is called with
──────────────────────────────────────────────────────────────────────────
RH + 2 BYTE Command code = 0
RH + 18 DWORD Pointer to character after equal sign
on CONFIG.SYS line that loaded driver
(this information is read-only)
RH + 22 BYTE Drive number for first unit of this
block driver (0 = A, 1 = B, and so
forth) (MS-DOS version 3 only)
──────────────────────────────────────────────────────────────────────────
It returns:
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
RH + 13 BYTE Number of units (block devices only)
RH + 14 DWORD Address of first free memory above
driver (break address)
RH + 18 DWORD BPB pointer array (block devices
only)
──────────────────────────────────────────────────────────────────────────
Function 01H (1): Media Check
The media-check function applies only to block devices, and in
character-device drivers it should do nothing except set the done flag.
This function is called when a drive-access call other than a simple file
read or write is pending. MS-DOS passes to the function the media
descriptor byte for the disk that it assumes is in the drive (Figure
14-8). If feasible, the media-check routine returns a code indicating
whether the disk has been changed since the last transfer. If the
media-check routine can assert that the disk has not been changed, MS-DOS
can bypass rereading the FAT before a directory access, which improves
overall performance.
Code Meaning
──────────────────────────────────────────────────────────────────────────
0F0H 3.5", 2-sided, 18-sector
0F8H fixed disk
0F9H 3.5", 2-sided, 9-sector
0F9H 5.25", 2-sided, 15-sector
0FCH 5.25", 1-sided, 9-sector
0FDH 5.25", 2-sided, 9-sector
0FEH 5.25", 1-sided, 8-sector
0FFH 5.25", 2-sided, 8-sector
──────────────────────────────────────────────────────────────────────────
Figure 14-8. Current valid MS-DOS codes for the media descriptor byte of
the request header, assuming bit 13 in the attribute word of the driver
header is zero.
MS-DOS responds to the results of the media-check function in the
following ways:
■ If the disk has not been changed, MS-DOS proceeds with the disk access.
■ If the disk has been changed, MS-DOS invalidates all buffers associated
with this unit, including buffers containing data waiting to be written
(this data is simply lost), performs a BUILD BPB call, and then reads
the disk's FAT and directory.
■ If the disk-change status is unknown, the action taken by MS-DOS
depends upon the state of its internal buffers. If data that needs to
be written out is present in the buffers, MS-DOS assumes no disk change
has occurred and writes the data (taking the risk that, if the disk
really was changed, the file structure on the new disk may be damaged).
If the buffers are empty or have all been previously flushed to the
disk, MS-DOS assumes that the disk was changed, and then proceeds as
described above for the disk-changed return code.
If bit 11 of the device-header attribute word is set (that is, the driver
supports the optional open/close/removable-media functions), the host
system is MS-DOS version 3.0 or later, and the function returns the
disk-changed code (-1), the function must also return the segment and
offset of the ASCIIZ volume label for the previous disk in the drive. (If
the driver does not have the volume label, it can return a pointer to the
ASCIIZ string NO NAME.) If MS-DOS determines that the disk was changed
with unwritten data still present in its buffers, it issues a
critical-error 0FH (invalid disk change). Application programs can trap
this critical error and prompt the user to replace the original disk.
The media-check function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code
RH + 2 BYTE Command code = 1
RH + 13 BYTE Media descriptor byte
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
RH + 14 BYTE Media-change code:
-1 if disk changed
0 if don't know whether disk changed
1 if disk not changed
RH + 15 DWORD Pointer to previous volume label, if
device attribute bit 11 = 1 and disk
has been changed (MS-DOS versions 3.0
and later)
──────────────────────────────────────────────────────────────────────────
Function 02H (2): Build BIOS Parameter Block (BPB)
The build BPB function applies only to block devices, and in
character-device drivers should do nothing except set the done flag. The
kernel uses this function to get a pointer to the valid BPB (see Figure
14-7) for the current disk and calls it when the disk-changed code is
returned by the media-check routine or the don't-know code is returned and
there are no dirty buffers (buffers with changed data that have not yet
been written to disk). Thus, a call to this function indicates that the
disk has been legally changed.
The build BPB function receives a pointer to a one-sector buffer in the
request header. If bit 13 in the driver header's attribute word is zero,
the buffer contains the first sector of the FAT (which includes the media
identification byte) and should not be altered by the driver. If bit 13 is
set, the driver can use the buffer as scratch space.
The build BPB function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code
RH + 2 BYTE Command code = 2
RH + 13 BYTE Media descriptor byte
RH + 14 DWORD Buffer address
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
RH + 18 DWORD Pointer to new BPB
──────────────────────────────────────────────────────────────────────────
Under MS-DOS versions 3.0 and later, if bit 11 of the header's device
attribute word is set, this routine should also read the volume label off
the disk and save it.
Function 03H (3): I/O-Control Read
The IOCTL read function allows the device driver to pass information
directly to the application program. This function is called only if bit
14 is set in the device attribute word. MS-DOS performs no error check on
IOCTL I/O calls.
The IOCTL read function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code (block devices)
RH + 2 BYTE Command code = 3
RH + 13 BYTE Media descriptor byte
RH + 14 DWORD Transfer address
RH + 18 WORD Byte/sector count
RH + 20 WORD Starting sector number (block
devices)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
RH + 18 WORD Actual bytes or sectors transferred
──────────────────────────────────────────────────────────────────────────
Function 04H (4): Read
The read function transfers data from the device into the specified memory
buffer. If an error is encountered during the read, the function must set
the error status and, in addition, report the number of bytes or sectors
successfully transferred; it is not sufficient to simply report an error.
The read function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code (block devices)
RH + 2 BYTE Command code = 4
RH + 13 BYTE Media descriptor byte
RH + 14 DWORD Transfer address
RH + 18 WORD Byte/sector count
RH + 20 WORD Starting sector number (block
devices)
──────────────────────────────────────────────────────────────────────────
For block-device read operations in MS-DOS version 4, if the logical unit
is larger than 32 MB and bit 1 of the driver's attribute word is set, the
following request structure is used instead:
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code
RH + 2 BYTE Command code = 4
RH + 13 BYTE Media descriptor byte
RH + 14 DWORD Transfer address
RH + 18 WORD Sector count
RH + 20 WORD Contains -1 to signal use of 32-bit
sector number
RH + 26 DWORD 32-bit starting sector number
──────────────────────────────────────────────────────────────────────────
The read function returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
RH + 18 WORD Actual bytes or sectors transferred
RH + 22 DWORD Pointer to volume label if error 0FH
is returned (MS-DOS versions 3.0 and
later)
──────────────────────────────────────────────────────────────────────────
Under MS-DOS versions 3.0 and later, this routine can use the count of
open files maintained by the open and close functions (0DH and 0EH) and
the media descriptor byte to determine whether the disk has been illegally
changed.
Function 05H (5): Nondestructive Read
The nondestructive read function applies only to character devices, and in
block devices it should do nothing except set the done flag. It returns
the next character that would be obtained with a read function (command
code 4), without removing that character from the driver's internal
buffer. MS-DOS uses this function to check the console driver for pending
Control-C characters during other operations.
The nondestructive read function is called with
──────────────────────────────────────────────────────────────────────────
RH + 2 BYTE Command code = 5
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
If busy bit = 0, at least one
character is waiting
If busy bit = 1, no characters are
waiting
RH + 13 BYTE Character (if busy bit = 0)
──────────────────────────────────────────────────────────────────────────
Function 06H (6): Input Status
The input-status function applies only to character devices, and in
block-device drivers it should do nothing except set the done flag. This
function returns the current input status for the device, allowing MS-DOS
to test whether characters are waiting in a type-ahead buffer. If the
character device does not have a type-ahead buffer, the input-status
routine should always return the busy bit equal to zero, so that MS-DOS
will not wait forever to call the read (04H) or nondestructive read (05H)
function.
The input-status function is called with
──────────────────────────────────────────────────────────────────────────
RH + 2 BYTE Command code = 6
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status:
If busy bit = 1, read request goes to
physical device.
If busy bit = 0, characters already
in device buffer and read request
returns quickly.
──────────────────────────────────────────────────────────────────────────
Function 07H (7): Flush Input Buffers
The flush-input-buffers function applies only to character devices, and in
block-device drivers it should do nothing except set the done flag. This
function causes any data waiting in the input buffer to be discarded.
The flush-input-buffers function is called with
──────────────────────────────────────────────────────────────────────────
RH + 2 BYTE Command code = 7
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
──────────────────────────────────────────────────────────────────────────
Function 08H (8): Write
The write function transfers data from the specified memory buffer to the
device. If an error is encountered during the write, the write function
must set the error status and, in addition, report the number of bytes or
sectors successfully transferred; it is not sufficient to simply report an
error.
The write function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code (block devices)
RH + 2 BYTE Command code = 8
RH + 13 BYTE Media descriptor byte
RH + 14 DWORD Transfer address
RH + 18 WORD Byte/sector count
RH + 20 WORD Starting sector number (block
devices)
──────────────────────────────────────────────────────────────────────────
For block-device write operations in MS-DOS version 4, if the logical unit
is larger than 32 MB and bit 1 of the driver's attribute word is set, the
following request structure is used instead:
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code
RH + 2 BYTE Command code = 8
RH + 13 BYTE Media descriptor byte
RH + 14 DWORD Transfer address
RH + 18 WORD Sector count
RH + 20 WORD Contains -1 to signal use of 32-bit
sector number
RH + 26 DWORD 32-bit starting sector number
──────────────────────────────────────────────────────────────────────────
The write function returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
RH + 18 WORD Actual bytes or sectors transferred
RH + 22 DWORD Pointer to volume label if error 0FH
returned (MS-DOS versions 3.0 and
later)
──────────────────────────────────────────────────────────────────────────
Under MS-DOS versions 3.0 and later, this routine can use the reference
count of open files maintained by the open and close functions (0DH and
0EH) and the media descriptor byte to determine whether the disk has been
illegally changed.
Function 09H (9): Write with Verify
The write-with-verify function transfers data from the specified memory
buffer to the device. If feasible, it should perform a read-after-write
verification of the data to confirm that the data was written correctly.
Otherwise, Function 09H is exactly like Function 08H.
Function 0AH (10): Output Status
The output-status function applies only to character devices, and in
block-device drivers it should do nothing except set the done flag. This
function returns the current output status for the device.
The output-status function is called with
──────────────────────────────────────────────────────────────────────────
RH + 2 BYTE Command code = 10 (0AH)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status:
If busy bit = 1, write request waits
for completion of current request.
If busy bit = 0, device idle and
write request starts immediately.
──────────────────────────────────────────────────────────────────────────
Function 0BH (11): Flush Output Buffers
The flush-output-buffers function applies only to character devices, and
in block-device drivers it should do nothing except set the done flag.
This function empties the output buffer, if any, and discards any pending
output requests.
The flush-output-buffers function is called with
──────────────────────────────────────────────────────────────────────────
RH + 2 BYTE Command code = 11 (0BH)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
──────────────────────────────────────────────────────────────────────────
Function 0CH (12): I/O-Control Write
The IOCTL write function allows an application program to pass control
information directly to the driver. This function is called only if bit 14
is set in the device attribute word. MS-DOS performs no error check on
IOCTL I/O calls.
The IOCTL write function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code (block devices)
RH + 2 BYTE Command code = 12 (0CH)
RH + 13 BYTE Media descriptor byte
RH + 14 DWORD Transfer address
RH + 18 WORD Byte/sector count
RH + 20 WORD Starting sector number (block
devices)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
RH + 18 WORD Actual bytes or sectors transferred
──────────────────────────────────────────────────────────────────────────
Function 0DH (13): Device Open
The device-open function is supported only under MS-DOS versions 3.0 and
later and is called only if bit 11 is set in the device attribute word of
the device header.
On block devices, the device-open function can be used to manage local
buffering and to increment a reference count of the number of open files
on the device. This capability must be used with care, however, because
programs that access files through FCBs frequently fail to close them,
thus invalidating the open-files count. One way to protect against this
possibility is to reset the open-files count to zero, without flushing the
buffers, whenever the answer to a media-change call is yes and a
subsequent build BPB call is made to the driver.
On character devices, the device-open function can be used to send a
device-initialization string (which can be set into the driver by an
application program by means of an IOCTL write function) or to deny
simultaneous access to a character device by more than one process. Note
that the predefined handles for the CON, AUX, and PRN devices are always
open.
The device-open function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code (block devices)
RH + 2 BYTE Command code = 13 (0DH)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
──────────────────────────────────────────────────────────────────────────
Function 0EH (14): Device Close
The device-close function is supported only under MS-DOS versions 3.0 and
later and is called only if bit 11 is set in the device attribute word of
the device header.
On block devices, this function can be used to manage local buffering and
to decrement a reference count of the number of open files on the device;
when the count reaches zero, all files have been closed and the driver
should flush buffers because the user may change disks.
On character devices, the device-close function can be used to send a
device-dependent post-I/O string such as a formfeed. (This string can be
set into the driver by an application program by means of an IOCTL write
function.) Note that the predefined handles for the CON, PRN, and AUX
devices are never closed.
The device-close function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code (block devices)
RH + 2 BYTE Command code = 14 (0EH)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
──────────────────────────────────────────────────────────────────────────
Function 0FH (15): Removable Media
The removable-media function is supported only under MS-DOS versions 3.0
and later and only on block devices; in character-device drivers it should
do nothing except set the done flag. This function is called only if bit
11 is set in the device attribute word in the device header.
The removable-media function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code
RH + 2 BYTE Command code = 15 (0FH)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status:
If busy bit = 1, medium nonremovable
If busy bit = 0, medium removable
──────────────────────────────────────────────────────────────────────────
Function 10H (16): Output Until Busy
The output-until-busy function is supported only under MS-DOS versions 3.0
and later, and only on character devices; in block-device drivers it
should do nothing except set the done flag. This function transfers data
from the specified memory buffer to a device, continuing to transfer bytes
until the device is busy. It is called only if bit 13 of the device
attribute word is set in the device header.
This function is an optimization included specifically for the use of
print spoolers. It is not an error for this function to return a number of
bytes transferred that is less than the number of bytes requested.
The output-until-busy function is called with
──────────────────────────────────────────────────────────────────────────
RH + 2 BYTE Command code = 16 (10H)
RH + 14 DWORD Transfer address
RH + 18 WORD Byte count
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
RH + 18 WORD Actual bytes transferred
──────────────────────────────────────────────────────────────────────────
Function 13H (19) Generic IOCTL
The generic IOCTL function is supported only under MS-DOS versions 3.2 and
later and is called only if bit 6 is set in the device attribute word of
the device header. This function corresponds to the MS-DOS generic IOCTL
service supplied to application programs by Int 21H Function 44H
Subfunctions 0CH and 0DH.
The generic IOCTL function is passed a category (major) code, a function
(minor) code, the contents of the SI and DI registers at the point of the
IOCTL call, and the segment and offset of a data buffer. This buffer in
turn contains other information whose format depends on the major and
minor IOCTL codes passed in the request header. The driver must interpret
the major and minor codes in the request header and the contents of the
additional buffer to determine which operation it will carry out, then set
the done flag in the request-header status word, and return any other
applicable information in the request header or the data buffer.
Services that the generic IOCTL function may invoke, if the driver
supports them, include configuration of the driver for nonstandard disk
formats, reading and writing entire disk tracks of data, and formatting
and verifying tracks. The generic IOCTL function has been designed to be
open-ended, so that it can be used to easily extend the device-driver
definition under future versions of MS-DOS.
The generic IOCTL function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit number (block devices)
RH + 2 BYTE Command code = 19 (13H)
RH + 13 BYTE Category (major) code
RH + 14 BYTE Function (minor) code
RH + 15 WORD SI register contents
RH + 17 WORD DI register contents
RH + 19 DWORD Address of generic IOCTL data packet
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
──────────────────────────────────────────────────────────────────────────
Function 17H (23): Get Logical Device
The get-logical-device function is supported only under MS-DOS versions
3.2 and later and only on block devices; in character-device drivers it
should do nothing except set the done bit in the status word. This
function is called only if bit 6 is set in the device attribute word of
the device header. It corresponds to the get-logical-device-map service
supplied to application programs through Int 21H Function 44H Subfunction
0EH.
The get-logical-device function returns a code for the last drive letter
used to reference the device; if only one drive letter is assigned to the
device, the returned unit code should be zero. Thus, this function can be
used to determine whether more than one drive letter is assigned to the
same physical device.
The get-logical-device function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code
RH + 2 BYTE Command code = 23 (17H)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Last unit referenced, or zero
RH + 3 WORD Status
──────────────────────────────────────────────────────────────────────────
Function 18H (24): Set Logical Device
The set-logical-device function is supported only under MS-DOS versions
3.2 and later and only on block devices; in character-device drivers it
should do nothing except set the done bit in the status word. This
function is called only if bit 6 is set in the device attribute word of
the device header. It corresponds to the set-logical-device-map service
supplied to application programs by MS-DOS through Int 21H Function 44H
Subfunction 0FH.
The set-logical-device function informs the driver of the next
logical-drive identifier that will be used to reference the physical
device. The unit code passed by the MS-DOS kernel in this case is
zero-based relative to the number of logical drives supported by this
particular driver. For example, if the driver supports two floppy-disk
units (A and B), only one physical floppy-disk drive exists in the system,
and the set-logical-device function is called with a unit number of 1, the
driver is being informed that the next read or write request from the
kernel will be directed to drive B.
The set-logical-device function is called with
──────────────────────────────────────────────────────────────────────────
RH + 1 BYTE Unit code
RH + 2 BYTE Command code = 24 (18H)
──────────────────────────────────────────────────────────────────────────
It returns
──────────────────────────────────────────────────────────────────────────
RH + 3 WORD Status
──────────────────────────────────────────────────────────────────────────
The Processing of a Typical I/O Request
An application program requests an I/O operation from MS-DOS by loading
registers with the appropriate values and executing an Int 21H. This
results in the following sequence of actions:
1. MS-DOS inspects its internal tables and determines which device driver
should receive the I/O request.
2. MS-DOS creates a request-header data packet in a reserved area of
memory. (Disk I/O requests are transformed from file and record
information into logical-sector requests by MS-DOS's interpretation of
the disk directory and FAT.)
3. MS-DOS calls the device driver's strat entry point, passing the
address of the request header in the ES:BX registers.
4. The device driver saves the address of the request header in a local
variable and performs a FAR RETURN.
5. MS-DOS calls the device driver's intr entry point.
6. The interrupt routine saves all registers, retrieves the address of
the request header that was saved by the strategy routine, extracts
the function code, and branches to the appropriate command-code
subroutine to perform the function.
7. If a data transfer on a block device was requested, the driver's read
or write subroutine translates the logical-sector number into a head,
track, and physical-sector address for the requested unit and then
performs the I/O operation. Because a multiple-sector transfer can be
requested in a single request header, a single request by MS-DOS to
the driver can result in multiple read or write commands to the disk
controller.
8. When the requested function is complete, the interrupt routine sets
the status word and any other required information into the request
header, restores all registers to their state at entry, and performs a
FAR RETURN.
9. MS-DOS translates the driver's return status into the appropriate
return code and carry-flag status for the MS-DOS Int 21H function that
was requested and returns control to the application program.
Note that a single request by an application program can result in MS-DOS
passing many request headers to the driver. For example, attempting to
open a file in a subdirectory on a previously unaccessed disk drive might
require the following actions:
■ Reading the disk's boot sector to get the BPB
■ Reading from one to many sectors of the root directory to find the
entry for the subdirectory and obtain its starting-cluster number
■ Reading from one to many sectors of both the FAT and the subdirectory
itself to find the entry for the desired file
The CLOCK Driver: A Special Case
MS-DOS uses the CLOCK device for marking file control blocks and directory
entries with the date and time, as well as for providing the date and time
services to application programs. This device has a unique type of
interaction with MS-DOS──a 6-byte sequence is read from or written to the
driver that obtains or sets the current date and time. The sequence has
the following format:
┌─────────┬─────────┬─────────┬─────────┬─────────┬─────────┐
│ 0 │ 1 │ 2 │ 3 │ 4 │ 5 │
│ Days │ Days │ Minutes │ Hours │Seconds/ │ Seconds │
│low byte │high byte│ │ │ 100 │ │
└─────────┴─────────┴─────────┴─────────┴─────────┴─────────┘
The value passed for days is a 16-bit integer representing the number of
days elapsed since January 1, 1980.
The clock driver can have any logical-device name because MS-DOS uses the
CLOCK bit in the device attribute word of the driver's device header to
identify the device, rather than its name. On IBM PC systems, the clock
device has the logical-device name CLOCK$.
Writing and Installing a Device Driver
Now that we have discussed the structure and capabilities of installable
device drivers for the MS-DOS environment, we can discuss the mechanical
steps of assembling and linking them.
Assembly
Device drivers for MS-DOS always have an origin of zero but are otherwise
assembled, linked, and converted into an executable module as though they
were .COM files. (Although MS-DOS is also capable of loading installable
drivers in the .EXE file format, this introduces unnecessary complexity
into writing and debugging drivers and offers no significant advantages.
In addition, it is not possible to use .EXE-format drivers with some IBM
versions of MS-DOS because the .EXE loader is located in COMMAND.COM,
which is not present when the installable device drivers are being
loaded.) The driver should not have a declared stack segment and must, in
general, follow the other restrictions outlined in Chapter 3 for
memory-image (.COM) programs. A driver can be loaded anywhere, so beware
that you do not make any assumptions in your code about the driver's
location in physical memory. Figure 14-9 presents a skeleton example that
you can follow as you read the next few pages.
──────────────────────────────────────────────────────────────────────────
name driver
page 55,132
title DRIVER.ASM Device-Driver Skeleton
;
; DRIVER.ASM MS-DOS device-driver skeleton
;
; The driver command-code routines are stubs only and have
; no effect but to return a nonerror "done" status.
;
; Copyright 1988 Ray Duncan
;
_TEXT segment word public 'CODE'
assume cs:_TEXT,ds:_TEXT,es:NOTHING
org 0
MaxCmd equ 24 ; maximum allowed command code:
; 12 for MS-DOS 2
; 16 for MS-DOS 3.0-3.1
; 24 for MS-DOS 3.2-3.3
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII linefeed
eom equ '$' ; end-of-message signal
Header: ; device-driver header
dd -1 ; link to next device driver
dw 0c840h ; device attribute word
dw Strat ; "strategy" routine entry point
dw Intr ; "interrupt" routine entry point
db 'SKELETON' ; logical-device name
RHPtr dd ? ; pointer to request header, passed
; by MS-DOS kernel to strategy routine
Dispatch: ; interrupt-routine command-code
; dispatch table:
dw Init ; 0 = initialize driver
dw MediaChk ; 1 = media check
dw BuildBPB ; 2 = build BPB
dw IoctlRd ; 3 = IOCTL read
dw Read ; 4 = read
dw NdRead ; 5 = nondestructive read
dw InpStat ; 6 = input status
dw InpFlush ; 7 = flush input buffers
dw Write ; 8 = write
dw WriteVfy ; 9 = write with verify
dw OutStat ; 10 = output status
dw OutFlush ; 11 = flush output buffers
dw IoctlWt ; 12 = IOCTL write
dw DevOpen ; 13 = device open (MS-DOS 3.0+)
dw DevClose ; 14 = device close (MS-DOS 3.0+)
dw RemMedia ; 15 = removable media (MS-DOS 3.0+)
dw OutBusy ; 16 = output until busy (MS-DOS 3.0+)
dw Error ; 17 = not used
dw Error ; 18 = not used
dw GenIOCTL ; 19 = generic IOCTL (MS-DOS 3.2+)
dw Error ; 20 = not used
dw Error ; 21 = not used
dw Error ; 22 = not used
dw GetLogDev ; 23 = get logical device (MS-DOS 3.2+)
dw SetLogDev ; 24 = set logical device (MS-DOS 3.2+)
Strat proc far ; device-driver strategy routine,
; called by MS-DOS kernel with
; ES:BX = address of request header
; save pointer to request header
mov word ptr cs:[RHPtr],bx
mov word ptr cs:[RHPtr+2],es
ret ; back to MS-DOS kernel
Strat endp
Intr proc far ; device-driver interrupt routine,
; called by MS-DOS kernel immediately
; after call to strategy routine
push ax ; save general registers
push bx
push cx
push dx
push ds
push es
push di
push si
push bp
push cs ; make local data addressable
pop ds ; by setting DS = CS
les di,[RHPtr] ; let ES:DI = request header
; get BX = command code
mov bl,es:[di+2]
xor bh,bh
cmp bx,MaxCmd ; make sure it's legal
jle Intr1 ; jump, function code is ok
call Error ; set error bit, "unknown command" code
jmp Intr2
Intr1: shl bx,1 ; form index to dispatch table
; and branch to command-code routine
call word ptr [bx+Dispatch]
les di,[RHPtr] ; ES:DI = addr of request header
Intr2: or ax,0100h ; merge 'done' bit into status and
mov es:[di+3],ax ; store status into request header
pop bp ; restore general registers
pop si
pop di
pop es
pop ds
pop dx
pop cx
pop bx
pop ax
ret ; back to MS-DOS kernel
; Command-code routines are called by the interrupt routine
; via the dispatch table with ES:DI pointing to the request
; header. Each routine should return AX = 0 if function was
; completed successfully or AX = (8000h + error code) if
; function failed.
MediaChk proc near ; function 1 = media check
xor ax,ax
ret
MediaChk endp
BuildBPB proc near ; function 2 = build BPB
xor ax,ax
ret
BuildBPB endp
IoctlRd proc near ; function 3 = IOCTL read
xor ax,ax
ret
IoctlRd endp
Read proc near ; function 4 = read (input)
xor ax,ax
ret
Read endp
NdRead proc near ; function 5 = nondestructive read
xor ax,ax
ret
NdRead endp
InpStat proc near ; function 6 = input status
xor ax,ax
ret
InpStat endp
InpFlush proc near ; function 7 = flush input buffers
xor ax,ax
ret
InpFlush endp
Write proc near ; function 8 = write (output)
xor ax,ax
ret
Write endp
WriteVfy proc near ; function 9 = write with verify
xor ax,ax
ret
endp
OutStat proc near ; function 10 = output status
xor ax,ax
ret
OutStat endp
OutFlush proc near ; function 11 = flush output buffers
xor ax,ax
ret
OutFlush endp
IoctlWt proc near ; function 12 = IOCTL write
xor ax,ax
ret
IoctlWt endp
DevOpen proc near ; function 13 = device open
xor ax,ax
ret
DevOpen endp
DevClose proc near ; function 14 = device close
xor ax,ax
ret
DevClose endp
RemMedia proc near ; function 15 = removable media
xor ax,ax
ret
RemMedia endp
OutBusy proc near ; function 16 = output until busy
xor ax,ax
ret
OutBusy endp
GenIOCTL proc near ; function 19 = generic IOCTL
xor ax,ax
ret
GenIOCTL endp
GetLogDev proc near ; function 23 = get logical device
xor ax,ax
ret
GetLogDev endp
SetLogDev proc near ; function 24 = set logical device
xor ax,ax
ret
SetLogDev endp
Error proc near ; bad command code in request header
mov ax,8003h ; error bit + "unknown command" code
ret
endp
Init proc near ; function 0 = initialize driver
push es ; save address of request header
push di
mov ax,cs ; convert load address to ASCII
mov bx,offset Ident1
call hexasc
mov ah,9 ; display driver sign-on message
mov dx,offset Ident
int 21h
pop di ; restore request-header address
pop es
; set address of free memory
; above driver (break address)
mov word ptr es:[di+14],offset Init
mov word ptr es:[di+16],cs
xor ax,ax ; return status
ret
Init endp
hexasc proc near ; converts word to hex ASCII
; call with AX = value,
; DS:BX = address for string
; returns AX, BX destroyed
push cx ; save registers
push dx
mov dx,4 ; initialize character counter
mov cx,4 ; isolate next four bits
rol ax,cl
mov cx,ax
and cx,0fh
add cx,'0' ; convert to ASCII
cmp cx,'9' ; is it 0-9?
jbe hexasc2 ; yes, jump
add cx,'A'-'9'-1 ; add fudge factor for A-F
hexasc2: ; store this character
mov [bx],cl
inc bx ; bump string pointer
dec dx ; count characters converted
jnz hexasc1 ; loop, not four yet
pop dx ; restore registers
pop cx
ret ; back to caller
hexasc endp
Ident db cr,lf,lf
db 'Advanced MS-DOS Example Device Driver'
db cr,lf
db 'Device driver header at: '
Ident1 db 'XXXX:0000'
db cr,lf,lf,eom
Intr endp
_TEXT ends
end
──────────────────────────────────────────────────────────────────────────
Figure 14-9. DRIVER.ASM: A functional skeleton from which you can
implement your own working device driver.
The driver's device header must be located at the beginning of the file
(offset 0000H). Both words in the link field in the header should be set
to -1. The attribute word must be set up correctly for the device type and
other options. The offsets to the strategy and interrupt routines must be
relative to the same segment base as the device header itself. If the
driver is for a character device, the name field should be filled in
properly with the device's logical name. The logical name can be any legal
8-character filename, padded with spaces and without a colon. Beware of
accidentally duplicating the names of existing character devices, unless
you are intentionally superseding a resident driver.
MS-DOS calls the strategy and interrupt routines for the device by means
of an intersegment call (CALL FAR) when the driver is first loaded and
installed and again whenever an application program issues an I/O request
for the device. MS-DOS uses the ES:BX registers to pass the strat routine
a double-word pointer to the request header; this address should be saved
internally in the driver so that it is available for use during the
subsequent call to the intr routine.
The command-code routines for function codes 0 through 12 (0CH) must be
present in every installable device driver, regardless of device type.
Functions 13 (0DH) and above are optional for drivers used with MS-DOS
versions 3.0 and later and can be handled in one of the following ways:
■ Don't implement them, and leave the associated bits in the device
header cleared. The resulting driver will work in either version 2 or
version 3 but does not take full advantage of the augmented
functionality of version 3.
■ Implement them, and test the MS-DOS version during the initialization
sequence, setting bits 6 and 11 of the device header appropriately.
Write all command-code routines so that they test this bit and adjust
to accommodate the host version of MS-DOS. Such a driver requires more
work and testing but will take full advantage of both the version 2 and
the version 3 environments.
■ Implement them, and assume that all the version 3 facilities are
available. With this approach, the resulting driver may not work
properly under version 2.
Remember that device drivers must preserve the integrity of MS-DOS. The
driver must preserve all registers, including flags (especially the
direction flag and interrupt enable bits), and if the driver makes heavy
use of the stack, it should switch to an internal stack of adequate depth
(the MS-DOS stack has room for only 40 to 50 bytes when a driver is
called).
If you install a new CON driver, be sure to set the bits for standard
input and standard output in the device attribute word in the device
header.
You'll recall that one file can contain multiple drivers. In this case,
the device-header link field of each driver should point to the segment
offset of the next, all using the same segment base, and the link field
for the last driver in the file should be set to -1,-1. The initialization
routines for all the drivers in the file should return the same break
address.
Linking
Use the standard MS-DOS linker to transform the .OBJ file that is output
from the assembler into a relocatable .EXE module. Then, use the EXE2BIN
utility (see Chapter 4) to convert the .EXE file into a memory-image
program. The extension on the final driver file can be anything, but .BIN
and .SYS are most commonly used in MS-DOS systems, and it is therefore
wise to follow one of these conventions.
Installation
After the driver is assembled, linked, and converted to a .BIN or .SYS
file, copy it to the root directory of a bootable disk. If it is a
character-device driver, do not use the same name for the file as you used
for the logical device listed in the driver's header, or you will not be
able to delete, copy, or rename the file after the driver is loaded.
Use your favorite text editor to add the line
DEVICE=[D:][PATH]FILENAME.EXT
to the CONFIG.SYS file on the bootable disk. (In this line, D: is an
optional drive designator and FILENAME.EXT is the name of the file
containing your new device driver. You can include a path specification in
the entry if you prefer not to put the driver file in your root
directory.) Now restart your computer system to load the modified
CONFIG.SYS file.
During the MS-DOS boot sequence, the SYSINIT module (which is part of
IO.SYS) reads and processes the CONFIG.SYS file. It loads the driver into
memory and inspects the device header. If the driver is a character-device
driver, SYSINIT links it into the device chain ahead of the other
character devices; if it is a block-device driver, SYSINIT places it
behind all previously linked block devices and the resident block devices
(Figures 14-10, 14-11, and 14-12). It accomplishes the linkage by
updating the link field in the device header to point to the segment and
offset of the next driver in the chain. The link field of the last driver
in the chain contains -1,-1.
Next, SYSINIT calls the strat routine with a request header that contains
a command code of zero, and then it calls the intr routine. The driver
executes its initialization routine and returns the break address, telling
MS-DOS how much memory to reserve for this driver. Now MS-DOS can proceed
to the next entry in the CONFIG.SYS file.
You cannot supersede a built-in block-device driver──you can only add
supplemental block devices. However, you can override the default system
driver for a character device (such as CON) with an installed driver by
giving it the same logical-device name in the device header. When
processing a character I/O request, MS-DOS always scans the list of
installed drivers before it scans the list of default devices and takes
the first match.
NUL
│
▼
CON
│
▼
AUX
│
▼
PRN
│
▼
CLOCK
│
▼
Any other resident block
or character devices
Figure 14-10. MS-DOS device-driver chain before any installable device
drivers have been loaded.
NUL
│
▼
Installable character-
device drivers
│
▼
CON
│
▼
AUX
│
▼
PRN
│
▼
CLOCK
│
▼
Any other resident block
or character devices
│
▼
Installable block-
device drivers
Figure 14-11. MS-DOS device-driver chain after installable device drivers
have been loaded.
Address Attribute Strategy Interrupt Type Units Name
routine routine
──────────────────────────────────────────────────────────────────────────
00E3:0111 8004 0FD5 0FE0 C NUL
0070:0148 8013 008E 0099 C CON
0070:01DD 8000 008E 009F C AUX
0070:028E 8000 008E 00AE C PRN
0070:0300 8008 008E 00C3 C CLOCK
0070:03CC 0000 008E 00C9 B 02
0070:01EF 8000 008E 009F C COM1
0070:02A0 8000 008E 00AE C LPT1
0070:06F0 8000 008E 00B4 C LPT2
0070:0702 8000 008E 00BA C LPT3
0070:0714 8000 008E 00A5 C COM2
End of
device chain
──────────────────────────────────────────────────────────────────────────
Figure 14-12. Example listing of device chain under MS-DOS version 2.1,
"plain vanilla" IBM PC with no fixed disks or user device drivers.
(C=character device, B=block device)
Debugging a Device Driver
The most important thing to remember when testing new device drivers is to
maintain adequate backups and a viable fallback position. Don't modify the
CONFIG.SYS file and install the new driver on your fixed disk before it is
proven! Be prudent──create a bootable floppy disk and put the modified
CONFIG.SYS file and the new driver on that for debugging. When everything
is working properly, copy the finished product to its permanent storage
medium.
The easiest way to test a new device driver is to write a simple
assembly-language front-end routine that sets up a simulated request
packet and then performs FAR CALLs to the strat and intr entry points,
exactly as MS-DOS would. You can then link the driver and the front end
together into a .COM or .EXE file that can be run under the control of
CodeView or another debugger. This arrangement makes it easy to trace each
of the command-code routines individually, to observe the results of the
I/O, and to examine the status codes returned in the request header.
Tracing the installed driver when it is linked into the MS-DOS system in
the normal manner is more difficult. Breakpoints must be chosen carefully,
to yield the maximum possible information per debugging run. Because
current versions of MS-DOS maintain only one request header internally,
the request header that was being used by the driver you are tracing will
be overwritten as soon as your debugger makes an output request to display
information. You will find it helpful to add a routine to your
initialization subroutine that displays the driver's load address on the
console when you boot MS-DOS; you can then use this address to inspect the
device-driver header and set breakpoints within the body of the driver.
Debugging a device driver can also be somewhat sticky when interrupt
handling is involved, especially if the device uses the same
interrupt-request priority level (IRQ level) as other peripherals in the
system. Cautious, conservative programming is needed to avoid unexpected
and unreproducible interactions with other device drivers and interrupt
handlers. If possible, prove out the basic logic of the driver using
polled I/O, rather than interrupt-driven I/O, and introduce interrupt
handling only when you know the rest of the driver's logic to be solid.
Typical device-driver errors or problems that can cause system crashes or
strange system behavior include the following:
■ Failure to set the linkage address of the last driver in a file to -1
■ Overflow of the MS-DOS stack by driver-initialization code, corrupting
the memory image of MS-DOS (can lead to unpredictable behavior during
boot; remedy is to use a local stack)
■ Incorrect break-address reporting by the initialization routine (can
lead to a system crash if the next driver loaded overwrites vital parts
of the driver)
■ Improper BPBs supplied by the build BPB routine, or incorrect BPB
pointer array supplied by the initialization routine (can lead to many
confusing problems, ranging from out-of-memory errors to system boot
failure)
■ Incorrect reporting of the number of bytes or sectors successfully
transferred at the time an I/O error occurs (can manifest itself as a
system crash after you enter R to the Abort, Retry, Ignore? prompt)
Although the interface between the DOS kernel and the device driver is
fairly simple, it is also quite strict. The command-code routines must
perform exactly as they are defined, or the system will behave
erratically. Even a very subtle discrepancy in the action of a
command-code routine can have unexpectedly large global effects.
────────────────────────────────────────────────────────────────────────────
Chapter 15 Filters
A filter is, essentially, a program that operates on a stream of
characters. The source and destination of the character stream can be
files, another program, or almost any character device. The transformation
applied by the filter to the character stream can range from an operation
as simple as character substitution to one as elaborate as generating
splines from sets of coordinates.
The standard MS-DOS package includes three simple filters: SORT, which
alphabetically sorts text on a line-by-line basis; FIND, which searches a
text stream to match a specified string; and MORE, which displays text one
screenful at a time.
System Support for Filters
The operation of a filter program relies on two MS-DOS features that first
appeared in version 2.0: standard devices and redirectable I/O.
The standard devices are represented by five handles that are originally
established by COMMAND.COM. Each process inherits these handles from its
immediate parent. Thus, the standard device handles are already open when
a process acquires control of the system, and it can use them with
Interrupt 21H Functions 3FH and 40H for read and write operations
without further preliminaries. The default assignments of the standard
device handles are as follows:
Handle Name Default device
──────────────────────────────────────────────────────────────────────────
0 stdin (standard input) CON
1 stdout (standard output) CON
2 stderr (standard error) CON
3 stdaux (standard auxiliary) AUX
4 stdprn (standard printer) PRN
──────────────────────────────────────────────────────────────────────────
The CON device is assigned by default to the system's keyboard and video
display. AUX and PRN are respectively associated by default with COM1 (the
first physical serial port) and LPT1 (the first parallel printer port).
You can use the MODE command to redirect LPT1 to one of the serial ports;
the MODE command will also redirect PRN.
When executing a program by entering its name at the COMMAND.COM prompt,
you can redirect the standard input, the standard output, or both from
their default device (CON) to another file, a character device, or a
process. You do this by including one of the special characters <, >, >>,
and | in the command line, in the form shown on the following page.
Symbol Effect
──────────────────────────────────────────────────────────────────────────
< file Takes standard input from the specified file instead of
the keyboard.
< device Takes standard input from the named device instead of
the keyboard.
> file Sends standard output to the specified file instead of
the display.
>> file Appends standard output to the current contents of the
specified file instead of sending it to the display.
> device Sends standard output to the named device instead of
the display.
p1 | p2 Routes standard output of program p1 to become the
standard input of program p2. (Output of p1 is said to
be piped to p2.)
──────────────────────────────────────────────────────────────────────────
For example, the command
C>SORT <MYFILE.TXT >PRN <Enter>
causes the SORT filter to read its input from the file MYFILE.TXT, sort
the lines alphabetically, and write the resulting text to the character
device PRN (the logical name for the system's list device).
The redirection requested by the <, >, >>, and | characters takes place at
the level of COMMAND.COM and is invisible to the program it affects. Any
other process can achieve a similar effect by redirecting the standard
input and standard output with Int 21H Function 46H before calling the
EXEC function (Int 21H Function 4BH) to run a child process.
Note that if a program circumvents MS-DOS to perform its input and output,
either by calling ROM BIOS functions or by manipulating the keyboard or
video controller directly, redirection commands placed in the program's
command line do not have the expected effect.
How Filters Work
By convention, a filter program reads its text from the standard input
device and writes the results of its operations to the standard output
device. When it reaches the end of the input stream, the filter simply
terminates. As a result, filters are both flexible and simple.
Filter programs are flexible because they do not know, and do not care
about, the source of the data they process or the destination of their
output. Thus, any character device that has a logical name within the
system (CON, AUX, COM1, COM2, PRN, LPT1, LPT2, LPT3, and so on), any file
on any block device (local or network) known to the system, or any other
program can supply a filter's input or accept its output. If necessary,
you can concatenate several functionally simple filters with pipes to
perform very complex operations.
Although flexible, filters are also simple because they rely on their
parent processes to supply standard input and standard output handles that
have already been appropriately redirected. The parent must open or create
any necessary files, check the validity of logical character-device names,
and load and execute the preceding or following process in a pipe. The
filter concerns itself only with the transformation it applies to the
data.
Building a Filter
Creating a new filter for MS-DOS is a straightforward process. In its
simplest form, a filter need only use the handle-oriented read (Interrupt
21H Function 3FH) and write (Interrupt 21H Function 40H) functions to
get characters or lines from standard input and send them to standard
output, performing any desired alterations on the text stream on a
character-by-character or line-by-line basis.
Figures 15-1 and 15-2 contain prototype character-oriented filters in
both assembly language and C. In these examples, the translate routine,
which is called for each character transferred from the standard input to
the standard output, does nothing at all. As a result, both filters
function rather like a very slow COPY command. You can quickly turn these
primitive filters into useful programs by substituting your own translate
routine.
If you try out these programs, you'll notice that the C prototype filter
runs much faster than its MASM equivalent. This is because the C runtime
library is performing hidden blocking and deblocking of the input and
output stream, whereas the MASM filter is doing exactly what it appears to
be doing: making two calls to MS-DOS for each character processed. You can
easily restore the MASM filter's expected speed advantage by adapting it
to read and write lines instead of single characters.
──────────────────────────────────────────────────────────────────────────
name proto
page 55,132
title PROTO.ASM--prototype filter
;
; PROTO.ASM: prototype character-oriented filter
;
; Copyright 1988 Ray Duncan
;
stdin equ 0 ; standard input handle
stdout equ 1 ; standard output handle
stderr equ 2 ; standard error handle
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII linefeed
_TEXT segment word public 'CODE'
assume cs:_TEXT,ds:_DATA,ss:STACK
main proc far ; entry point from MS-DOS
mov ax,_DATA ; set DS = our data segment
mov ds,ax
main1: ; read char from stdin...
mov dx,offset char ; DS:DX = buffer address
mov cx,1 ; CX = length to read
mov bx,stdin ; BX = standard input handle
mov ah,3fh ; function 3fh = read
int 21h ; transfer to MS-DOS
jc main3 ; if error, terminate
cmp ax,1 ; any character read?
jne main2 ; if end of file, terminate
call translate ; translate character
; write char to stdout...
mov dx,offset char ; DS:DX = buffer address
mov cx,1 ; CX = length to write
mov bx,stdout ; BX = standard output handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
jc main3 ; if error, terminate
cmp ax,1 ; was character written?
jne main3 ; if disk full, terminate
jmp main1 ; get another character
main2: ; end of file reached
mov ax,4c00h ; function 4ch = terminate
; return code = 0
int 21h ; transfer to MS-DOS
main3: ; error or disk full
mov ax,4c01h ; function 4ch = terminate
; return code = 1
int 21h ; transfer to MS-DOS
main endp
;
; Perform any necessary translation on character
; from standard input stored in variable 'char'.
; This example simply leaves character unchanged.
;
translate proc near
ret ; does nothing
translate endp
_TEXT ends
_DATA segment word public 'DATA'
char db 0 ; storage for input character
_DATA ends
STACK segment para stack 'STACK'
dw 64 dup (?)
STACK ends
end main ; defines program entry point
──────────────────────────────────────────────────────────────────────────
Figure 15-1. PROTO.ASM, the source code for a prototype
character-oriented MASM filter.
──────────────────────────────────────────────────────────────────────────
/*
PROTO.C: prototype character-oriented filter
Copyright 1988 Ray Duncan
*/
#include <stdio.h>
main(int argc, char *argv[])
{
char ch;
while((ch=getchar()) != EOF) /* read a character */
{
ch = translate(ch); /* translate it if necessary */
putchar(ch); /* write the character */
}
exit(0); /* terminate at end of file */
}
/*
Perform any necessary translation on character
from input file. This example simply returns
the same character.
*/
int translate(char ch)
{
return (ch);
}
──────────────────────────────────────────────────────────────────────────
Figure 15-2. PROTO.C, the source code for a prototype character-oriented
C filter.
The CLEAN Filter
As a more practical example of MS-DOS filters, let's look at a simple but
very useful filter called CLEAN. Figures 15-3 and 15-4 show the
assembly-language and C source code for this filter. CLEAN processes a
text stream by stripping the high bit from all characters, expanding tabs
to spaces, and throwing away all control codes except carriage returns,
linefeeds, and formfeeds. Consequently, CLEAN can transform almost any
kind of word-processed document file into a plain ASCII text file.
──────────────────────────────────────────────────────────────────────────
name clean
page 55,132
title CLEAN--Text-file filter
;
; CLEAN.ASM Filter to turn document files into
; normal text files.
;
; Copyright 1988 Ray Duncan
;
; Build: C>MASM CLEAN;
; C>LINK CLEAN;
;
; Usage: C>CLEAN <infile >outfile
;
; All text characters are passed through with high
; bit stripped off. Formfeeds, carriage returns,
; and linefeeds are passed through. Tabs are expanded
; to spaces. All other control codes are discarded.
;
tab equ 09h ; ASCII tab code
lf equ 0ah ; ASCII linefeed
ff equ 0ch ; ASCII formfeed
cr equ 0dh ; ASCII carriage return
blank equ 020h ; ASCII space code
eof equ 01ah ; Ctrl-Z end-of-file
tabsiz equ 8 ; width of tab stop
bufsiz equ 128 ; size of input and
; output buffers
stdin equ 0000 ; standard input handle
stdout equ 0001 ; standard output handle
stderr equ 0002 ; standard error handle
_TEXT segment word public 'CODE'
assume cs:_TEXT,ds:_DATA,es:_DATA,ss:STACK
clean proc far ; entry point from MS-DOS
push ds ; save DS:0000 for final
xor ax,ax ; return to MS-DOS, in case
push ax ; function 4ch can't be used
mov ax,_DATA ; make data segment addressable
mov ds,ax
mov es,ax
mov ah,30h ; check version of MS-DOS
int 21h
cmp al,2 ; MS-DOS 2.0 or later?
jae clean1 ; jump if version OK
; MS-DOS 1, display error
; message and exit...
mov dx,offset msg1 ; DS:DX = message address
mov ah,9 ; function 9 = display string
int 21h ; transfer to MS-DOS
ret ; then exit the old way
clean1: call init ; initialize input buffer
clean2: call getc ; get character from input
jc clean9 ; exit if end of stream
and al,07fh ; strip off high bit
cmp al,blank ; is it a control char?
jae clean4 ; no, write it
cmp al,eof ; is it end of file?
je clean8 ; yes, write EOF and exit
cmp al,tab ; is it a tab?
je clean6 ; yes, expand it to spaces
cmp al,cr ; is it a carriage return?
je clean3 ; yes, go process it
cmp al,lf ; is it a linefeed?
je clean3 ; yes, go process it
cmp al,ff ; is it a formfeed?
jne clean2 ; no, discard it
clean3: mov column,0 ; if CR, LF, or FF,
jmp clean5 ; reset column to zero
clean4: inc column ; if non-control character,
; increment column counter
clean5: call putc ; write char to stdout
jnc clean2 ; if disk not full,
; get another character
; write failed...
mov dx,offset msg2 ; DS:DX = error message
mov cx,msg2_len ; CX = message length
mov bx,stderr ; BX = standard error handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
mov ax,4c01h ; function 4ch = terminate
; return code = 1
int 21h ; transfer to MS-DOS
clean6: mov ax,column ; tab code detected
cwd ; tabsiz - (column MOD tabsiz)
mov cx,tabsiz ; is number of spaces needed
idiv cx ; to move to next tab stop
sub cx,dx
add column,cx ; also update column counter
clean7: push cx ; save spaces counter
mov al,blank ; write an ASCII space
call putc
pop cx ; restore spaces counter
loop clean7 ; loop until tab stop
jmp clean2 ; get another character
clean8: call putc ; write EOF mark
clean9: call flush ; write last output buffer
mov ax,4c00h ; function 4ch = terminate
; return code = 0
int 21h ; transfer to MS-DOS
clean endp
getc proc near ; get character from stdin
; returns carry = 1 if
; end of input, else
; AL = char, carry = 0
mov bx,iptr ; get input buffer pointer
cmp bx,ilen ; end of buffer reached?
jne getc1 ; not yet, jump
; more data is needed...
mov bx,stdin ; BX = standard input handle
mov cx,bufsiz ; CX = length to read
mov dx,offset ibuff ; DS:DX = buffer address
mov ah,3fh ; function 3fh = read
int 21h ; transfer to MS-DOS
jc getc2 ; jump if read failed
or ax,ax ; was anything read?
jz getc2 ; jump if end of input
mov ilen,ax ; save length of data
xor bx,bx ; reset buffer pointer
getc1: mov al,[ibuff+bx] ; get character from buffer
inc bx ; bump buffer pointer
mov iptr,bx ; save updated pointer
clc ; return character in AL
ret ; and carry = 0 (clear)
getc2: stc ; end of input stream
ret ; return carry = 1 (set)
getc endp
putc proc near ; send character to stdout,
; returns carry = 1 if
; error, else carry = 0
mov bx,optr ; store character into
mov [obuff+bx],al ; output buffer
inc bx ; bump buffer pointer
cmp bx,bufsiz ; buffer full?
jne putc1 ; no, jump
mov bx,stdout ; BX = standard output handle
mov cx,bufsiz ; CX = length to write
mov dx,offset obuff ; DS:DX = buffer address
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
jc putc2 ; jump if write failed
cmp ax,cx ; was write complete?
jne putc2 ; jump if disk full
xor bx,bx ; reset buffer pointer
putc1: mov optr,bx ; save buffer pointer
clc ; write successful,
ret ; return carry = 0 (clear)
putc2: stc ; write failed or disk full,
ret ; return carry = 1 (set)
putc endp
init proc near ; initialize input buffer
mov bx,stdin ; BX = standard input handle
mov cx,bufsiz ; CX = length to read
mov dx,offset ibuff ; DS:DX = buffer address
mov ah,3fh ; function 3fh = read
int 21h ; transfer to MS-DOS
jc init1 ; jump if read failed
mov ilen,ax ; save actual bytes read
init1: ret
init endp
flush proc near ; flush output buffer
mov cx,optr ; CX = bytes to write
jcxz flush1 ; exit if buffer empty
mov dx,offset obuff ; DS:DX = buffer address
mov bx,stdout ; BX = standard output handle
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
flush1: ret
flush endp
_TEXT ends
_DATA segment word public 'DATA'
ibuff db bufsiz dup (0) ; input buffer
obuff db bufsiz dup (0) ; output buffer
iptr dw 0 ; ibuff pointer
ilen dw 0 ; bytes in ibuff
optr dw 0 ; obuff pointer
column dw 0 ; current column counter
msg1 db cr,lf
db 'clean: need MS-DOS version 2 or greater.'
db cr,lf,'$'
msg2 db cr,lf
db 'clean: disk is full.'
db cr,lf
msg2_len equ $-msg2
_DATA ends
STACK segment para stack 'STACK'
dw 64 dup (?)
STACK ends
end clean
──────────────────────────────────────────────────────────────────────────
Figure 15-3. CLEAN.ASM, the source code for the MASM version of the CLEAN
filter.
──────────────────────────────────────────────────────────────────────────
/*
CLEAN.C Filter to turn document files into
normal text files.
Copyright 1988 Ray Duncan
Compile: C>CL CLEAN.C
Usage: C>CLEAN <infile >outfile
All text characters are passed through with high bit stripped
off. Formfeeds, carriage returns, and linefeeds are passed
through. Tabs are expanded to spaces. All other control codes
are discarded.
*/
#include <stdio.h>
#define TAB_WIDTH 8 /* width of a tab stop */
#define TAB '\x09' /* ASCII tab character */
#define LF '\x0A' /* ASCII linefeed */
#define FF '\x0C' /* ASCII formfeed */
#define CR '\x0D' /* ASCII carriage return */
#define BLANK '\x20' /* ASCII space code */
#define EOFMK '\x1A' /* Ctrl-Z end of file */
main(int argc, char *argv[])
{
char c; /* character from stdin */
int col = 0; /* column counter */
while((c = getchar()) != EOF) /* read input character */
{
c &= 0x07F; /* strip high bit */
switch(c) /* decode character */
{
case LF: /* if linefeed or */
case CR: /* carriage return, */
col=0; /* reset column count */
case FF: /* if formfeed, carriage */
wchar(c); /* return, or linefeed, */
break; /* pass character through */
case TAB: /* if tab, expand to spaces*/
do wchar(BLANK);
while((++col % TAB_WIDTH) != 0);
break;
default: /* discard other control */
if(c >= BLANK) /* characters, pass text */
{ /* characters through */
wchar(c);
col++; /* bump column counter */
}
break;
}
}
wchar(EOFMK); /* write end-of-file mark */
exit(0);
}
/*
Write a character to the standard output. If
write fails, display error message and terminate.
*/
wchar(char c)
{
if((putchar(c) == EOF) && (c != EOFMK))
{
fputs("clean: disk full",stderr);
exit(1);
}
}
──────────────────────────────────────────────────────────────────────────
Figure 15-4. CLEAN.C, the source code for the C version of the CLEAN
filter.
When using the CLEAN filter, you must specify the source and destination
files with redirection parameters in the command line; otherwise, CLEAN
will simply read the keyboard and write to the display. For example, to
filter the document file MYFILE.DOC and leave the result in the file
MYFILE.TXT, you would enter the following command:
C>CLEAN <MYFILE.DOC >MYFILE.TXT <Enter>
(Note that the original file, MYFILE.DOC, is unchanged.)
One valuable application of this filter is to rescue assembly-language
source files. If you accidentally edit such a source file in document
mode, the resulting file may cause the assembler to generate spurious or
confusing error messages. CLEAN lets you turn the source file back into
something the assembler can cope with, without losing the time you spent
to edit it.
Another handy application for CLEAN is to list a word-processed document
in raw form on the printer, using a command such as
C>CLEAN <MYFILE.DOC >PRN <Enter>
Contrasting the C and assembly-language versions of this filter provides
some interesting statistics. The C version contains 79 lines and compiles
to a 5889-byte .EXE file, whereas the assembly-language version contains
265 lines and builds an 1107-byte .EXE file. The size and execution-speed
advantages of implementing such tools in assembly language is obvious,
even compared with such an excellent compiler as the Microsoft C
Optimizing Compiler. However, you must balance performance considerations
against the time and expense required for programming, particularly when a
program will not be used very often.
────────────────────────────────────────────────────────────────────────────
Chapter 16 Compatibility and Portability
At the beginning of this book, we surveyed the history of MS-DOS and saw
that new versions come along nearly every year, loosely coupled to the
introduction of new models of personal computers. We then focused on each
of the mainstream issues of MS-DOS applications programming: the user
interface; mass storage; memory management; control of "child" processes;
and special classes of programs, such as filters, interrupt handlers, and
device drivers.
It's now time to close the circle and consider two global concerns of
MS-DOS programming: compatibility and portability. For your programs to
remain useful in a constantly evolving software and hardware environment,
you must design them so that they perform reliably on any reasonable
machine configuration and exploit available system resources; in addition,
you should be able to upgrade them easily for new versions of MS-DOS, for
new machines, and, for that matter, for completely new environments such
as MS OS/2.
Degrees of Compatibility
If we look at how existing MS-DOS applications use the operating system
and hardware, we find that we can assign them to one of four categories:
■ MS-DOS─compatible applications
■ ROM BIOS─compatible applications
■ Hardware-compatible applications
■ "Ill-behaved" applications
MS-DOS─compatible applications use only the documented MS-DOS function
calls and do not call the ROM BIOS or access the hardware directly. They
use ANSI escape sequences for screen control, and their input and output
is redirectable. An MS-DOS─compatible application will run on any machine
that supports MS-DOS, regardless of the machine configuration. Because of
the relatively poor performance of MS-DOS's built-in display and serial
port drivers, few popular programs other than compilers, assemblers, and
linkers fall into this category.
ROM BIOS─compatible applications use the documented MS-DOS and ROM BIOS
function calls but do not access the hardware directly. As recently as
three years ago, this strategy might have significantly limited a
program's potential market. Today, the availability of high-quality
IBM-compatible ROM BIOSes from companies such as Phoenix has ensured the
dominance of the IBM ROM BIOS standard; virtually no machines are being
sold in which a program cannot rely as much on the ROM BIOS interface as
it might on the MS-DOS interface. However, as we noted in Chapters 6 and
7, the ROM BIOS display and serial drivers are still not adequate to the
needs of high-performance interactive applications, so the popular
programs that fall into this category are few.
Hardware-compatible applications generally use MS-DOS functions for mass
storage, memory management, and the like, and use a mix of MS-DOS and ROM
BIOS function calls and direct hardware access for their user interfaces.
The amount of hardware dependence in such programs varies widely. For
example, some programs only write characters and attributes into the video
controller's regen buffer and use the ROM BIOS to switch modes and
position the cursor; others bypass the ROM BIOS video driver altogether
and take complete control of the video adapter. As this book is written,
the vast majority of the popular MS-DOS "productivity" applications (word
processors, databases, telecommunications programs, and so on) can be
placed somewhere in this category.
"Ill-behaved" applications are those that rely on undocumented MS-DOS
function calls or data structures, interception of MS-DOS or ROM BIOS
interrupts, or direct access to mass storage devices (bypassing the MS-DOS
file system). These programs tend to be extremely sensitive to their
environment and typically must be "adjusted" in order to work with each
new MS-DOS version or PC model. Virtually all popular terminate-
and-stay-resident (TSR) utilities, network programs, and disk
repair/optimization packages are in this category.
Writing Well-Behaved MS-DOS Applications
Your choice of MS-DOS functions, ROM BIOS functions, or direct hardware
access to solve a particular problem must always be balanced against
performance needs; and, of course, the user is the final judge of a
program's usefulness and reliability. Nevertheless, you can follow some
basic guidelines, outlined below, to create well-behaved applications that
are likely to run properly under future versions of MS-DOS and under
multitasking program managers that run on top of MS-DOS, such as Microsoft
Windows.
Program structure
Design your programs as .EXE files with separate code, data, and stack
segments; shun the use of .COM files. Use the Microsoft conventions for
segment names and attributes discussed in Chapter 3. Inspect the
environment block at runtime to locate your program's overlays or data
files; don't "hard-wire" a directory location into the program.
Check host capabilities
Obtain the MS-DOS version number with Int 21H Function 30H during your
program's initialization and be sure that all of the functions your
program requires are actually available. If you find that the host MS-DOS
version is inadequate, be careful about which functions you call to
display an error message and to terminate.
Use the enhanced capabilities of MS-DOS versions 3 and 4 when your program
is running under those versions. For example, you can specify a sharing
mode when opening a file with Int 21H Function 3DH, you can create
temporary or unique files with Int 21H Functions 5AH and 5BH, and you
can obtain extended error information (including a recommended recovery
strategy) with Int 21H Function 59H. Section 2 of this book contains
version-dependency information for each MS-DOS function.
Input and output
Use the handle file functions exclusively and extend full path support
throughout your application (being sure to allow for the maximum possible
path length during user input of filenames). Use buffered I/O whenever
possible. The device drivers in MS-DOS versions 2.0 and later can handle
strings as long as 64 KB, and performance will be improved if you write
fewer, larger records as opposed to many short ones.
Avoid the use of FCBs, the Int 25H or Int 26H functions, or the ROM BIOS
disk driver. If you must use FCBs, close them when you are done with them
and don't move them around while they are open. Avoid reopening FCBs that
are already open or reclosing FCBs that have already been closed──these
seemingly harmless practices can cause problems when network software is
running.
Memory management
During your program's initialization, release any memory that is not
needed by the program. (This is especially important for .COM programs.)
If your program requires extra memory for buffers or tables, allocate that
memory dynamically when it is needed and release it as soon as it is no
longer required. Use expanded memory, when it is available, to minimize
your program's demands on conventional memory.
As a general rule, don't touch any memory that is not owned by your
program. To set or inspect interrupt vectors, use Int 21H Functions 25H
and 35H rather than editing the interrupt vector table directly. If you
alter the contents of interrupt vectors, save their original values and
restore them before the program exits.
Process management
To isolate your program from dependencies on PSP structure and relocation
information, use the EXEC function (Int 21H Function 4BH) when loading
overlays or other programs. Terminate your program with Int 21H Function
4CH, passing a zero return code if the program executes successfully and
a nonzero code if an error is encountered. Your program's parent can then
test this return code with Int 21H Function 4DH or, in a batch file, with
the IF ERRORLEVEL statement.
Exception handling
Install Ctrl-C (Int 23H) and critical-error (Int 24H) handlers so that
your program cannot be terminated unexpectedly by the user's entry of
Ctrl-C or Ctrl-Break or by a hardware I/O failure. This is particularly
important if your program uses expanded memory or installs its own
interrupt handlers.
ROM BIOS and Hardware-Compatible Applications
When you feel the need to introduce ROM BIOS or hardware dependence for
performance reasons, keep it isolated to small, well-documented procedures
that can be easily modified when the hardware changes. Use macros and
equates to hide hardware characteristics and to avoid spreading "magic
numbers" throughout your program.
Check host capabilities
If you use ROM BIOS functions in your program, you must check the machine
model at runtime to be sure that the functions your program needs are
actually available. There is a machine ID byte at F000:FFFEH whose value
is interpreted as follows:
──────────────────────────────────────────────────────────────────────────
F8H PS/2 Models 70 and 80
F9H PC Convertible
FAH PS/2 Model 30
FBH PC/XT (later models)
FCH PC/AT, PC/XT-286, PS/2 Models 50 and 60
FDH PCjr
FEH PC/XT (early models)
FFH PC "Classic"
──────────────────────────────────────────────────────────────────────────
In some cases, submodels can be identified; see Int 15H Function C0H on
page 573. Section 3 of this book contains version-dependency information
for each ROM BIOS function.
When writing your own direct video drivers, you must determine the type
and capabilities of the video adapter by a combination of Int 10H calls,
reading ports, and inspection of the ROM BIOS data area at 0040:0000H and
the memory reserved for the EGA or VGA ROM BIOS, among other things. The
techniques required are beyond the scope of this book but are well
explained in Programmer's Guide to PC and PS/2 Video Systems (Microsoft
Press, 1987).
Avoid unstable hardware
Some areas of IBM personal computer architecture have remained remarkably
stable from the original IBM PC, based on a 4.77 MHz 8088, to today's PS/2
Model 80, based on a 20 MHz 80386. IBM's track record for upward
compatibility in its video and serial communications controllers has been
excellent; in many cases, the same hardware-dependent code that was
written for the original IBM PC runs perfectly well on an IBM PS/2 Model
80. Other areas of relative hardware stability are:
■ Sound control via port 61H
■ The 8253 timer chip's channels 0 and 2 (ports 40H, 42H, and 43H)
■ The game adapter at port 201H
■ Control of the interrupt system via the 8259 PIC's mask register at
port 21H
However, direct sound generation and manipulation of the 8253 timer or
8259 PIC are quite likely to cause problems if your program is run under a
multitasking program manager such as Microsoft Windows or DesqView.
Keyboard mapping, the keyboard controller, and the floppy and fixed disk
controllers are areas of relative hardware instability. Programs that
bypass MS-DOS for keyboard or disk access are much less likely to function
properly across the different PC models and are also prone to interfere
with each other and with well-behaved applications.
OS/2 Compatibility
MS-DOS is upwardly compatible in several respects with OS/2, Microsoft's
multitasking protected-mode virtual memory operating system for 80286 and
80386 computers. The OS/2 graphical user interface (the Presentation
Manager) is nearly identical to Microsoft Windows 2.0. OS/2 versions 1.0
and 1.1 use exactly the same disk formats as MS-DOS so that files may
easily be moved between MS-DOS and OS/2 systems. Most important, OS/2
includes a module called the "DOS Compatibility Environment" or "3.x Box,"
which can run one MS-DOS application at a time alongside protected-mode
OS/2 applications.
The 3.x Box traps Int 21H function calls and remaps them into OS/2
function calls, emulating an MS-DOS 3.3 environment with the file-sharing
module (SHARE.EXE) loaded but returning a major version number of 10
instead of 3 for Int 21H Function 30H. The 3.x Box also supports most ROM
BIOS calls, either by emulating their function or by interlocking the
device and then calling the original ROM BIOS routine. In addition, the
3.x Box maintains the ROM BIOS data area, provides timer ticks to
applications via Int 1CH, and supports certain undocumented MS-DOS
services and data structures so that most TSR utilities can function
properly. Nevertheless, the 3.x Box's emulation of MS-DOS is not perfect,
and you must be aware of certain constraints on MS-DOS applications
running under OS/2.
The most significant restriction on an MS-DOS application is that it does
not receive any CPU cycles when it is in the background. That is, when a
protected-mode application has been "selected," so that the user can
interact with it, the MS-DOS application is frozen. If the MS-DOS
application has captured any interrupt vectors (such as the serial port or
timer tick), these interrupts will not be serviced until the application
is again selected and in the foreground. OS/2 must freeze MS-DOS
applications when they are in the background because they execute in real
mode and are thus not subject to hardware memory protection; nothing else
ensures that they will not interfere with a protected-mode process that
has control of the screen and keyboard.
Use of FCBs is restricted in the 3.x Box, as it is under MS-DOS 3 or 4
with SHARE.EXE loaded. A file cannot be opened with an FCB if any other
process is using it. The number of FCBs that can be simultaneously opened
is limited to 16 or to the number specified in a CONFIG.SYS FCBS=
directive. Even when the handle file functions are used, these functions
may fail unexpectedly due to the activity of other processes (for example,
if a protected-mode process has already opened the file with "deny all"
sharing mode); most MS-DOS applications are not written with file sharing
in mind, and they do not handle such errors gracefully.
Direct writes to a fixed disk using Int 26H or Int 13H are not allowed.
This prevents the file system from being corrupted, because protected-mode
applications running concurrently with the MS-DOS application may also be
writing to the same disk. Imagine the mess if a typical MS-DOS unerase
utility were to alter the root directory and FAT at the same time that a
protected-mode database program was updating its file and indexes!
MS-DOS applications that attempt to reprogram the 8259 to move the
interrupt vector table or that modify interrupt vectors already belonging
to an OS/2 device driver are terminated by the operating system. MS-DOS
applications can change the 8259's interrupt-mask register, disable and
reenable interrupts at their discretion, and read or write any I/O port.
The obvious corollary is that an MS-DOS program running in the 3.x Box can
crash the entire OS/2 system at any time; this is the price for allowing
real-mode applications to run at all.
Porting MS-DOS Applications to OS/2
The application program interface (API) provided by OS/2 to protected-mode
programs is quite different from the familiar Int 21H interface of MS-DOS
and the OS/2 3.x Box. However, the OS/2 API is functionally a proper
superset of MS-DOS. This makes it easy to convert well-behaved MS-DOS
applications to run in OS/2 protected mode, whence they can be enhanced to
take advantage of OS/2's virtual memory, multitasking, and interprocess
communication capabilities.
To give you a feeling for both the nature of the OS/2 API and the
practices that should be avoided in MS-DOS programming if portability to
OS/2 is desired, I will outline my own strategy for converting existing
MS-DOS assembly-language programs to OS/2. For the purposes of discussion,
I have divided the conversion process into five steps and have assigned
each an easily remembered buzzword:
1. Segmentation
2. Rationalization
3. Encapsulation
4. Conversion
5. Optimization
The first three stages can (and should) be performed and tested in the
MS-DOS environment; only the last two require OS/2 and the protected-mode
programming tools. As you read on, you may notice that an MS-DOS program
that follows the compatibility guidelines presented earlier in this
chapter requires relatively little work to make it run in protected mode.
This is the natural benefit of working with the operating system instead
of against it.
Segmentation
Most of the 80286's protected-mode capabilities revolve around a change in
the way memory is addressed. In real mode, the 80286 essentially emulates
an 8088/86 processor, and the value in a segment register corresponds
directly to a physical memory address. MS-DOS runs on the 80286 in real
mode.
When an 80286 is running in protected mode, as it does under OS/2, an
additional level of indirection is added to memory addressing.☼ A segment
register holds a selector, which is an index to a table of descriptors. A
descriptor defines the physical address and length of a memory segment,
its characteristics (executable, read-only data, or read/write data) and
access rights, and whether the segment is currently resident in RAM or has
been swapped out to disk. Each time a program loads a segment register or
accesses memory, the 80286 hardware checks the associated descriptor and
the program's privilege level, generating a fault if the selector or
memory operation is not valid. The fault acts like a hardware interrupt,
allowing the operating system to regain control and take the appropriate
action.
This scheme of memory addressing in protected mode has two immediate
consequences for application programs. The first is that application
programs can no longer perform arithmetic on the contents of segment
registers (because selectors are magic numbers and have no direct
relationship to physical memory addresses) or use segment registers for
storage of temporary values. A program must not load a segment register
with anything but a legitimate selector provided by the OS/2 loader or
resulting from an OS/2 memory allocation function call. The second
consequence is that a program must strictly segregate machine code
("text") from data, placing them in separate segments with distinct
selectors (because a selector that is executable is not writable, and vice
versa).
Accordingly, the first step in converting a program for OS/2 is to turn it
into a .EXE-type program that uses the Microsoft segment, class, and group
conventions described in Chapter 3. At minimum, the program must have one
code segment and one data segment, and should declare a group──with the
special name DGROUP──that contains the "near" data segment, stack, and
local heap (if any). At the same time, you should remove or rewrite any
code that performs direct manipulation of segment values.
After restructuring and segmentation, reassemble and link your program and
check to be sure it still works as expected under MS-DOS. Changing or
adding segmentation often uncovers hidden addressing assumptions in the
code, so it is best to track these problems down before making other
substantive changes to the program.
Rationalization
Once you've successfully segmented your program so that it can be linked
and executed as a .EXE file under MS-DOS, the next step is to rationalize
your code. By rationalization I mean converting your program into a
completely well-behaved MS-DOS application.
First, you must ruthlessly eliminate any elements that manipulate the
peripheral device adapters directly, alter interrupt priorities, edit the
system interrupt-vector table, or depend on CPU speed or characteristics
(such as timing loops). In protected mode, control of the interrupt system
is completely reserved to the operating system and its device drivers, I/O
ports may be read or written by an application only under very specific
conditions, and timing loops burn up CPU cycles that can be used by other
processes.
As I mentioned earlier in this chapter, display routines constitute the
most common area of hardware dependence in an MS-DOS application. Direct
manipulation of the video adapter and its regen buffer poses obvious
difficulties in a multitasking, protected-memory environment such as OS/2.
For porting purposes, you must convert all routines that write text to the
display, modify character attributes, or affect cursor shape or position
into Int 21H Function 40H calls using ANSI escape sequences or into ROM
BIOS Int 10H calls. Similarly, you must convert all hardware-dependent
keyboard operations to Int 21H Function 3FH or ROM BIOS Int 16H calls.
Once all hardware dependence has been expunged from your program, your
next priority is to make it well-behaved in its use of system memory.
Under MS-DOS an application is typically handed all remaining memory in
the system to do with as it will; under OS/2 the converse is true: A
process is initially allocated only enough memory to hold its code,
declared data storage, and stack. You can make the MS-DOS loader behave
like the OS/2 loader by linking your application with the /CPARMAXALLOC
switch. Alternatively, your program can give up all extra memory during
its initialization with Int 21H Function 4AH, as recommended earlier in
this chapter.
After your program completes its initialization sequence, it should
dynamically obtain and release any additional memory it may require for
buffers and tables with MS-DOS Int 21H Functions 48H and 49H. To ensure
compatibility with protected mode, limit the size of any single allocated
block to 65,536 bytes or less, even though MS-DOS allows larger blocks to
be allocated.
Finally, you must turn your attention to file and device handling. Replace
any calls to FCB file functions with their handle-based equivalents,
because OS/2 does not support FCBs in protected mode at all. Check
pathnames for validity within the application; although MS-DOS and the 3.x
Box silently truncate a name or extension, OS/2 refuses to open or create
a file in protected mode if the name or extension is too long and returns
an error instead. Replace any use of the predefined handles for the
standard auxiliary and standard list devices with explicit opens of COM1,
PRN, LPT1, and so on, using the resulting handle for read and write
operations. OS/2 does not supply processes with standard handles for the
serial communications port or printer.
Encapsulation
When you reach this point, with a well-behaved, segmented MS-DOS
application in hand, the worst of a port to OS/2 is behind you. You are
now ready to prepare your program for true conversion to protected-mode
operation by encapsulating, in individual subroutines, every part of the
program that is specific to the host operating system. The objective here
is to localize the program's "knowledge" of the environment into small
procedures that can be subsequently modified without affecting the
remainder of the program.
As an example of encapsulation, consider a typical call by an MS-DOS
application to write a string to the standard output device (Figure
16-1). In order to facilitate conversion to OS/2, you would replace every
instance of such a write to a file or device with a call to a small
subroutine that "hides" the mechanics of the actual operating-system
function call, as illustrated in Figure 16-2.
Another candidate for encapsulation, which does not necessarily involve an
operating-system function call, is the application's code to gain access
to command-line parameters, environment-block variables, and the name of
the file it was loaded from. Under MS-DOS, this information is divided
between the program segment prefix (PSP) and the environment block, as we
saw in Chapters 3 and 12; under OS/2, there is no such thing as a PSP,
and the program filename and command-line information are appended to the
environment block.
──────────────────────────────────────────────────────────────────────────
stdin equ 0 ; standard input handle
stdout equ 1 ; standard output handle
stderr equ 2 ; standard error handle
msg db 'This is a sample message'
msg_len equ $-msg
.
.
.
mov dx,seg msg ; DS:DX = message address
mov ds,dx
mov dx,offset DGROUP:msg
mov cx,msg_len ; CX = message length
mov bx,stdout ; BX = handle
mov ah,40h ; AH = function 40h write
int 21h ; transfer to MS-DOS
jc error ; jump if error
cmp ax,msg_len ; all characters written?
jne diskfull ; no, device is full
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 16-1. Typical in-line code for an MS-DOS function call. This
particular sequence writes a string to the standard output device. Since
the standard output might be redirected to a file without the program's
knowledge, it must also check that all of the requested characters were
actually written; if the returned length is less than the requested
length, this usually indicates that the standard output has been
redirected to a disk file and that the disk is full.
──────────────────────────────────────────────────────────────────────────
stdin equ 0 ; standard input handle
stdout equ 1 ; standard output handle
stderr equ 2 ; standard error handle
msg db 'This is a sample message'
msg_len equ $-msg
.
.
.
mov dx,seg msg ; DS:DX = message address
mov ds,dx
mov dx,offset DGROUP:msg
mov cx,msg_len ; CX = message length
mov bx,stdout ; BX = handle
call write ; perform the write
jc error ; jump if error
cmp ax,msg_len ; all characters written?
jne diskfull ; no, device is full
.
.
.
write proc near ; write to file or device
; Call with:
; BX = handle
; CX = length of data
; DS:DX = address of data
; returns:
; if successful, carry clear
; and AX = bytes written
; if error, carry set
; and AX = error code
mov ah,40h ; function 40h = write
int 21h ; transfer to MS-DOS
ret ; return status in CY and AX
write endp
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 16-2. Code from Figure 16-1 after "encapsulation." The portion of
the code that is operating-system dependent has been isolated inside a
subroutine that is called from other points within the application.
When you have completed the encapsulation of system services and access to
the PSP and environment, subject your program once more to thorough
testing under MS-DOS. This is your last chance, while you are still
working in a familiar milieu and have access to your favorite debugging
tools, to detect any subtle errors you may have introduced during the
three conversion steps discussed thus far.
Conversion
Next, you must rewrite each system-dependent procedure you created during
the encapsulation stage to conform to the OS/2 protected-mode API. In
contrast to MS-DOS functions, which are actuated through software
interrupts and pass parameters in registers, OS/2 API functions are
requested through a far call to a named entry point. Parameters are passed
on the stack, along with the addresses of variables within the calling
program's data segment that will receive any results returned by the
function. The status of an operation is returned in register AX──zero if
the function succeeded, an error code otherwise. All other registers are
preserved.
Although it is not my intention here to provide a detailed introduction to
OS/2 programming, Figure 16-3 illustrates the final form of our previous
example, after conversion for OS/2. Note especially the addition of the
extrn statement, the wlen variable, and the simulation of the MS-DOS
function status. This code may not be elegant, but it serves the purpose
of limiting the necessary changes to a very small portion of the source
file. Some OS/2 functions (such as DosOpen) require parameters that have
no counterpart under MS-DOS; you can usually select reasonable values for
these extra parameters that will make their existence temporarily
invisible to the remainder of the application.
──────────────────────────────────────────────────────────────────────────
stdin equ 0 ; standard input handle
stdout equ 1 ; standard output handle
stderr equ 2 ; standard error handle
extrn DosWrite:far
msg db 'This is a sample message'
msg_len equ $-msg
wlen dw ? ; receives actual number
; of bytes written
.
.
.
mov dx,seg msg ; DS:DX = message address
mov ds,dx
mov dx,offset DGROUP:msg
mov cx,msg_len ; CX = message length
mov bx,stdout ; BX = handle
call write ; perform the write
jc error ; jump if error
cmp ax,msg_len ; all characters written?
jne diskfull ; no, device is full
.
.
.
write proc near ; write to file or device
; call with:
; BX = handle
; CX = length of data
; DS:DX = address of data
; returns:
; if successful, carry clear
; and AX = bytes written
; if error, carry set
; and AX = error code
push bx ; handle
push ds ; address of data
push dx
push cx ; length of data
push ds ; receives length written
mov ax,offset DGROUP:wlen
push ax
call DosWrite ; transfer to OS/2
or ax,ax ; did write succeed?
jnz write1 ; jump, write failed
mov ax,wlen ; no error, OR cleared CY
ret ; and AX := bytes written
write1: stc ; write error, return CY set
ret ; and AX = error number
write endp
.
.
.
──────────────────────────────────────────────────────────────────────────
Figure 16-3. Code from Figure 16-2 after "conversion." The MS-DOS
function call has been replaced with the equivalent OS/2 function call.
Since the knowledge of the operating system has been hidden inside the
subroutine by the previous encapsulation step, the surrounding program's
requests for write operations should run unchanged. Note that the OS/2
function had to be declared as an external name with the "far" attribute,
and that a variable named wlen was added to the data segment of the
application to receive the actual number of bytes written.
Figures 16-4, 16-5, and 16-6 list the OS/2 services that are equivalent
to selected MS-DOS and ROM BIOS Int 21H, Int 10H, and Int 16H calls.
MS-DOS functions related to FCBs and PSPs are not included in these tables
because OS/2 does not support either of these structures. The MS-DOS
terminate-and-stay-resident functions are also omitted. Because OS/2 is a
true multitasking system, a process doesn't need to terminate in order to
stay resident while another process is running.
╓┌─┌───────────────────┌────────────────────────────────┌────────────────────╖
MS-DOS Description OS/2 function
──────────────────────────────────────────────────────────────────────────
Int 21H Function
0 Terminate process DosExit
1 Character input with echo KbdCharIn
2 Character output VioWrtTTY
3 Auxiliary input DosRead
MS-DOS Description OS/2 function
──────────────────────────────────────────────────────────────────────────
3 Auxiliary input DosRead
4 Auxiliary output DosWrite
5 Printer output DosWrite
6 Direct console I/O KbdCharIn,
VioWrtTTY
7 Unfiltered input without echo KbdCharIn
8 Character input without echo KbdCharIn
9 Display string VioWrtTTY
0AH (10) Buffered keyboard input KbdStringIn
0BH (11) Check input status KbdPeek
0CH (12) Reset buffer and input KbdFlushBuffer,
KbdCharIn
0DH (13) Disk reset DosBufReset
0EH (14) Select disk DosSelectDisk
19H (25) Get current disk DosQCurDisk
1BH (27) Get default drive data DosQFSInfo
1CH (28) Get drive data DosQFSInfo
2AH (42) Get date DosGetDateTime
2BH (43) Set date DosSetDateTime
MS-DOS Description OS/2 function
──────────────────────────────────────────────────────────────────────────
2BH (43) Set date DosSetDateTime
2CH (44) Get time DosGetDateTime
2DH (45) Set time DosSetDateTime
2EH (46) Set verify flag DosSetVerify
30H (48) Get MS-DOS version DosGetVersion
36H (54) Get drive allocation DosQFSInfo
information
38H (56) Get or set country DosGetCtryInfo
information
39H (57) Create directory DosMkdir
3AH (58) Delete directory DosRmdir
3BH (59) Set current directory DosChdir
3CH (60) Create file DosOpen
3DH (61) Open file DosOpen
3EH (62) Close file DosClose
3FH (63) Read file or device DosRead
40H (64) Write file or device DosWrite
41H (65) Delete file DosDelete
42H (66) Set file pointer DosChgFilePtr
MS-DOS Description OS/2 function
──────────────────────────────────────────────────────────────────────────
42H (66) Set file pointer DosChgFilePtr
43H (67) Get or set file attributes DosQFileMode,
DosSetFileMode
44H (68) I/O control (IOCTL) DosDevIOCtl
45H (69) Duplicate handle DosDupHandle
46H (70) Redirect handle DosDupHandle
47H (71) Get current directory DosQCurDir
48H (72) Allocate memory block DosAllocSeg
49H (73) Release memory block DosFreeSeg
4AH (74) Resize memory block DosReAllocSeg
4BH (75) Execute program DosExecPgm
4CH (76) Terminate process with DosExit
return code
4DH (77) Get return code DosCWait
4EH (78) Find first file DosFindFirst
4FH (79) Find next file DosFindNext
54H (84) Get verify flag DosQVerify
56H (86) Rename file DosMove
57H (87) Get or set file date and time DosQFileInfo,
MS-DOS Description OS/2 function
──────────────────────────────────────────────────────────────────────────
57H (87) Get or set file date and time DosQFileInfo,
DosSetFileInfo
59H (89) Get extended error DosErrClass
information
5BH (91) Create new file DosOpen
5CH (92) Lock or unlock file region DosFileLocks
65H (101) Get extended country DosGetCtryInfo
information
66H (102) Get or set code page DosGetCp,
DosSetCp
67H (103) Set handle count DosSetMaxFH
68H (104) Commit file DosBufReset
6CH (108) Extended open file DosOpen
──────────────────────────────────────────────────────────────────────────
Figure 16-4. Table of selected MS-DOS function calls and their OS/2
counterparts. Note that OS/2 functions are typically more powerful and
flexible than the corresponding MS-DOS functions, and that this is not a
complete list of OS/2 services.
ROM BIOS Description OS/2 function
──────────────────────────────────────────────────────────────────────────
Int 10H Function
0 Select display mode VioSetMode
1 Set cursor type VioSetCurType
2 Set cursor position VioSetCurPos
3 Get cursor position VioGetCurPos
6 Initialize or scroll window up VioScrollUp
7 Initialize or scroll window down VioScrollDn
8 Read character and attribute VioReadCellStr
9 Write character and attribute VioWrtNCell
0AH (10) Write character VioWrtNChar
0EH (14) Write character in teletype mode VioWrtTTY
0FH (15) Get display mode VioGetMode
10H (16) Set palette, border color, etc. VioSetState
13H (19) Write string in teletype mode VioWrtTTY
──────────────────────────────────────────────────────────────────────────
Figure 16-5. Table of ROM BIOS Int 10H video-display driver functions
used by MS-DOS applications and their OS/2 equivalents. This is not a
complete list of OS/2 video services.
ROM BIOS Description OS/2 function
──────────────────────────────────────────────────────────────────────────
Int 16H Function
0 Read keyboard character KbdCharIn
1 Get keyboard status KbdPeek
2 Get keyboard flags KbdGetStatus
──────────────────────────────────────────────────────────────────────────
Figure 16-6. Table of ROM BIOS Int 16H keyboard driver functions used by
MS-DOS applications and their OS/2 equivalents. This is not a complete
list of OS/2 keyboard services.
Optimization
Once your program is running in protected mode, it is time to unravel some
of the changes made for purposes of conversion and to introduce various
optimizations. Three obvious categories should be considered:
1. Modifying the program's user-interface code for the more powerful OS/2
keyboard and display API functions.
2. Incorporating 80286-specific machine instructions where appropriate.
3. Revamping the application to exploit the OS/2 facilities that are
unique to protected mode. (Of course, the application benefits from
OS/2's virtual memory capabilities automatically; it can allocate
memory until physical memory and disk swapping space are exhausted.)
Modifying subroutines that encapsulate user input and output to take
advantage of the additional functionality available under OS/2 is
straight-forward, and the resulting performance improvements can be quite
dramatic. For example, the OS/2 video driver offers a variety of services
that are far superior to the screen support in MS-DOS and the ROM BIOS,
including high-speed display of strings and attributes at any screen
position, "reading back" selected areas of the display into a buffer, and
scrolling in all four directions.
The 80286-specific machine instructions can be very helpful in reducing
code size and increasing execution speed. The most useful instructions are
the shifts and rotates by an immediate count other than one, the
three-operand multiply where one of the operands is an immediate (literal)
value, and the push immediate value instruction (particularly handy for
setting up OS/2 function calls). For example, in Figure 16-3, the
sequence
mov ax,offset DGROUP:wlen
push ax
could be replaced by the single instruction
push offset DGROUP:wlen
Restructuring an application to take full advantage of OS/2's
protected-mode capabilities requires close study of both the application
and the OS/2 API, but such study can pay off with sizable benefits in
performance, ease of maintenance, and code sharing. Often, for instance,
different parts of an application are concerned with I/O devices of vastly
different speeds, such as the keyboard, disk, and video display. It both
simplifies and enhances the application to separate these elements into
subprocesses (called threads in OS/2) that execute asynchronously,
communicate through shared data structures, and synchronize with each
other, when necessary, using semaphores.
As another example, when several applications are closely related and
contain many identical or highly similar procedures, OS/2 allows you to
centralize those procedures in a dynamic link library. Routines in a
dynamic link library are bound to a program at its load time (rather than
by LINK, as in the case of traditional runtime libraries) and are shared
by all the processes that need them. This reduces the size of each
application .EXE file and allows more efficient use of memory. Best of
all, dynamic link libraries drastically simplify code maintenance; the
routines in the libraries can be debugged or improved at any time, and the
applications that use them will automatically benefit the next time they
are executed.
────────────────────────────────────────────────────────────────────────────
SECTION 2 MS-DOS FUNCTIONS REFERENCE
────────────────────────────────────────────────────────────────────────────
Notes to the Reader
This section documents the services that the MS-DOS kernel provides to
application programs via software interrupts 20H─2FH. Each MS-DOS function
is described in the same format:
■ A heading containing the function's name, software interrupt and
function number, and an icon indicating the MS-DOS version in which the
function was first supported. You can assume that the function is
available in all subsequent MS-DOS versions unless explicitly noted
otherwise.
■ A synopsis of the actions performed by the function and the
circumstances under which it would be used.
■ A summary of the function's arguments.
■ The results and/or error indicators returned by the function. A
comprehensive list of error codes can be found in the entry for Int 21H
Function 59H.
■ Notes describing special uses or dependencies of the function.
■ A skeleton example of the function's use, written in assembly language.
Version icons used in the synopsis, arguments, results, or Notes sections
refer to specific minor or major versions, unless they include a + sign to
indicate a version and all subsequent versions.
For purposes of clarity, the examples may include instructions that would
not be necessary if the code were inserted into a working program. For
example, most of the examples explicitly set the segment registers when
passing the address of a filename or buffer to MS-DOS; in real
applications, the segment registers are usually initialized once at entry
to the program and left alone thereafter.
Int 21H Function Summary by Number
╓┌─┌───────┌────────┌───────────────────────────────────────┌───────┌────────╖
Hex Dec Function name Vers F/H☼
Hex Dec Function name Vers F/H☼
──────────────────────────────────────────────────────────────────────────
00H 0 Terminate Process 1.0+
01H 1 Character Input with Echo 1.0+
02H 2 Character Output 1.0+
03H 3 Auxiliary Input 1.0+
04H 4 Auxiliary Output 1.0+
05H 5 Printer Output 1.0+
06H 6 Direct Console I/O 1.0+
07H 7 Unfiltered Character Input Without Echo 1.0+
08H 8 Character Input Without Echo 1.0+
09H 9 Display String 1.0+
0AH 10 Buffered Keyboard Input 1.0+
0BH 11 Check Input Status 1.0+
0CH 12 Flush Input Buffer and Then Input 1.0+
0DH 13 Disk Reset 1.0+
0EH 14 Select Disk 1.0+
0FH 15 Open File 1.0+ F
10H 16 Close File 1.0+ F
11H 17 Find First File 1.0+ F
12H 18 Find Next File 1.0+ F
Hex Dec Function name Vers F/H☼
──────────────────────────────────────────────────────────────────────────
12H 18 Find Next File 1.0+ F
13H 19 Delete File 1.0+ F
14H 20 Sequential Read 1.0+ F
15H 21 Sequential Write 1.0+ F
16H 22 Create File 1.0+ F
17H 23 Rename File 1.0+ F
18H 24 Reserved
19H 25 Get Current Disk 1.0+
1AH 26 Set DTA Address 1.0+
1BH 27 Get Default Drive Data 1.0+
1CH 28 Get Drive Data 2.0+
1DH 29 Reserved
1EH 30 Reserved
1FH 31 Reserved
20H 32 Reserved
21H 33 Random Read 1.0+ F
22H 34 Random Write 1.0+ F
23H 35 Get File Size 1.0+ F
24H 36 Set Relative Record Number 1.0+ F
Hex Dec Function name Vers F/H☼
──────────────────────────────────────────────────────────────────────────
24H 36 Set Relative Record Number 1.0+ F
25H 37 Set Interrupt Vector 1.0+
26H 38 Create New PSP 1.0+
27H 39 Random Block Read 1.0+ F
28H 40 Random Block Write 1.0+ F
29H 41 Parse Filename 1.0+
2AH 42 Get Date 1.0+
2BH 43 Set Date 1.0+
2CH 44 Get Time 1.0+
2DH 45 Set Time 1.0+
2EH 46 Set Verify Flag 1.0+
2FH 47 Get DTA Address 2.0+
30H 48 Get MS-DOS Version Number 2.0+
31H 49 Terminate and Stay Resident 2.0+
32H 50 Reserved
33H 51 Get or Set Break Flag, Get Boot Drive 2.0+
34H 52 Reserved
35H 53 Get Interrupt Vector 2.0+
36H 54 Get Drive Allocation Information 2.0+
Hex Dec Function name Vers F/H☼
──────────────────────────────────────────────────────────────────────────
36H 54 Get Drive Allocation Information 2.0+
37H 55 Reserved
38H 56 Get or Set Country Information 2.0+
39H 57 Create Directory 2.0+
3AH 58 Delete Directory 2.0+
3BH 59 Set Current Directory 2.0+
3CH 60 Create File 2.0+ H
3DH 61 Open File 2.0+ H
3EH 62 Close File 2.0+ H
3FH 63 Read File or Device 2.0+ H
40H 64 Write File or Device 2.0+ H
41H 65 Delete File 2.0+ H
42H 66 Set File Pointer 2.0+ H
43H 67 Get or Set File Attributes 2.0+
44H 68 IOCTL (I/O Control) 2.0+
45H 69 Duplicate Handle 2.0+
46H 70 Redirect Handle 2.0+
47H 71 Get Current Directory 2.0+
48H 72 Allocate Memory Block 2.0+
Hex Dec Function name Vers F/H☼
──────────────────────────────────────────────────────────────────────────
48H 72 Allocate Memory Block 2.0+
49H 73 Release Memory Block 2.0+
4AH 74 Resize Memory Block 2.0+
4BH 75 Execute Program (EXEC) 2.0+
4CH 76 Terminate Process with Return Code 2.0+
4DH 77 Get Return Code 2.0+
4EH 78 Find First File 2.0+ H
4FH 79 Find Next File 2.0+ H
50H 80 Reserved
51H 81 Reserved
52H 82 Reserved
53H 83 Reserved
54H 84 Get Verify Flag 2.0+
55H 85 Reserved
56H 86 Rename File 2.0+
57H 87 Get or Set File Date and Time 2.0+ H
58H 88 Get or Set Allocation Strategy 3.0+
59H 89 Get Extended Error Information 3.0+
5AH 90 Create Temporary File 3.0+ H
Hex Dec Function name Vers F/H☼
──────────────────────────────────────────────────────────────────────────
5AH 90 Create Temporary File 3.0+ H
5BH 91 Create New File 3.0+ H
5CH 92 Lock or Unlock File Region 3.0+ H
5DH 93 Reserved
5EH 94 Get Machine Name, Get or Set Printer 3.1+
Setup
5FH 95 Device Redirection 3.1+
60H 96 Reserved
61H 97 Reserved
62H 98 Get PSP Address 3.0+
63H 99 Get DBCS Lead Byte Table 2.25
only
64H 100 Reserved
65H 101 Get Extended Country Information 3.3+
66H 102 Get or Set Code Page 3.3+
67H 103 Set Handle Count 3.3+
68H 104 Commit File 3.3+ H
69H 105 Reserved
6AH 106 Reserved
Hex Dec Function name Vers F/H☼
──────────────────────────────────────────────────────────────────────────
6AH 106 Reserved
6BH 107 Reserved
6CH 108 Extended Open File 4.0+ H
──────────────────────────────────────────────────────────────────────────
Int 21H Function Summary by Category
╓┌─┌───────┌────────┌───────────────────────────────────────┌───────┌────────╖
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
Character I/O
01H 1 Character Input with Echo 1.0+
02H 2 Character Output 1.0+
03H 3 Auxiliary Input 1.0+
04H 4 Auxiliary Output 1.0+
05H 5 Printer Output 1.0+
06H 6 Direct Console I/O 1.0+
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
06H 6 Direct Console I/O 1.0+
07H 7 Unfiltered Character Input Without Echo 1.0+
08H 8 Character Input Without Echo 1.0+
09H 9 Display String 1.0+
0AH 10 Buffered Keyboard Input 1.0+
0BH 11 Check Input Status 1.0+
0CH 12 Flush Input Buffer and Then Input 1.0+
File Operations
0FH 15 Open File 1.0+ F
10H 16 Close File 1.0+ F
11H 17 Find First File 1.0+ F
12H 18 Find Next File 1.0+ F
13H 19 Delete File 1.0+ F
16H 22 Create File 1.0+ F
17H 23 Rename File 1.0+ F
23H 35 Get File Size 1.0+ F
29H 41 Parse Filename 1.0+ F
3CH 60 Create File 2.0+ H
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
3CH 60 Create File 2.0+ H
3DH 61 Open File 2.0+ H
3EH 62 Close File 2.0+ H
41H 65 Delete File 2.0+ H
43H 67 Get or Set File Attributes 2.0+
45H 69 Duplicate Handle 2.0+
46H 70 Redirect Handle 2.0+
4EH 78 Find First File 2.0+ H
4FH 79 Find Next File 2.0+ H
56H 86 Rename File 2.0+
57H 87 Get or Set File Date and Time 2.0+ H
5AH 90 Create Temporary File 3.0+ H
5BH 91 Create New File 3.0+ H
67H 103 Set Handle Count 3.3+
68H 104 Commit File 3.3+ H
6CH 108 Extended Open File 4.0+ H
Record Operations
14H 20 Sequential Read 1.0+ F
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
14H 20 Sequential Read 1.0+ F
15H 21 Sequential Write 1.0+ F
1AH 26 Set DTA Address 1.0+
21H 33 Random Read 1.0+ F
22H 34 Random Write 1.0+ F
24H 36 Set Relative Record Number 1.0+ F
27H 39 Random Block Read 1.0+ F
28H 40 Random Block Write 1.0+ F
2FH 47 Get DTA Address 2.0+
3FH 63 Read File or Device 2.0+ H
40H 64 Write File or Device 2.0+ H
42H 66 Set File Pointer 2.0+ H
5CH 92 Lock or Unlock File Region 3.0+ H
Directory Operations
39H 57 Create Directory 2.0+
3AH 58 Delete Directory 2.0+
3BH 59 Set Current Directory 2.0+
47H 71 Get Current Directory 2.0+
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
47H 71 Get Current Directory 2.0+
Disk Management
0DH 13 Disk Reset 1.0+
0EH 14 Select Disk 1.0+
19H 25 Get Current Disk 1.0+
1BH 27 Get Default Drive Data 1.0+
1CH 28 Get Drive Data 2.0+
2EH 46 Set Verify Flag 1.0+
36H 54 Get Drive Allocation Information 2.0+
54H 84 Get Verify Flag 2.0+
Process Management
00H 0 Terminate Process 1.0+
26H 38 Create New PSP 1.0+
31H 49 Terminate and Stay Resident 2.0+
4BH 75 Execute Program (EXEC) 2.0+
4CH 76 Terminate Process with Return Code 2.0+
4DH 77 Get Return Code 2.0+
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
4DH 77 Get Return Code 2.0+
62H 98 Get PSP Address 3.0+
Memory Management
48H 72 Allocate Memory Block 2.0+
49H 73 Release Memory Block 2.0+
4AH 74 Resize Memory Block 2.0+
58H 88 Get or Set Allocation Strategy 3.0+
Network Functions
5EH 94 Get Machine Name, Get or Set Printer 3.1+
Setup
5FH 95 Device Redirection 3.1+
Time and Date
2AH 42 Get Date 1.0+
2BH 43 Set Date 1.0+
2CH 44 Get Time 1.0+
2DH 45 Set Time 1.0+
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
2DH 45 Set Time 1.0+
Miscellaneous System Functions
25H 37 Set Interrupt Vector 1.0+
30H 48 Get MS-DOS Version Number 2.0+
33H 51 Get or Set Break Flag, Get Boot Drive 2.0+
35H 53 Get Interrupt Vector 2.0+
38H 56 Get or Set Country Information 2.0+
44H 68 IOCTL (I/O Control) 2.0+
59H 89 Get Extended Error Information 3.0+
63H 99 Get Lead Byte Table 2.25
only
65H 101 Get Extended Country Information 3.3+
66H 102 Get or Set Code Page 3.3+
Reserved Functions
18H 24 Reserved
1DH 29 Reserved
1EH 30 Reserved
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
1EH 30 Reserved
1FH 31 Reserved
20H 32 Reserved
32H 50 Reserved
34H 52 Reserved
37H 55 Reserved
50H 80 Reserved
51H 81 Reserved
52H 82 Reserved
53H 83 Reserved
55H 85 Reserved
5DH 93 Reserved
60H 96 Reserved
61H 97 Reserved
64H 100 Reserved
69H 105 Reserved
6AH 106 Reserved
6BH 107 Reserved
──────────────────────────────────────────────────────────────────────────
Hex Dec Function name Vers F/H
──────────────────────────────────────────────────────────────────────────
──────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 20H [1.0]
Terminate process
────────────────────────────────────────────────────────────────────────────
Terminates the current process. This is one of several methods that a
program can use to perform a final exit. MS-DOS then takes the following
actions:
■ All memory belonging to the process is released.
■ File buffers are flushed and any open handles for files or devices owned
by the process are closed.
■ The termination handler vector (Int 22H) is restored from PSP:000AH.
■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH.
■ [2.0+] The critical-error handler vector (Int 24H) is restored from
PSP:0012H.
■ Control is transferred to the termination handler.
If the program is returning to COMMAND.COM, control transfers to the
resident portion, and the transient portion is reloaded if necessary. If a
batch file is in progress, the next line of the file is fetched and
interpreted; otherwise, a prompt is issued for the next user command.
Call with:
CS = segment address of program segment prefix
Returns:
Nothing
Notes:
■ Any files that have been written to using FCBs should be closed before
performing this exit call; otherwise, data may be lost.
■ Other methods of performing a final exit are:
∙ Int 21H Function 00H
∙ Int 21H Function 31H
∙ Int 21H Function 4CH
∙ Int 27H
■ [2.0+] Int 21H Functions 31H and 4CH are the preferred methods for
termination, since they allow a return code to be passed to the parent
process.
■ [3.0+] If the program is running on a network, it should remove all
locks it has placed on file regions before terminating.
Example:
Terminate the current program, returning control to the program's parent.
.
.
.
int 20h ; transfer to MS-DOS
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 00H
Terminate process
────────────────────────────────────────────────────────────────────────────
Terminates the current process. This is one of several methods that a
program can use to perform a final exit. MS-DOS then takes the following
actions:
■ All memory belonging to the process is released.
■ File buffers are flushed and any open handles for files or devices owned
by the process are closed.
■ The termination handler vector (Int 22H) is restored from PSP:000AH.
■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH.
■ [2.0+] The critical-error handler vector (Int 24H) is restored from
PSP:0012H.
■ Control is transferred to the termination handler.
If the program is returning to COMMAND.COM, control transfers to the
resident portion, and the transient portion is reloaded if necessary. If a
batch file is in progress, the next line of the file is fetched and
interpreted; otherwise, a prompt is issued for the next user command.
Call with:
AH = 00H
CS = segment address of program segment prefix
Returns:
Nothing
Notes:
■ Any files that have been written to using FCBs should be closed before
performing this exit call; otherwise, data may be lost.
■ Other methods of performing a final exit are:
∙ Int 20H
∙ Int 21H Function 31H
∙ Int 21H Function 4CH<21H4CH>
∙ Int 27H
■ [2.0+] Int 21H Functions 31H and 4CH are the preferred methods for
termination, since they allow a return code to be passed to the parent
process.
■ [3.0+] If the program is running on a network, it should remove all
locks it has placed on file regions before terminating.
Example:
Terminate the current program, returning control to the program's parent.
.
.
.
mov ah,0 ; function number
int 21h ; transfer to MS-DOS
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 01H
Character input with echo
────────────────────────────────────────────────────────────────────────────
[1] Inputs a character from the keyboard, then echoes it to the display.
If no character is ready, waits until one is available.
[2.0+] Reads a character from the standard input device and echoes it to
the standard output device. If no character is ready, waits until one is
available. Input can be redirected. (If input has been redirected, there
is no way to detect EOF.)
Call with:
AH = 01H
Returns:
AL = 8-bit input data
Notes:
■ If the standard input is not redirected, and the character read is a
Ctrl-C, an Int 23H is executed. If the standard input is redirected, a
Ctrl-C is detected at the console, and BREAK is ON, an Int 23H is
executed.
■ To read extended ASCII codes (such as the special function keys F1 to
F10) on the IBM PC and compatibles, you must call this function twice.
The first call returns the value 00H to signal the presence of an
extended code.
■ See also Int 21H Functions 06H, 07H, and 08H, which provide character
input with various combinations of echo and/or Ctrl-C sensing.
■ [2.0+] You can also read the keyboard by issuing a read (Int 21H
Function 3FH) using the predefined handle for the standard input
(0000H), if input has not been redirected, or a handle obtained by
opening the logical device CON.
Example:
Read one character from the keyboard into register AL, echo it to the
display, and store it in the variable char.
char db 0 ; input character
.
.
.
mov ah,1 ; function number
int 21h ; transfer to MS-DOS
mov char,al ; save character
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 02H
Character output
────────────────────────────────────────────────────────────────────────────
[1] Outputs a character to the currently active video display.
[2.0+] Outputs a character to the standard output device. Output can be
redirected. (If output is redirected, there is no way to detect disk
full.)
Call with:
AH = 02H
DL = 8-bit data for output
Returns:
Nothing
Notes:
■ If a Ctrl-C is detected at the keyboard after the requested character is
output, an Int 23H is executed.
■ If the standard output has not been redirected, a backspace code (08H)
causes the cursor to move left one position. If output has been
redirected, the backspace code does not receive any special treatment.
■ [2.0+] You can also send strings to the display by performing a write
(Int 21H Function 40H) using the predefined handle for the standard
output (0001H), if output has not been redirected, or a handle obtained
by opening the logical device CON.
Example:
Send the character "*" to the standard output device.
.
.
.
mov ah,2 ; function number
mov dl,'*' ; character to output
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 03H
Auxiliary input
────────────────────────────────────────────────────────────────────────────
[1] Reads a character from the first serial port.
[2.0+] Reads a character from the standard auxiliary device. The default
is the first serial port (COM1).
Call with:
AH = 03H
Returns:
AL = 8-bit input data
Notes:
■ In most MS-DOS systems, the serial device is unbuffered and is not
interrupt-driven. If the auxiliary device sends data faster than your
program can process it, characters may be lost.
■ At startup on the IBM PC, PC-DOS initializes the first serial port to
2400 baud, no parity, 1 stop bit, and 8 data bits. Other implementations
of MS-DOS may initialize the serial device differently.
■ There is no way for a user program to read the status of the auxiliary
device or to detect I/O errors (such as lost characters) through this
function call. On the IBM PC, more precise control can be obtained by
calling ROM BIOS Int 14H or by driving the communications controller
directly.
■ If a Ctrl-C is detected at the keyboard, an Int 23H is executed.
■ [2.0+] You can also input from the auxiliary device by requesting a read
(Int 21H Function 3FH) using the predefined handle for the standard
auxiliary device (0003H) or using a handle obtained by opening the
logical device AUX.
Example:
Read a character from the standard auxiliary input and store it in the
variable char.
char db 0 ; input character
.
.
.
mov ah,3 ; function number
int 21h ; transfer to MS-DOS
mov char,al ; save character
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 04H
Auxiliary output
────────────────────────────────────────────────────────────────────────────
[1] Outputs a character to the first serial port.
[2.0+] Outputs a character to the standard auxiliary device. The default
is the first serial port (COM1).
Call with:
AH = 04H
DL = 8-bit data for output
Returns:
Nothing
Notes:
■ If the output device is busy, this function waits until the device is
ready to accept a character.
■ There is no way to poll the status of the auxiliary device using this
function. On the IBM PC, more precise control can be obtained by calling
ROM BIOS Int 14H or by driving the communications controller directly.
■ If a Ctrl-C is detected at the keyboard, an Int 23H is executed.
■ [2.0+] You can also send strings to the auxiliary device by performing a
write (Int 21H Function 40H) using the predefined handle for the
standard auxiliary device (0003H) or using a handle obtained by opening
the logical device AUX.
Example:
Output a "*'' character to the auxiliary device.
.
.
.
mov ah,4 ; function number
mov dl,'*' ; character to output
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 05H
Printer output
────────────────────────────────────────────────────────────────────────────
[1] Sends a character to the first list device (PRN or LPT1).
[2.0+] Sends a character to the standard list device. The default device
is the printer on the first parallel port (LPT1), unless explicitly
redirected by the user with the MODE command.
Call with:
AH = 05H
DL = 8-bit data for output
Returns:
Nothing
Notes:
■ If the printer is busy, this function waits until the printer is ready
to accept the character.
■ There is no standardized way to poll the status of the printer under
MS-DOS.
■ If a Ctrl-C is detected at the keyboard, an Int 23H is executed.
■ [2.0+] You can also send strings to the printer by performing a write
(Int 21H Function 40H) using the predefined handle for the standard
printer device (0004H) or using a handle obtained by opening the logical
device PRN or LPT1.
Example:
Output the character "*'' to the list device.
.
.
.
mov ah,5 ; function number
mov dl,'*' ; character to output
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 06H
Direct console I/O
────────────────────────────────────────────────────────────────────────────
Used by programs that need to read and write all possible characters and
control codes without any interference from the operating system.
[1] Reads a character from the keyboard or writes a character to the
display.
[2.0+] Reads a character from the standard input device or writes a
character to the standard output device. I/O may be redirected. (If I/O
has been redirected, there is no way to detect EOF or disk full.)
Call with:
AH = 06H
DL = function requested
00H─FEH if output request
0FFH if input request
Returns:
If called with DL = 00H─0FEH
Nothing
If called with DL = FFH and a character is ready
Zero flag = clear
AL = 8-bit input data
If called with DL = FFH and no character is ready
Zero flag = set
Notes:
■ No special action is taken upon entry of a Ctrl-C when this service is
used.
■ To read extended ASCII codes (such as the special function keys F1 to
F10) on the IBM PC and compatibles, you must call this function twice.
The first call returns the value 00H to signal the presence of an
extended code.
■ See also Int 21H Functions 01H, 07H, and 08H, which provide character
input with various combinations of echo and/or Ctrl-C sensing, and
Functions 02H and 09H, which may be used to write characters to the
standard output.
■ [2.0+] You can also read the keyboard by issuing a read (Int 21H
Function 3FH) using the predefined handle for the standard input
(0000H), if input has not been redirected, or a handle obtained by
opening the logical device CON.
■ [2.0+] You can also send characters to the display by issuing a write
(Int 21H Function 40H) using the predefined handle for the standard
output (0001H), if output has not been redirected, or a handle obtained
by opening the logical device CON.
Examples:
Send the character "*" to the standard output device.
.
.
.
mov ah,6 ; function number
mov dl,'*' ; character to output
int 21h ; transfer to MS-DOS
.
.
.
Read a character from the standard input device and save it in the
variable char. If no character is ready, wait until one is available.
char db 0 ; input character
.
.
.
wait: mov ah,6 ; function number
mov dl,0ffh ; parameter for read
int 21h ; transfer to MS-DOS
jz wait ; wait until char ready
mov char,al ; save the character
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 07H
Unfiltered character input without echo
────────────────────────────────────────────────────────────────────────────
[1] Reads a character from the keyboard without echoing it to the display.
If no character is ready, waits until one is available.
[2.0+] Reads a character from the standard input device without echoing it
to the standard output device. If no character is ready, waits until one
is available. Input may be redirected. (If input has been redirected,
there is no way to detect EOF.)
Call with:
AH = 07H
Returns:
AL = 8-bit input data
Notes:
■ No special action is taken upon entry of a Ctrl-C when this function is
used. If Ctrl-C checking is required, use Int 21H Function 08H instead.
■ To read extended ASCII codes (such as the special function keys F1 to
F10) on the IBM PC and compatibles, you must call this function twice.
The first call returns the value 00H to signal the presence of an
extended code.
■ See also Int 21H Functions 01H, 06H, and 08H, which provide character
input with various combinations of echo and/or Ctrl-C sensing.
■ [2.0+] You can also read the keyboard by issuing a read (Int 21H
Function 3FH) using the predefined handle for the standard input
(0000H), if input has not been redirected, or a handle obtained by
opening the logical device CON.
Example:
Read a character from the standard input without echoing it to the
display, and store it in the variable char.
char db 0 ; input character
.
.
.
mov ah,7 ; function number
int 21h ; transfer to MS-DOS
mov char,al ; save character
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 08H
Character input without echo
────────────────────────────────────────────────────────────────────────────
[1] Reads a character from the keyboard without echoing it to the display.
If no character is ready, waits until one is available.
[2.0+] Reads a character from the standard input device without echoing it
to the standard output device. If no character is ready, waits until one
is available. Input may be redirected. (If input has been redirected,
there is no way to detect EOF.)
Call with:
AH = 08H
Returns:
AL = 8-bit input data
Notes:
■ If the standard input is not redirected, and the character read is a
Ctrl-C, an Int 23H is executed. If the standard input is redirected, a
Ctrl-C is detected at the console, and BREAK is ON, an Int 23H is
executed. To avoid possible interruption by a Ctrl-C, use Int 21H
Function 07H instead.
■ To read extended ASCII codes (such as the special function keys F1 to
F10) on the IBM PC and compatibles, you must call this function twice.
The first call returns the value 00H to signal the presence of an
extended code.
■ See also Int 21H Functions 01H, 06H, and 07H, which provide character
input with various combinations of echo and/or Ctrl-C sensing.
■ [2.0+] You can also read the keyboard by issuing a read (Int 21H
Function 3FH) using the predefined handle for the standard input
(0000H), if input has not been redirected, or a handle obtained by
opening the logical device CON.
Example:
Read a character from the standard input without echoing it to the
display, allowing possible detection of Ctrl-C, and store the character in
the variable char.
char db 0
.
.
.
mov ah,8 ; function number
int 21h ; transfer to MS-DOS
mov char,al ; save character
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 09H
Display string
────────────────────────────────────────────────────────────────────────────
[1] Sends a string of characters to the display.
[2.0+] Sends a string of characters to the standard output device. Output
may be redirected. (If output has been redirected, there is no way to
detect disk full.)
Call with:
AH = 09H
DS:DX = segment:offset of string
Returns:
Nothing
Notes:
■ The string must be terminated with the character $ (24H), which is not
transmitted. Any other ASCII codes, including control codes, can be
embedded in the string.
■ See Int 21H Functions 02H and 06H for single-character output to the
video display or standard output device.
■ If a Ctrl-C is detected at the keyboard, an Int 23H is executed.
■ [2.0+] You can also send strings to the display by performing a write
(Int 21H Function 40H) using the predefined handle for the standard
output (0001H), if it has not been redirected, or a handle obtained by
opening the logical device CON.
Example:
Send the string Hello World, followed by a carriage return and line feed,
to the standard output device.
cr equ 0dh
lf equ 0ah
msg db 'Hello World',cr,lf,'$'
.
.
.
mov ah,9 ; function number
mov dx,seg msg ; address of string
mov ds,dx
mov dx,offset msg
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 0AH (10)
Buffered keyboard input
────────────────────────────────────────────────────────────────────────────
[1] Reads a line from the keyboard and places it in a user-designated
buffer. The characters are echoed to the display.
[2.0+] Reads a string of bytes from the standard input device, up to and
including an ASCII carriage return (0DH), and places them in a
user-designated buffer. The characters are echoed to the standard output
device. Input may be redirected. (If input has been redirected, there is
no way to detect EOF.)
Call with:
AH = 0AH
DS:DX = segment:offset of buffer
Returns:
Nothing (data placed in buffer)
Notes:
■ The buffer used by this function has the following format:
Byte Contents
────────────────────────────────────────────────────────────────────────
0 maximum number of characters to read, set by program
1 number of characters actually read (excluding carriage
return), set
by MS-DOS
2+ string read from keyboard or standard input, terminated by
a carriage return (0DH)
────────────────────────────────────────────────────────────────────────
■ If the buffer fills to one fewer than the maximum number of characters
it can hold, subsequent input is ignored and the bell is sounded until a
carriage return is detected.
■ This input function is buffered with type-ahead capability, and all of
the standard keyboard editing commands are active.
■ If the standard input is not redirected, and a Ctrl-C is detected at the
console, an Int 23H is executed. If the standard input is redirected, a
Ctrl-C is detected at the console, and BREAK is ON, an Int 23H is
executed.
■ See Int 21H Functions 01H, 06H, 07H, and 08H for single-character input
from the keyboard or standard input device.
■ [2.0+] You can also read strings from the keyboard by performing a read
(Int 21H Function 3FH) using the predefined handle for the standard
input (0000H), if it has not been redirected, or a handle obtained by
opening the logical device CON.
Example:
Read a string that is a maximum of 80 characters long from the standard
input device, placing it in the buffer named buff.
buff db 81 ; maximum length of input
db 0 ; actual length of input
db 81 dup (0) ; actual input placed here
.
.
.
mov ah,0ah ; function number
mov dx,seg buff ; input buffer address
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 0BH (11)
Check input status
────────────────────────────────────────────────────────────────────────────
[1] Checks whether a character is available from the keyboard.
[2.0+] Checks whether a character is available from the standard input
device. Input can be redirected.
Call with:
AH = 0BH
Returns:
AL = 00H if no character is available
FFH if at least one character is available
Notes:
■ [1] If a Ctrl-C is detected, an Int 23H is executed.
■ [2.0+] If the standard input is not redirected, and a Ctrl-C is detected
at the console, an Int 23H is executed. If the standard input is
redirected, a Ctrl-C is detected at the console, and BREAK is ON, an Int
23H is executed.
■ If a character is waiting, this function will continue to return a true
flag until the character is consumed with a call to Int 21H Function
01H, 06H, 07H, 08H, 0AH, or 3FH.
■ This function is equivalent to IOCTL Int 21H Function 44H Subfunction
06H.
Example:
Test whether a character is available from the standard input.
.
.
.
mov ah,0bh ; function number
int 21h ; transfer to MS-DOS
or al,al ; character waiting?
jnz ready ; jump if char ready
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 0CH (12)
Flush input buffer and then input
────────────────────────────────────────────────────────────────────────────
[1] Clears the type-ahead buffer and then invokes one of the keyboard
input functions.
[2.0+] Clears the standard input buffer and then invokes one of the
character input functions. Input can be redirected.
Call with:
AH = 0CH
AL = number of input function to be invoked after resetting
buffer (must be 01H, 06H, 07H, 08H, or 0AH)
(if AL = 0AH)
DS:DX = segment:offset of input buffer
Returns:
(if called with AL = 01H, 06H, 07H, or 08H)
AL = 8-bit input data
(if called with AL = 0AH)
Nothing (data placed in buffer)
Notes:
■ The function exists to allow a program to defeat MS-DOS's type-ahead
feature. It discards any characters that are waiting in MS-DOS's
internal type-ahead buffer, forcing the specified input function to wait
for a character (usually a keyboard entry) that is truly entered after
the program's request.
■ The presence or absence of Ctrl-C checking during execution of this
function depends on the function number placed in register AL.
■ A function number in AL other than 01H, 06H, 07H, 08H, or 0AH simply
flushes the input buffer and returns control to the calling program.
Example:
Clear the type-ahead buffer, then wait for a character to be entered,
echoing it and then returning it in AL. Store the character in the
variable char.
char db 0
.
.
.
mov ah,0ch ; function number
mov al,1 ; subfunction = input char
int 21h ; transfer to MS-DOS
mov char,al ; save character
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 0DH (13)
Disk reset
────────────────────────────────────────────────────────────────────────────
Flushes all file buffers. All data that has been logically written by user
programs, but has been temporarily buffered within MS-DOS, is physically
written to the disk.
Call with:
AH = 0DH
Returns:
Nothing
Notes:
■ This function does not update the disk directory for any files that are
still open. If your program fails to properly close all files before the
disk is removed, and files have changed size, the data forced out to the
disk by this function may still be inaccessible because the directory
entries will not be correct.
■ [3.3+] Int 21H Function 68H (Commit File) should be used in preference
to this function, since it also updates the disk directory.
Example:
Flush all MS-DOS internal disk buffers.
.
.
.
mov ah,0dh ; function number
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 0EH (14)
Select disk
────────────────────────────────────────────────────────────────────────────
Selects the specified drive to be the current, or default, disk drive and
returns the total number of logical drives in the system.
Call with:
AH = 0EH
DL = drive code (0 = A, 1 = B, etc.)
Returns:
AL = number of logical drives in system
Notes:
■ [1] 16 drive designators (0 through 0FH) are available.
■ [2] 63 drive designators (0 through 3FH) are available.
■ [3.0+] 26 drive designators (0 through 19H) are available.
■ To preserve upward compatibility, new applications should limit
themselves to the drive letters A─Z (0 = A, 1 = B, etc.).
■ Logical drives means the total number of block devices: floppy disks,
simulated disk drives (RAMdisks), and hard-disk drives. A single
physical hard-disk drive is frequently partitioned into two or more
logical drives.
■ [1] [2] In single-drive IBM PC─compatible systems, the value 2 is
returned in AL, because PC-DOS supports two logical drives (A: and B:)
on the single physical floppy-disk drive. The actual number of physical
drives in the system can be determined with ROM BIOS Int 11H.
■ [3.0+] The value returned in AL is either 5 or the drive code
corresponding to the LASTDRIVE entry (if any) in CONFIG.SYS, whichever
is greater.
Example:
Make drive B the current (default) disk drive. Save the total number of
logical drives in the system in the variable drives.
drives db 0
.
.
.
mov ah,0eh ; function number
mov dl,1 ; drive 1 = B
int 21h ; transfer to MS-DOS
mov drives,al ; save total drives
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 0FH (15)
Open file
────────────────────────────────────────────────────────────────────────────
Opens a file and makes it available for subsequent read/write operations.
Call with:
AH = 0FH
DS:DX = segment:offset of file control block
Returns:
If function successful (file found)
AL = 00H
and FCB filled in by MS-DOS as follows:
drive field (offset 00H) = 1 for drive A, 2 for drive B, etc.
current block field (offset 0CH) = 00H
record size field (offset 0EH) = 0080H
[2.0+] size field (offset 10H) = file size from directory
[2.0+] date field (offset 14H) = date stamp from directory
[2.0+] time field (offset 16H) = time stamp from directory
If function unsuccessful (file not found)
AL = 0FFH
Notes:
■ If your program is going to use a record size other than 128 bytes, it
should set the record-size field at FCB offset 0EH after the file is
successfully opened and before any other disk operation.
■ If random access is to be performed, the calling program must also set
the FCB relative-record field (offset 21H) after successfully opening
the file.
■ For format of directory time and date, see Int 21H Function 57H.
■ [2.0+] Int 21H Function 3DH, which allows full access to the
hierarchical directory structure, should be used in preference to this
function.
■ [3.0+] If the program is running on a network, the file is opened for
read/write access in compatibility sharing mode.
Example:
Attempt to open the file named QUACK.DAT on the default disk drive.
myfcb db 0 ; drive = default
db 'QUACK ' ; filename, 8 characters
db 'DAT' ; extension, 3 characters
db 25 dup (0) ; remainder of FCB
.
.
.
mov ah,0fh ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if open failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 10H (16)
Close file
────────────────────────────────────────────────────────────────────────────
Closes a file, flushes all MS-DOS internal disk buffers associated with
the file to disk, and updates the disk directory if the file has been
modified or extended.
Call with:
AH = 10H
DS:DX = segment:offset of file control block
Returns:
If function successful (directory update successful)
AL = 00H
If function unsuccessful (file not found in directory)
AL = FFH
Notes:
■ [1] [2] MS-DOS versions 1 and 2 do not reliably detect a floppy-disk
change, and an error can occur if the user changes disks while a file is
still open on that drive. In the worst case, the directory and file
allocation table of the newly inserted disk can be damaged or destroyed.
■ [2.0+] Int 21H Function 3EH should be used in preference to this
function.
Example:
Close the file that was previously opened using the file control block
named myfcb.
myfcb db 0 ; drive = default
db 'QUACK ' ; filename, 8 characters
db 'DAT' ; extension, 3 characters
db 25 dup (0) ; remainder of FCB
.
.
.
mov ah,10h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if close failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 11H (17)
Find first file
────────────────────────────────────────────────────────────────────────────
Searches the current directory on the designated drive for a matching
filename.
Call with:
AH = 11H
DS:DX = segment:offset of file control block
Returns:
If function successful (matching filename found)
AL = 00H
and buffer at current disk transfer area (DTA) address filled in as an
unopened normal FCB or extended FCB, depending on which type of FCB was
input to function
If function unsuccessful (no matching filename found)
AL = FFH
Notes:
■ Use Int 21H Function 1AH to set the DTA to point to a buffer of
adequate size before calling this function.
■ The wildcard character ? is allowed in the filename in all versions of
MS-DOS. In versions 3.0 and later, the wildcard character * may also be
used in a filename. If ? or * is used, this function returns the first
matching filename.
■ An extended FCB must be used to search for files that have the system,
hidden, read-only, directory, or volume-label attributes.
■ If an extended FCB is used, its attribute byte determines the type of
search that will be performed. If the attribute byte contains 00H, only
ordinary files are found. If the volume-label attribute bit is set, only
volume labels will be returned (if any are present). If any other
attribute or combination of attributes is set (such as hidden, system,
or read-only), those files and all ordinary files will be matched.
■ [2.0+] Int 21H Function 4EH, which allows full access to the
hierarchical directory structure, should be used in preference to this
function.
Example:
Search for the first file with the extension .COM in the current
directory.
buff db 37 dup (0) ; receives search result
myfcb db 0 ; drive = default
db '????????' ; wildcard filename
db 'COM' ; extension = COM
db 25 dup (0) ; remainder of FCB
.
.
.
; set DTA address
mov ah,1ah ; function number
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
; search for first match
mov ah,11h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if no match
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 12H (18)
Find next file
────────────────────────────────────────────────────────────────────────────
Given that a previous call to Int 21H Function 11H has been successful,
returns the next matching filename (if any).
Call with:
AH = 12H
DS:DX = segment:offset of file control block
Returns:
If function successful (matching filename found)
AL = 00H
and buffer at current disk transfer area (DTA) address set up as an
unopened normal FCB or extended FCB, depending on which type of FCB was
originally input to Int 21H Function 11H
If function unsuccessful (no more matching filenames found)
AL = FFH
Notes:
■ This function assumes that the FCB used as input has been properly
initialized by a previous call to Int 21H Function 11H (and possible
subsequent calls to Int 21H Function 12H) and that the filename or
extension being searched for contained at least one wildcard character.
■ As with Int 21H Function 11H, it is important to use Int 21H Function
1AH to set the DTA to a buffer of adequate size before calling this
function.
■ [2.0+] Int 21H Functions 4EH and 4FH, which allow full access to the
hierarchical directory structure, should be used in preference to this
function.
Example:
Assuming a previous successful call to function 11H, search for the next
file with the extension .COM in the current directory. If the DTA has not
been changed since the previous search, another call to Function 1AH is
not necessary.
buff db 37 dup (0) ; receives search result
my_fcb db 0 ; drive = default
db '????????' ; wildcard filename
db 'COM' ; extension = COM
db 25 dup (0) ; remainder of FCB
.
.
.
; set DTA address
mov ah,1ah ; function number
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
; search for next match
mov ah,12h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if no match
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 13H (19)
Delete file
────────────────────────────────────────────────────────────────────────────
Deletes all matching files from the current directory on the default or
specified disk drive.
Call with:
AH = 13H
DS:DX = segment:offset of file control block
Returns:
If function successful (file or files deleted)
AL = 00H
If function unsuccessful (no matching files were found, or at least one
matching file was read-only)
AL = FFH
Notes:
■ The wildcard character ? is allowed in the filename; if ? is present and
there is more than one matching filename, all matching files will be
deleted.
■ [2.0+] Int 21H Function 41H, which allows full access to the
hierarchical directory structure, should be used in preference to this
function.
■ [3.0+] If the program is running on a network, the user must have Create
rights to the directory containing the file to be deleted.
Example:
Delete the file MYFILE.DAT from the current disk drive and directory.
myfcb db 0 ; drive = default
db 'MYFILE ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
.
.
.
mov ah,13h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump, delete failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 14H (20)
Sequential read
────────────────────────────────────────────────────────────────────────────
Reads the next sequential block of data from a file, then increments the
file pointer appropriately.
Call with:
AH = 14H
DS:DX = segment:offset of previously opened file control block
Returns:
AL = 00H if read successful
01H if end of file
02H if segment wrap
03H if partial record read at end of file
Notes:
■ The record is read into memory at the current disk transfer area (DTA)
address, specified by the most recent call to Int 21H Function 1AH. If
the size of the record and the location of the buffer are such that a
segment overflow or wraparound would occur, the function fails with a
return code of 02H.
■ The number of bytes of data to be read is specified by the record-size
field (offset 0EH) of the file control block (FCB).
■ The file location of the data that will be read is specified by the
combination of the current block field (offset 0CH) and current record
field (offset 20H) of the file control block (FCB). These fields are
also automatically incremented by this function.
■ If a partial record is read at the end of file, it is padded to the
requested record length with zeros.
■ [3.0+] If the program is running on a network, the user must have Read
access rights to the directory containing the file to be read.
Example:
Read 1024 bytes of data from the file specified by the previously opened
file control block myfcb.
myfcb db 0 ; drive = default
db 'QUACK ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
.
.
.
mov ah,14h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
; set record size
mov word ptr myfcb+0eH,1024
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if read failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 15H (21)
Sequential write
────────────────────────────────────────────────────────────────────────────
Writes the next sequential block of data into a file, then increments the
file pointer appropriately.
Call with:
AH = 15H
DS:DX = segment:offset of previously opened file control block
Returns:
AL = 00H if write successful
01H if disk is full
02H if segment wrap
Notes:
■ The record is written (logically, not necessarily physically) to the
disk from memory at the current disk transfer area (DTA) address,
specified by the most recent call to Int 21H Function 1AH. If the size
of the record and the location of the buffer are such that a segment
overflow or wraparound would occur, the function fails with a return
code of 02H.
■ The number of bytes of data to be written is specified by the
record-size field (offset 0EH) of the file control block (FCB).
■ The file location of the data that will be written is specified by the
combination of the current block field (offset 0CH) and current record
field (offset 20H) of the file control block (FCB). These fields are
also automatically incremented by this function.
■ [3.0+] If the program is running on a network, the user must have Write
access rights to the directory containing the file to be written.
Example:
Write 1024 bytes of data to the file specified by the previously opened
file control block myfcb.
myfcb db 0 ; drive = default
db 'QUACK ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
.
.
.
mov ah,15h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
; set record size
mov word ptr myfcb+0eh,1024
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if write failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 16H (22)
Create file
────────────────────────────────────────────────────────────────────────────
Creates a new directory entry in the current directory or truncates any
existing file with the same name to zero length. Opens the file for
subsequent read/write operations.
Call with:
AH = 16H
DS:DX = segment:offset of unopened file control block
Returns:
If function successful (file was created or truncated)
AL = 00H
and FCB filled in by MS-DOS as follows:
drive field (offset 00H) = 1 for drive A, 2 for drive B, etc.
current block field (offset 0CH) = 00H
record size field (offset 0EH) = 0080H
[2.0+] size field (offset 10H) = file size from directory
[2.0+] date field (offset 14H) = date stamp from directory
[2.0+] time field (offset 16H) = time stamp from directory
If function unsuccessful (directory full)
AL = FFH
Notes:
■ Since an existing file with the specified name is truncated to zero
length (i.e., all data in that file is irretrievably lost), this
function must be used with caution.
■ If this function is called with an extended file control block (FCB),
the new file may be assigned a special attribute, such as hidden or
system, during its creation by setting the appropriate bit in the
extended FCB's attribute byte.
■ Since this function also opens the file, a subsequent call to Int 21H
Function 0FH is not required.
■ For format of directory time and date, see Int 21H Function 57H.
■ [2.0+] Int 21H Functions 3CH, 5AH, 5BH, and 6CH, which provide full
access to the hierarchical directory structure, should be used in
preference to this function.
■ [3.0+] If the program is running on a network, the user must have Create
rights to the directory that will contain the new file.
Example:
Create a file in the current directory using the name in the file control
block myfcb.
myfcb db 0 ; drive = default
db 'QUACK ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
.
.
.
mov ah,16h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if create failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 17H (23)
Rename file
────────────────────────────────────────────────────────────────────────────
Alters the name of all matching files in the current directory on the disk
in the specified drive.
Call with:
AH = 17H
DS:DX = segment:offset of "special" file control block
Returns:
If function successful (one or more files renamed)
AL = 00H
If function unsuccessful (no matching files, or new filename matched an
existing file)
AL = FFH
Notes:
■ The special file control block has a drive code, filename, and extension
in the usual position (bytes 0 through 0BH) and a second filename
starting 6 bytes after the first (offset 11H).
■ The ? wildcard character can be used in the first filename. Every file
matching the first file specification will be renamed to match the
second file specification.
■ If the second file specification contains any ? wildcard characters, the
corresponding letters in the first filename are left unchanged.
■ The function terminates if the new name to be assigned to a file matches
that of an existing file.
■ [2.0+] An extended FCB can be used with this function to rename a
directory.
■ [2.0+] Int 21H Function 56H, which allows full access to the
hierarchical directory structure, should be used in preference to this
function.
Example:
Rename the file OLDNAME.DAT to NEWNAME.DAT.
myfcb db 0 ; drive = default
db 'OLDNAME ' ; old file name, 8 chars
db 'DAT' ; old extension, 3 chars
db 6 dup (0) ; reserved area
db 'NEWNAME ' ; new file name, 8 chars
db 'DAT' ; new extension, 3 chars
db 14 dup (0) ; reserved area
.
.
.
mov ah,17h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if rename failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 18H (24)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 19H (25)
Get current disk
────────────────────────────────────────────────────────────────────────────
Returns the drive code of the current, or default, disk drive.
Call with:
AH = 19H
Returns:
AL = drive code (0 = A, 1 = B, etc.)
Notes:
■ To set the default drive, use Int 21H Function 0EH.
■ Some other Int 21H functions use drive codes beginning at 1 (that is,
1 = A, 2 = B, etc.) and reserve drive code zero for the default drive.
Example:
Get the current disk drive and save the code in the variable cdrive.
cdrive db 0 ; current drive code
.
.
.
mov ah,19h ; function number
int 21h ; transfer to MS-DOS
mov cdrive,al ; save drive code
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 1AH (26)
Set DTA address
────────────────────────────────────────────────────────────────────────────
Specifies the address of the disk transfer area (DTA) to be used for
subsequent FCB-related function calls.
Call with:
AH = 1AH
DS:DX = segment:offset of disk transfer area
Returns:
Nothing
Notes:
■ If this function is never called by the program, the DTA defaults to a
128-byte buffer at offset 0080H in the program segment prefix.
■ In general, it is the programmer's responsibility to ensure that the
buffer area specified is large enough for any disk operation that will
use it. The only exception to this is that MS-DOS will detect and abort
disk transfers that would cause a segment wrap.
■ Int 21H Function 2FH can be used to determine the current disk transfer
address.
■ The only handle-type operations that rely on the DTA address are the
directory search functions, Int 21H Functions 4EH and 4FH.
Example:
Set the current disk transfer area address to the buffer labeled buff.
buff db 128 dup (?)
.
.
.
mov ah,1ah ; function number
mov dx,seg buff ; address of disk
mov ds,dx ; transfer area
mov dx,offset buff
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 1BH (27)
Get default drive data
────────────────────────────────────────────────────────────────────────────
Obtains selected information about the default disk drive and a pointer to
the media identification byte from its file allocation table.
Call with:
AH = 1BH
Returns:
If function successful
AL = sectors per cluster
DS:BX = segment:offset of media ID byte
CX = size of physical sector (bytes)
DX = number of clusters for default drive
If function unsuccessful (invalid drive or critical error)
AL = FFH
Notes:
■ The media ID byte has the following meanings:
0F0H 3.5-inch double-sided, 18 sectors
or "other"
0F8H fixed disk
0F9H 5.25-inch double-sided, 15 sectors
or 3.5-inch double-sided, 9 sectors
0FCH 5.25-inch single-sided, 9 sectors
0FDH 5.25-inch double-sided, 9 sectors
0FEH 5.25-inch single-sided, 8 sectors
0FFH 5.25-inch double-sided, 8 sectors
■ To obtain information about disks other than the one in the default
drive, use Int 21H Function 1CH or 36H.
■ [1] The address returned in DS:BX points to a copy of the first sector
of the actual FAT, with the media ID byte in the first byte.
■ [2.0+] The address returned in DS:BX points only to a copy of the media
ID byte from the disk's FAT; the memory above that address cannot be
assumed to contain the FAT or any other useful information. If direct
access to the FAT is required, use Int 25H to read it into memory.
Example:
Determine whether the current disk drive is fixed or removable.
.
.
.
mov ah,1bh ; function number
int 21h ; transfer to MS-DOS
; check media ID byte
cmp byte ptr [bx],0f8h
je fixed ; jump if fixed disk
jmp floppy ; else assume floppy
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 1CH (28)
Get drive data
────────────────────────────────────────────────────────────────────────────
Obtains allocation information about the specified disk drive and a
pointer to the media identification byte from its file allocation table.
Call with:
AH = 1CH
DL = drive code (0 = default, 1 = A, etc.)
Returns:
If function successful
AL = sectors per cluster
DS:BX = segment:offset of media ID byte
CX = size of physical sector (bytes)
DX = number of clusters for default or specified drive
If function unsuccessful (invalid drive or critical error)
AL = FFH
Notes:
■ The media ID byte has the following meanings:
0F0H 3.5-inch double-sided, 18 sectors
or "other"
0F8H fixed disk
0F9H 5.25-inch double-sided, 15 sectors
or 3.5-inch double-sided, 9 sectors
0FCH 5.25-inch single-sided, 9 sectors
0FDH 5.25-inch double-sided, 9 sectors
0FEH 5.25-inch single-sided, 8 sectors
0FFH 5.25-inch double-sided, 8 sectors
■ In general, this call is identical to Int 21H Function 1BH, except for
the ability to designate a specific disk drive. See also Int 21H
Function 36H, which returns similar information.
■ [1] The address returned in DS:BX points to a copy of the first sector
of the actual FAT, with the media ID byte in the first byte.
■ [2.0+] The address returned in DS:BX points only to a copy of the media
ID byte from the disk's FAT; the memory above that address cannot be
assumed to contain the FAT or any other useful information. If direct
access to the FAT is required, use Int 25H to read it into memory.
Example:
Determine whether disk drive C is fixed or removable.
.
.
.
mov ah,1ch ; function number
mov dl,3 ; drive code 3 = C
int 21h ; transfer to MS-DOS
; check media ID byte
cmp byte ptr ds:[bx],0f8h
je fixed ; jump if fixed disk
jmp floppy ; else assume floppy
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 1DH (29)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 1EH (30)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 1FH (31)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 20H (32)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 21H (33)
Random read
────────────────────────────────────────────────────────────────────────────
Reads a selected record from a file into memory.
Call with:
AH = 21H
DS:DX = segment:offset of previously opened file control block
Returns:
AL = 00H if read successful
01H if end of file
02H if segment wrap, read canceled
03H if partial record read at end of file
Notes:
■ The record is read into memory at the current disk transfer area
address, specified by the most recent call to Int 21H Function 1AH. It
is the programmer's responsibility to ensure that this area is large
enough for any record that will be transferred. If the size and location
of the buffer are such that a segment overflow or wraparound would
occur, the function fails with a return code of 02H.
■ The file location of the data to be read is determined by the
combination of the relative-record field (offset 21H) and the
record-size field (offset 0EH) of the FCB. The default record size is
128 bytes.
■ The current block field (offset 0CH) and current record field (offset
20H) are updated to agree with the relative-record field as a side
effect of the function.
■ The relative-record field of the FCB is not incremented by this
function; it is the responsibility of the application to update the FCB
appropriately if it wishes to read successive records. Compare with Int
21H Function 27H, which can read multiple records with one function
call and automatically increments the relative-record field.
■ If a partial record is read at end of file, it is padded to the
requested record length with zeros.
■ [3.0+] If the program is running on a network, the user must have Read
access rights to the directory containing the file to be read.
Example:
Open the file MYFILE.DAT, set the record length to 1024 bytes, then read
record number 4 from the file into the buffer named buff.
myfcb db 0 ; drive = default
db 'MYFILE ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
buff db 1024 dup (?) ; receives read data
.
.
.
; open the file
mov ah,0fh ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check open status
jnz error ; jump if no file
; set DTA address
mov ah,1ah ; function number
mov dx,offset buff ; read buffer address
int 21h ; transfer to MS-DOS
; set record size
mov word ptr myfcb+0eh,1024
; set record number
mov word ptr myfcb+21h,4
mov word ptr myfcb+23h,0
; read the record
mov ah,21h ; function number
mov dx,offset myfcb ; address of FCB
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if read failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 22H (34)
Random write
────────────────────────────────────────────────────────────────────────────
Writes data from memory into a selected record in a file.
Call with:
AH = 22H
DS:DX = segment:offset of previously opened file control block
Returns:
AL = 00H if write successful
01H if disk full
02H if segment wrap, write canceled
Notes:
■ The record is written (logically, not necessarily physically) to the
file from memory at the current disk transfer address, specified by the
most recent call to Int 21H Function 1AH. If the size and location of
the buffer are such that a segment overflow or wraparound would occur,
the function fails with a return code of 02H.
■ The file location of the data to be written is determined by the
combination of the relative-record field (offset 21H) and the
record-size field (offset 0EH) of the FCB. The default record size is
128 bytes.
■ The current block field (offset 0CH) and current record field (offset
20H) are updated to agree with the relative-record field as a side
effect of the function.
■ The relative-record field of the FCB is not incremented by this
function; it is the responsibility of the application to update the FCB
appropriately if it wishes to write successive records. Compare with Int
21H Function 28H, which can write multiple records with one function
call and automatically increments the relative-record field.
■ If a record is written beyond the current end of file, the space between
the old end of file and the new record is allocated but not initialized.
■ [3.0+] If the program is running on a network, the user must have Write
access rights to the directory containing the file to be written.
Example:
Open the file MYFILE.DAT, set the record length to 1024 bytes, write
record number 4 into the file from the buffer named buff, then close the
file.
myfcb db 0 ; drive = default
db 'MYFILE ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
buff db 1024 dup (?) ; buffer for write
.
.
.
; open the file
mov ah,0fh ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if no file
; set DTA address
mov dx,offset buff ; buffer address
mov ah,1ah ; function number
int 21h ; transfer to MS-DOS
; set record size
mov word ptr myfcb+0eh,1024
; set record number
mov word ptr myfcb+21h,4
mov word ptr myfcb+23h,0
; write the record
mov ah,22h ; function number
mov dx,offset myfcb ; address of FCB
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if write failed
; close the file
mov ah,10h ; function number
mov dx,offset myfcb ; address of FCB
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if close failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 23H (35)
Get file size
────────────────────────────────────────────────────────────────────────────
Searches for a matching file in the current directory; if one is found,
updates the FCB with the file's size in terms of number of records.
Call with:
AH = 23H
DS:DX = segment:offset of unopened file control block
Returns:
If function successful (matching file found)
AL = 00H
and FCB relative-record field (offset 21H) set to the number of records in
the file, rounded up if necessary to the next complete record
If function unsuccessful (no matching file found)
AL = FFH
Notes:
■ An appropriate value must be placed in the FCB record-size field (offset
0EH) before calling this function. There is no default record size for
this function. Compare with the FCB-related open and create functions
(Int 21H Functions 0FH and 16H), which initialize the FCB for a
default record size of 128 bytes.
■ The record-size field can be set to 1 to find the size of the file in
bytes.
■ Because record numbers are zero based, this function can be used to
position the FCB's file pointer to the end of file.
Example:
Determine the size in bytes of the file MYFILE.DAT and leave the result in
registers DX:AX.
myfcb db 0 ; drive = default
db 'MYFILE ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
.
.
.
mov ah,23h ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
; record size = 1 byte
mov word ptr myfcb+0eh,1
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if no file
; get file size in bytes
mov ax,word ptr myfcb+21h
mov dx,word ptr myfcb+23h
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 24H (36)
Set relative record number
────────────────────────────────────────────────────────────────────────────
Sets the relative-record-number field of a file control block (FCB) to
correspond to the current file position as recorded in the opened FCB.
Call with:
AH = 24H
DS:DX = segment:offset of previously opened file control block
Returns:
AL is destroyed (other registers not affected)
FCB relative-record field (offset 21H) updated
Notes:
■ This function is used when switching from sequential to random I/O
within a file. The contents of the relative-record field (offset 21H)
are derived from the record size (offset 0EH), current block (offset
0CH), and current record (offset 20H) fields of the file control block.
■ All four bytes of the FCB relative-record field (offset 21H) should be
initialized to zero before calling this function.
Example:
After a series of sequential record transfers have been performed using
the file control block myfcb, obtain the current relative-record position
in the file and leave the record number in DX.
myfcb db 0 ; drive = default
db 'MYFILE ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
.
.
.
mov dx,seg myfcb ; make FCB addressable
mov ds,dx
; initialize relative
; record field to zero
mov word ptr myfcb+21h,0
mov word ptr myfcb+23h,0
; now set record number
mov ah,24h ; function number
mov dx,offset myfcb ; address of FCB
int 21h ; transfer to MS-DOS
; load record number in DX
mov dx,word ptr myfcb+21h
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 25H (37)
Set interrupt vector
────────────────────────────────────────────────────────────────────────────
Initializes a CPU interrupt vector to point to an interrupt handling
routine.
Call with:
AH = 25H
AL = interrupt number
DS:DX = segment:offset of interrupt handling routine
Returns:
Nothing
Notes:
■ This function should be used in preference to direct editing of the
interrupt-vector table by well-behaved applications.
■ Before an interrupt vector is modified, its original value should be
obtained with Int 21H Function 35H and saved, so that it can be restored
using this function before program termination.
Example:
Install a new interrupt handler, named zdiv, for "divide by zero" CPU
exceptions.
.
.
.
mov ah,25h ; function number
mov al,0 ; interrupt number
mov dx,seg zdiv ; address of handler
mov ds,dx
mov dx,offset zdiv
int 21h ; transfer to MS-DOS
.
.
.
zdiv: ; int 00h handler
iret ; (does nothing)
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 26H (38)
Create new PSP
────────────────────────────────────────────────────────────────────────────
Copies the program segment prefix (PSP) of the currently executing program
to a specified segment address in free memory, then updates the new PSP to
make it usable by another program.
Call with:
AH = 26H
DX = segment of new program segment prefix
Returns:
Nothing
Notes:
■ After the executing program's PSP is copied into the new segment, the
memory size information in the new PSP is updated appropriately and the
current contents of the termination (Int 22H), Ctrl-C handler (Int 23H),
and critical-error handler (Int 24H) vectors are saved starting at
offset 0AH.
■ This function does not load another program or in itself cause one to be
executed.
■ [2.0+] Int 21H Function 4BH (EXEC), which can be used to load and
execute programs or overlays in either .COM or .EXE format, should be
used in preference to this function.
Example:
Create a new program segment prefix 64 KB above the currently executing
program. This example assumes that the running program was loaded as a
.COM file so that the CS register points to its PSP throughout its
execution. If the running program was loaded as a .EXE file, the address
of the PSP must be obtained with Int 21H Function 62H (under MS-DOS 3.0
or later) or by saving the original contents of the DS or ES registers at
entry.
.
.
.
mov ah,26h ; function number
mov dx,cs ; PSP segment of
; this program
add dx,1000h ; add 64 KB as
; paragraph address
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 27H (39)
Random block read
────────────────────────────────────────────────────────────────────────────
Reads one or more sequential records from a file into memory, starting at
a designated file location.
Call with:
AH = 27H
CX = number of records to read
DS:DX = segment:offset of previously opened file control block
Returns:
AL = 00H if all requested records read
01H if end of file
02H if segment wrap
03H if partial record read at end of file
CX = actual number of records read
Notes:
■ The records are read into memory at the current disk transfer area
address, specified by the most recent call to Int 21H Function 1AH. It
is the programmer's responsibility to ensure that this area is large
enough for the group of records that will be transferred. If the size
and location of the buffer are such that a segment overflow or
wraparound would occur, the function fails with a return code of 02H.
■ The file location of the data to be read is determined by the
combination of the relative-record field (offset 21H) and the
record-size field (offset 0EH) of the FCB. The default record size is
128 bytes.
■ After the disk transfer is performed, the current block (offset 0CH),
current record (offset 20H), and relative-record (offset 21H) fields of
the FCB are updated to point to the next record in the file.
■ If a partial record is read at the end of file, the remainder of the
record is padded with zeros.
■ Compare with Int 21H Function 21H, which transfers only one record per
function call and does not update the FCB relative-record field.
■ [3.0+] If the program is running on a network, the user must have Read
access rights to the directory containing the file to be read.
Example:
Read four 1024-byte records starting at record number 8 into the buffer
named buff, using the file control block myfcb.
myfcb db 0 ; drive = default
db 'MYFILE ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
buff db 4096 dup (?) ; buffer for data
.
.
.
; set DTA address
mov ah,1ah ; function number
mov dx,seg buff ; address of buffer
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
; set relative-record number
mov word ptr myfcb+21h,8
mov word ptr myfcb+23h,0
; set record size
mov word ptr myfcb+0eh,1024
; read the records
mov ah,27h ; function number
mov cx,4 ; number of records
mov dx,offset myfcb ; address of FCB
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if read error
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 28H (40)
Random block write
────────────────────────────────────────────────────────────────────────────
Writes one or more sequential records from memory to a file, starting at a
designated file location.
Call with:
AH = 28H
CX = number of records to write
DS:DX = segment:offset of previously opened file control block
Returns:
AL = 00H if all requested records written
01H if disk full
02H if segment wrap
CX = actual number of records written
Notes:
■ The records are written (logically, not necessarily physically) to disk
from memory at the current disk transfer area address, specified by the
most recent call to Int 21H Function 1AH. If the size and location of
the buffer are such that a segment overflow or wraparound would occur,
the function fails with a return code of 02H.
■ The file location of the data to be written is determined by the
combination of the relative-record field (offset 21H) and the
record-size field (offset 0EH) of the FCB. The default record size is
128 bytes.
■ After the disk transfer is performed, the current block (offset 0CH),
current record (offset 20H), and relative-record (offset 21H) fields of
the FCB are updated to point to the next record in the file.
■ If this function is called with CX = 0, no data is written to the disk
but the file is extended or truncated to the length specified by
combination of the record-size (offset 0EH) and the relative-record
(offset 21H) fields of the FCB.
■ Compare with Int 21H Function 22H, which transfers only one record per
function call and does not update the FCB relative-record field.
■ [3.0+] If the program is running on a network, the user must have Write
access rights to the directory containing the file to be written.
Example:
Write four 1024-byte records, starting at record number 8, to disk from
the buffer named buff, using the file control block myfcb.
myfcb db 0 ; drive = default
db 'MYFILE ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
buff db 4096 dup (?) ; buffer for data
.
.
.
; set DTA address
mov ah,1ah ; function number
mov dx,seg buff ; address of buffer
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
; set relative-record number
mov word ptr myfcb+21h,8
mov word ptr myfcb+23h,0
; set record size
mov word ptr myfcb+0eh,1024
; write the records
mov ah,28h ; function number
mov cx,4 ; number of records
mov dx,offset myfcb ; address of FCB
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if write error
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 29H (41)
Parse filename
────────────────────────────────────────────────────────────────────────────
Parses a text string into the various fields of a file control block
(FCB).
Call with:
AH = 29H
AL = flags to control parsing
Bit 3 = 1 if extension field in FCB will be
modified only if an extension is
specified in the string being parsed.
= 0 if extension field in FCB will be
modified regardless; if no extension is
present in the parsed string, FCB
extension is set to ASCII blanks.
Bit 2 = 1 if filename field in FCB will be
modified only if a filename is
specified in the string being parsed.
= 0 if filename field in FCB will be
modified regardless; if no filename is
present in the parsed string, FCB
filename is set to ASCII blanks.
Bit 1 = 1 if drive ID byte in FCB will be
modified only if a drive was specified
in the string being parsed.
= 0 if the drive ID byte in FCB will be
modified regardless; if no drive
specifier is present in the parsed
string, FCB drive-code field is set to
0 (default).
Bit 0 = 1 if leading separators will be scanned
off (ignored).
= 0 if leading separators will not be
scanned off.
DS:SI = segment:offset of string
ES:DI = segment:offset of file control block
Returns:
AL = 00H if no wildcard characters encountered
01H if parsed string contained wildcard characters
FFH if drive specifier invalid
DS:SI = segment:offset of first character after parsed filename
ES:DI = segment:offset of formatted unopened file control block
Notes:
■ This function regards the following as separator characters:
[1] : . ; , = + tab space / " [ ]
[2.0+] : . ; , = + tab space
■ This function regards all control characters and the following as
terminator characters:
: . ; , = + tab space < > | / " [ ]
■ If no valid filename is present in the string to be parsed, upon return
ES:DI + 1 points to an ASCII blank.
■ If the * wildcard character occurs in a filename or extension, it and
all remaining characters in the corresponding field in the FCB are set
to ?.
■ This function (and file control blocks in general) cannot be used with
file specifications that include a path.
Example:
Parse the string fname into the file control block myfcb.
fname db 'D:QUACK.DAT',0 ; filename to be parsed
myfcb db 37 dup (0) ; becomes file control block
.
.
.
mov ah,29h ; function number
mov al,01h ; skip leading separators
mov si,seg fname ; address of filename
mov ds,si
mov si,offset fname
mov di,seg myfcb ; address of FCB
mov es,di
mov di,offset myfcb
int 21h ; transfer to MS-DOS
cmp al,0ffh ; check status
je error ; jump, drive invalid
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 2AH (42)
Get date
────────────────────────────────────────────────────────────────────────────
Obtains the system day of the month, day of the week, month, and year.
Call with:
AH = 2AH
Returns:
CX = year (1980 through 2099)
DH = month (1 through 12)
DL = day (1 through 31)
Under MS-DOS versions 1.1 and later
AL = day of the week (0 = Sunday, 1 = Monday, etc.)
Notes:
■ This function's register format is the same as that required for Int 21H
Function 2BH (Set Date).
■ This function can be used together with Int 21H Function 2BH to find
the day of the week for an arbitrary date. The current date is first
obtained with Function 2AH and saved. The date of interest is then set
with Function 2BH, and the day of the week for that date is obtained
with a subsequent call to Function 2AH. Finally, the current date is
restored with an additional call to Function 2BH, using the values
obtained with the original Function 2AH call.
Example:
Obtain the current date and save its components in the variables year,
day, and month.
year dw 0
month db 0
day db 0
.
.
.
mov ah,2ah ; function number
int 21h ; transfer to MS-DOS
mov year,cx ; save year (word)
mov month,dh ; save month (byte)
mov day,dl ; save day (byte)
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 2BH (43)
Set date
────────────────────────────────────────────────────────────────────────────
Initializes the system clock driver to a specific date. The system time is
not affected.
Call with:
AH = 2BH
CX = year (1980 through 2099)
DH = month (1 through 12)
DL = day (1 through 31)
Returns:
AL = 00H if date set successfully
FFH if date not valid (ignored)
Note:
■ This function's register format is the same as that required for Int 21H
Function 2AH (Get Date).
Example:
Set the system date according to the contents of the variables year, day,
and month.
year dw 0
month db 0
day db 0
.
.
.
mov ah,2bh ; function number
mov cx,year ; get year (word)
mov dh,month ; get month (byte)
mov dl,day ; get day (byte)
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if date invalid
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 2CH (44)
Get time
────────────────────────────────────────────────────────────────────────────
Obtains the time of day from the system real-time clock driver, converted
to hours, minutes, seconds, and hundredths of seconds.
Call with:
AH = 2CH
Returns:
CH = hours (0 through 23)
CL = minutes (0 through 59)
DH = seconds (0 through 59)
DL = hundredths of seconds (0 through 99)
Notes:
■ This function's register format is the same as that required for Int 21H
Function 2DH (Set Time).
■ On most IBM PC─compatible systems, the real-time clock does not have a
resolution of single hundredths of seconds. On such machines, the values
returned by this function in register DL are discontinuous.
Example:
Obtain the current time and save its two major components in the variables
hours and minutes.
hours db 0
minutes db 0
.
.
.
mov ah,2ch ; function number
int 21h ; transfer to MS-DOS
mov hours,ch ; save hours (byte)
mov minutes,cl ; save minutes (byte)
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 2DH (45)
Set time
────────────────────────────────────────────────────────────────────────────
Initializes the system real-time clock to a specified hour, minute,
second, and hundredth of second. The system date is not affected.
Call with:
AH = 2DH
CH = hours (0 through 23)
CL = minutes (0 through 59)
DH = seconds (0 through 59)
DL = hundredths of seconds (0 through 99)
Returns:
AL = 00H if time set successfully
FFH if time not valid (ignored)
Note:
■ This function's register format is the same as that required for Int 21H
Function 2CH (Get Time).
Example:
Set the system time according to the contents of the variables hours and
minutes. Force the current seconds and hundredths of seconds to zero.
hours db 0
minutes db 0
.
.
.
mov ah,2dh ; function number
mov ch,hours ; get hours (byte)
mov cl,minutes ; get minutes (byte)
mov dx,0 ; force seconds and
; hundredths to zero
int 21h ; transfer to MS-DOS
or al,al ; check status
jnz error ; jump if time invalid
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [1.0]
Function 2EH (46)
Set verify flag
────────────────────────────────────────────────────────────────────────────
Turns off or turns on the operating-system flag for automatic
read-after-write verification of data.
Call with:
AH = 2EH
AL = 00H if turning off verify flag
01H if turning on verify flag
DL = 00H (MS-DOS versions 1 and 2)
Returns:
Nothing
Notes:
■ Because read-after-write verification slows disk operations, the default
setting of the verify flag is OFF.
■ If a particular disk unit's device driver does not support
read-after-write verification, this function has no effect.
■ The current state of the verify flag can be determined using Int 21H
Function 54H.
■ The state of the verify flag is also controlled by the MS-DOS commands
VERIFY OFF and VERIFY ON.
Example:
Save the current state of the system verify flag in the variable vflag,
then force all subsequent disk writes to be verified.
vflag db 0 ; previous verify flag
.
.
.
; get verify flag
mov ah,54h ; function number
int 21h ; transfer to MS-DOS
mov vflag,al ; save current flag state
; set verify flag
mov ah,2eh ; function number
mov al,1 ; AL = 1 for verify on
mov dl,0 ; DL must be zero
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 2FH (47)
Get DTA address
────────────────────────────────────────────────────────────────────────────
Obtains the current address of the disk transfer area (DTA) for FCB file
read/write operations.
Call with:
AH = 2FH
Returns:
ES:BX = segment:offset of disk transfer area
Note:
■ The disk transfer area address is set with Int 21H Function 1AH. The
default DTA is a 128-byte buffer at offset 80H in the program segment
prefix.
Example:
Obtain the current disk transfer area address and save it in the variable
olddta.
olddta dd ? ; save disk transfer address
.
.
.
mov ah,2fh ; function number
int 21h ; transfer to MS-DOS
; save it as DWORD pointer
mov word ptr olddta,bx
mov word ptr olddta+2,es
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 30H (48)
Get MS-DOS version number
────────────────────────────────────────────────────────────────────────────
Returns the version number of the host MS-DOS operating system. This
function is used by application programs to determine the capabilities of
their environment.
Call with:
AH = 30H
AL = 00H
Returns:
If running under MS-DOS version 1
AL = 00H
If running under MS-DOS versions 2.0 or later
AL = major version number (MS-DOS 3.10 = 3, etc.)
AH = minor version number (MS-DOS 3.10 = 0AH, etc.)
BH = Original Equipment Manufacturer's (OEM's) serial number
(OEM-dependent──usually 00H for IBM's PC-DOS, 0FFH or
other values for MS-DOS)
BL:CX = 24-bit user serial number (optional, OEM-dependent)
Notes:
■ Because this function was not defined under MS-DOS version 1, it should
always be called with AL = 00H. In an MS-DOS version 1 environment, AL
will be returned unchanged.
■ Care must be taken not to exit in an unacceptable fashion if an MS-DOS
version 1 environment is detected. For example, Int 21H Function 4CH
(Terminate Process with Return Code), Int 21H Function 40H (Write to
File or Device), and the standard error handle are not available in
MS-DOS version 1. In such cases a program should display an error
message using Int 21H Function 09H and then terminate with Int 20H or
Int 21H Function 00H.
Example:
Get the MS-DOS version number, terminating the current process with an
error message if not running under MS-DOS version 2.0 or later.
cr equ 0dh ; ASCII carriage return
lf equ 0ah ; ASCII line feed
msg db cr,lf
db 'Wrong MS-DOS version'
db cr,lf,'$'
.
.
.
mov ax,3000h ; function number
int 21h ; transfer to MS-DOS
cmp al,2 ; version 2 or later?
jae label1 ; yes, jump
; display error message
mov ah,09 ; function number
mov dx,offset msg ; message address
int 21h ; transfer to MS-DOS
; terminate process
mov ah,0 ; function number
int 21h ; transfer to MS-DOS
label1: .
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 31H (49)
Terminate and stay resident
────────────────────────────────────────────────────────────────────────────
Terminates execution of the currently executing program, passing a return
code to the parent process, but reserves part or all of the program's
memory so that it will not be overlaid by the next transient program to be
loaded. MS-DOS then takes the following actions:
■ File buffers are flushed and any open handles for files or devices owned
by the process are closed.
■ The termination handler vector (Int 22H) is restored from PSP:000AH.
■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH.
■ [2.0+] The critical-error handler vector (Int 24H) is restored from
PSP:0012H.
■ Control is transferred to the termination handler.
If the program is returning to COMMAND.COM, control transfers to the
resident portion, and the transient portion is reloaded if necessary. If a
batch file is in progress, the next line of the file is fetched and
interpreted; otherwise, a prompt is issued for the next user command.
Call with:
AH = 31H
AL = return code
DX = amount of memory to reserve (in paragraphs)
Returns:
Nothing
Notes:
■ This function call is typically used to allow user-written utilities,
drivers, or interrupt handlers to be loaded as ordinary .COM or .EXE
programs and then remain resident. Subsequent entrance to the code is
via a hardware or software interrupt.
■ This function attempts to set the initial memory allocation block to the
length in paragraphs specified in register DX. If other memory blocks
have been requested by the application using Int 21H Function 48H, they
will not be released by this function.
■ Other methods of performing a final exit are:
∙ Int 20H
∙ Int 21H Function 00H
∙ Int 21H Function 4CH
∙ Int 27H
■ The return code may be retrieved by a parent process with Int 21H
Function 4DH (Get Return Code). It can also be tested in a batch file
with an IF ERRORLEVEL statement. By convention, a return code of zero
indicates successful execution, and a nonzero return code indicates an
error.
■ This function should not be called by .EXE programs that are loaded at
the high end of the transient program area (that is, linked with the
/HIGH switch) because doing so reserves the memory that is normally used
by the transient part of COMMAND.COM. If COMMAND.COM cannot be reloaded,
the system will fail.
■ [2.0+] This function should be used in preference to Int 27H because it
supports return codes, allows larger amounts of memory to be reserved,
and does not require CS to contain the segment of the program segment
prefix.
■ [3.0+] If the program is running on a network, it should remove all
locks it has placed on file regions before terminating.
Example:
Exit with a return code of 1 but stay resident, reserving 16 KB of memory
starting at the program segment prefix of the process.
.
.
.
mov ah,31h ; function number
mov al,1 ; return code for parent
mov dx,0400h ; paragraphs to reserve
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 32H (50)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 33H (51)
Get or set break flag, get boot drive
────────────────────────────────────────────────────────────────────────────
Obtains or changes the status of the operating system's break flag, which
influences Ctrl-C checking during function calls. Also returns the system
boot drive in version 4.0.
Call with:
If getting break flag
AH = 33H
AL = 00H
If setting break flag
AH = 33H
AL = 01H
DL = 00H if turning break flag OFF
01H if turning break flag ON
[4] If getting boot drive
AH = 33H
AL = 05H
Returns:
If called with AL = 00H or 01H
DL = 00H break flag is OFF
01H break flag is ON
[4] If called with AL = 05H
DL = boot drive (1 = A, 2 = B, etc.)
Notes:
■ When the system break flag is on, the keyboard is examined for a Ctrl-C
entry whenever any operating-system input or output is requested; if
Ctrl-C is detected, control is transferred to the Ctrl-C handler (Int
23H). When the break flag is off, MS-DOS only checks for a Ctrl-C entry
when executing the traditional character I/O functions (Int 21H
Functions 01H through 0CH).
■ The break flag is not part of the local environment of the currently
executing program; it affects all programs. An application that alters
the flag should first save the flag's original status, then restore the
flag before terminating.
Example:
Save the current state of the system break flag in the variable brkflag,
then turn the break flag off to disable Ctrl-C checking during most MS-DOS
function calls.
brkflag db 0 ; save break flag
.
.
.
; get current break flag
mov ah,33h ; function number
mov al,0 ; AL = 0 to get flag
int 21h ; transfer to MS-DOS
mov brkflag,dl ; save current flag
; now set break flag
mov ah,33h ; function number
mov al,1 ; AL = 1 to set flag
mov dl,0 ; set break flag OFF
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 34H (52)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 35H (53)
Get interrupt vector
────────────────────────────────────────────────────────────────────────────
Obtains the address of the current interrupt-handler routine for the
specified machine interrupt.
Call with:
AH = 35H
AL = interrupt number
Returns:
ES:BX = segment:offset of interrupt handler
Note:
■ Together with Int 21H Function 25H (Set Interrupt Vector), this
function is used by well-behaved application programs to modify or
inspect the machine interrupt vector table.
Example:
Obtain the address of the current interrupt handler for hardware interrupt
level 0 (divide by zero) and save it in the variable oldint0.
oldint0 dd ? ; previous handler address
.
.
.
mov ah,35h ; function number
mov al,0 ; interrupt level
int 21h ; transfer to MS-DOS
; save old handler address
mov word ptr oldint0,bx
mov word ptr oldint0+2,es
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 36H (54)
Get drive allocation information
────────────────────────────────────────────────────────────────────────────
Obtains selected information about a disk drive, from which the drive's
capacity and remaining free space can be calculated.
Call with:
AH = 36H
DL = drive code (0 = default, 1 = A, etc.)
Returns:
If function successful
AX = sectors per cluster
BX = number of available clusters
CX = bytes per sector
DX = clusters per drive
If function unsuccessful (drive invalid)
AX = FFFFH
Notes:
■ This function regards "lost" clusters as being in use and does not
report them as part of the number of available clusters, even though
they are not assigned to a file.
■ Similar information is returned by Int 21H Functions 1BH and 1CH.
Example:
Calculate the capacity of disk drive C in bytes, leaving the result in the
variable drvsize. (This code assumes that the product of sectors/cluster *
bytes/sector will not overflow 16 bits.)
drvsize dd ? ; drive C size in bytes
.
.
.
mov ah,36h ; function number
mov dl,3 ; drive C = 3
int 21h ; transfer to MS-DOS
mul cx ; sectors/cluster
; * bytes/sector
mul dx ; * total clusters
; result now in DX:AX
; store low word
mov word ptr drvsize,ax
; store high word
mov word ptr drvsize+2,dx
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 37H (55)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 38H (56)
Get or set country information
────────────────────────────────────────────────────────────────────────────
[2] Obtains internationalization information for the current country.
[3.0+] Obtains internationalization information for the current or
specified country or sets the current country code.
Call with:
If getting country information (MS-DOS version 2)
AH = 38H
AL = 0 to get "current" country information
DS:DX = segment:offset of buffer for returned information
If getting country information (MS-DOS versions 3.0 and later)
AH = 38H
AL = 0 to get "current" country information
1─FEH to get information for countries with code < 255
FFH to get information for countries with code >=
255
BX = country code, if AL = FFH
DS:DX = segment:offset of buffer for returned information
If setting current country code (MS-DOS versions 3.0 and later)
AH = 38H
AL = 1─FEH country code for countries with code < 255
FFH for countries with code >= 255
BX = country code, if AL = 0FFH
DX = FFFFH
Returns:
If function successful
Carry flag = clear
and, if getting internationalization information
BX = country code
DS:DX = segment:offset of buffer holding internationalization
information
and buffer filled in as follows:
(for PC-DOS 2.0 and 2.1)
Byte(s) Contents
00H─01H date format
0 = USA m d y
1 = Europe d m y
2 = Japan y m d
02H─03H ASCIIZ currency symbol
04H─05H ASCIIZ thousands separator
06H─07H ASCIIZ decimal separator
08H─1FH reserved
(for MS-DOS versions 2.0 and later, PC-DOS versions 3.0 and later)
Byte(s) Contents
00H─01H date format
0 = USA m d y
1 = d m y
Europe
2 = Japan y m d
02H─06H ASCIIZ currency symbol string
07H─08H ASCIIZ thousands separator character
09H─0AH ASCIIZ decimal separator character
0BH─0CH ASCIIZ date separator character
0DH─0EH ASCIIZ time separator character
0FH currency format
bit 0 = 0 if currency symbol precedes value
= 1 if currency symbol follows value
bit 1 = 0 if no space between value and currency
symbol
= 1 if one space between value and
currency symbol
bit 2 = 0 if currency symbol and decimal are
separate
= 1 if currency symbol replaces decimal
separator
10H number of digits after decimal in currency
11H time format
bit 0 = 0 if 12-hour clock
= 1 if 24-hour clock
12H─15H case-map call address
16H─17H ASCIIZ data-list separator
18H─21H reserved
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The default country code is determined by the COUNTRY= directive in
CONFIG.SYS or by the KEYBxx keyboard driver file if one is loaded.
Otherwise, the default country code is OEM-dependent.
■ The previous contents of register CX may be destroyed by the Get Country
Information subfunction.
■ The case-map call address is the segment:offset of a FAR procedure that
performs country-specific mapping on character values from 80H through
0FFH. The procedure must be called with the character to be mapped in
register AL. If an alternate value exists for that character, it is
returned in AL; otherwise, AL is unchanged. In general, lowercase
characters are mapped to their uppercase equivalents, and accented or
otherwise modified vowels are mapped to their plain vowel equivalents.
■ [3.0+] The value in register DX is used by MS-DOS to select between the
Set Country and Get Country Information subfunctions.
■ [3.3+] Int 21H Function 65H (Get Extended Country Information) returns
a superset of the information supplied by this function.
Examples:
Obtain internationalization information for the current country in the
buffer ctrybuf.
ctrybuf db 34 dup (0)
.
.
.
mov ah,38h ; function number
mov al,0 ; get current country
mov dx,seg ctrybuf ; address of buffer
mov ds,dx ; for country information
mov dx,offset ctrybuf
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
If the program is running under PC-DOS 3.3 and the current country code is
49 (West Germany), ctrybuf is filled in with the following information:
dw 0001h ; date format
db 'DM',0,0,0 ; ASCIIZ currency symbol
db '.',0 ; ASCIIZ thousands separator
db ',',0 ; ASCIIZ decimal separator
db '.',0 ; ASCIIZ date separator
db '.',0 ; ASCIIZ time separator
db 02h ; currency format
db 02h ; digits after decimal
db 01h ; time format
dd 026ah:176ch ; case-map call address
db ';',0 ; ASCIIZ data-list separator
db 10 dup (0) ; reserved
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 39H (57)
Create directory
────────────────────────────────────────────────────────────────────────────
Creates a directory using the specified drive and path.
Call with:
AH = 39H
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ The function fails if:
∙ any element of the pathname does not exist.
∙ a directory with the same name at the end of the same path already
exists.
∙ the parent directory for the new directory is the root directory and
is full.
∙ [3.0+] the program is running on a network and the user running the
program has insufficient access rights.
Example:
Create a directory named MYSUB in the root directory on drive C.
dname db 'C:\MYSUB',0
.
.
.
mov ah,39h ; function number
mov dx,seg dname ; address of pathname
mov ds,dx
mov dx,offset dname
int 21h ; transfer to MS-DOS
jc error ; jump if create failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 3AH (58)
Delete directory
────────────────────────────────────────────────────────────────────────────
Removes a directory using the specified drive and path.
Call with:
AH = 3AH
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ The function fails if:
∙ any element of the pathname does not exist.
∙ the specified directory is also the current directory.
∙ the specified directory contains any files.
∙ [3.0+] the program is running on a network and the user running the
program has insufficient access rights.
Example:
Remove the directory named MYSUB in the root directory on drive C.
dname db 'C:\MYSUB',0
.
.
.
mov ah,3ah ; function number
mov dx,seg dname ; address of pathname
mov ds,dx
mov dx,offset dname
int 21h ; transfer to MS-DOS
jc error ; jump if delete failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 3BH (59)
Set current directory
────────────────────────────────────────────────────────────────────────────
Sets the current, or default, directory using the specified drive and
path.
Call with:
AH = 3BH
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The function fails if any element of the pathname does not exist.
■ Int 21H Function 47H can be used to obtain the name of the current
directory before using Int 21H Function 3BH to select another, so that
the original directory can be restored later.
Example:
Change the current directory for drive C to the directory \MYSUB.
dname db 'C:\MYSUB',0
.
.
.
mov ah,3bh ; function number
mov dx,seg dname ; address of pathname
mov ds,dx
mov dx,offset dname
int 21h ; transfer to MS-DOS
jc error ; jump if bad path
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 3CH (60)
Create file
────────────────────────────────────────────────────────────────────────────
Given an ASCIIZ pathname, creates a new file in the designated or default
directory on the designated or default disk drive. If the specified file
already exists, it is truncated to zero length. In either case, the file
is opened and a handle is returned that can be used by the program for
subsequent access to the file.
Call with:
AH = 3CH
CX = file attribute (bits may be combined)
Bit(s) Significance (if set)
0 read-only
1 hidden
2 system
3 volume label
4 reserved (0)
5 archive
6─15 reserved (0)
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
AX = handle
If function failed
Carry flag = set
AX = error code
Notes:
■ The function fails if:
∙ any element of the pathname does not exist.
∙ the file is being created in the root directory and the root directory
is full.
∙ a file with the same name and the read-only attribute already exists
in the specified directory.
∙ [3.0+] the program is running on a network and the user running the
program has insufficient access rights.
■ A file is usually given a normal (0) attribute when it is created. The
file's attribute can subsequently be modified with Int 21H Function
43H.
■ [3.0+] A volume label can be created using an attribute of 0008H, if one
does not already exist. When files are created, bit 3 of the attribute
parameter should always be clear (0).
■ [3.0+] See the entries for Int 21H Functions 5AH and 5BH, which may
also be used to create files.
■ [4.0+] Int 21H Function 6CH combines the services of Functions 3CH,
3DH, and 5BH.
Example:
Create and open, or truncate to zero length and open, the file
C:\MYDIR\MYFILE.DAT, and save the handle for subsequent access to the
file.
fname db 'C:\MYDIR\MYFILE.DAT',0
fhandle dw ?
.
.
.
mov ah,3ch ; function number
xor cx,cx ; normal attribute
mov dx,seg fname ; address of pathname
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if create failed
mov fhandle,ax ; save file handle
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 3DH (61)
Open file
────────────────────────────────────────────────────────────────────────────
Given an ASCIIZ pathname, opens the specified file in the designated or
default directory on the designated or default disk drive. A handle is
returned which can be used by the program for subsequent access to the
file.
Call with:
AH = 3DH
AL = access mode
Bit(s) Significance
0─2 access mode
000 = read access
001 = write access
010 = read/write access
3 reserved (0)
4─6 sharing mode (MS-DOS versions 3.0 and later)
000 = compatibility mode
001 = deny all
010 = deny write
011 = deny read
100 = deny none
7 inheritance flag (MS-DOS versions 3.0 and later)
0 = child process inherits handle
1 = child does not inherit handle
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
AX = handle
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ Any normal, system, or hidden file with a matching name will be opened
by this function. If the file is read-only, the success of the operation
also depends on the access code in bits 0─2 of register AL. After
opening the file, the file read/write pointer is set to offset zero (the
first byte of the file).
■ The function fails if:
∙ any element of the pathname does not exist.
∙ the file is opened with an access mode of read/write and the file has
the read-only attribute.
∙ [3.0+] SHARE.EXE is loaded and the file has already been opened by one
or more other processes in a sharing mode that is incompatible with
the current program's request.
■ The file's date and time stamp can be accessed after a successful open
call with Int 21H Function 57H.
■ The file's attributes (hidden, system, read-only, or archive) can be
obtained with Int 21H Function 43H.
■ When a file handle is inherited by a child process or is duplicated with
Int 21H Function 45H or 46H, all sharing and access restrictions are
also inherited.
■ [2] Only bits 0─2 of register AL are significant; the remaining bits
should be zero for upward compatibility.
■ [3.0+] Bits 4─7 of register AL control access to the file by other
programs. (Bits 4─6 have no effect unless SHARE.EXE is loaded.)
■ [3.0+] A file-sharing error causes a critical-error exception (Int 24H)
with an error code of 02H. Int 21H Function 59H can be used to obtain
information about the sharing violation.
■ [4.0+] Int 21H Function 6CH combines the services of Functions 3CH,
3DH, and 5BH.
Example:
Open the file C:\MYDIR\MYFILE.DAT for both reading and writing, and save
the handle for subsequent access to the file.
fname db 'C:\MYDIR\MYFILE.DAT',0
fhandle dw ?
.
.
.
mov ah,3dh ; function number
mov al,2 ; mode = read/write
mov dx,seg fname ; address of pathname
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if open failed
mov fhandle,ax ; save file handle
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 3EH (62)
Close file
────────────────────────────────────────────────────────────────────────────
Given a handle that was obtained by a previous successful open or create
operation, flushes all internal buffers associated with the file to disk,
closes the file, and releases the handle for reuse. If the file was
modified, the time and date stamp and file size are updated in the file's
directory entry.
Call with:
AH = 3EH
BX = handle
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ If you accidentally call this function with a zero handle, the standard
input device is closed, and the keyboard appears to go dead. Make sure
you always call the close function with a valid, nonzero handle.
Example:
Close the file whose handle is saved in the variable fhandle.
fhandle dw 0
.
.
.
mov ah,3eh ; function number
mov bx,fhandle ; file handle
int 21h ; transfer to MS-DOS
jc error ; jump if close failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 3FH (63)
Read file or device
────────────────────────────────────────────────────────────────────────────
Given a valid file handle from a previous open or create operation, a
buffer address, and a length in bytes, transfers data at the current
file-pointer position from the file into the buffer and then updates the
file pointer position.
Call with:
AH = 3FH
BX = handle
CX = number of bytes to read
DS:DX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AX = bytes transferred
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ If reading from a character device (such as the standard input) in
cooked mode, at most one line of input will be read (i.e., up to a
carriage return character or the specified length, whichever comes
first).
■ If the carry flag is returned clear but AX = 0, then the file pointer
was already at end of file when the program requested the read.
■ If the carry flag is returned clear but AX < CX, then a partial record
was read at end of file or there is an error.
■ [3.0+] If the program is running on a network, the user must have Read
access rights to the directory and file.
Example:
Using the file handle from a previous open or create operation, read 1024
bytes at the current file pointer into the buffer named buff.
buff db 1024 dup (?) ; buffer for read
fhandle dw ? ; contains file handle
.
.
.
mov ah,3fh ; function number
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
mov bx,fhandle ; file handle
mov cx,1024 ; length to read
int 21h ; transfer to MS-DOS
jc error ; jump, read failed
cmp ax,cx ; check length of read
jl done ; jump, end of file
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 40H (64)
Write file or device
────────────────────────────────────────────────────────────────────────────
Given a valid file handle from a previous open or create operation, a
buffer address, and a length in bytes, transfers data from the buffer into
the file and then updates the file pointer position.
Call with:
AH = 40H
BX = handle
CX = number of bytes to write
DS:DX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AX = bytes transferred
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ If the carry flag is returned clear but AX < CX, then a partial record
was written or there is an error. This can be caused by a Ctrl-Z (1AH)
embedded in the data if the destination is a character device in cooked
mode or by a disk full condition if the destination is a file.
■ If the function is called with CX = 0, the file is truncated or extended
to the current file pointer position.
■ [3.0+] If the program is running on a network, the user must have Write
access rights to the directory and file.
Example:
Using the handle from a previous open or create operation, write 1024
bytes to disk at the current file pointer from the buffer named buff.
buff db 1024 dup (?) ; buffer for write
fhandle dw ? ; contains file handle
.
.
.
mov ah,40h ; function number
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
mov bx,fhandle ; file handle
mov cx,1024 ; length to write
int 21h ; transfer to MS-DOS
jc error ; jump, write failed
cmp ax,1024 ; entire record written?
jne error ; no, jump
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 41H (65)
Delete file
────────────────────────────────────────────────────────────────────────────
Deletes a file from the specified or default disk and directory.
Call with:
AH = 41H
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ This function deletes a file by replacing the first character of its
filename in the directory with the character e (E5H) and marking the
file's clusters as "free" in the disk's file allocation table. The
actual data stored in those clusters is not overwritten.
■ Only one file at a time may be deleted with this function. Unlike the
FCB-related Delete File function (Int 21H Function 13H), the * and ?
wildcard characters are not allowed in the file specification.
■ The function fails if:
∙ any element of the pathname does not exist.
∙ the designated file exists but has the read-only attribute. (Int 21H
Function 43H can be used to examine and modify a file's attribute
before attempting to delete it.)
∙ [3.0+] the program is running on a network, and the user running the
program has insufficient access rights.
Example:
Delete the file named MYFILE.DAT from the directory \MYDIR on drive C.
fname db 'C:\MYDIR\MYFILE.DAT',0
.
.
.
mov ah,41h ; function number
mov dx,seg fname ; filename address
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if delete failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 42H (66)
Set file pointer
────────────────────────────────────────────────────────────────────────────
Sets the file location pointer relative to the start of file, end of file,
or current file position.
Call with:
AH = 42H
AL = method code
00H absolute offset from start of file
01H signed offset from current file pointer
02H signed offset from end of file
BX = handle
CX = most significant half of offset
DX = least significant half of offset
Returns:
If function successful
Carry flag = clear
DX = most significant half of resulting file pointer
AX = least significant half of resulting file pointer
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ This function uses a method code and a double-precision (32-bit) value
to set the file pointer. The next record read or written in the file
will begin at the new file pointer location. No matter what method is
used in the call to this function, the file pointer returned in DX:AX is
always the resulting absolute byte offset from the start of file.
■ Method 02H may be used to find the size of the file by calling Int 21H
Function 42H with an offset of 0 and examining the pointer location
that is returned.
■ Using methods 01H or 02H, it is possible to set the file pointer to a
location that is before the start of file. If this is done, no error is
returned by this function, but an error will be encountered upon a
subsequent attempt to read or write the file.
Examples:
Using the file handle from a previous open or create operation, set the
current file pointer position to 1024 bytes after the start of file.
fhandle dw ?
.
.
.
mov ah,42h ; function number
mov al,0 ; method = absolute
mov bx,fhandle ; file handle
mov cx,0 ; upper half of offset
mov dx,1024 ; lower half of offset
int 21h ; transfer to MS-DOS
jc error ; jump, function failed
.
.
.
The following subroutine accepts a record number, record size, and handle
and sets the file pointer appropriately.
; call this routine with BX = handle
; AX = record number
; CX = record size
; returns all registers unchanged
;
setptr proc near
push ax ; save record number
push cx ; save record size
push dx ; save whatever's in DX
mul cx ; size * record number
mov cx,ax ; upper part to CX
xchg cx,dx ; lower part to DX
mov ax,4200h ; function number & method
int 21h ; transfer to MS-DOS
pop dx ; restore previous DX
pop cx ; restore record size
pop ax ; restore record number
ret ; back to caller
setptr endp
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 43H (67)
Get or set file attributes
────────────────────────────────────────────────────────────────────────────
Obtains or alters the attributes of a file (read-only, hidden, system, or
archive) or directory.
Call with:
AH = 43H
AL = 00H to get attributes
01H to set attributes
CX = file attribute, if AL = 01H (bits can be combined)
Bit(s) Significance (if set)
0 read-only
1 hidden
2 system
3─4 reserved (0)
5 archive
6─15 reserved (0)
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
CX = file attribute
Bit(s) Significance (if set)
0 read-only
1 hidden
2 system
3 volume label
4 directory
5 archive
6─15 reserved (0)
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ Bits 3 and 4 of register CX must always be clear (0) when this function
is called; in other words, you cannot change an existing file into a
directory or volume label. However, you can assign the "hidden"
attribute to an existing directory with this function.
■ [3.0+] If the program is running on a network, the user must have Create
access rights to the directory containing the file whose attribute is to
be modified.
Example:
Change the attribute of the file D:\MYDIR\MYFILE.DAT to read-only, so that
it cannot be accidentally modified or deleted by other application
programs.
rdonly equ 01h ; file attributes
hidden equ 02h
system equ 04h
volume equ 08h
subdir equ 10h
archive equ 20h
fname db 'D:\MYDIR\MYFILE.DAT',0
.
.
.
mov ah,43h ; function number
mov al,01h ; subfunction = modify
mov cx,rdonly ; read-only attribute
mov dx,seg fname ; filename address
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if modify failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68)
IOCTL (I/O control)
────────────────────────────────────────────────────────────────────────────
Provides a direct path of communication between an application program and
a device driver. Allows a program to obtain hardware-dependent information
and to request operations that are not supported by other MS-DOS function
calls.
The IOCTL subfunctions and the MS-DOS versions in which they first became
available are:
╓┌─┌──────────────┌──────────────────────────────────────────┌───────────────╖
Subfunction Name MS-DOS version
──────────────────────────────────────────────────────────────────────────
00H Get Device Information 2.0
01H Set Device Information 2.0
02H Receive Control Data from Character 2.0
Device Driver
03H Send Control Data to Character Device 2.0
Driver
04H Receive Control Data from Block Device 2.0
Driver
05H Send Control Data to Block Device Driver 2.0
06H Check Input Status 2.0
07H Check Output Status 2.0
08H Check If Block Device Is Removable 3.0
Subfunction Name MS-DOS version
──────────────────────────────────────────────────────────────────────────
08H Check If Block Device Is Removable 3.0
09H Check If Block Device Is Remote 3.1
0AH (10) Check If Handle Is Remote 3.1
0BH (11) Change Sharing Retry Count 3.1
0CH (12) Generic I/O Control for Character Devices
CL = 45H: Set Iteration Count 3.2
CL = 4AH: Select Code Page 3.3
CL = 4CH: Start Code Page Preparation 3.3
CL = 4DH: End Code Page Preparation 3.3
CL = 5FH: Set Display Information 4.0
CL = 65H: Get Iteration Count 3.2
CL = 6AH: Query Selected Code Page 3.3
CL = 6BH: Query Prepare List 3.3
CL = 7FH: Get Display Information 4.0
0DH (13) Generic I/O Control for Block Devices
CL = 40H: Set Device Parameters 3.2
CL = 41H: Write Track 3.2
CL = 42H: Format and Verify Track 3.2
CL = 47H: Set Access Flag 4.0
Subfunction Name MS-DOS version
──────────────────────────────────────────────────────────────────────────
CL = 47H: Set Access Flag 4.0
CL = 60H: Get Device Parameters 3.2
CL = 61H: Read Track 3.2
CL = 62H: Verify Track 3.2
CL = 67H: Get Access Flag 4.0
0EH (14) Get Logical Drive Map 3.2
0FH (15) Set Logical Drive Map 3.2
──────────────────────────────────────────────────────────────────────────
Only IOCTL Subfunctions 00H, 06H, and 07H may be used for handles
associated with files. Subfunctions 00H─08H are not supported on network
devices.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68) Subfunction 00H
IOCTL: get device information
────────────────────────────────────────────────────────────────────────────
Returns a device information word for the file or device associated with
the specified handle.
Call with:
AH = 44H
AL = 00H
BX = handle
Returns:
If function successful
Carry flag = clear
DX = device information word
For a file:
Bit(s) Significance
0─5 drive number (0 = A, 1 = B, etc.)
6 0 if file has been written
1 if file has not been written
7 0, indicating a file
8─15 reserved
For a device:
Bit(s) Significance
0 1 if standard input
1 1 if standard output
2 1 if NUL device
3 1 if clock device
4 reserved
5 0 if handle in ASCII mode
1 if handle in binary mode
6 0 if end of file on input
7 1, indicating a device
8─13 reserved
14 0 if IOCTL subfunctions 02H and 03H not
supported
1 if IOCTL subfunctions 02H and 03H supported
15 reserved
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ Bits 8─15 of DX correspond to the upper 8 bits of the device-driver
attribute word.
■ Bit 5 of the device information word for a handle associated with a
character device signifies whether MS-DOS considers that handle to be in
binary ("raw") mode or ASCII ("cooked") mode. In ASCII mode, MS-DOS
filters the character stream and may take special action when the
characters Ctrl-C, Ctrl-S, Ctrl-P, Ctrl-Z, and carriage return are
detected. In binary mode, all characters are treated as data, and the
exact number of characters requested is always read or written.
Example:
See Int 21H Function 44H Subfunction 01H.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68) Subfunction 01H
IOCTL: set device information
────────────────────────────────────────────────────────────────────────────
Sets certain flags for a handle associated with a character device. This
subfunction may not be used for a handle that is associated with a file.
Call with:
AH = 44H
AL = 01H
BX = handle
DX = device information word
Bit(s) Significance
0 1 if standard input
1 1 if standard output
2 1 if NUL device
3 1 if clock device
4 reserved (0)
5 0 to select ASCII mode
1 to select binary mode
6 reserved (0)
7 1, indicating a device
8─15 reserved (0)
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ If register DH does not contain 00H, control returns to the program with
the carry flag set and error code 0001H (invalid function) in register
AX.
■ Bit 5 of the information word for a handle associated with a character
device signifies whether MS-DOS considers that handle to be in binary
("raw") or ASCII ("cooked") mode. See Notes for Int 21H Function 44H
Subfunction 00H.
Example:
Place the standard output handle into binary ("raw") mode. This speeds up
output by disabling checking for Ctrl-C, Ctrl-S, and Ctrl-P between each
character.
.
.
.
; get device information
mov ax,4400h ; function & subfunction
mov bx,1 ; standard output handle
int 21h ; transfer to MS-DOS
mov dh,0 ; force DH = 0
or dl,20h ; set binary mode bit
; set device information
mov ax,4401h ; function & subfunction
int 21h ; transfer to MS-DOS
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68) Subfunction 02H
IOCTL: read control data from character device driver
────────────────────────────────────────────────────────────────────────────
Reads control data from a character-device driver. The length and contents
of the data are specific to each device driver and do not follow any
standard format. This function does not necessarily result in any input
from the physical device.
Call with:
AH = 44H
AL = 02H
BX = handle
CX = number of bytes to read
DS:DX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AX = bytes read
and buffer contains control data from driver
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ If supported by the driver, this subfunction can be used to obtain
hardware-dependent status and availability information that is not
supported by other MS-DOS function calls.
■ Character-device drivers are not required to support IOCTL Subfunction
02H. A program can test bit 14 of the device information word returned
by IOCTL Subfunction 00H to determine whether the driver supports this
subfunction. If Subfunction 02H is requested and the driver does not
have the ability to process control data, control returns to the program
with the carry flag set and error code 0001H (invalid function) in
register AX.
Example:
Read a control string from the standard list driver into the buffer buff.
stdprn equ 4 ; standard list handle
buflen equ 64 ; length of buffer
ctllen dw ? ; length of control string
buff db buflen dup (0) ; receives control string
.
.
.
mov ax,4402h ; function & subfunction
mov bx,stdprn ; standard list handle
mov cx,buflen ; buffer length
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
jc error ; jump if read failed
mov ctllen,ax ; save control string length
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68) Subfunction 03H
IOCTL: write control data to character-device driver
────────────────────────────────────────────────────────────────────────────
Transfers control data from an application to a character-device driver.
The length and contents of the data are specific to each device driver and
do not follow any standard format. This function does not necessarily
result in any output to the physical device.
Call with:
AH = 44H
AL = 03H
BX = handle
CX = number of bytes to write
DS:DX = segment:offset of data
Returns:
If function successful
Carry flag = clear
AX = bytes transferred
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ If supported by the driver, this subfunction can be used to request
hardware-dependent operations (such as setting baud rate for a serial
port) that are not supported by other MS-DOS function calls.
■ Character-device drivers are not required to support IOCTL Subfunction
03H. A program can test bit 14 of the device information word returned
by IOCTL Subfunction 00H to determine whether the driver supports this
subfunction. If Subfunction 03H is requested and the driver does not
have the ability to process control data, control returns to the program
with the carry flag set and error code 0001H (invalid function) in
register AX.
Example:
Write a control string from the buffer buff to the standard list device
driver. The length of the string is assumed to be in the variable ctllen.
stdprn equ 4 ; standard list handle
buflen equ 64 ; length of buffer
ctllen dw ? ; length of control data
buff db buflen dup (?) ; contains control data
.
.
.
mov ax,4403h ; function & subfunction
mov bx,stdprn ; standard list handle
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
mov cx,ctllen ; length of control data
int 21h ; transfer to MS-DOS
jc error ; jump if write failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68) Subfunction 04H
IOCTL: read control data from block-device driver
────────────────────────────────────────────────────────────────────────────
Transfers control data from a block-device driver directly into an
application program's buffer. The length and contents of the data are
specific to each device driver and do not follow any standard format. This
function does not necessarily result in any input from the physical
device.
Call with:
AH = 44H
AL = 04H
BL = drive code (0 = default, 1 = A, 2 = B, etc.)
CX = number of bytes to read
DS:DX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AX = bytes transferred
and buffer contains control data from device driver
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ When supported by the driver, this subfunction can be used to obtain
hardware-dependent status and availability information that is not
provided by other MS-DOS function calls.
■ Block-device drivers are not required to support IOCTL Subfunction 04H.
If this subfunction is requested and the driver does not have the
ability to process control data, control returns to the program with the
carry flag set and error code 0001H (invalid function) in register AX.
Example:
Read a control string from the block-device driver for drive C into the
buffer buff.
buflen equ 64 ; length of buffer
ctllen dw ? ; length of control string
buff db buflen dup (0) ; receives control string
.
.
.
mov ax,4404h ; function & subfunction
mov bl,3 ; drive C = 3
mov cx,buflen ; buffer length
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
int 21h ; transfer to MS-DOS
jc error ; jump if read failed
mov ctllen,ax ; save control string length
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68) Subfunction 05H
IOCTL: write control data to block-device driver
────────────────────────────────────────────────────────────────────────────
Transfers control data from an application program directly to a
block-device driver. The length and contents of the control data are
specific to each device driver and do not follow any standard format. This
function does not necessarily result in any output to the physical device.
Call with:
AH = 44H
AL = 05H
BL = drive code (0 = default, 1 = A, 2 = B, etc.)
CX = number of bytes to write
DS:DX = segment:offset of data
Returns:
If function successful
Carry flag = clear
AX = bytes transferred
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ When supported by the driver, this subfunction can be used to request
hardware-dependent operations (such as tape rewind or disk eject) that
are not provided by other MS-DOS function calls.
■ Block-device drivers are not required to support IOCTL Subfunction 05H.
If this subfunction is requested and the driver does not have the
ability to process control data, control returns to the program with the
carry flag set and error code 0001H (invalid function) in register AX.
Example:
Write a control string from the buffer buff to the block-device driver for
drive C. The length of the string is assumed to be in the variable ctllen.
buflen equ 64 ; length of buffer
ctllen dw ? ; length of control data
buff db buflen dup (?) ; contains control data
.
.
.
mov ax,4405h ; function & subfunction
mov bl,3 ; drive C = 3
mov dx,seg buff ; buffer address
mov ds,dx
mov dx,offset buff
mov cx,ctllen ; length of control data
int 21h ; transfer to MS-DOS
jc error ; jump if write failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68) Subfunction 06H
IOCTL: check input status
────────────────────────────────────────────────────────────────────────────
Returns a code indicating whether the device or file associated with a
handle is ready for input.
Call with:
AH = 44H
AL = 06H
BX = handle
Returns:
If function successful
Carry flag = clear
and, for a device:
AL = 00H if device not ready
FFH if device ready
or, for a file:
AL = 00H if file pointer at EOF
FFH if file pointer not at EOF
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ This function can be used to check the status of character devices, such
as the serial port, that do not have their own "traditional" MS-DOS
status calls.
Example:
Check whether a character is ready from the standard auxiliary device
(usually COM1).
stdaux equ 3 ; standard auxiliary handle
.
.
.
mov ax,4406h ; function & subfunction
mov bx,stdaux ; standard auxiliary handle
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
or al,al ; test status flag
jnz ready ; jump if character ready
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 44H (68) Subfunction 07H
IOCTL: check output status
────────────────────────────────────────────────────────────────────────────
Returns a code indicating whether the device associated with a handle is
ready for output.
Call with:
AH = 44H
AL = 07H
BX = handle
Returns:
If function successful
Carry flag = clear
and, for a device:
AL = 00H if device not ready
FFH if device ready
or, for a file:
AL = FFH
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ When used with a handle for a file, this function always returns a ready
status, even if the disk is full or no disk is in the drive.
Example:
Check whether the standard auxiliary device (usually COM1) can accept a
character for output.
stdaux equ 3 ; standard auxiliary handle
.
.
.
mov ax,4407h ; function & subfunction
mov bx,stdaux ; standard auxiliary handle
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
or al,al ; test status flag
jnz ready ; jump if not busy
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.0]
Function 44H (68) Subfunction 08H
IOCTL: check if block device is removable
────────────────────────────────────────────────────────────────────────────
Checks whether the specified block device contains a removable storage
medium, such as a floppy disk.
Call with:
AH = 44H
AL = 08H
BL = drive number (0 = default, 1 = A, 2 = B, etc.)
Returns:
If function successful
Carry flag = clear
AL = 00H if medium is removable
01H if medium is not removable
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ If a file is not found as expected on a particular drive, a program can
use this subfunction to determine whether the user should be prompted to
insert another disk.
■ This subfunction may not be used for a network drive.
■ Block drivers are not required to support Subfunction 08H. If this
subfunction is requested and the block device cannot supply the
information, control returns to the program with the carry flag set and
error code 0001H (invalid function) in register AX.
Example:
Check whether drive C is removable.
.
.
.
mov ax,4408h ; function & subfunction
mov bl,3 ; drive 3 = C
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
and al,1 ; test type of medium
jnz fixed ; jump if not removable
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 44H (68) Subfunction 09H
IOCTL: check if block device is remote
────────────────────────────────────────────────────────────────────────────
Checks whether the specified block device is local (attached to the
computer running the program) or remote (redirected to a network server).
Call with:
AH = 44H
AL = 09H
BL = drive number (0 = default, 1 = A, 2 = B, etc.)
Returns:
If function successful
Carry flag = clear
DX = device attribute word
bit 12 = 0 if drive is local
1 if drive is remote
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ Use of this subfunction should be avoided. Application programs should
not distinguish between files on local and remote devices.
Example:
Check whether drive D is mounted on the machine running the program or is
a network drive.
.
.
.
mov ax,4409h ; function & subfunction
mov bl,4 ; drive 4 = D
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
and dx,1000h ; test local/remote bit
jnz remote ; jump if network drive
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 44H (68) Subfunction 0AH (10)
IOCTL: check if handle is remote
────────────────────────────────────────────────────────────────────────────
Checks whether the specified handle refers to a file or device that is
local (located on the PC that is running the program) or remote (located
on a network server).
Call with:
AH = 44H
AL = 0AH
BX = handle
Returns:
If function successful
Carry flag = clear
DX = attribute word for file or device
bit 15 = 0 if local
1 if remote
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ Application programs should not ordinarily attempt to distinguish
between files on local and remote devices.
■ If the network has not been started, control returns to the calling
program with the carry flag set and error code 0001H (invalid function)
in register AX.
Example:
Check if the handle saved in the variable fhandle is associated with a
file or device on the machine running the program or on a network server.
fhandle dw ? ; device handle
.
.
.
mov ax,440ah ; function & subfunction
mov bx,fhandle ; file/device handle
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
and dx,8000h ; test local/remote bit
jnz remote ; jump if network handle
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 44H (68) Subfunction 0BH (11)
IOCTL: change sharing retry count
────────────────────────────────────────────────────────────────────────────
Sets the number of times MS-DOS retries a disk operation after a failure
caused by a file-sharing violation before it returns an error to the
requesting process. This subfunction is not available unless the
file-sharing module (SHARE.EXE) is loaded.
Call with:
AH = 44H
AL = 0BH
CX = delays per retry (default = 1)
DX = number of retries (default = 3)
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The length of a delay is a machine-dependent value determined by the CPU
type and clock speed. Each delay consists of the following instruction
sequence:
xor cx,cx
loop $
which executes 65,536 times before falling out of the loop.
■ The sharing retry count affects the behavior of the system as a whole
and is not a local parameter for the process. If a program changes the
sharing retry count, it should restore the default values before
terminating.
Example:
Change the number of automatic retries for a file-sharing violation to
five.
.
.
.
mov ax,440bh ; function & subfunction
mov cx,1 ; delays per retry
mov dx,5 ; number of retries
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.2]
Function 44H (68) Subfunction 0CH (12)
IOCTL: generic I/O control for character devices
────────────────────────────────────────────────────────────────────────────
Provides a general-purpose mechanism for communication between application
programs and character-device drivers.
Call with:
AH = 44H
AL = 0CH
BX = handle
CH = category (major) code:
00H = unknown
01H = COM1, COM2, COM3, or COM4 (3.3)
03H = CON (keyboard and display) (3.3)
05H = LPT1, LPT2, or LPT3 (3.2)
CL = function (minor) code:
45H = Set Iteration Count (3.2)
4AH = Select Code Page (3.3)
4CH = Start Code Page Preparation (3.3)
4DH = End Code Page Preparation (3.3)
5FH = Set Display Information (4.0)
65H = Get Iteration Count (3.2)
6AH = Query Selected Code Page (3.3)
6BH = Query Prepare List (3.3)
7FH = Get Display Information (4.0)
DS:DX = segment:offset of parameter block
Returns:
If function successful
Carry flag = clear
and, if called with CL = 65H, 6AH, 6BH, or 7FH
DS:DX = segment:offset of parameter block
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ If the minor code is 45H (Set Iteration Count) or 65H (Get Iteration
Count), the parameter block is simply a 2-byte buffer containing or
receiving the iteration count for the printer. This call is valid only
for printer drivers that support Output Until Busy, and determines the
number of times the device driver will wait for the device to signal
ready before returning from the output call.
■ The parameter block for minor code 4DH (End Code Page Preparation) has
the following format:
dw 2 ; length of following data
dw 0 ; (reserved)
■ For MS-DOS version 3.3, the parameter block for minor codes 4AH (Select
Code Page) and 6AH (Query Code Page) has the following format:
dw 2 ; length of following data
dw ? ; code page ID
For MS-DOS version 4.0, minor codes 4AH and 6AH also set or get the
double-byte character set (DBCS) lead byte table, and the following format
is used:
dw (n+2)*2+1 ; length of following data
dw ? ; code page ID
db start,end ; DBCS lead byte range 1
.
.
.
db start,end ; DBCS lead byte range n
db 0,0
■ The parameter block for minor code 4CH (Start Code Page Preparation) has
the following format:
dw 0 ; font type
; bit 0 = 0 downloaded
; = 1 cartridge
; bits 1-15 = reserved (0)
dw (n+1)*2 ; length of remainder of
; parameter block
dw n ; number of code pages in
; the following list
dw ? ; code page 1
dw ? ; code page 2
.
.
.
dw ? ; code page n
■ The parameter block for minor code 6BH (Query Prepare List) has the
following format, assuming n hardware code pages and m prepared code
pages (n <= 12, m <= 12):
dw (n+m+2)*2 ; length of following data
dw n ; number of hardware code pages
dw ? ; hardware code page 1
dw ? ; hardware code page 2
.
.
.
dw ? ; hardware code page n
dw m ; number of prepared code pages
dw ? ; prepared code page 1
dw ? ; prepared code page 2
.
.
.
dw ? ; prepared code page m
■ After a minor code 4CH (Start Code Page Preparation) call, the data
defining the code page font is written to the driver using one or more
calls to the IOCTL Write Control Data subfunction (Interrupt 21H,
Function 44H, Subfunction 03H). The format of the data is device- and
driver-specific. After the font data has been written to the driver, a
minor code 4DH (End Code Page Preparation) call must be issued. If no
data is written to the driver between the minor code 4CH and 4DH calls,
the driver interprets the newly prepared code pages as hardware code
pages.
■ A special variation of the minor code 4CH (Start Code Page Preparation)
call, called "Refresh," is required to actually load the peripheral
device with the prepared code pages. The refresh operation is obtained
by requesting minor code 4CH with each code page position in the
parameter block set to -1, followed by an immediate call for minor code
4DH (End Code Page Preparation).
■ [4.0+] For minor codes 5FH (Set Display Information) and 7FH (Get
Display Information), the parameter block is formatted as follows:
db 0 ; level (0 in MS-DOS 4.0)
db 0 ; reserved (must be 0)
dw 14 ; length of following data
dw ? ; control flags
; bit 0 = 0 intensity
; = 1 blink
; bits 1-15 = reserved (0)
db ? ; mode type (1 = text, 2 = APA)
db 0 ; reserved (must be 0)
dw ? ; colors
; 0 = monochrome compatible
; 1 = 2 colors
; 2 = 4 colors
; 4 = 16 colors
; 8 = 256 colors
dw ? ; pixel columns
dw ? ; pixel rows
dw ? ; character columns
dw ? ; character rows
Example:
Get the current code page for the standard list device.
stdprn equ 4 ; standard list handle
pars dw 2 ; length of data
dw ? ; receives code page
.
.
.
mov ax,440ch ; function & subfunction
mov bx,stdprn ; standard list handle
mov ch,5 ; LPTx category
mov cl,6ah ; query code page
mov dx,seg pars ; parameter block address
mov ds,dx
mov dx,offset pars
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.2]
Function 44H Subfunction 0DH (13)
IOCTL: generic I/O control for block devices
────────────────────────────────────────────────────────────────────────────
Provides a general-purpose mechanism for communication between application
programs and block-device drivers. Allows a program to inspect or change
device parameters for a logical drive and to read, write, format, and
verify disk tracks in a hardware-independent manner.
Call with:
AH = 44H
AL = 0DH
BL = drive code (0 = default, 1 = A, 2 = B, etc.)
CH = category (major) code:
08H = disk drive
CL = function (minor) code:
40H = Set Device Parameters
41H = Write Track
42H = Format and Verify Track
47H = Set Access Flag (4.0)
60H = Get Device Parameters
61H = Read Track
62H = Verify Track
67H = Get Access Flag (4.0)
DS:DX = segment:offset of parameter block
Returns:
If function successful
Carry flag = clear
and, if called with CL = 60H or 61H
DS:DX = segment:offset of parameter block
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The minor code 40H (Set Device Parameters) function must be used before
an attempt to write, read, format, or verify a track on a logical drive.
In general, the following sequence applies to any of these operations:
∙ Get the current parameters (minor code 60H). Examine and save them.
∙ Set the new parameters (minor code 40H).
∙ Perform the task.
∙ Retrieve the original parameters and restore them with minor code 40H.
■ For minor codes 40H (Set Device Parameters) and 60H (Get Device
Parameters), the parameter block is formatted as follows:
Special-functions field: offset 00H, length = 1 byte
Bit(s) Value Meaning
0 0 device BPB field contains a new default BPB
1 use current BPB
1 0 use all fields in parameter block
1 use track layout field only
2 0 sectors in track may be different sizes (should
always be avoided)
1 sectors in track are all same size; sector numbers
range from 1 to the total number of sectors in the
track (should always be used)
3─7 0 reserved
Device type field: offset 01H, length 1 byte
Value Meaning
0 320/360 KB, 5.25-inch disk
1 1.2 MB, 5.25-inch disk
2 720 KB, 3.5-inch disk
3 single-density, 8-inch disk
4 double-density, 8-inch disk
5 fixed disk
6 tape drive
7 other type of block device
Device attributes field: offset 02H, length 1 word
Bit(s) Value Meaning
0 0 removable storage medium
1 nonremovable storage medium
1 0 door lock not supported
1 door lock supported
2─15 0 reserved
Number of cylinders field: offset 04H, length 1 word
Maximum number of cylinders supported on the block device
Media type field: offset 06H, length 1 byte
Value Meaning
0 1.2 MB, 5.25-inch disk
1 320/360 KB, 5.25-inch disk
Device BPB field: offset 07H, length 31 bytes
For format of the device BPB, see separate Note below.
If bit 0 = 0 in special-functions field, this field contains the new
default BPB for the device.
If bit 0 = 1 in special-functions field, the BPB in this field is
returned by the device driver in response to subsequent Build BPB
requests.
Track layout field: offset 26H, variable-length table
Length Meaning
Word number of sectors in track
Word number of first sector in track
Word size of first sector in track
.
.
.
Word number of last sector in track
Word size of last sector in track
■ The device BPB field is a 31-byte data structure that describes the
current disk and its control areas. The field is formatted as follows:
Byte(s) Meaning
00H─01H bytes per sector
02H sectors per cluster (allocation unit)
03─04H reserved sectors, beginning at sector 0
05H number of file allocation tables (FATs)
06H─07H maximum number of root-directory entries
08H─09H number of sectors
0AH media descriptor
0BH─0CH sectors per FAT
0DH─0EH sectors per track
0FH─10H number of heads
11H─14H number of hidden sectors
15H─18H large number of sectors (if bytes 08H─09H=0)
19H─1EH reserved
■ When minor code 40H (Set Device Parameters) is used, the number of
cylinders should not be altered, or some or all of the volume may become
inaccessible.
■ For minor codes 41H (Write Track) and 61H (Read Track), the parameter
block is formatted as follows:
Byte(s) Meaning
00H special-functions field (must be 0)
01H─02H head
03H─04H cylinder
05H─06H starting sector
07H─08H sectors to transfer
09H─0CH transfer buffer address
■ For minor codes 42H (Format and Verify Track) and 62H (Verify Track),
the parameter block is formatted as follows:
Byte(s) Meaning
00H special-functions field
Bit(s) Significance
0 0 = Format/Verify track
1 = Format status call (MS-DOS 4.0 only)
1─7 reserved (0)
01H─02H head
03H─04H cylinder
In MS-DOS 4.0, this function may be called with bit 0 of the
special-functions field set after a minor code 40H call (Set Device
Parameters) to determine whether the driver supports the specified
number of tracks and sectors per track. A status is returned in the
special-functions field which is interpreted as follows:
Value Meaning
0 specified number of tracks and sectors per track supported
1 this function not supported by the ROM BIOS
2 specified number of tracks and sectors per track not
supported
3 no disk in drive
■ For minor codes 47H (Set Access Flag) and 67H (Get Access Flag), the
parameter block is formatted as follows:
Byte Meaning
00H special-functions field (must be 0)
01H disk access flag
When the disk access flag is zero, access to the medium is blocked by
the driver. The flag is set to zero when the driver detects an
unformatted medium or a medium with an invalid boot record. When the
access flag is nonzero, read/write operations to the medium are allowed
by the driver. A formatting program must clear the disk access flag with
minor code 47H before it requests minor code 42H (Format and Verify
Track).
Example:
Get the device parameter block for disk drive C.
dbpb db 128 dup (0) ; device parameter block
.
.
.
mov ax,440dh ; function & subfunction
mov bl,3 ; drive C = 3
mov ch,8 ; disk category
mov cl,60h ; get device parameters
mov dx,seg dbpb ; buffer address
mov ds,dx
mov dx,offset dbpb
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.2]
Function 44H (68) Subfunction 0EH (14)
IOCTL: get logical drive map
────────────────────────────────────────────────────────────────────────────
Returns the logical drive code that was most recently used to access the
specified block device.
Call with:
AH = 44H
AL = 0EH
BL = drive code (0 = default, 1 = A, 2 = B, etc.)
Returns:
If function successful
Carry flag = clear
AL = mapping code
00H if only one logical drive code assigned to the
block device
01H─1AH logical drive code (1 = A, 2 = B, etc.) mapped
to the block device
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ If a drive has not been assigned a logical mapping with Function 44H
Subfunction 0FH, the logical and physical drive codes are the same.
Example:
Check whether drive A has more than one logical drive code.
.
.
.
mov ax,440eh ; function & subfunction
mov bl,1 ; drive 1 = A
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
or al,al ; test drive code
jz label1 ; jump, no drive aliases
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.2]
Function 44H (68) Subfunction 0FH (15)
IOCTL: set logical drive map
────────────────────────────────────────────────────────────────────────────
Sets the next logical drive code that will be used to reference a block
device.
Call with:
AH = 44H
AL = 0FH
BL = drive code (0 = default, 1 = A, 2 = B, etc.)
Returns:
If function successful
Carry flag = clear
AL = mapping code
00H if only one logical drive code assigned to the
block device
01H─1AH logical drive code (1 = A, 2 = B, etc.) mapped
to the block device
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ When a physical block device is aliased to more than one logical drive
code, this function can be used to inform the driver which drive code
will next be used to access the device.
Example:
Notify the floppy-disk driver that the next access will be for logical
drive B.
.
.
.
mov ax,440fh ; function & subfunction
mov bl,2 ; drive 2 = B
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 45H (69)
Duplicate handle
────────────────────────────────────────────────────────────────────────────
Given a handle for a currently open device or file, returns a new handle
that refers to the same device or file at the same position.
Call with:
AH = 45H
BX = handle to be duplicated
Returns:
If function successful
Carry flag = clear
AX = new handle
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ A seek, read, or write operation that moves the file pointer for one of
the two handles also moves the file pointer associated with the other.
■ This function can be used to efficiently update the directory for a file
that has changed in length, without incurring the overhead of closing
and then reopening the file. The handle for the file is simply
duplicated with this function and the duplicate is closed, leaving the
original handle open for further read/write operations.
■ [3.3] See also Int 21H Function 68H (Commit File).
Example:
Duplicate the handle stored in the variable fhandle, then close the
duplicate. This ensures that all buffered data is physically written to
disk and that the directory entry for the corresponding file is updated,
but leaves the original handle open for subsequent file operations.
fhandle dw 0 ; file handle
.
.
.
; get duplicate handle
mov ah,45h ; function number
mov bx,fhandle ; original file handle
int 21h ; transfer to MS-DOS
jc error ; jump if dup failed
; now close dup'd handle
mov bx,ax ; put handle into BX
mov ah,3eh ; function number
int 21h ; transfer to MS-DOS jc error
; jump if close failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 46H (70)
Redirect handle
────────────────────────────────────────────────────────────────────────────
Given two handles, makes the second handle refer to the same device or
file at the same location as the first handle. The second handle is then
said to be redirected.
Call with:
AH = 46H
BX = handle for file or device
CX = handle to be redirected
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ If the handle passed in CX already refers to an open file, that file is
closed first.
■ A seek, read, or write operation that moves the file pointer for one of
the two handles also moves the file pointer associated with the other.
■ This function is commonly used to redirect the standard input and output
handles to another file or device before a child process is executed
with Int 21H Function 4BH.
Example:
Redirect the standard output to the list device, so that all output
directed to the console will appear on the printer instead. Later, restore
the original meaning of the standard output handle.
stdin equ 0
stdout equ 1
stderr equ 2
stdaux equ 3
stdprn equ 4
dhandle dw 0 ; duplicate handle
.
.
.
; get dup of stdout
mov ah,45h ; function number
mov bx,stdout ; standard output handle
int 21h ; transfer to MS-DOS
jc error ; jump if dup failed
mov dhandle,ax ; save dup'd handle
;
; redirect standard output
; to standard list device
mov ah,46h ; function number
mov bx,stdprn ; standard list handle
mov cx,stdout ; standard output handle
int 21h ; transfer to MS-DOS
jc error ; jump if redirect failed
.
.
.
; restore standard output
; to original meaning
mov ah,46h ; function number
mov bx,dhandle ; saved duplicate handle
mov cx,stdout ; standard output handle
int 21h ; transfer to MS-DOS
jc error ; jump if redirect failed
; close duplicate handle
; because no longer needed
mov ah,3eh ; function number
mov bx,dhandle ; saved duplicate handle
int 21h ; transfer to MS-DOS
jc error ; jump if close failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 47H (71)
Get current directory
────────────────────────────────────────────────────────────────────────────
Obtains an ASCIIZ string that describes the path from the root to the
current directory, and the name of that directory.
Call with:
AH = 47H
DL = drive code (0 = default, 1 = A, etc.)
DS:SI = segment:offset of 64-byte buffer
Returns:
If function successful
Carry flag = clear
and buffer is filled in with full pathname from root of current directory.
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The returned path name does not include the drive identifier or a
leading backslash (\). It is terminated with a null (00H) byte.
Consequently, if the current directory is the root directory, the first
byte in the buffer will contain 00H.
■ The function fails if the drive code is invalid.
■ The current directory may be set with Int 21H Function 3BH.
Example:
Get the name of the current directory for drive C into the buffer named
dbuff.
dbuff db 64 dup (0) ; receives path string
.
.
.
mov ah,47h ; function number
mov dl,03 ; drive C = 3
mov si,seg dbuff ; buffer address
mov ds,si
mov si,offset dbuff
int 21h ; transfer to MS-DOS
jc error ; jump if error
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 48H (72)
Allocate memory block
────────────────────────────────────────────────────────────────────────────
Allocates a block of memory and returns a pointer to the beginning of the
allocated area.
Call with:
AH = 48H
BX = number of paragraphs of memory needed
Returns:
If function successful
Carry flag = clear
AX = base segment address of allocated block
If function unsuccessful
Carry flag = set
AX = error code
BX = size of largest available block (paragraphs)
Notes:
■ If the function succeeds, the base address of the newly allocated block
is AX:0000.
■ The default allocation strategy used by MS-DOS is "first fit"; that is,
the memory block at the lowest address that is large enough to satisfy
the request is allocated. The allocation strategy can be altered with
Int 21H Function 58H.
■ When a .COM program is loaded, it ordinarily already "owns" all of the
memory in the transient program area, leaving none for dynamic
allocation. The amount of memory initially allocated to a .EXE program
at load time depends on the MINALLOC and MAXALLOC fields in the .EXE
file header. See Int 21H Function 4AH.
Example:
Request a 64 KB block of memory for use as a buffer.
bufseg dw ? ; segment base of new block
.
.
.
mov ah,48h ; function number
mov bx,1000h ; block size (paragraphs)
int 21h ; transfer to MS-DOS
jc error ; jump if allocation failed
mov bufseg,ax ; save segment of new block
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 49H (73)
Release memory block
────────────────────────────────────────────────────────────────────────────
Releases a memory block and makes it available for use by other programs.
Call with:
AH = 49H
ES = segment of block to be released
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ This function assumes that the memory block being released was
previously obtained by a successful call to Int 21H Function 48H.
■ The function will fail or can cause unpredictable system errors if:
∙ the program releases a memory block that does not belong to it.
∙ the segment address passed in register ES is not a valid base address
for an existing memory block.
Example:
Release the memory block that was previously allocated in the example for
Int 21H Function 48H (page 438).
bufseg dw ? ; segment base of block
.
.
.
mov ah,49h ; function number
mov es,bufseg ; base segment of block
int 21h ; transfer to MS-DOS
jc error ; jump if release failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 4AH (74)
Resize memory block
────────────────────────────────────────────────────────────────────────────
Dynamically shrinks or extends a memory block, according to the needs of
an application program.
Call with:
AH = 4AH
BX = desired new block size in paragraphs
ES = segment of block to be modified
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
BX = maximum block size available (paragraphs)
Notes:
■ This function modifies the size of a memory block that was previously
allocated with a call to Int 21H Function 48H.
■ If the program is requesting an increase in the size of an allocated
block, and this function fails, the maximum possible size for the
specified block is returned in register BX. The program can use this
value to determine whether it should terminate, or continue in a
degraded fashion with less memory.
■ A program that uses EXEC (Int 21H Function 4BH) to load and execute a
child program must call this function first to make memory available for
the child, passing the address of its PSP in register ES and the amount
of memory needed for its own code, data, and stacks in register BX.
Example:
Resize the memory block that was allocated in the example for Int 21H
Function 48H (page 438), shrinking it to 32 KB.
bufseg dw ? ; segment base of block
.
.
.
mov ah,4ah ; function number
mov bx,0800h ; new size (paragraphs)
mov es,bufseg ; segment base of block
int 21h ; transfer to MS-DOS
jc error ; jump, resize failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 4BH (75)
Execute program (EXEC)
────────────────────────────────────────────────────────────────────────────
Allows an application program to run another program, regaining control
when it is finished. Can also be used to load overlays, although this use
is uncommon.
Call with:
AH = 4BH
AL = subfunction
00H = Load and Execute Program
03H = Load Overlay
ES:BX = segment:offset of parameter block
DS:DX = segment:offset of ASCIIZ program pathname
Returns:
If function successful
Carry flag = clear
[2] all registers except for CS:IP may be destroyed
[3.0+] registers are preserved in the usual fashion
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The parameter block format for Subfunction 00H (Load and Execute
Program) is as follows:
Bytes Contents
00H─01H segment pointer to environment block
02H─03H offset of command line tail
04H─05H segment of command line tail
06H─07H offset of first FCB to be copied into new PSP + 5CH
08H─09H segment of first FCB
0AH─0BH offset of second FCB to be copied into new PSP + 6CH
0CH─0DH segment of second FCB
■ The parameter block format for Subfunction 03H (Load Overlay) is as
follows:
Bytes Contents
00H─01H segment address where overlay is to be loaded
02H─03H relocation factor to apply to loaded image
■ The environment block must be paragraph-aligned. It consists of a
sequence of ASCIIZ strings in the form:
db 'COMSPEC=A:\COMMAND.COM',0
The entire set of strings is terminated by an extra null (00H) byte.
■ The command tail format consists of a count byte, followed by an ASCII
string, terminated by a carriage return (which is not included in the
count). The first character in the string should be an ASCII space (20H)
for compatibility with the command tail passed to programs by
COMMAND.COM. For example:
db 6,' *.DAT',0dh
■ Before a program uses Int 21H Function 4BH to run another program, it
must release all memory it is not actually using with a call to Int 21H
Function 4AH, passing the segment address of its own PSP and the number
of paragraphs to retain.
■ Ordinarily, all active handles of the parent program are inherited by
the child program, although the parent can prevent this in MS-DOS 3.0
and later by setting the inheritance bit when the file or device is
opened. Any redirection of the standard input and/or output in the
parent process also affects the child process.
■ The environment block can be used to pass information to the child
process. If the environment block pointer in the parameter block is
zero, the child program inherits an exact copy of the parent's
environment. In any case, the segment address of the child's environment
is found at offset 002CH in the child's PSP.
■ After return from the EXEC function call, the termination type and
return code of the child program may be obtained with Int 21H Function
4DH.
Example:
See Chapter 12.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 4CH (76)
Terminate process with return code
────────────────────────────────────────────────────────────────────────────
Terminates the current process, passing a return code to the parent
process. This is one of several methods that a program can use to perform
a final exit. MS-DOS then takes the following actions:
■ All memory belonging to the process is released.
■ File buffers are flushed and any open handles for files or devices owned
by the process are closed.
■ The termination handler vector (Int 22H) is restored from PSP:000AH.
■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH.
■ [2.0+] The critical-error handler vector (Int 24H) is restored from
PSP:0012H.
■ Control is transferred to the termination handler.
If the program is returning to COMMAND.COM, control transfers to the
resident portion and the transient portion is reloaded if necessary. If a
batch file is in progress, the next line of the file is fetched and
interpreted; otherwise, a prompt is issued for the next user command.
Call with:
AH = 4CH
AL = return code
Returns:
Nothing
Notes:
■ [2.0+] This is the preferred method of termination for application
programs because it allows a return code to be passed to the parent
program and does not rely on the contents of any segment register. Other
methods of performing a final exit are:
∙ Int 20H
∙ Int 21H Function 00H
∙ Int 21H Function 31H
∙ Int 27H
■ Any files that have been opened using FCBs and modified by the program
should be closed before program termination; otherwise, data may be
lost.
■ The return code can be retrieved by the parent process with Int 21H
Function 4DH (Get Return Code). It can also be tested in a batch file
with an IF ERRORLEVEL statement. By convention, a return code of zero
indicates successful execution, and a non-zero return code indicates an
error.
■ [3.0+] If the program is running on a network, it should remove all
locks it has placed on file regions before terminating.
Example:
Terminate the current process, passing a return code of 1 to the parent
process.
.
.
.
mov ah,4ch ; function number
mov al,01h ; return code
int 21h ; transfer to MS-DOS
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 4DH (77)
Get return code
────────────────────────────────────────────────────────────────────────────
Used by a parent process, after the successful execution of an EXEC call
(Int 21H Function 4BH), to obtain the return code and termination type of
a child process.
Call with:
AH = 4DH
Returns:
AH = exit type
00H if normal termination by Int 20H, Int 21H Function
00H, or Int 21H Function 4CH
01H if termination by user's entry of CtrlDC
02H if termination by critical-error handler
03H if termination by Int 21H Function 31H or Int 27H
AL = return code passed by child process (0 if child terminated
by Int 20H, Int 21H Function 00H, or Int 27H)
Notes:
■ This function will yield the return code of a child process only once. A
subsequent call without an intervening EXEC (Int 21H Function 4BH) will
not necessarily return any valid information.
■ This function does not set the carry flag to indicate an error. If no
previous child process has been executed, the values returned in AL and
AH are undefined.
Example:
Get the return code and termination kind of child process that was
previously executed with Int 21H Function 4BH (EXEC).
retcode dw ? ; return code, termination type
.
.
.
mov ah,4dh ; function number
int 21h ; transfer to MS-DOS
mov retcode,ax ; save child process info
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 4EH (78)
Find first file
────────────────────────────────────────────────────────────────────────────
Given a file specification in the form of an ASCIIZ string, searches the
default or specified directory on the default or specified drive for the
first matching file.
Call with:
AH = 4EH
CX = search attribute (bits may be combined)
Bit(s) Significance (if set)
0 read-only
1 hidden
2 system
3 volume label
4 directory
5 archive
6─15 reserved (0)
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful (matching file found)
Carry flag = clear
and search results returned in current disk transfer area as follows:
Byte(s) Description
00H─14H reserved (0)
15H attribute of matched file or directory
16H─17H file time
bits 00H─04H = 2-second increments (0─29)
bits 05H─0AH = minutes (0─59)
bits 0BH─0FH = hours (0─23)
18H─19H file date
bits 00H─04H = day (1─31)
bits 05H─08H = month (1─12)
bits 09H─0FH = year (relative to 1980)
1AH─1DH file size
1EH─2AH ASCIIZ filename and extension
If function unsuccessful (no matching files)
Carry flag = set
AX = error code
Notes:
■ This function assumes that the DTA has been previously set by the
program with Int 21H Function 1AH to point to a buffer of adequate
size.
■ The * and ? wildcard characters are allowed in the filename. If wildcard
characters are present, this function returns only the first matching
filename.
■ If the attribute is 0, only ordinary files are found. If the volume
label attribute bit is set, only volume labels will be returned (if any
are present). Any other attribute or combination of attributes (hidden,
system, and directory) results in those files and all normal files being
matched.
Example:
Find the first .COM file in the directory \MYDIR on drive C.
fname db 'C:\MYDIR\*.COM',0
dbuff db 43 dup (0) ; receives search results
.
.
.
; set DTA address
mov ah,1ah ; function number
mov dx,seg dbuff ; result buffer address
mov ds,dx
mov dx,offset dbuff
int 21h ; transfer to MS-DOS
; search for first match
mov ah,4eh ; function number
mov cx,0 ; normal attribute
mov dx,seg fname ; address of filename
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if no match
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 4FH (79)
Find next file
────────────────────────────────────────────────────────────────────────────
Assuming a previous successful call to Int 21H Function 4EH, finds the
next file in the default or specified directory on the default or
specified drive that matches the original file specification.
Call with:
AH = 4FH
Assumes DTA points to working buffer used by previous successful Int 21H
Function 4EH or 4FH.
Returns:
If function successful (matching file found)
Carry flag = clear
and search results returned in current disk transfer area as described for
Int 21H Function 4EH
If function unsuccessful (no more matching files)
Carry flag = set
AX = error code
Notes:
■ Use of this call assumes that the original file specification passed to
Int 21H Function 4EH contained one or more * or ? wildcard characters.
■ When this function is called, the current disk transfer area (DTA) must
contain information from a previous successful call to Int 21H Function
4EH or 4FH.
Example:
Continuing the search operation in the example for Int 21H Function 4EH,
find the next .COM file (if any) in the directory \MYDIR on drive C.
fname db 'C:\MYDIR\*.COM',0
dbuff db 43 dup (0) ; receives search results
.
.
.
; search for next match
mov ah,4fh ; function number
int 21h ; transfer to MS-DOS
jc error ; jump if no more files
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 50H (80)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 51H (81)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 52H (82)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 53H (83)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 54H (84)
Get verify flag
────────────────────────────────────────────────────────────────────────────
Obtains the current value of the system verify (read-after-write) flag.
Call with:
AH = 54H
Returns:
AL = current verify flag value
00H if verify off
01H if verify on
Notes:
■ Because read-after-write verification slows disk operations, the default
state of the system verify flag is OFF.
■ The state of the system verify flag can be changed through a call to Int
21H Function 2EH or by the MS-DOS commands VERIFY ON and VERIFY OFF.
Example:
Obtain the state of the system verify flag.
.
.
.
mov ah,54h ; function number
int 21h ; transfer to MS-DOS
cmp al,01h ; check verify state
je label1 ; jump if verify on
; else assume verify off
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 55H (85)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 56H (86)
Rename file
────────────────────────────────────────────────────────────────────────────
Renames a file and/or moves its directory entry to a different directory
on the same disk. In MS-DOS version 3.0 and later, this function can also
be used to rename directories.
Call with:
AH = 56H
DS:DX = segment:offset of current ASCIIZ pathname
ES:DI = segment:offset of new ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The function fails if:
∙ any element of the pathname does not exist.
∙ a file with the new pathname already exists.
∙ the current pathname specification contains a different disk drive
than does the new pathname.
∙ the file is being moved to the root directory, and the root directory
is full.
∙ [3.0+] the program is running on a network and the user has
insufficient access rights to either the existing file or the new
directory.
■ The * and ? wildcard characters are not allowed in either the current or
new pathname specifications.
Example:
Change the name of the file MYFILE.DAT in the directory \MYDIR on drive C
to MYTEXT.DAT. At the same time, move the file to the directory \SYSTEM on
the same drive.
oldname db 'C:\MYDIR\MYFILE.DAT',0
newname db 'C:\SYSTEM\MYTEXT.DAT',0
.
.
.
mov ah,56h ; function number
mov dx,seg oldname ; old filename address
mov ds,dx
mov dx,offset oldname
mov di,seg newname ; new filename address
mov es,di
mov di,offset newname
int 21h ; transfer to MS-DOS
jc error ; jump if rename failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.0]
Function 57H (87)
Get or set file date and time
────────────────────────────────────────────────────────────────────────────
Obtains or modifies the date and time stamp in a file's directory entry.
Call with:
If getting date and time
AH = 57H
AL = 00H
BX = handle
If setting date and time
AH = 57H
AL = 01H
BX = handle
CX = time
bits 00H─04H = 2-second increments (0─29)
bits 05H─0AH = minutes (0─59)
bits 0BH─0FH = hours (0─23)
DX = date
bits 00H─04H = day (1─31)
bits 05H─08H = month (1─12)
bits 09H─0FH = year (relative to 1980)
Returns:
If function successful
Carry flag = clear
and, if called with AL = 00H
CX = time
DX = date
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The file must have been previously opened or created via a successful
call to Int 21H Function 3CH, 3DH, 5AH, 5BH, or 6CH.
■ If the 16-bit date for a file is set to zero, that file's date and time
are not displayed on directory listings.
■ A date and time set with this function will prevail, even if the file is
modified afterwards before the handle is closed.
Example:
Get the date that the file MYFILE.DAT was created or last modified, and
then decompose the packed date into its constituent parts in the variables
month, day, and year.
fname db 'MYFILE.DAT',0
month dw 0
day dw 0
year dw 0
.
.
.
; first open the file
mov ah,3dh ; function number
mov al,0 ; read-only mode
mov dx,seg fname ; filename address
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if open failed
; get file date/time
mov bx,ax ; copy handle to BX
mov ah,57h ; function number
mov al,0 ; 0 = get subfunction
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
mov day,dx ; decompose date
and day,01fh ; isolate day
mov cl,5
shr dx,cl
mov month,dx ; isolate month
and month,0fh
mov cl,4
shr dx,cl ; isolate year
and dx,03fh ; relative to 1980
add dx,1980 ; correct to real year
mov year,dx ; save year
; now close file,
; handle still in BX
mov ah,3eh ; function number
int 21h ; transfer to MS-DOS
jc error ; jump if close failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.0]
Function 58H (88)
Get or set allocation strategy
────────────────────────────────────────────────────────────────────────────
Obtains or changes the code indicating the current MS-DOS strategy for
allocating memory blocks.
Call with:
If getting strategy code
AH = 58H
AL = 00H
If setting strategy code
AH = 58H
AL = 01H
BX = desired strategy code
00H = first fit
01H = best fit
02H = last fit
Returns:
If function successful
Carry flag = clear
and, if called with AL = 00H
AX = current strategy code
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The memory allocation strategies are:
∙ First fit: MS-DOS searches the available memory blocks from low
addresses to high addresses, assigning the first one large enough to
satisfy the block allocation request.
∙ Best fit: MS-DOS searches all available memory blocks and assigns the
smallest available block that will satisfy the request, regardless of
its position.
∙ Last fit: MS-DOS searches the available memory blocks from high
addresses to low addresses, assigning the highest one large enough to
satisfy the block allocation request.
■ The default MS-DOS memory allocation strategy is First Fit (code 0).
Example:
Save the code indicating the current memory allocation strategy in the
variable strat, then change the system's memory allocation strategy to
"best fit."
strat dw 0 ; previous strategy code
.
.
.
; get current strategy
mov ah,58h ; function number
mov al,0 ; 0 = get strategy
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
mov strat,ax ; save strategy code
; now set new strategy
mov ah,58h ; function number
mov al,1 ; 1 = set strategy
mov bx,1 ; 1 = best fit
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.0]
Function 59H (89)
Get extended error information
────────────────────────────────────────────────────────────────────────────
Obtains detailed error information after a previous unsuccessful Int 21H
function call, including the recommended remedial action.
Call with:
AH = 59H
BX = 00H
Returns:
AX = extended error code
01H function number invalid
02H file not found
03H path not found
04H too many open files
05H access denied
06H handle invalid
07H memory control blocks destroyed
08H insufficient memory
09H memory block address invalid
0AH (10) environment invalid
0BH (11) format invalid
0CH (12) access code invalid
0DH (13) data invalid
0EH (14) unknown unit
0FH (15) disk drive invalid
10H (16) attempted to remove current directory
11H (17) not same device
12H (18) no more files
13H (19) disk write-protected
14H (20) unknown unit
15H (21) drive not ready
16H (22) unknown command
17H (23) data error (CRC)
18H (24) bad request structure length
19H (25) seek error
1AH (26) unknown media type
1BH (27) sector not found
1CH (28) printer out of paper
1DH (29) write fault
1EH (30) read fault
1FH (31) general failure
20H (32) sharing violation
21H (33) lock violation
22H (34) disk change invalid
23H (35) FCB unavailable
24H (36) sharing buffer exceeded
25H─31H reserved
32H (50) unsupported network request
33H (51) remote machine not listening
34H (52) duplicate name on network
35H (53) network name not found
36H (54) network busy
37H (55) device no longer exists on network
38H (56) netBIOS command limit exceeded
39H (57) error in network adapter hardware
3AH (58) incorrect response from network
3BH (59) unexpected network error
3CH (60) remote adapter incompatible
3DH (61) print queue full
3EH (62) not enough space for print file
3FH (63) print file canceled
40H (64) network name deleted
41H (65) network access denied
42H (66) incorrect network device type
43H (67) network name not found
44H (68) network name limit exceeded
45H (69) netBIOS session limit exceeded
46H (70) file sharing temporarily paused
47H (71) network request not accepted
48H (72) print or disk redirection paused
49H─4FH reserved
50H (80) file already exists
51H (81) reserved
52H (82) cannot make directory
53H (83) fail on Int 24H (critical error)
54H (84) too many redirections
55H (85) duplicate redirection
56H (86) invalid password
57H (87) invalid parameter
58H (88) network device fault
59H (89) function not supported by network
5AH (90) required system component not installed
BH = error class
01H if out of resource (such as storage or handles)
02H if not error, but temporary situation (such as
locked region in file) that can be expected to
end
03H if authorization problem
04H if internal error in system software
05H if hardware failure
06H if system software failure not the fault of the
active process (such as missing configuration
files)
07H if application program error
08H if file or item not found
09H if file or item of invalid type or format
0AH (10) if file or item locked
0BH (11) if wrong disk in drive, bad spot on disk, or
storage medium problem
0CH (12) if item already exists
0DH (13) unknown error
BL = recommended action
01H retry reasonable number of times, then prompt
user to select abort or ignore
02H retry reasonable number of times with delay
between retries, then prompt user to select
abort or ignore
03H get corrected information from user (typically
caused by incorrect filename or drive
specification)
04H abort application with cleanup (i.e., terminate
the program in as orderly a manner as possible:
releasing locks, closing files, etc.)
05H perform immediate exit without cleanup
06H ignore error
07H retry after user intervention to remove cause of
error
CH = error locus
01H unknown
02H block device (disk or disk emulator)
03H network
04H serial device
05H memory
and, for MS-DOS 3.0 and later,
ES:DI = ASCIIZ volume label of disk to insert, if AX = 0022H
(invalid disk change)
Notes:
■ This function may be called after any other Int 21H function call that
returned an error status, in order to obtain more detailed information
about the error type and the recommended action. If the previous Int 21H
function call had no error, 0000H is returned in register AX. This
function may also be called during the execution of a critical-error
(Int 24H) handler.
■ The contents of registers CL, DX, SI, DI, BP, DS, and ES are destroyed
by this function.
■ Note that extended error codes 13H─1FH (19─31) and 34 (22H) correspond
exactly to the error codes 0─0CH (0─12) and 0FH (15) returned by Int
24H.
■ You should not code your programs to recognize only specific error
numbers if you wish to ensure upward compatibility, because new error
codes are added in each version of MS-DOS.
Example:
Attempt to open the file named NOSUCH.DAT using a file control block; if
the open request fails, get the extended error code.
myfcb db 0 ; drive = default
db 'NOSUCH ' ; filename, 8 chars
db 'DAT' ; extension, 3 chars
db 25 dup (0) ; remainder of FCB
.
.
.
label1: ; open the file
mov ah,0fh ; function number
mov dx,seg myfcb ; address of FCB
mov ds,dx
mov dx,offset myfcb
int 21h ; transfer to MS-DOS
or al,al ; check open status
jz success ; jump if opened OK
; open failed, get
; extended error info
mov ah,59h ; function number
xor bx,bx ; BX must = 0
int 21h ; transfer to MS-DOS
or ax,ax ; double check for error
jz success ; jump if no error
cmp bl,2 ; should we retry?
jle label1 ; yes, jump
jmp error ; no, give up
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.0]
Function 5AH (90)
Create temporary file
────────────────────────────────────────────────────────────────────────────
Creates a file with a unique name, in the current or specified directory
on the default or specified disk drive, and returns a handle that can be
used by the program for subsequent access to the file. The name generated
for the file is also returned in a buffer specified by the program.
Call with:
AH = 5AH
CX = attribute (bits may be combined)
Bit(s) Significance (if set)
0 read-only
1 hidden
2 system
3─4 reserved (0)
5 archive
6─15 reserved (0)
DS:DX = segment:offset of ASCIIZ path
Returns:
If function successful
Carry flag = clear
AX = handle
DS:DX = segment:offset of complete ASCIIZ pathname
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The ASCIIZ path supplied to this function should be followed by at least
13 additional bytes of buffer space. MS-DOS adds a backslash (\) to the
supplied path, if necessary, then appends a null-terminated filename
that is a function of the current time.
■ Files created with this function are not automatically deleted when the
calling program terminates.
■ The function fails if
∙ any element of the pathname does not exist.
∙ the file is being created in the root directory, and the root
directory is full.
■ See also Int 21H Functions 3CH, 5BH, and 6CH, which provide
additional facilities for creating files.
■ [3.0+] If the program is running on a network, the file is created and
opened for read/write access in compatibility sharing mode.
Example:
Create a temporary file with a unique name and normal attribute in
directory \TEMP of drive C. Note that you must allow room for MS-DOS to
append the generated filename to the supplied path. The complete file
specification should be used to delete the temporary file before your
program terminates.
fname db 'C:\TEMP\' ; pathname for temp file
db 13 dup (0) ; receives filename
fhandle dw ? ; file handle
.
.
.
mov ah,5ah ; function number
mov cx,0 ; normal attribute
mov dx,seg fname ; address of pathname
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if create failed
mov fhandle,ax ; save file handle
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.0]
Function 5BH (91)
Create new file
────────────────────────────────────────────────────────────────────────────
Given an ASCIIZ pathname, creates a file in the designated or default
directory on the designated or default drive, and returns a handle that
can be used by the program for subsequent access to the file. If a file
with the same name already exists, the function fails.
Call with:
AH = 5BH
CX = attribute (bits may be combined)
Bit(s) Significance (if set)
0 read-only
1 hidden
2 system
3 volume label
4 reserved (0)
5 archive
6─15 reserved (0)
DS:DX = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
AX = handle
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The function fails if:
∙ any element of the specified path does not exist.
∙ a file with the identical pathname (i.e., the same filename and
extension in the same location in the directory structure) already
exists.
∙ the file is being created in the root directory, and the root
directory is full.
∙ [3.0+] the program is running on a network, and the user has
insufficient access rights to the directory that will contain the
file.
■ The file is usually given a normal attribute (0) when it is created, and
is opened for both read and write operations. The attribute can
subsequently be modified with Int 21H Function 43H.
■ See also Int 21H Functions 3CH, 5AH, and 6CH, which provide
alternative ways of creating files.
■ This function may be used to implement semaphores in a network or
multitasking environment. If the function succeeds, the program has
acquired the semaphore. To release the semaphore, the program simply
deletes the file.
Example:
Create and open a file named MYFILE.DAT in the directory \MYDIR on drive
C; MS-DOS returns an error if a file with the same name already exists in
that location.
fname db 'C:\MYDIR\MYFILE.DAT',0
fhandle dw ? ; file handle
.
.
.
mov ah,5bh ; function number
xor cx,cx ; normal attribute
mov dx,seg fname ; filename address
mov ds,dx
mov dx,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if create failed
mov fhandle,ax ; save file handle
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.0]
Function 5CH (92)
Lock or unlock file region
────────────────────────────────────────────────────────────────────────────
Locks or unlocks the specified region of a file. This function is not
available unless the file-sharing module (SHARE.EXE) is loaded.
Call with:
AH = 5CH
AL = 00H if locking region
01H if unlocking region
BX = handle
CX = high part of region offset
DX = low part of region offset
SI = high part of region length
DI = low part of region length
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ This function is useful for file and record synchronization in a
multitasking environment or network. Access to the file as a whole is
controlled by the attribute and file-sharing parameters passed in open
or create calls and by the file's attributes, which are stored in its
directory entry.
■ The beginning location in the file to be locked or unlocked is supplied
as a positive double precision integer, which is a byte offset into the
file. The length of the region to be locked or unlocked is similarly
supplied as a positive double precision integer.
■ For every call to lock a region of a file, there must be a subsequent
unlock call with exactly the same file offset and length.
■ Locking beyond the current end of file is not an error.
■ Duplicate handles created with Int 21H Function 45H, or handles
redirected to the file with Int 21H Function 46H, are allowed access to
locked regions within the same process.
■ Programs that are loaded with the EXEC call (Int 21H Function 4BH)
inherit the handles of their parent but not any active locks.
■ If a process terminates without releasing active locks on a file, the
result is undefined. Therefore, programs using this function should
install their own Int 23H and Int 24H handlers so that they cannot be
terminated unexpectedly.
Example:
Assume that a file was previously opened and that its handle was saved in
the variable fhandle. Lock a 4096 byte region of the file, starting at
32,768 bytes from the beginning of the file, so that it cannot be accessed
by other programs.
fhandle dw ? ; file handle
.
.
.
mov ah,5ch ; function number
mov al,0 ; subfunction 0 = lock
mov bx,fhandle ; file handle
mov cx,0 ; upper part of offset
mov dx,32768 ; lower part of offset
mov si,0 ; upper part of length
mov di,4096 ; lower part of length
int 21h ; transfer to MS-DOS
jc error ; jump if lock failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 5DH (93)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 5EH (94) Subfunction 00H
Get machine name
────────────────────────────────────────────────────────────────────────────
Returns the address of an ASCIIZ (null-terminated) string identifying the
local computer. This function call is only available when Microsoft
Networks is running.
Call with:
AH = 5EH
AL = 00H
DS:DX = segment:offset of buffer to receive string
Returns:
If function successful
Carry flag = clear
CH = 00H if name not defined
<> 00H if name defined
CL = netBIOS name number (if CH <> 0)
DX:DX = segment:offset of identifier (if CH <> 0 )
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The computer identifier is a 15-byte string, padded with spaces and
terminated with a null (00H) byte.
■ The effect of this call is unpredictable if the file-sharing support
module is not loaded.
Example:
Get the machine name of the local computer into the buffer named mname.
mname db 16 dup (?)
.
.
.
mov ax,5e00h ; function & subfunction
mov dx,seg mname ; address of buffer
mov ds,dx
mov dx,offset mname
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
or ch,ch ; make sure name exists
jz error ; jump if no name defined
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 5EH (94) Subfunction 02H
Set printer setup string
────────────────────────────────────────────────────────────────────────────
Specifies a string to be sent in front of all files directed to a
particular network printer, allowing users at different network nodes to
specify individualized operating modes on the same printer. This function
call is only available when Microsoft Networks is running.
Call with:
AH = 5EH
AL = 02H
BX = redirection list index
CX = length of setup string
DS:SI = segment:offset of setup string
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The redirection list index passed in register BX is obtained with
Function 5FH Subfunction 02H (Get Redirection List Entry).
■ See also Function 5EH Subfunction 03H, which may be used to obtain the
existing setup string for a particular network printer.
Example:
Initialize the setup string for the printer designated by redirection list
index 2 so that the device is put into boldface mode before printing a
file requested by this network node.
setup db 01bh,045h ; selects boldface mode
.
.
.
mov ax,5e02h ; function & subfunction
mov bx,2 ; redirection list index 2
mov cx,2 ; length of setup string
mov si,seg setup ; address of setup string
mov ds,si
mov si,offset setup
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 5EH (94) Subfunction 03H
Get printer setup string
────────────────────────────────────────────────────────────────────────────
Obtains the printer setup string for a particular network printer. This
function call is only available when Microsoft Networks is running.
Call with:
AH = 5EH
AL = 03H
BX = redirection list index
ES:DI = segment:offset of buffer to receive setup string
Returns:
If function successful
Carry flag = clear
CX = length of printer setup string
ES:DI = address of buffer holding setup string
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The redirection list index passed in register BX is obtained with
Function 5FH Subfunction 02H (Get Redirection List Entry).
■ See also Int 21H Function 5EH Subfunction 02H, which is used to specify
a setup string for a network printer.
Example:
Get the setup string for this network node associated with the printer
designated by redirection list index 2.
setup db 64 dup (?) ; receives setup string
.
.
.
mov ax,5e03h ; function & subfunction
mov bx,2 ; redirection list index 2
mov di,seg setup ; address of buffer
mov es,di
mov di,offset setup
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 5FH (95) Subfunction 02H
Get redirection list entry
────────────────────────────────────────────────────────────────────────────
Allows inspection of the system redirection list, which associates local
logical names with network files, directories, or printers. This function
call is only available when Microsoft Networks is running and the
file-sharing module (SHARE.EXE) has been loaded.
Call with:
AH = 5FH
AL = 02H
BX = redirection list index
DS:SI = segment:offset of 16-byte buffer to receive local device
name
ES:DI = segment:offset of 128-byte buffer to receive network name
Returns:
If function successful
Carry flag = clear
BH = device status flag
bit 0 = 0 if device valid
= 1 if not valid
BL = device type
03H if printer
04H if drive
CX = stored parameter value
DX = destroyed
BP = destroyed
DS:SI = segment:offset of ASCIIZ local device name
ES:DI = segment:offset of ASCIIZ network name
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ The parameter returned in CX is a value that was previously passed to
MS-DOS in register CX with Int 21H Function 5FH Subfunction 03H
(Redirect Device). It represents data that is private to the
applications which store and retrieve it and has no meaning to MS-DOS.
Example:
Get the local and network names for the device specified by the first
redirection list entry.
local db 16 dup (?) ; receives local device name
network db 128 dup (?) ; receives network name
.
.
.
mov ax,5f02h ; function & subfunction
mov bx,0 ; redirection list entry 0
mov si,seg local ; local name buffer addr
mov ds,si
mov si,offset local
mov di,seg network ; network name buffer addr
mov es,di
mov di,offset network
int 21h ; transfer to MS-DOS
jc error ; jump if call failed
or bh,bh ; check device status
jnz error ; jump if device not valid
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 5FH (95) Subfunction 03H
Redirect device
────────────────────────────────────────────────────────────────────────────
Establishes redirection across the network by associating a local device
name with a network name. This function call is only available when
Microsoft Networks is running and the file-sharing module (SHARE.EXE) has
been loaded.
Call with:
AH = 5FH
AL = 03H
BL = device type
03H if printer
04H if drive
CX = parameter to save for caller
DS:SI = segment:offset of ASCIIZ local device name
ES:DI = segment:offset of ASCIIZ network name, followed by ASCIIZ
password
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The local name can be a drive designator (a letter followed by a colon,
such as "D:"), a printer name, or a null string. Printer names must be
one of the following: PRN, LPT1, LPT2, or LPT3. If a null string
followed by a password is used, MS-DOS attempts to grant access to the
network directory with the specified password.
■ The parameter passed in CX can be retrieved by later calls to Int 21H
Function 5FH Subfunction 02H. It represents data that is private to the
applications which store and retrieve it and has no meaning to MS-DOS.
Example:
Redirect the local drive E to the directory \FORTH on the server named
LMI, using the password FRED.
locname db 'E:',0 ; local drive
netname db '\\LMI\FORTH',0
db 'FRED',0
.
.
.
mov ax,5f03h ; function & subfunction
mov bl,4 ; code 4 = disk drive
mov si,seg locname ; address of local name
mov ds,si
mov si,offset locname
mov di,seg netname ; address of network name
mov es,di
mov di,offset netname
int 21h ; transfer to MS-DOS
jc error ; jump if redirect failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.1]
Function 5FH (95) Subfunction 04H
Cancel device redirection
────────────────────────────────────────────────────────────────────────────
Cancels a previous redirection request by removing the association of a
local device name with a network name. This function call is only
available when Microsoft Networks is running and the file-sharing module
(SHARE.EXE) has been loaded.
Call with:
AH = 5FH
AL = 04H
DS:SI = segment:offset of ASCIIZ local device name
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ The supplied name can be a drive designator (a letter followed by a
colon, such as "D:"), a printer name, or a string starting with two
backslashes (\\). Printer names must be one of the following: PRN, LPT1,
LPT2, or LPT3. If the string with two backslashes is used, the
connection between the local machine and the network directory is
terminated.
Example:
Cancel the redirection of local drive E to the network server.
locname db 'E:',0
.
.
. mov ax,5f04h ; function & subfunction
mov si,seg locname ; address of local name
mov ds,si
mov si,offset locname
int 21h ; transfer to MS-DOS
jc error ; jump if cancel failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 60H (96)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 61H (97)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [3.0]
Function 62H (98)
Get PSP address
────────────────────────────────────────────────────────────────────────────
Obtains the segment (paragraph) address of the program segment prefix
(PSP) for the currently executing program.
Call with:
AH = 62H
Returns:
BX = segment address of program segment prefix
Notes:
■ Before a program receives control from MS-DOS, its program segment
prefix is set up to contain certain vital information, such as:
∙ the segment address of the program's environment block
∙ the command line originally entered by the user
∙ the original contents of the terminate, Ctrl-C, and critical-error
handler vectors
∙ the top address of available RAM
■ The segment address of the PSP is normally passed to the program in
registers DS and ES when it initially receives control from MS-DOS. This
function allows a program to conveniently recover the PSP address at any
point during its execution, without having to save it at program entry.
Example:
Get the segment base of the program segment prefix, then copy the command
tail from the PSP into the local buffer named buff.
ctail equ 080H ; PSP offset, command tail
buff db 80 dup (?) ; copy of command tail
.
.
.
; get PSP address
mov ah,62H ; function number
int 21h ; transfer to MS-DOS
; copy command tail
mov ds,bx ; PSP segment to DS
mov si,offset ctail ; offset of command tail
mov di,seg buff ; local buffer address
mov es,di
mov di,offset buff
mov cl,[si] ; length of command tail
inc cl ; include count byte
xor ch,ch
cld
rep movsb ; copy to local buffer
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [2.25 only]
Function 63H (99)
Get DBCS lead byte table
────────────────────────────────────────────────────────────────────────────
Obtains the address of the system table of legal lead byte ranges for
double-byte character sets (DBCS), or sets or obtains the interim console
flag. Int 21H Function 63H is available only in MS-DOS version 2.25; it
is not supported in MS-DOS versions 3.0 and later.
Call with:
AH = 63H
AL = subfunction
00H if getting address of DBCS lead byte table
01H if setting or clearing interim console flag
02H if obtaining value of interim console flag
If AL = 01H
DL = 00H if clearing interim console flag
01H if setting interim console flag
Returns:
If function successful
Carry flag = clear
and, if called with AL = 00H
DS:SI = segment:offset of DBCS lead byte table
or, if called with AL = 02H
DL = value of interim console flag
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The DBCS lead byte table consists of a variable number of two byte
entries, terminated by two null (00H) bytes. Each pair defines the
beginning and ending value for a range of lead bytes. The value of a
legal lead byte is always in the range 80─0FFH.
■ Entries in the lead byte table must be in ascending order. If no legal
lead bytes are defined in a given system, the table consists only of the
two null bytes.
■ If the interim console flag is set, Int 21H Functions 07H (Unfiltered
Character Input), 08H (Character Input without Echo), and 0BH
(Keyboard Status) will support interim characters.
■ Unlike most other MS-DOS services, this function call does not
necessarily preserve any registers except SS:SP.
■ [4.0] The address of the DBCS lead byte table can also be obtained with
Int 21H Function 65H.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 64H (100)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [3.3]
Function 65H (101)
Get extended country information
────────────────────────────────────────────────────────────────────────────
Obtains information about the specified country and/or code page.
Call with:
AH = 65H
AL = subfunction
01H = Get General Internationalization Information
02H = Get Pointer to Uppercase Table
04H = Get Pointer to Filename Uppercase Table
06H = Get Pointer to Collating Table
07H = Get Pointer to Double-Byte Character Set (DBCS)
Vector (MS-DOS versions 4.0 and later)
BX = code page of interest (-1 = active CON device)
CX = length of buffer to receive information (must be >= 5)
DX = country ID (-1 = default)
ES:DI = address of buffer to receive information
Returns:
If function successful
Carry flag = clear
and requested data placed in calling program's buffer
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The information returned by this function is a superset of the
information returned by Int 21H Function 38H.
■ This function may fail if either the country code or the code page
number is invalid, or if the code page does not match the country code.
■ The function fails if the specified buffer length is less than five
bytes. If the buffer to receive the information is at least five bytes
long but is too short for the requested information, the data is
truncated and no error is returned.
■ The format of the data returned by Subfunction 01H is:
Byte(s) Contents
00H information ID code (1)
01H─02H length of following buffer
03H─04H country ID
05H─06H code page number
07H─08H date format
0 = USA m d y
1 = Europe d m y
2 = Japan y m d
09H─0DH ASCIIZ currency symbol
0EH─0FH ASCIIZ thousands separator
10H─11H ASCIIZ decimal separator
12H─13H ASCIIZ date separator
14H─15H ASCIIZ time separator
16H currency format flags
bit 0 =>0 if currency symbol precedes value
=>1 if currency symbol follows value
bit 1 =>0 if no space between value and
currency symbol
=>1 if one space between value and
currency symbol
bit 2 =>0 if currency symbol and decimal are
separate
=>1 if currency symbol replaces decimal
separator
17H number of digits after decimal in currency
18H time format
bit 0 = 0 if 12-hour clock
= 1 if 24-hour clock
19H─1CH case-map routine call address
1DH─1EH ASCIIZ data list separator
1FH─28H reserved
■ The format of the data returned by Subfunctions 02H, 04H, 06H, and 07H
is:
Byte(s) Contents
00H information ID code (2, 4, or 6)
01H─05H double-word pointer to table
■ The uppercase and filename uppercase tables are a maximum of 130 bytes
long. The first two bytes contain the size of the table; the following
bytes contain the uppercase equivalents, if any, for character codes
80H─FFH. The main use of these tables is to map accented or otherwise
modified vowels to their plain vowel equivalents. Text translated with
the help of this table can be sent to devices that do not support the
IBM graphics character set, or used to create filenames that do not
require a special keyboard configuration for entry.
■ The collating table is a maximum of 258 bytes long. The first two bytes
contain the table length, and the subsequent bytes contain the values to
be used for the corresponding character codes (0─FFH) during a sort
operation. This table maps uppercase and lowercase ASCII characters to
the same collating codes so that sorts will be case-insensitive, and it
maps accented vowels to their plain vowel equivalents.
■ [4.0+] Subfunction 07H returns a pointer to a variable length table of
that defines ranges for double-byte character set (DBCS) lead bytes. The
table is terminated by a pair of zero bytes, unless it must be truncated
to fit in the buffer, and has the following format:
dw length
db start1,end1
db start2,end2
.
.
.
db 0,0
For example:
dw 4
db 81h,9fh
db 0e0h,0fch
db 0,0
■ In some cases a truncated translation table may be presented to the
program by MS-DOS. Applications should always check the length at the
beginning of the table, to make sure it contains a translation code for
the particular character of interest.
Examples:
Obtain the extended country information associated with the default
country and code page 437.
buffer db 41 dup (0) ; receives country info
.
.
.
mov ax,6501h ; function & subfunction
mov bx,437 ; code page
mov cx,41 ; buffer length
mov dx,-1 ; default country
mov di,seg buffer ; buffer address
mov es,di
mov di,offset buffer
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
In this case, MS-DOS filled the following extended country information
into the buffer:
buffer db 1 ; info ID code
dw 38 ; length of following buffer
dw 1 ; country ID (USA)
dw 437 ; code page number
dw 0 ; date format
db '$',0,0,0,0 ; currency symbol
db ',',0 ; thousands separator
db '.',0 ; decimal separator
db '-',0 ; date separator
db ':',0 ; time separator
db 0 ; currency format flags
db 2 ; digits in currency
db 0 ; time format
dd 026ah:176ch ; case map entry point
db ',',0 ; data list separator
db 10 dup (0) ; reserved
Obtain the pointer to the uppercase table associated with the default
country and code page 437.
buffer db 5 dup (0) ; receives pointer info
.
.
.
mov ax,6502h ; function number
mov bx,437 ; code page
mov cx,5 ; length of buffer
mov dx,-1 ; default country
mov di,seg buffer ; buffer address
mov es,di
mov di,offset buffer
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
In this case, MS-DOS filled the following values into the buffer:
buffer db 2 ; info ID code
dw 0204h ; offset of uppercase table
dw 1140h ; segment of uppercase table
and the table at 1140:0204H contains the following data:
0 1 2 3 4 5 6 7 8 9 A B C D E F 0123456789ABCDE
1140:0200 80 00 80 9A 45 41 8E 41 8F 80 45 45 ....EA.A..E
1140:0210 45 49 49 49 8E 8F 90 92 92 4F 99 4F 55 55 59 99 EIII.....O.OUUY
1140:0220 9A 9B 9C 9D 9E 9F 41 49 4F 55 A5 A5 A6 A7 A8 A9 ......AIOU.....
1140:0230 AA AB AC AD AE AF B0 B1 B2 B3 B4 B5 B6 B7 B8 B9 ...............
1140:0240 BA BB BC BD BE BF C0 C1 C2 C3 C4 C5 C6 C7 C8 C9 ...............
1140:0250 CA CB CC CD CE CF D0 D1 D2 D3 D4 D5 D6 D7 D8 D9 ...............
1140:0260 DA DB DC DD DE DF E0 E1 E2 E3 E4 E5 E6 E7 E8 E9 ...............
1140:0270 EA EB EC ED EE EF F0 F1 F2 F3 F4 F5 F6 F7 F8 F9 ...............
1140:0280 FA FB FC FD FE FF ......
────────────────────────────────────────────────────────────────────────────
Int 21H [3.3]
Function 66H (102)
Get or set code page
────────────────────────────────────────────────────────────────────────────
Obtains or selects the current code page.
Call with:
AH = 66H
AL = subfunction
01H = Get Code Page
02H = Select Code Page
BX = code page to select, if AL = 02H
Returns:
If function successful
Carry flag = clear
and, if called with AL = 01H
BX = active code page
DX = default code page
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ When the Select Code Page subfunction is used, MS-DOS gets the new code
page from the COUNTRY.SYS file. The device must be previously prepared
for code page switching with the appropriate DEVICE= directive in the
CONFIG.SYS file and NLSFUNC and MODE CP PREPARE commands (placed in the
AUTOEXEC.BAT file, usually).
Example:
Force the active code page to be the same as the system's default code
page, that is, restore the code page that was active when the system was
first booted.
.
.
.
; get current and
; default code page
mov ax,6601h ; function number
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
; set code page
mov bx,dx ; active = default
mov ax,6602h ; function number
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.3]
Function 67H (103)
Set handle count
────────────────────────────────────────────────────────────────────────────
Sets the maximum number of files and devices that may be opened
simultaneously using handles by the current process.
Call with:
AH = 67H
BX = number of desired handles
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ This function call controls the size of the table that relates handle
numbers for the current process to MS-DOS's internal, global table for
all of the open files and devices in the system. The default table is
located in the reserved area of the process's PSP and is large enough
for 20 handles.
■ The function fails if the requested number of handles is greater than 20
and there is not sufficient free memory in the system to allocate a new
block to hold the enlarged table.
■ If the number of handles requested is larger than the available entries
in the system's global table for file and device handles (controlled by
the FILES entry in CONFIG.SYS), no error is returned. However, a
subsequent attempt to open a file or device, or create a new file, will
fail if all the entries in the system's global file table are in use,
even if the requesting process has not used up all its own handles.
Example:
Set the maximum handle count for the current process to thirty, so that
the process can have as many as 30 files or devices opened simultaneously.
(Five of the handles are already assigned to the standard devices when the
process starts up.) Note that a FILES=30 (or greater value) entry in the
CONFIG.SYS file would also be required for the process to successfully
open 30 files or devices.
.
.
.
mov ah,67h ; function number
mov bx,30 ; maximum number of handles
int 21h ; transfer to MS-DOS
jc error ; jump if function failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H [3.3]
Function 68H (104)
Commit file
────────────────────────────────────────────────────────────────────────────
Forces all data in MS-DOS's internal buffers associated with a specified
handle to be physically written to the device. If the handle refers to a
file, and the file has been modified, the time and date stamp and file
size in the file's directory entry are updated.
Call with:
AH = 68H
BX = handle
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The effect of this function is equivalent to closing and reopening a
file, or to duplicating a handle for the file with Int 21H Function
45H and then closing the duplicate. However, this function has the
advantage that it will not fail due to lack of handles, and the
application does not risk losing control of the file in multitasking or
network environments.
■ If this function is requested for a handle associated with a character
device, a success flag is returned, but there is no other effect.
Example:
Assume that the file MYFILE.DAT has been previously opened and that the
handle for that file is stored in the variable fhandle. Call the Commit
File function to ensure that any data in MS-DOS's internal buffers
associated with the handle is written out to disk and that the directory
and file allocation table are up to date.
fname db 'MYFILE.DAT',0 ; ASCIIZ filename
fhandle dw ? ; file handle
.
.
.
mov ah,68h ; function number
mov bx,fhandle ; file handle
int 21h ; transfer to MS-DOS
jc error ; jump if commit failed
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 69H (105)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 6AH (106)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H
Function 6BH (107)
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 21H [4.0]
Function 6CH (108)
Extended open file
────────────────────────────────────────────────────────────────────────────
Given an ASCIIZ pathname, opens, creates or replaces a file in the
designated or default directory on the designated or default disk drive.
Returns a handle that can be used by the program for subsequent access to
the file.
Call with:
AH = 6CH
AL = 00H
BX = open mode
Bit(s) Significance
0─2 access type
000 = read-only
001 = write-only
010 = read/write
3 reserved (0)
4─6 sharing mode
000 = compatibility
001 = deny read/write (deny all)
010 = deny write
011 = deny read
100 = deny none
7 inheritance
0 = child process inherits handle
1 = child does not inherit handle
8─12 reserved (0)
13 critical error handling
0 = execute Int 24H
1 = return error to process
14 write-through
0 = writes may be buffered and deferred
1 = physical write at request time
15 reserved (0)
CX = file attribute (bits may be combined; ignored if open)
Bit(s) Significance (if set)
0 read-only
1 hidden
2 system
3 volume label
4 reserved (0)
5 archive
6─15 reserved (0)
DX = open flag
Bits Significance
0─3 action if file exists
0000 = fail
0001 = open file
0010 = replace file
4─7 action if file doesn't exist
0000 = fail
0001 = create file
8─15 reserved (0)
DS:SI = segment:offset of ASCIIZ pathname
Returns:
If function successful
Carry flag = clear
AX = handle
CX = action taken
1 = file existed and was opened
2 = file did not exist and was created
3 = file existed and was replaced
If function failed
Carry flag = set
AX = error code
Notes:
■ The function fails if:
∙ any element of the pathname does not exist.
∙ the file is being created in the root directory and the root directory
is full.
∙ the file is being created and a file with the same name and the
read-only attribute already exists in the specified directory.
∙ the program is running on a network and the user running the program
has insufficient access rights.
■ A file is usually given a normal (0) attribute when it is created. The
file's attribute can subsequently be modified with Int 21H Function
43H.
■ This function combines the capabilities of Int 21H Functions 3CH, 3DH,
and 5BH. It was added to MS-DOS for compatibility with the DosOpen
function of OS/2.
Example:
Create the file MYFILE.DAT, if it does not already exist, in directory
\MYDIR on drive C, and save the handle for subsequent access to the file.
fname db 'C:\MYDIR\MYFILE.DAT',0
fhandle dw ?
.
.
.
mov ax,6c00h ; function number
mov bx,4042h ; read/write, deny none,
; write-through mode
xor cx,cx ; normal attribute
mov dx,0010h ; create if doesn't exist,
; fail if exists
mov si,seg fname ; address of pathname
mov ds,si
mov si,offset fname
int 21h ; transfer to MS-DOS
jc error ; jump if open failed
mov fhandle,ax ; save file handle
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 22H [1.0]
Terminate handler address
────────────────────────────────────────────────────────────────────────────
The machine interrupt vector for Int 22H (memory locations 0000:0088H
through 0000:008BH) contains the address of the routine that receives
control when the currently executing program terminates via Int 20H, Int
27H, or Int 21H Functions 00H, 31H, or 4CH. The address in this vector
is also copied into offsets 0AH through 0DH of the program segment prefix
(PSP) when a program is loaded but before it begins executing, and is
restored from the PSP (in case it was modified by the application) as part
of MS-DOS's termination handling.
This interrupt should never be issued directly.
────────────────────────────────────────────────────────────────────────────
Int 23H [1.0]
Ctrl-C handler address
────────────────────────────────────────────────────────────────────────────
The machine interrupt vector for Int 23H (memory locations 0000:008CH
though 0000:008FH) contains the address of the routine which receives
control when a Ctrl-C is detected during any character I/O function and,
if the Break flag is ON, during most other MS-DOS function calls. The
address in this vector is also copied into locations 0EH through 11H of
the program segment prefix (PSP) when a program is loaded but before it
begins executing, and is restored from the PSP (in case it was modified by
the application) as part of MS-DOS's termination handling.
This interrupt should never be issued directly.
Notes:
■ The initialization code for an application can use Int 21H Function
25H to reset the Interrupt 23H vector to point to its own routine for
Ctrl-C handling. In this way, the program can avoid being terminated
unexpectedly as the result of the user's entry of a Ctrl-C or
Ctrl-Break.
■ When a Ctrl-C is detected and the program's Int 23H handler receives
control, all registers are set to their values at the point of the
original function call. The handler can then do any of the following:
∙ Set a local flag for later inspection by the application, or take any
other appropriate action, and perform an IRET. All registers must be
preserved. The MS-DOS function in progress will be restarted from
scratch and will proceed to completion, control finally returning to
the application in the normal manner.
∙ Take appropriate action and then perform a RET FAR to give control
back to MS-DOS. The state of the carry flag is used by MS-DOS to
determine what action to take. If the carry flag is set, the
application will be terminated; if the carry flag is clear, the
application will continue in the normal manner.
∙ Retain control by transferring to an error-handling routine within the
application and then resume execution or take other appropriate
action, never performing a RET FAR or IRET to end the
interrupt-handling sequence. This option will cause no harm to the
system.
■ Any MS-DOS function call may be used within the body of an Int 23H
handler.
Example:
See Chapter 5.
────────────────────────────────────────────────────────────────────────────
Int 24H [1.0]
Critical-error handler address
────────────────────────────────────────────────────────────────────────────
The machine interrupt vector for Int 24H (memory locations 0000:0090H
through 0000:0093H) contains the address of the routine that receives
control when a critical error (usually a hardware error) is detected. This
address is also copied into locations 12H through 15H of the program
segment prefix (PSP) when a program is loaded but before it begins
executing, and is restored from the PSP (in case it was modified by the
application) as part of MS-DOS's termination handling.
This interrupt should never be issued directly.
Notes:
■ On entry to the critical-error interrupt handler, bit 7 of register AH
is clear (0) if the error was a disk I/O error; otherwise, it is set
(1). BP:SI contains the address of a device-driver header from which
additional information can be obtained. Interrupts are disabled. The
registers will be set up for a retry operation, and an error code will
be in the lower half of the DI register, with the upper half undefined.
The lower byte of DI contains:
00H write-protect error
01H unknown unit
02H drive not ready
03H unknown command
04H data error (CRC)
05H bad request structure length
06H seek error
07H unknown media type
08H sector not found
09H printer out of paper
0AH write fault
0BH read fault
0CH general failure
0DH reserved
0EH reserved
0FH invalid disk change (MS-DOS version 3 only)
Note that these are the same error codes returned by the device driver
in the request header. Also, upon entry, the stack is set up as shown in
Figure 8-8, page 149.
■ When a disk I/O error occurs, MS-DOS automatically retries the operation
before issuing a critical-error Int 24H. The number of retries varies
in different versions of MS-DOS, but is typically in the range three to
five.
■ Int 24H handlers must preserve the SS, SP, DS, ES, BX, CX, and DX
registers. Only Int 21H Functions 01H─0CH and 59H can be used by an
Int 24H handler; other function calls will destroy the MS-DOS stack and
its ability to retry or ignore an error.
■ When the Int 24H handler issues an IRET, it should return an action code
in AL that will be interpreted by DOS as follows:
0 ignore the error
1 retry the operation
2 terminate the program
3 [3.0+] fail the function call in progress
■ If the Int 24H handler returns control directly to the application
program rather than to MS-DOS, it must restore the program's registers,
removing all but the last three words from the stack, and issue an IRET.
Control returns to the instruction immediately following the function
call that caused the error. This option leaves MS-DOS in an unstable
state until a call to an Int 21H function higher than Function 0CH is
made.
Example:
See Chapter 8.
────────────────────────────────────────────────────────────────────────────
Int 25H [1.0]
Absolute disk read
────────────────────────────────────────────────────────────────────────────
Provides a direct linkage to the MS-DOS BIOS module to read data from a
logical disk sector into memory.
Call with:
For access to partitions <= 32 MB
AL = drive number (0 = A, 1 = B, etc)
CX = number of sectors to read
DX = starting sector number
DS:BX = segment:offset of buffer
For access to partitions > 32 MB (MS-DOS 4.0 and later)
AL = drive number (0 = A, 1 = B, etc)
CX = -1
DS:BX = segment:offset of parameter block (see Notes)
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code (see Notes)
Notes:
■ All registers except the segment registers may be destroyed.
■ When this function returns, the CPU flags originally pushed on the stack
by the INT 25H instruction are still on the stack. The stack must be
cleared by a POPF or ADD SP,2 to prevent uncontrolled stack growth and
to make accessible any other values that were pushed on the stack before
the call to INT 25H.
■ Logical sector numbers are obtained by numbering each disk sector
sequentially from cylinder 0, head 0, sector 1, and continuing until the
last sector on the disk is counted. The head number is incremented
before the track number. Logically adjacent sectors may not be
physically adjacent, due to interleaving that occurs at the
device-adapter level for some disk types.
■ The error code is interpreted as follows: The lower byte (AL) is the
same error code that is returned in the lower byte of DI when an Int 24H
is issued. The upper byte (AH) contains:
01H if bad command
02H if bad address mark
04H if requested sector not found
08H if direct memory access (DMA) failure
10H if data error (bad CRC)
20H if controller failed
40H if seek operation failed
80H if attachment failed to respond
■ [4.0+] When accessing partitions larger than 32 MB under MS-DOS version
4, this function uses a parameter block with the following format:
Bytes Description
00H─03H 32-bit sector number
04H─05H number of sectors to read
06H─07H offset of buffer
08H─09H segment of buffer
Example:
Read logical sector 1 of drive A into the memory area named buff. (On most
MS-DOS floppy disks, this sector contains the beginning of the file
allocation table.)
buff db 512 dup (?) ; receives data from disk
.
.
.
mov al,0 ; drive A
mov cx,1 ; number of sectors
mov dx,1 ; beginning sector number
mov bx,seg buff ; buffer address
mov ds,bx
mov bx,offset buff
int 25h ; request disk read
jc error ; jump if read failed
add sp,2 ; clear stack
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 26H [1.0]
Absolute disk write
────────────────────────────────────────────────────────────────────────────
Provides a direct linkage to the MS-DOS BIOS module to write data from
memory to a logical disk sector.
Call with:
For access to partitions <= 32 MB
AL = drive number (0 = A, 1 = B, etc)
CX = number of sectors to write
DX = starting sector number
DS:BX = segment:offset of buffer
For access to partitions > 32 MB (MS-DOS 4.0 and later)
AL = drive number (0 = A, 1 = B, etc)
CX = -1
DS:BX = segment:offset of parameter block (see Notes)
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code (see Notes)
Notes:
■ All registers except the segment registers may be destroyed.
■ When this function returns, the CPU flags originally pushed onto the
stack by the INT 26H instruction are still on the stack. The stack must
be cleared by a POPF or ADD SP,2 to prevent uncontrolled stack growth
and to make accessible any other values that were pushed on the stack
before the call to INT 26H.
■ Logical sector numbers are obtained by numbering each disk sector
sequentially from cylinder 0, head 0, sector 1, and continuing until the
last sector on the disk is counted. The head number is incremented
before the track number. Logically adjacent sectors may not be
physically adjacent, due to interleaving that occurs at the
device-adapter level for some disk types.
■ The error code is interpreted as follows: The lower byte (AL) is the
same error code that is returned in the lower byte of DI when an Int
24H is issued. The upper byte (AH) contains:
01H if bad command
02H if bad address mark
03H if write-protect fault
04H if requested sector not found
08H if direct memory access (DMA) failure
10H if data error (bad CRC)
20H if controller failed
40H if seek operation failed
80H if attachment failed to respond
■ [4.0+] When accessing partitions larger than 32 MB under MS-DOS version
4, this function uses a parameter block with the following format:
Bytes Description
00H─03H 32-bit sector number
04H─05H number of sectors to read
06H─07H offset of buffer
08H─09H segment of buffer
Example:
Write the contents of the memory area named buff into logical sector 3 of
drive C.
Warning: Verbatim use of the following code could damage the file system
on your fixed disk. There is, unfortunately, no way to provide a really
safe example of this function.
buff db 512 dup (?) ; contains data for write
.
.
.
mov al,2 ; drive C
mov cx,1 ; number of sectors
mov dx,3 ; beginning sector number
mov bx,seg buff ; buffer address
mov ds,bx
mov bx,offset buff
int 26h ; request disk write
jc error ; jump if write failed
add sp,2 ; clear stack
.
.
.
────────────────────────────────────────────────────────────────────────────
Int 27H [1.0]
Terminate and stay resident
────────────────────────────────────────────────────────────────────────────
Terminates execution of the currently executing program, but reserves part
or all of its memory so that it will not be overlaid by the next transient
program to be loaded. MS-DOS then takes the following actions:
■ File buffers are flushed and any open handles for files or devices owned
by the process are closed.
■ The termination handler vector (Int 22H) is restored from PSP:000AH.
■ The Ctrl-C handler vector (Int 23H) is restored from PSP:000EH.
■ [2.0+] The critical-error handler vector (Int 24H) is restored from
PSP:0012H.
■ Control is transferred to the termination handler.
If the program is returning to COMMAND.COM, control transfers to the
resident portion and the transient portion is reloaded if necessary. If a
batch file is in progress, the next line of the file is fetched and
interpreted; otherwise a prompt is issued for the next user command.
Call with:
DX = offset of the last byte plus one (relative to the program
segment prefix)
of program to be protected
CS = segment of program segment prefix
Returns:
Nothing
Notes:
■ This function call is typically used to allow user-written utilities,
drivers, or interrupt handlers to be loaded as ordinary .COM or .EXE
programs, then remain resident. Subsequent entrance to the code is via a
hardware or software interrupt.
■ This function attempts to set the initial memory allocation block to the
length in bytes specified in register DX. If other memory blocks have
been requested by the application via Int 21H Function 48H, they will
not be released by this function.
■ Other methods of performing a final exit are:
∙ Int 20H
∙ Int 21H Function 00H
∙ Int 21H Function 31H
∙ Int 21H Function 4CH
■ This function should not be called by .EXE programs that are loaded at
the high end of the transient program area (i.e., linked with the /HIGH
switch), because doing so reserves the memory normally used by the
transient part of COMMAND.COM. If COMMAND.COM cannot be reloaded, the
system will fail.
■ This function does not work correctly when DX contains values in the
range 0FFF1H─0FFFFH. In this case, MS-DOS discards the high bit of the
value in DX, resulting in the reservation of 32 KB less memory than was
requested by the program.
■ [2.0+] Int 21H Function 31H should be used in preference to this
function, because it supports return codes, allows larger amounts of
memory to be reserved, and does not require CS to contain the segment of
the program segment prefix.
■ [3.0+] If the program is running on a network, it should remove all
locks it has placed on file regions before terminating.
Example:
Terminate and stay resident, reserving enough memory to contain the entire
program.
.
.
.
mov dx,offset pend ; DX = bytes to reserve
int 27h ; terminate, stay resident
.
.
.
pend equ $ ; offset, end of program
end
────────────────────────────────────────────────────────────────────────────
Int 28H
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 29H
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 2AH
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 2BH
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 2CH
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 2DH
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 2EH
Reserved
────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 2FH [3.0]
Multiplex interrupt
────────────────────────────────────────────────────────────────────────────
Provides a general-purpose avenue of communication with another process or
with MS-DOS extensions, such as the print spooler, ASSIGN, SHARE, and
APPEND. The multiplex number in register AH specifies the process or
extension being communicated with. The range 00H─BFH is reserved for
MS-DOS; applications may use the range C0H─FFH.
────────────────────────────────────────────────────────────────────────────
Int 2FH [3.0]
Function 01H
Print spooler
────────────────────────────────────────────────────────────────────────────
Submits a file to the print spooler, removes a file from the print
spooler's queue of pending files, or obtains the status of the printer.
The print spooler, which is contained in the file PRINT.COM, was first
added to MS-DOS in version 2.0, but the application program interface to
the spooler was not documented until MS-DOS version 3.
Call with:
AH = 01H
AL = subfunction
00H = Get Installed State
01H = Submit File to be Printed
02H = Remove File from Print Queue
03H = Cancel All Files in Queue
04H = Hold Print Jobs for Status Read
05H = Release Hold
DS:DX = segment:offset of packet (Subfunction 01H)
segment:offset of ASCIIZ pathname (Subfunction 02H)
Returns:
If function successful
Carry flag = clear
and, if called with AL = 00H
AL = print spooler state
00H if not installed, ok to install
01H if not installed, not ok to install
FFH if installed
or, if called with AL = 04H
DX = error count
DS:SI = segment:offset of print queue file list
If function unsuccessful
Carry flag = set
AX = error code
Notes:
■ The packet passed to Subfunction 01H consists of five bytes. The first
byte contains the level, which should be 00H for current versions of
MS-DOS. The following four bytes contain the segment:offset of an ASCIIZ
pathname, which may not include wildcard characters. If the specified
file exists, it is added to the print queue.
■ The * and ? wildcard characters may be included in a pathname passed to
Subfunction 02H, making it possible to delete multiple files from the
print queue with one call.
■ The address returned by Subfunction 04H points to a list of 64-byte
entries, each containing an ASCIIZ pathname. The first pathname in the
list is the file currently being printed. The last entry in the list is
a null string (a single 00H byte).
────────────────────────────────────────────────────────────────────────────
Int 2FH [3.2]
Function 02H
ASSIGN
────────────────────────────────────────────────────────────────────────────
Returns a code indicating whether the resident portion of the ASSIGN
utility has been loaded.
Call with:
AH = 02H
AL = subfunction
00H = Get Installed State
Returns:
If function successful
Carry flag = clear
AL = ASSIGN installed status
00H if not installed, ok to install
01H if not installed, not ok to install
FFH if installed
If function unsuccessful
Carry flag = set
AX = error code
────────────────────────────────────────────────────────────────────────────
Int 2FH [3.2]
Function 10H (16)
SHARE
────────────────────────────────────────────────────────────────────────────
Returns a code indicating whether the SHARE.EXE file-sharing module has
been loaded.
Call with:
AH = 10H
AL = subfunction
00H = Get Installed State
Returns:
If function successful
Carry flag = clear
AL = SHARE installed status
00H if not installed, ok to install
01H if not installed, not ok to install
FFH if installed
If function unsuccessful
Carry flag = set
AX = error code
────────────────────────────────────────────────────────────────────────────
Int 2FH [3.3]
Function B7H (183)
APPEND
────────────────────────────────────────────────────────────────────────────
Allows an application to test whether APPEND has been installed. If APPEND
is resident, returns the APPEND version, state, and the path used to
search for data files.
Call with:
AH = B7H
AL = subfunction
00H = Get Installed State
02H = Get Append Version (4.0)
04H = Get Append Path Pointer (4.0)
06H = Get Append Function State (4.0)
07H = Set Append Function State (4.0)
11H = Set Return Found Name State (4.0, see Note)
BX = APPEND state (if AL = 07H)
Bit(s) Significance (if set)
0 APPEND enabled
1─12 Reserved (0)
13 /PATH switch active
14 /E switch active
15 /X switch active
Returns:
If function successful
Carry flag = clear
and, if called with AL = 00H
AL = APPEND installed status
00H if not installed, ok to install
01H if not installed, not ok to install
FFH if installed
or, if called with AL = 02H (MS-DOS 4.0)
AX = FFFFH if MS-DOS 4.0 APPEND
or, if called with AL = 04H (MS-DOS 4.0)
ES:DI = segment:offset of active APPEND path
or, if called with AL = 06H (MS-DOS 4.0)
BX = APPEND state (see above)
If function unsuccessful
Carry flag = set
AX = error code
Note:
■ If the Return Found Name State is set with Subfunction 11H, the fully
qualified filename is returned to the next application to call Int 21H
Function 3DH, 43H, or 6CH. The name is placed at the same address as the
ASCIIZ parameter string for the Int 21H function, so the application
must be sure to provide a buffer of adequate size. The Return Found Name
State is reset after APPEND processes one Int 21H function call.
────────────────────────────────────────────────────────────────────────────
SECTION 3 IBM ROM BIOS AND MOUSE FUNCTIONS REFERENCE
────────────────────────────────────────────────────────────────────────────
Notes to the Reader
In the headers for ROM BIOS video driver (Int 10H) function calls, the
following icons are used:
[MDA] Monochrome Display Adapter
[CGA] Color/Graphics Adapter
[PCjr] PCjr system board video controller
[EGA] Enhanced Graphics Adapter
[MCGA] Multi-Color Graphics Array (PS/2 Models 25 & 30)
[VGA] Video Graphics Array (PS/2 Models 50 and above)
In the remainder of this section, the following icons are used:
[PC] Original IBM PC, PC/XT, and PCjr, unless
otherwise noted.
[AT] PC/AT and PC/XT-286, unless otherwise noted.
[PS/2] All PS/2 models (including Models 25 and 30),
unless otherwise noted.
ROM BIOS functions that are unique to the PC Convertible have been
omitted.
Some functions are supported only in very late revisions of a particular
machine's ROM BIOS (such as Int 1AH Functions 00H and 01H on the PC/XT).
In general, such functions are not given an icon for that machine since a
program could not safely assume that they were available based on the
machine ID byte(s).
Summary of ROM BIOS and Mouse Function Calls
╓┌─┌──────┌────────────┌─────────────┌───────────────────────────────────────╖
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
10H Video Driver
10H 00H Set Video Mode
10H 01H Set Cursor Type
10H 02H Set Cursor Position
10H 03H Get Cursor Position
10H 04H Get Light Pen Position
10H 05H Set Display Page
10H 06H Initialize or Scroll Window Up
10H 07H Initialize or Scroll Window Down
10H 08H Read Character and Attribute at Cursor
10H 09H Write Character and Attribute at Cursor
10H 0AH (10) Write Character at Cursor
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
10H 0AH (10) Write Character at Cursor
10H 0BH (11) Set Palette, Background, or Border
10H 0CH (12) Write Graphics Pixel
10H 0DH (13) Read Graphics Pixel
10H 0EH (14) Write Character in Teletype Mode
10H 0FH (15) Get Video Mode
10H 10H (16) 00H Set Palette Register
10H 10H (16) 01H Set Border Color
10H 10H (16) 02H Set Palette and Border
10H 10H (16) 03H Toggle Blink/Intensity Bit
10H 10H (16) 07H Get Palette Register
10H 10H (16) 08H Get Border Color
10H 10H (16) 09H Get Palette and Border
10H 10H (16) 10H (16) Set Color Register
10H 10H (16) 12H (18) Set Block of Color Registers
10H 10H (16) 13H (19) Set Color Page State
10H 10H (16) 15H (21) Get Color Register
10H 10H (16) 17H (23) Get Block of Color Registers
10H 10H (16) 1AH (26) Get Color Page State
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
10H 10H (16) 1AH (26) Get Color Page State
10H 10H (16) 1BH (27) Set Gray-Scale Values
10H 11H (17) 00H Load User Font
10H 11H (17) 01H Load ROM 8-by-14 Font
10H 11H (17) 02H Load ROM 8-by-8 Font
10H 11H (17) 03H Set Block Specifier
10H 11H (17) 04H Load ROM 8-by-16 Font
10H 11H (17) 10H (16) Load User Font, Reprogram Controller
10H 11H (17) 11H (17) Load ROM 8-by-14 Font, Reprogram
Controller
10H 11H (17) 12H (18) Load ROM 8-by-8 Font, Reprogram
Controller
10H 11H (17) 14H (20) Load ROM 8-by-16 Font, Reprogram
Controller
10H 11H (17) 20H (32) Set Int 1FH Pointer
10H 11H (17) 21H (33) Set Int 43H for User's Font
10H 11H (17) 22H (34) Set Int 43H for ROM 8-by-14 Font
10H 11H (17) 23H (35) Set Int 43H for ROM 8-by-8 Font
10H 11H (17) 24H (36) Set Int 43H for Rom 8-by-16 Font
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
10H 11H (17) 24H (36) Set Int 43H for Rom 8-by-16 Font
10H 11H (17) 30H (48) Get Font Information
10H 12H (18) 10H (16) Get Configuration Information
10H 12H (18) 20H (32) Select Alternate PrintScreen
10H 12H (18) 30H (48) Set Scan Lines
10H 12H (18) 31H (49) Enable/Disable Palette Loading
10H 12H (18) 32H (50) Enable/Disable Video
10H 12H (18) 33H (51) Enable/Disable Gray-Scale Summing
10H 12H (18) 34H (52) Enable/Disable Cursor Emulation
10H 12H (18) 35H (53) Switch Active Display
10H 12H (18) 36H (54) Enable/Disable Screen Refresh
10H 13H (19) Write String in Teletype Mode
10H 1AH (26) Get or Set Display Combination Code
10H 1BH (27) Get Functionality/State Information
10H 1CH (28) Save or Restore Video State
11H Get Equipment Configuration
12H Get Conventional Memory Size
13H Disk Driver
13H 00H Reset Disk System
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
13H 00H Reset Disk System
13H 01H Get Disk System Status
13H 02H Read Sector
13H 03H Write Sector
13H 04H Verify Sector
13H 05H Format Track
13H 06H Format Bad Track
13H 07H Format Drive
13H 08H Get Drive Parameters
13H 09H Initialize Fixed Disk Characteristics
13H 0AH (10) Read Sector Long
13H 0BH (11) Write Sector Long
13H 0CH (12) Seek
13H 0DH (13) Reset Fixed Disk System
13H 0EH (14) Read Sector Buffer
13H 0FH (15) Write Sector Buffer
13H 10H (16) Get Drive Status
13H 11H (17) Recalibrate Drive
13H 12H (18) Controller RAM Diagnostic
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
13H 12H (18) Controller RAM Diagnostic
13H 13H (19) Controller Drive Diagnostic
13H 14H (20) Controller Internal Diagnostic
13H 15H (21) Get Disk Type
13H 16H (22) Get Disk Change Status
13H 17H (23) Set Disk Type
13H 18H (24) Set Media Type for Format
13H 19H (25) Park Heads
13H 1AH (26) Format ESDI Drive
14H Serial Communications Port Driver
14H 00H Initialize Communications Port
14H 01H Write Character to Communications Port
14H 02H Read Character from Communications Port
14H 03H Get Communications Port Status
14H 04H Extended Initialize Communications Port
14H 05H Extended Communications Port Control
15H I/O Subsystem Extensions
15H 00H Turn On Cassette Motor
15H 01H Turn Off Cassette Motor
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
15H 01H Turn Off Cassette Motor
15H 02H Read Cassette
15H 03H Write Cassette
15H 0FH (15) Format ESDI Drive Periodic Interrupt
15H 21H (33) 00H Read POST Error Log
15H 21H (33) 01H Write POST Error Log
15H 4FH (79) Keyboard Intercept
15H 80H (128) Device Open
15H 81H (129) Device Close
15H 82H (130) Process Termination
15H 83H (131) Event Wait
15H 84H (132) Read Joystick
15H 85H (133) SysReq Key
15H 86H (134) Delay
15H 87H (135) Move Extended Memory Block
15H 88H (136) Get Extended Memory Size
15H 89H (137) Enter Protected Mode
15H 90H (144) Device Wait
15H 91H (145) Device Post
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
15H 91H (145) Device Post
15H C0H (192) Get System Environment
15H C1H (193) Get Address of Extended BIOS Data Area
15H C2H (194) 00H Enable/Disable Pointing Device
15H C2H (194) 01H Reset Pointing Device
15H C2H (194) 02H Set Sample Rate
15H C2H (194) 03H Set Resolution
15H C2H (194) 04H Get Pointing Device Type
15H C2H (194) 05H Initialize Pointing Device Interface
15H C2H (194) 06H Set Scaling or Get Status
15H C2H (194) 07H Set Pointing Device Handler Address
15H C3H (195) Set Watchdog Time-Out
15H C4H (196) Programmable Option Select
16H Keyboard Driver
16H 00H Read Character from Keyboard
16H 01H Get Keyboard Status
16H 02H Get Keyboard Flags
16H 03H Set Repeat Rate
16H 04H Set Keyclick
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
16H 04H Set Keyclick
16H 05H Push Character and Scan Code
16H 10H (16) Read Character from Enhanced Keyboard
16H 11H (17) Get Enhanced Keyboard Status
16H 12H (18) Get Enhanced Keyboard Flags
17H Parallel Port Printer Driver
17H 00H Write Character to Printer
17H 01H Initialize Printer Port
17H 02H Get Printer Status
18H ROM BASIC
19H Reboot System
1AH Real-time (CMOS) Clock Driver
1AH 00H Get Tick Count
1AH 01H Set Tick Count
1AH 02H Get Time
1AH 03H Set Time
1AH 04H Get Date
1AH 05H Set Date
1AH 06H Set Alarm
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
1AH 06H Set Alarm
1AH 07H Reset Alarm
1AH 0AH (10) Get Day Count
1AH 0BH (11) Set Day Count
1AH 80H (128) Set Sound Source
33H Microsoft Mouse Driver
33H 00H Reset Mouse and Get Status
33H 01H Show Mouse Pointer
33H 02H Hide Mouse Pointer
33H 03H Get Mouse Position and Button Status
33H 04H Set Mouse Pointer Position
33H 05H Get Button Press Information
33H 06H Get Button Release Information
33H 07H Set Horizontal Limits for Pointer
33H 08H Set Vertical Limits for Pointer
33H 09H Set Graphics Pointer Shape
33H 0AH (10) Set Text Pointer Type
33H 0BH (11) Read Mouse Motion Counters
33H 0CH (12) Set User-defined Mouse Event Handler
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
33H 0CH (12) Set User-defined Mouse Event Handler
33H 0DH (13) Turn On Light Pen Emulation
33H 0EH (14) Turn Off Light Pen Emulation
33H 0FH (15) Set Mickeys to Pixels Ratio
33H 10H (16) Set Mouse Pointer Exclusion Area
33H 13H (19) Set Double Speed Threshold
33H 14H (20) Swap User-defined Mouse Event Handlers
33H 15H (21) Get Mouse Save State Buffer Size
33H 16H (22) Save Mouse Driver State
33H 17H (23) Restore Mouse Driver State
33H 18H (24) Set Alternate Mouse Event Handler
33H 19H (25) Get Address of Alternate Mouse Event
Handler
33H 1AH (26) Set Mouse Sensitivity
33H 1BH (27) Get Mouse Sensitivity
33H 1CH (28) Set Mouse Interrupt Rate
33H 1DH (29) Select Pointer Page
33H 1EH (30) Get Pointer Page
33H 1FH (31) Disable Mouse Driver
Int Function Subfunction Name
──────────────────────────────────────────────────────────────────────────
33H 1FH (31) Disable Mouse Driver
33H 20H (32) Enable Mouse Driver
33H 21H (33) Reset Mouse Driver
33H 22H (34) Set Language for Mouse Driver Messages
33H 23H (35) Get Language Number
33H 24H (36) Get Mouse Information
──────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 00H
Set video mode
────────────────────────────────────────────────────────────────────────────
Selects the current video display mode. Also selects the active video
controller, if more than one video controller is present.
Call with:
AH = 00H
AL = video mode (see Notes)
Returns:
Nothing
Notes:
■ The video modes applicable to the various IBM machine models and video
adapters are as follows:
╓┌─┌─────┌───────────┌────────┌───────────┌─────┌─────┌────┌─────┌─────┌─────╖
Mode Resolution Colors Text/ MDA CGA PCjr EGA MCGA VGA
graphics
──────────────────────────────────────────────────────────────────────────
00H 40-by-25 16 text * * * * *
color burst
off
Mode Resolution Colors Text/ MDA CGA PCjr EGA MCGA VGA
graphics
──────────────────────────────────────────────────────────────────────────
off
01H 40-by-25 16 text * * * * *
02H 80-by-25 16 text * * * * *
color burst
off
03H 80-by-25 16 text * * * * *
04H 320-by-200 4 graphics * * * * *
05H 320-by-200 4 graphics * * * * *
color burst
off
06H 640-by-200 2 graphics * * * * *
07H 80-by-25 2☼ text * * *
08H 160-by-200 16 graphics *
09H 320-by-200 16 graphics *
0AH 640-by-200 4 graphics *
0BH reserved
0CH reserved
0DH 320-by-200 16 graphics * *
Mode Resolution Colors Text/ MDA CGA PCjr EGA MCGA VGA
graphics
──────────────────────────────────────────────────────────────────────────
0DH 320-by-200 16 graphics * *
0EH 640-by-200 16 graphics * *
0FH 640-by-350 2☼ graphics * *
10H 640-by-350 4 graphics *☼
10H 640-by-350 16 graphics *☼ *
11H 640-by-480 2 graphics * *
12H 640-by-480 16 graphics *
13H 320-by-200 256 graphics * *
──────────────────────────────────────────────────────────────────────────
■ The presence or absence of color burst is only significant when a
composite monitor is being used. For RGB monitors, there is no
functional difference between modes 00H and 01H or modes 02H and 03H. On
the CGA, two palettes are available in mode 04H and one in mode 05H.
■ On the PC/AT, PCjr, and PS/2, if bit 7 of AL is set, the display buffer
is not cleared when a new mode is selected. On the PC or PC/XT, this
capability is available only when an EGA or VGA (which have their own
ROM BIOS) is installed.
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 01H
Set cursor type
────────────────────────────────────────────────────────────────────────────
Selects the starting and ending lines for the blinking hardware cursor in
text display modes.
Call with:
AH = 01H
CH bits 0─4 = starting line for cursor
CL bits 0─4 = ending line for cursor
Returns:
Nothing
Notes:
■ In text display modes, the video hardware causes the cursor to blink,
and the blink cannot be disabled. In graphics modes, the hardware cursor
is not available.
■ The default values set by the ROM BIOS are:
Display Start End
────────────────────────────────────────────────────────────────────────
monochrome mode 07H 11 12
text modes 00H─03H 6 7
────────────────────────────────────────────────────────────────────────
■ On the EGA, MCGA, and VGA in text modes 00H─03H, the ROM BIOS accepts
cursor start and end values as though the character cell were 8 by 8 and
remaps the values as appropriate for the true character cell dimensions.
This mapping is called cursor emulation.
■ You can turn off the cursor in several ways. On the MDA, CGA, and VGA,
setting register CH = 20H causes the cursor to disappear. Techniques
that involve setting illegal starting and ending lines for the current
display mode are unreliable. An alternative is to position the cursor to
a nondisplayable address, such as (x,y)=(0,25).
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 02H
Set cursor position
────────────────────────────────────────────────────────────────────────────
Positions the cursor on the display, using text coordinates.
Call with:
AH = 02H
BH = page
DH = row (y coordinate)
DL = column (x coordinate)
Returns:
Nothing
Notes:
■ A separate cursor is maintained for each display page, and each can be
set independently with this function regardless of the currently active
page. The number of available display pages depends on the video adapter
and current display mode. See Int 10H Function 05H.
■ Text coordinates (x,y)=(0,0) are the upper left corner of the screen.
■ The maximum value for each text coordinate depends on the video adapter
and current display mode, as follows:
Mode Maximum x Maximum y
────────────────────────────────────────────────────────────────────────
00H 39 24
01H 39 24
02H 79 24
03H 79 24
04H 39 24
05H 39 24
06H 79 24
07H 79 24
08H 19 24
09H 39 24
0AH 79 24
0BH reserved
0CH reserved
0DH 39 24
0EH 79 24
0FH 79 24
10H 79 24
11H 79 29
12H 79 29
13H 39 24
────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 03H
Get cursor position
────────────────────────────────────────────────────────────────────────────
Obtains the current position of the cursor on the display, in text
coordinates.
Call with:
AH = 03H
BH = page
Returns:
CH = starting line for cursor
CL = ending line for cursor
DH = row (y coordinate)
DL = column (x coordinate)
Note:
■ A separate cursor is maintained for each display page, and each can be
inspected independently with this function regardless of the currently
active page. The number of available display pages depends on the video
adapter and current display mode. See Int 10H Function 05H.
────────────────────────────────────────────────────────────────────────────
Int 10H [CGA] [PCjr] [EGA]
Function 04H
Get light pen position
────────────────────────────────────────────────────────────────────────────
Obtains the current status and position of the light pen.
Call with:
AH = 04H
Returns:
AH = 00H if light pen not down/not triggered
01H if light pen down/triggered
BX = pixel column (graphics x coordinate)
CH = pixel row (graphics y coordinate, modes 04H─06H)
CX = pixel row (graphics y coordinate, modes 0DH─13H)
DH = character row (text y coordinate)
DL = character column (text x coordinate)
Notes:
■ The range of text and graphics coordinates returned by this function
depends on the current display mode.
■ On the CGA, the graphics coordinates returned by this function are not
continuous. The y coordinate is always a multiple of two; the x
coordinate is either a multiple of four (for 320-by-200 graphics modes)
or a multiple of eight (for 640-by-200 graphics modes).
■ Careful selection of background and foreground colors is necessary to
obtain maximum sensitivity from the light pen across the full screen
width.
────────────────────────────────────────────────────────────────────────────
Int 10H [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 05H
Set display page
────────────────────────────────────────────────────────────────────────────
Selects the active display page for the video display.
Call with:
For CGA, EGA, MCGA, VGA
AH = 05H
AL = page
0─7 for modes 00H and 01H (CGA, EGA, MCGA, VGA)
0─3 for modes 02H and 03H (CGA)
0─7 for modes 02H and 03H (EGA, MCGA, VGA)
0─7 for mode 07H (EGA, VGA)
0─7 for mode 0DH (EGA, VGA)
0─3 for mode 0EH (EGA, VGA)
0─1 for mode 0FH (EGA, VGA)
0─1 for mode 10H (EGA, VGA)
For PCjr only
AH = 05H
AL = subfunction
80H = read CRT/CPU page registers
81H = set CPU page register
82H = set CRT page register
83H = set both CPU and CRT page registers
BH = CRT page (Subfunctions 82H and 83H)
BL = CPU page (Subfunctions 81H and 83H)
Returns:
If CGA, EGA, MCGA, or VGA adapter
Nothing
If PCjr and if function called with AL = 80H─83H
BH = CRT page register
BL = CPU page register
Notes:
■ Video mode and adapter combinations not listed above support one display
page (for example, a Monochrome Adapter in mode 7).
■ Switching between pages does not affect their contents. In addition,
text can be written to any video page with Int 10H Functions 02H, 09H,
and 0AH, regardless of the page currently being displayed.
■ On the PCjr, the CPU page determines the part of the physical memory
region 00000H─1FFFFH that will be hardware mapped onto 16 KB of memory
beginning at segment B800H. The CRT page determines the starting address
of the physical memory used by the video controller to refresh the
display. Smooth animation effects can be achieved by manipulation of
these registers. Programs that write directly to the B800H segment can
reach only the first 16 KB of the video refresh buffer. Programs
requiring direct access to the entire 32 KB buffer in modes 09H and 0AH
can obtain the current CRT page from the ROM BIOS variable PAGDAT at
0040:008AH.
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 06H
Initialize or scroll window up
────────────────────────────────────────────────────────────────────────────
Initializes a specified window of the display to ASCII blank characters
with a given attribute or scrolls up the contents of a window by a
specified number of lines.
Call with:
AH = 06H
AL = number of lines to scroll (if zero, entire window is
blanked)
BH = attribute to be used for blanked area
CH = y coordinate, upper left corner of window
CL = x coordinate, upper left corner of window
DH = y coordinate, lower right corner of window
DL = x coordinate, lower right corner of window
Returns:
Nothing
Notes:
■ In video modes that support multiple pages, this function affects only
the page currently being displayed.
■ If AL contains a value other than 00H, the area within the specified
window is scrolled up by the requested number of lines. Text that is
scrolled beyond the top of the window is lost. The new lines that appear
at the bottom of the window are filled with ASCII blanks carrying the
attribute specified by register BH.
■ To scroll down the contents of a window, see Int 10H Function 07H.
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 07H
Initialize or scroll window down
────────────────────────────────────────────────────────────────────────────
Initializes a specified window of the display to ASCII blank characters
with a given attribute, or scrolls down the contents of a window by a
specified number of lines.
Call with:
AH = 07H
AL = number of lines to scroll (if zero, entire window is
blanked)
BH = attribute to be used for blanked area
CH = y coordinate, upper left corner of window
CL = x coordinate, upper left corner of window
DH = y coordinate, lower right corner of window
DL = x coordinate, lower right corner of window
Returns:
Nothing
Notes:
■ In video modes that support multiple pages, this function affects only
the page currently being displayed.
■ If AL contains a value other than 00H, the area within the specified
window is scrolled down by the requested number of lines. Text that is
scrolled beyond the bottom of the window is lost. The new lines that
appear at the top of the window are filled with ASCII blanks carrying
the attribute specified by register BH.
■ To scroll up the contents of a window, see Int 10H Function 06H.
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 08H
Read character and attribute at cursor
────────────────────────────────────────────────────────────────────────────
Obtains the ASCII character and its attribute at the current cursor
position for the specified display page.
Call with:
AH = 08H
BH = page
Returns:
AH = attribute
AL = character
Note:
■ In video modes that support multiple pages, characters and their
attributes may be read from any page, regardless of the page currently
being displayed.
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 09H
Write character and attribute at cursor
────────────────────────────────────────────────────────────────────────────
Writes an ASCII character and its attribute to the display at the current
cursor position.
Call with:
AH = 09H
AL = character
BH = page
BL = attribute (text modes) or color (graphics modes)
CX = count of characters to write (replication factor)
Returns:
Nothing
Notes:
■ In graphics modes, the replication factor in CX produces a valid result
only for the current row. If more characters are written than there are
remaining columns in the current row, the result is unpredictable.
■ All values of AL result in some sort of display; control characters,
including bell, backspace, carriage return, and line feed, are not
recognized as special characters and do not affect the cursor position.
■ After a character is written, the cursor must be moved explicitly with
Int 10H Function 02H to the next position.
■ To write a character without changing the attribute at the current
cursor position, use Int 10H Function 0AH.
■ If this function is used to write characters in graphics mode and bit 7
of BL is set (1), the character will be exclusive-OR'd (XOR) with the
current display contents. This feature can be used to write characters
and then "erase" them.
■ For the CGA and PCjr in graphics modes 04H─06H, the bit patterns for
character codes 80H─FFH are obtained from a table whose address is
stored in the vector for Int 1FH. On the PCjr, the address of the table
for character codes 00H─7FH is stored in the vector for Int 44H.
Alternative character sets may be installed by loading them into memory
and updating this vector.
■ For the EGA, MCGA, and VGA in graphics modes, the address of the
character definition table is stored in the vector for Int 43H. See Int
10H Function 11H.
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 0AH (10)
Write character at cursor
────────────────────────────────────────────────────────────────────────────
Writes an ASCII character to the display at the current cursor position.
The character receives the attribute of the previous character displayed
at the same position.
Call with:
AH = 0AH
AL = character
BH = page
BL = color (graphics modes, PCjr only)
CX = count of characters to write (replication factor)
Returns:
Nothing
Notes:
■ In graphics modes, the replication factor in CX produces a valid result
only for the current row. If more characters are written than there are
remaining columns in the current row, the result is unpredictable.
■ All values of AL result in some sort of display; control characters,
including bell, backspace, carriage return, and line feed, are not
recognized as special characters and do not affect the cursor position.
■ After a character is written, the cursor must be moved explicitly with
Int 10H Function 02H to the next position.
■ To write a character and attribute at the current cursor position, use
Int 10H Function 09H.
■ If this function is used to write characters in graphics mode and bit 7
of BL is set (1), the character will be exclusive-OR'd (XOR) with the
current display contents. This feature can be used to write characters
and then "erase" them.
■ For the CGA and PCjr in graphics modes 04H─06H, the bit patterns for
character codes 80H─FFH are obtained from a table whose address is
stored in the vector for Int 1FH. On the PCjr, the address of the table
for character codes 00H─7FH is stored in the vector for Int 44H.
Alternative character sets may be installed by loading them into memory
and updating this vector.
■ For the EGA, MCGA, and VGA in graphics modes, the address of the
character definition table is stored in the vector for Int 43H. See Int
10H Function 11H.
────────────────────────────────────────────────────────────────────────────
Int 10H [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 0BH (11)
Set palette, background, or border
────────────────────────────────────────────────────────────────────────────
Selects a palette, background, or border color.
Call with:
To set the background color and border color for graphics modes or the
border color for text modes
AH = 0BH
BH = 00H
BL = color
To select the palette (320-by-200 4-color graphics modes)
AH = 0BH
BH = 01H
BL = palette (see Notes)
Returns:
Nothing
Notes:
■ In text modes, this function selects only the border color. The
background color of each individual character is controlled by the upper
4 bits of that character's attribute byte.
■ On the CGA and EGA, this function is valid for palette selection only in
320-by-200 4-color graphics modes.
■ In 320-by-200 4-color graphics modes, if register BH = 01H, the
following palettes may be selected:
Palette Pixel value Color
────────────────────────────────────────────────────────────────────────
0 0 same as background
1 green
2 red
3 brown or yellow
1 0 same as background
1 cyan
2 magenta
3 white
────────────────────────────────────────────────────────────────────────
■ On the CGA in 640-by-200 2-color graphics mode, the background color
selected with this function actually controls the display color for
nonzero pixels; zero pixels are always displayed as black.
■ On the PCjr in 640-by-200 2-color graphics mode, if BH = 00H and bit 0
of register BL is cleared, pixel value 1 is displayed as white; if bit 0
is set, pixel value 1 is displayed as black.
■ See also Int 10H Function 10H, which is used for palette programming on
the PCjr, EGA, MCGA, and VGA.
────────────────────────────────────────────────────────────────────────────
Int 10H [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 0CH (12)
Write graphics pixel
────────────────────────────────────────────────────────────────────────────
Draws a point on the display at the specified graphics coordinates.
Call with:
AH = 0CH
AL = pixel value
BH = page
CX = column (graphics x coordinate)
DX = row (graphics y coordinate)
Returns:
Nothing
Notes:
■ The range of valid pixel values and (x,y) coordinates depends on the
current video mode.
■ If bit 7 of AL is set, the new pixel value will be exclusive-OR'd (XOR)
with the current contents of the pixel.
■ Register BH is ignored for display modes that support only one page.
────────────────────────────────────────────────────────────────────────────
Int 10H [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 0DH (13)
Read graphics pixel
────────────────────────────────────────────────────────────────────────────
Obtains the current value of the pixel on the display at the specified
graphics coordinates.
Call with:
AH = 0DH
BH = page
CX = column (graphics x coordinate)
DX = row (graphics y coordinate)
Returns:
AL = pixel value
Notes:
■ The range of valid (x,y) coordinates and possible pixel values depends
on the current video mode.
■ Register BH is ignored for display modes that support only one page.
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 0EH (14)
Write character in teletype mode
────────────────────────────────────────────────────────────────────────────
Writes an ASCII character to the display at the current cursor position,
using the specified color (if in graphics modes), and then increments the
cursor position appropriately.
Call with:
AH = 0EH
AL = character
BH = page
BL = foreground color (graphics modes)
Returns:
Nothing
Notes:
■ The special ASCII codes for bell (07H), backspace (08H), carriage return
(0DH), and line feed (0AH) are recognized, and the appropriate action is
taken. All other characters are written to the display (even if they are
control characters), and the cursor is moved to the next position.
■ In video modes that support multiple pages, characters can be written to
any page, regardless of the page currently being displayed.
■ Line wrapping and scrolling are provided. If the cursor is at the end of
a line, it is moved to the beginning of the next line. If the cursor
reaches the end of the last line on the screen, the screen is scrolled
up by one line and the cursor is placed at the beginning of a new blank
line. The attribute for the entire new line is taken from the last
character that was written on the preceding line.
■ The default MS-DOS console driver (CON) uses this function to write text
to the screen. You cannot use this function to specify the attribute of
a character. One method of writing a character to the screen with a
specific attribute is to first write an ASCII blank (20H) with the
desired attribute at the current cursor location using Int 10H Function
09H and then write the actual character with Int 10H Function 0EH.
This technique, although somewhat clumsy, does not require the program
to explicitly handle line wrapping and scrolling.
■ See also Int 10H Function 13H.
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 0FH (15)
Get video mode
────────────────────────────────────────────────────────────────────────────
Obtains the current display mode of the active video controller.
Call with:
AH = 0FH
Returns:
AH = number of character columns on screen
AL = display mode (see Int 10H Function 00H)
BH = active display page
Note:
■ This function can be called to obtain the screen width before clearing
the screen with Int 10H Functions 06H or 07H.
────────────────────────────────────────────────────────────────────────────
Int 10H [PCjr] [EGA] [MCGA] [VGA]
Function 10H (16) Subfunction 00H
Set palette register
────────────────────────────────────────────────────────────────────────────
Sets the correspondence of a palette register to a displayable color.
Call with:
On the PCjr, EGA, or VGA
AH = 10H
AL = 00H
BH = color value
BL = palette register (00─0FH)
On the MCGA
AH = 10H
AL = 00H
BX = 0712H
Returns:
Nothing
Note:
■ On the MCGA, this function can only be called with BX = 0712H and
selects a color register set with eight consistent colors.
────────────────────────────────────────────────────────────────────────────
Int 10H [PCjr] [EGA] [VGA]
Function 10H (16) Subfunction 01H
Set border color
────────────────────────────────────────────────────────────────────────────
Controls the color of the screen border (overscan).
Call with:
AH = 10H
AL = 01H
BH = color value
Returns:
Nothing
────────────────────────────────────────────────────────────────────────────
Int 10H [PCjr] [EGA] [VGA]
Function 10H (16) Subfunction 02H
Set palette and border
────────────────────────────────────────────────────────────────────────────
Sets all palette registers and the border color (overscan) in one
operation.
Call with:
AH = 10H
AL = 02H
ES:DX = segment:offset of color list
Returns:
Nothing
Notes:
■ The color list is 17 bytes long. The first 16 bytes are the color values
to be loaded into palette registers 0─15, and the last byte is stored in
the border color register.
■ In 16-color graphics modes, the following default palette is set up:
Pixel value Color
────────────────────────────────────────────────────────────────────────
01H blue
02H green
03H cyan
04H red
05H magenta
06H brown
07H white
08H gray
09H light blue
0AH light green
0BH light cyan
0CH light red
0DH light magenta
0EH yellow
0FH intense white
────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 10H [PCjr] [EGA] [MCGA] [VGA]
Function 10H (16) Subfunction 03H
Toggle blink/intensity bit
────────────────────────────────────────────────────────────────────────────
Determines whether the most significant bit of a character attribute will
select blinking or intensified display.
Call with:
AH = 10H
AL = 03H
BL = blink/intensity toggle
0 = enable intensity
1 = enable blinking
Returns:
Nothing
────────────────────────────────────────────────────────────────────────────
Int 10H [VGA]
Function 10H (16) Subfunction 07H
Get palette register
────────────────────────────────────────────────────────────────────────────
Returns the color associated with the specified palette register.
Call with:
AH = 10H
AL = 07H
BL = palette register
Returns:
BH = color
────────────────────────────────────────────────────────────────────────────
Int 10H [VGA]
Function 10H (16) Subfunction 08H
Get border color
────────────────────────────────────────────────────────────────────────────
Returns the current border color (overscan).
Call with:
AH = 10H
AL = 08H
Returns:
BH = color
────────────────────────────────────────────────────────────────────────────
Int 10H [VGA]
Function 10H (16) Subfunction 09H
Get palette and border
────────────────────────────────────────────────────────────────────────────
Gets the contents of all palette registers and the border color (overscan)
in one operation.
Call with:
AH = 10H
AL = 09H
ES:DX = segment:offset of 17-byte buffer
Returns:
ES:DX = segment:offset of buffer
and buffer contains palette values in bytes 00H─0FH and border color in
byte 10H.
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 10H (16) Subfunction 10H (16)
Set color register
────────────────────────────────────────────────────────────────────────────
Programs an individual color register with a red-green-blue (RGB)
combination.
Call with:
AH = 10H
AL = 10H
BX = color register
CH = green value
CL = blue value
DH = red value
Returns:
Nothing
Note:
■ If gray-scale summing is enabled, the weighted gray-scale value is
calculated as described under Int 10H Function 10H Subfunction 1BH and
is stored into all three components of the color register. See also Int
10H Function 12H Subfunction 33H.
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 10H (16) Subfunction 12H (18)
Set block of color registers
────────────────────────────────────────────────────────────────────────────
Programs a group of consecutive color registers in one operation.
Call with:
AH = 10H
AL = 12H
BX = first color register
CX = number of color registers
ES:DX = segment:offset of color table
Returns:
Nothing
Notes:
■ The table consists of a series of 3-byte entries, one entry per color
register to be programmed. The bytes of an individual entry specify the
red, green, and blue values (in that order) for the associated color
register.
■ If gray-scale summing is enabled, the weighted gray-scale value for each
register is calculated as described under Int 10H Function 10H
Subfunction 1BH and is stored into all three components of the color
register. See also Int 10H Function 12H Subfunction 33H.
────────────────────────────────────────────────────────────────────────────
Int 10H [VGA]
Function 10H (16) Subfunction 13H (19)
Set color page state
────────────────────────────────────────────────────────────────────────────
Selects the paging mode for the color registers, or selects an individual
page of color registers.
Call with:
To select the paging mode
AH = 10H
AL = 13H
BH = paging mode
00H for 4 pages of 64 registers
01H for 16 pages of 16 registers
BL = 00H
To select a color register page
AH = 10H
AL = 13H
BH = page
BL = 01H
Returns:
Nothing
Note:
■ This function is not valid in mode 13H (320-by-200 256-color graphics).
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 10H (16) Subfunction 15H (21)
Get color register
────────────────────────────────────────────────────────────────────────────
Returns the contents of a color register as its red, green, and blue
components.
Call with:
AH = 10H
AL = 15H
BX = color register
Returns:
CH = green value
CL = blue value
DH = red value
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 10H (16) Subfunction 17H (23)
Get block of color registers
────────────────────────────────────────────────────────────────────────────
Allows the red, green, and blue components associated with each of a set
of color registers to be read in one operation.
Call with:
AH = 10H
AL = 17H
BX = first color register
CX = number of color registers
ES:DX = segment:offset of buffer to receive color list
Returns:
ES:DX = segment:offset of buffer and buffer contains color list
Note:
■ The color list returned in the caller's buffer consists of a series of
3-byte entries corresponding to the color registers. Each 3-byte entry
contains the register's red, green, and blue components in that order.
────────────────────────────────────────────────────────────────────────────
Int 10H [VGA]
Function 10H (16) Subfunction 1AH (26)
Get color page state
────────────────────────────────────────────────────────────────────────────
Returns the color register paging mode and current color page.
Call with:
AH = 10H
AL = 1AH
Returns:
BH = color page
BL = paging mode
00H if 4 pages of 64 registers
01H if 16 pages of 16 registers
Note:
■ See Int 10H Function 10H Subfunction 13H, which allows selection of the
paging mode or current color page.
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 10H (16) Subfunction 1BH (27)
Set gray-scale values
────────────────────────────────────────────────────────────────────────────
Transforms the red, green, and blue values of one or more color registers
into the gray-scale equivalents.
Call with:
AH = 10H
AL = 1BH
BX = first color register
CX = number of color registers
Returns:
Nothing
Note:
■ For each color register, the weighted sum of its red, green, and blue
values is calculated (30% red + 59% green + 11% blue) and written back
into all three components of the color register. The original red,
green, and blue values are lost.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [MCGA] [VGA]
Function 11H (17) Subfunctions 00H and 10H (16)
Load user font
────────────────────────────────────────────────────────────────────────────
Loads the user's font (character definition) table into the specified
block of character generator RAM.
Call with:
AH = 11H
AL = 00H or 10H (see Notes)
BH = points (bytes per character)
BL = block
CX = number of characters defined by table
DX = first character code in table
ES:BP = segment:offset of font table
Returns:
Nothing
Notes:
■ This function provides font selection in text (alphanumeric) display
modes. For font selection in graphics (all-points-addressable) modes,
see Int 10H Function 11H Subfunctions 20H─24H.
■ If AL = 10H, page 0 must be active. The points (bytes per character),
rows, and length of the refresh buffer are recalculated. The controller
is reprogrammed with the maximum scan line (points - 1), cursor start
(points - 2), cursor end (points - 1), vertical display end
((rows*points) - 1), and underline location (points - 1, mode 7 only).
If Subfunction 10H is called at any time other than immediately after a
mode set, the results are unpredictable.
■ On the MCGA, a Subfunction 00H call should be followed by a Subfunction
03H call so that the ROM BIOS will load the font into the character
generator's internal font pages.
■ Subfunction 10H is reserved on the MCGA. If it is called, Subfunction
00H is executed.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [VGA]
Function 11H (17) Subfunctions 01H and 11H (17)
Load ROM 8-by-14 font
────────────────────────────────────────────────────────────────────────────
Loads the ROM BIOS default 8-by-14 font table into the specified block of
character generator RAM.
Call with:
AH = 11H
AL = 01H or 11H (see Notes)
BL = block
Returns:
Nothing
Notes:
■ This function provides font selection in text (alphanumeric) display
modes. For font selection in graphics (all-points-addressable) modes,
see Int 10H Function 11H Subfunctions 20H─24H.
■ If AL = 11H, page 0 must be active. The points (bytes per character),
rows, and length of the refresh buffer are recalculated. The controller
is reprogrammed with the maximum scan line (points - 1), cursor start
(points - 2), cursor end (points - 1), vertical display end
((rows*points) - 1), and underline location (points - 1, mode 7 only).
If Subfunction 11H is called at any time other than immediately after a
mode set, the results are unpredictable.
■ Subfunctions 01H and 11H are reserved on the MCGA. If either is called,
Subfunction 04H is executed.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [MCGA] [VGA]
Function 11H (17) Subfunctions 02H and 12H (18)
Load ROM 8-by-8 font
────────────────────────────────────────────────────────────────────────────
Loads the ROM BIOS default 8-by-8 font table into the specified block of
character generator RAM.
Call with:
AH = 11H
AL = 02H or 12H (see Notes)
BL = block
Returns:
Nothing
Notes:
■ This function provides font selection in text (alphanumeric) display
modes. For font selection in graphics (all-points-addressable) modes,
see Int 10H Function 11H Subfunctions 20H─24H.
■ If AL = 12H, page 0 must be active. The points (bytes per character),
rows, and length of the refresh buffer are recalculated. The controller
is reprogrammed with the maximum scan line (points - 1), cursor start
(points - 2), cursor end (points - 1), vertical display end
((rows*points) - 1), and underline location (points - 1, mode 7 only).
If Subfunction 12H is called at any time other than immediately after a
mode set, the results are unpredictable.
■ On the MCGA, a Subfunction 02H call should be followed by a Subfunction
03H call, so that the ROM BIOS will load the font into the character
generator's internal font pages.
■ Subfunction 12H is reserved on the MCGA. If it is called, Subfunction
02H is executed.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [MCGA] [VGA]
Function 11H (17) Subfunction 03H
Set block specifier
────────────────────────────────────────────────────────────────────────────
Determines the character blocks selected by bit 3 of character attribute
bytes in alphanumeric (text) display modes.
Call with:
AH = 11H
AL = 03H
BL = character generator block select code (see Notes)
Returns:
Nothing
Notes:
■ On the EGA and MCGA, the bits of BL are used as follows:
Bits Significance
────────────────────────────────────────────────────────────────────────
0─1 character block selected by attribute bytes with bit 3 = 0
2─3 character block selected by attribute bytes with bit 3 = 1
4─7 not used (should be 0)
────────────────────────────────────────────────────────────────────────
■ On the VGA, the bits of BL are used as follows:
Bits Significance
────────────────────────────────────────────────────────────────────────
0,1,4 character block selected by attribute bytes with bit 3 = 0
2,3,5 character block selected by attribute bytes with bit 3 = 1
6─7 not used (should be 0)
────────────────────────────────────────────────────────────────────────
■ When using a 256-character set, both fields of BL should select the same
character block. In such cases, character attribute bit 3 controls the
foreground intensity. When using 512-character sets, the fields of BL
designate the blocks holding each half of the character set, and bit 3
of the character attribute selects the upper or lower half of the
character set.
■ When using a 512-character set, a call to Int 10H Function 10H
Subfunction 00H with BX = 0712H is recommended to set the color planes
to eight consistent colors.
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 11H (17) Subfunctions 04H and 14H (20)
Load ROM 8-by-16 font
────────────────────────────────────────────────────────────────────────────
Loads the ROM BIOS default 8-by-16 font table into the specified block of
character generator RAM.
Call with:
AH = 11H
AL = 04H or 14H (see Notes)
BL = block
Returns:
Nothing
Notes:
■ This function provides font selection in text (alphanumeric) display
modes. For font selection in graphics (all-points-addressable) modes,
see Int 10H Function 11H Subfunctions 20H─24H.
■ If AL = 14H, page 0 must be active. The points (bytes per character),
rows, and length of the refresh buffer are recalculated. The controller
is reprogrammed with the maximum scan line (points - 1), cursor start
(points - 2), cursor end (points - 1), vertical display end (rows*points
- 1 for 350- and 400-line modes, or rows *points *2 - 1 for 200-line
modes), and underline location (points - 1, mode 7 only).
If Subfunction 14H is called at any time other than immediately after a
mode set, the results are unpredictable.
■ On the MCGA, a Subfunction 04H call should be followed by a Subfunction
03H call so that the ROM BIOS will load the font into the character
generator's internal font pages.
■ Subfunction 14H is reserved on the MCGA. If it is called, Subfunction
04H is executed.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [MCGA] [VGA]
Function 11H (17) Subfunction 20H (32)
Set Int 1FH font pointer
────────────────────────────────────────────────────────────────────────────
Sets the Int 1FH pointer to the user's font table. This table is used for
character codes 80H─FFH in graphics modes 04H─06H.
Call with:
AH = 11H
AL = 20H
ES:BP = segment:offset of font table
Returns:
Nothing
Notes:
■ This function provides font selection in graphics
(all-points-addressable) display modes. For font selection in text
(alphanumeric) modes, see Int 10H Function 11H Subfunctions 00H─14H.
■ If this subfunction is called at any time other than immediately after a
mode set, the results are unpredictable.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [MCGA] [VGA]
Function 11H (17) Subfunction 21H (33)
Set Int 43H for user's font
────────────────────────────────────────────────────────────────────────────
Sets the vector for Int 43H to point to the user's font table and updates
the video ROM BIOS data area. The video controller is not reprogrammed.
Call with:
AH = 11H
AL = 21H
BL = character rows specifier
00H if user specified (see register DL)
01H = 14 (0EH) rows
02H = 25 (19H) rows
03H = 43 (2BH) rows
CX = points (bytes per character)
DL = character rows per screen (if BL = 00H)
ES:BP = segment:offset of user font table
Returns:
Nothing
Notes:
■ This function provides font selection in graphics
(all-points-addressable) display modes. For font selection in text
(alphanumeric) modes, see Int 10H Function 11H Subfunctions 00H─14H.
■ If this subfunction is called at any time other than immediately after a
mode set, the results are unpredictable.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [MCGA] [VGA]
Function 11H (17) Subfunction 22H (34)
Set Int 43H for ROM 8-by-14 font
────────────────────────────────────────────────────────────────────────────
Sets the vector for Int 43H to point to the ROM BIOS default 8-by-14 font
and updates the video ROM BIOS data area. The video controller is not
reprogrammed.
Call with:
AH = 11H
AL = 22H
BL = character rows specifier
00H if user specified (see register DL)
01H = 14 (0EH) rows
02H = 25 (19H) rows
03H = 43 (2BH) rows
DL = character rows per screen (if BL = 00H)
Returns:
Nothing
Notes:
■ This function provides font selection in graphics
(all-points-addressable) display modes. For font selection in text
(alphanumeric) modes, see Int 10H Function 11H Subfunctions 00H─14H.
■ If this subfunction is called at any time other than immediately after a
mode set, the results are unpredictable.
■ When this subfunction is called on the MCGA, Subfunction 24H is
substituted.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [MCGA] [VGA]
Function 11H (17) Subfunction 23H (35)
Set Int 43H for ROM 8-by-8 font
────────────────────────────────────────────────────────────────────────────
Sets the vector for Int 43H to point to the ROM BIOS default 8-by-8 font
and updates the video ROM BIOS data area. The video controller is not
reprogrammed.
Call with:
AH = 11H
AL = 23H
BL = character rows specifier
00H if user specified (see register DL)
01H = 14 (0EH) rows
02H = 25 (19H) rows
03H = 43 (2BH) rows
DL = character rows per screen (if BL = 00H)
Returns:
Nothing
Notes:
■ This function provides font selection in graphics
(all-points-addressable) display modes. For font selection in text
(alphanumeric) modes, see Int 10H Function 11H Subfunctions 00H─14H.
■ If this subfunction is called at any time other than immediately after a
mode set, the results are unpredictable.
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 11H (17) Subfunction 24H (36)
Set Int 43H for ROM 8-by-16 font
────────────────────────────────────────────────────────────────────────────
Sets the vector for Int 43H to point to the ROM BIOS default 8-by-16 font
and updates the video ROM BIOS data area. The video controller is not
reprogrammed.
Call with:
AH = 11H
AL = 24H
BL = row specifier
00H if user specified (see register DL)
01H = 14 (0EH) rows
02H = 25 (19H) rows
03H = 43 (2BH) rows
DL = character rows per screen (if BL = 00H)
Returns:
Nothing
Notes:
■ This function provides font selection in graphics
(all-points-addressable) display modes. For font selection in text
(alphanumeric) modes, see Int 10H Function 11H Subfunctions 00H─14H.
■ If this subfunction is called at any time other than immediately after a
mode set, the results are unpredictable.
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [MCGA] [VGA]
Function 11H (17) Subfunction 30H (48)
Get font information
────────────────────────────────────────────────────────────────────────────
Returns a pointer to the character definition table for a font and the
points (bytes per character) and rows for that font.
Call with:
AH = 11H
AL = 30H
BH = font code
00H = current Int 1FH contents
01H = current Int 43H contents
02H = ROM 8-by-14 font (EGA, VGA only)
03H = ROM 8-by-8 font (characters 00H─7FH)
04H = ROM 8-by-8 font (characters 80H─FFH)
05H = ROM alternate 9-by-14 font (EGA, VGA only)
06H = ROM 8-by-16 font (MCGA, VGA only)
07H = ROM alternate 9-by-16 font (VGA only)
Returns:
CX = points (bytes per character)
DL = rows (character rows on screen - 1)
ES:BP = segment:offset of font table
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [VGA]
Function 12H (18) Subfunction 10H (16)
Get configuration information
────────────────────────────────────────────────────────────────────────────
Obtains configuration information for the active video subsystem.
Call with:
AH = 12H
BL = 10H
Returns:
BH = display type
0 if color display
1 if monochrome display
BL = memory installed on EGA board
00H if 64 KB
01H if 128 KB
02H if 192 KB
03H if 256 KB
CH = feature bits (see Notes)
CL = switch setting (see Notes)
Notes:
■ The feature bits are set from Input Status register 0 in response to an
output on the specified Feature Control register bits:
Feature Feature control Input status
bit(s) output bit bit
────────────────────────────────────────────────────────────────────────
0 0 5
1 0 6
2 1 5
3 1 6
4─7 not used
────────────────────────────────────────────────────────────────────────
■ The bits in the switch settings byte indicate the state of the EGA's
configuration DIP switch (1 = off, 0 = on).
Bit(s) Significance
────────────────────────────────────────────────────────────────────────
0 configuration switch 1
1 configuration switch 2
2 configuration switch 3
3 configuration switch 4
4─7 not used
────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 10H [EGA] [VGA]
Function 12H (18) Subfunction 20H (32)
Select alternate printscreen
────────────────────────────────────────────────────────────────────────────
Selects an alternate print-screen routine for the EGA and VGA that works
properly if the screen length is not 25 lines. The ROM BIOS default
print-screen routine always prints 25 lines.
Call with:
AH = 12H
BL = 20H
Returns:
Nothing
────────────────────────────────────────────────────────────────────────────
Int 10H [VGA]
Function 12H (18) Subfunction 30H (48)
Set scan lines
────────────────────────────────────────────────────────────────────────────
Selects the number of scan lines for alphanumeric modes. The selected
value takes effect the next time Int 10H Function 00H is called to select
the display mode.
Call with:
AH = 12H
AL = scan line code
00H = 200 scan lines
01H = 350 scan lines
02H = 400 scan lines
BL = 30H
Returns:
If the VGA is active
AL = 12H
If the VGA is not active
AL = 00H
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 12H (18) Subfunction 31H (49)
Enable/disable default palette loading
────────────────────────────────────────────────────────────────────────────
Enables or disables loading of a default palette when a video display mode
is selected.
Call with:
AH = 12H
AL = 00H to enable default palette loading
01H to disable default palette loading
BL = 31H
Returns:
If function supported
AL = 12H
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 12H (18) Subfunction 32H (50)
Enable/disable video
────────────────────────────────────────────────────────────────────────────
Enables or disables CPU access to the video adapter's I/O ports and video
refresh buffer.
Call with:
AH = 12H
AL = 00H to enable access
01H to disable access
BL = 32H
Returns:
If function supported
AL = 12H
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 12H (18) Subfunction 33H (51)
Enable/disable gray-scale summing
────────────────────────────────────────────────────────────────────────────
Enables or disables gray-scale summing for the currently active display.
Call with:
AH = 12H
AL = 00H to enable gray-scale summing
01H to disable gray-scale summing
BL = 33H
Returns:
If function supported
AL = 12H
Note:
■ When enabled, gray-scale summing occurs during display mode selection,
palette programming, and color register loading.
────────────────────────────────────────────────────────────────────────────
Int 10H [VGA]
Function 12H (18) Subfunction 34H (52)
Enable/disable cursor emulation
────────────────────────────────────────────────────────────────────────────
Enables or disables cursor emulation for the currently active display.
When cursor emulation is enabled, the ROM BIOS automatically remaps Int
10H Function 01H cursor starting and ending lines for the current
character cell dimensions.
Call with:
AH = 12H
AL = 00H to enable cursor emulation
01H to disable cursor emulation
BL = 34H
Returns:
If function supported
AL = 12H
────────────────────────────────────────────────────────────────────────────
Int 10H [MCGA] [VGA]
Function 12H (18) Subfunction 35H (53)
Switch active display
────────────────────────────────────────────────────────────────────────────
Allows selection of one of two video adapters in the system when memory
usage or port addresses conflict between the two adapters.
Call with:
AH = 12H
AL = switching function
00H to disable initial video adapter
01H to enable system board video adapter
02H to disable active video adapter
03H to enable inactive video adapter
BL = 35H
ES:DX = segment:offset of 128-byte buffer (if AL = 00H, 02H, or
03H)
Returns:
If function supported
AL = 12H
and, if called with AL = 00H or 02H
Video adapter state information saved in caller's buffer
or, if called with AL = 03H
Video adapter state restored from information in caller's buffer
Notes:
■ This subfunction cannot be used unless both video adapters have a
disable capability (Int 10H Function 12H Subfunction 32H).
■ If there is no conflict between the system board video and the adapter
board video in memory or port usage, both video controllers can be
active simultaneously and this subfunction is not required.
────────────────────────────────────────────────────────────────────────────
Int 10H [VGA]
Function 12H (18) Subfunction 36H (54)
Enable/disable screen refresh
────────────────────────────────────────────────────────────────────────────
Enables or disables the video refresh for the currently active display.
Call with:
AH = 12H
AL = 00H to enable refresh
01H to disable refresh
BL = 36H
Returns:
If function supported
AL = 12H
────────────────────────────────────────────────────────────────────────────
Int 10H [MDA] [CGA] [PCjr] [EGA] [MCGA] [VGA]
Function 13H (19)
Write string in teletype mode
────────────────────────────────────────────────────────────────────────────
Transfers a string to the video buffer for the currently active display,
starting at the specified position.
Call with:
AH = 13H
AL = write mode
0 attribute in BL; string contains character codes
only; and cursor position is not updated after
write
1 attribute in BL;
string contains character codes only; and cursor
position is updated after write
2 string contains alternating character codes and
attribute bytes; and cursor position is not
updated after write
3 string contains alternating character codes and
attribute bytes; and cursor position is updated
after write
BH = page
BL = attribute, if AL = 00H or 01H
CX = length of character string
DH = y coordinate (row)
DL = x coordinate (column)
ES:BP = segment:offset of string
Returns:
Nothing
Notes:
■ This function is not available on the original IBM PC or PC/XT unless an
EGA video adapter (which contains its own ROM BIOS) is installed.
■ This function may be thought of as an extension to Int 10H Function
0EH. The control characters bell (07H), backspace (08H), line feed
(0AH), and carriage return (0DH) are recognized and handled
appropriately.
────────────────────────────────────────────────────────────────────────────
Int 10H [PS/2]
Function 1AH (26)
Get or set display combination code
────────────────────────────────────────────────────────────────────────────
Returns a code describing the installed display adapter(s) or updates the
ROM BIOS's variable describing the installed adapter(s).
Call with:
AH = 1AH
AL = subfunction
00H = get display combination code
01H = set display combination code
BH = inactive display code (if AL = 01H)
BL = active display code (if AL = 01H)
Returns:
If function supported
AL = 1AH
and, if called with AL = 00H
BH = inactive display code
BL = active display code
Note:
■ The display codes are interpreted as follows:
Code(s) Video subsystem type
────────────────────────────────────────────────────────────────────────
00H no display
01H MDA with 5151 monitor
02H CGA with 5153 or 5154 monitor
03H reserved
04H EGA with 5153 or 5154 monitor
05H EGA with 5151 monitor
06H PGA with 5175 monitor
07H VGA with analog monochrome monitor
08H VGA with analog color monitor
09H reserved
0AH MCGA with digital color monitor
0BH MCGA with analog monochrome monitor
0CH MCGA with analog color monitor
0DH─FEH reserved
FFH unknown
────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 10H [PS/2]
Function 1BH (27)
Get functionality/state information
────────────────────────────────────────────────────────────────────────────
Obtains information about the current display mode as well as a pointer to
a table describing the characteristics and capabilities of the video
adapter and monitor.
Call with:
AH = 1BH
BX = implementation type (always 00H)
ES:DI = segment:offset of 64-byte buffer
Returns:
If function supported
AL = 1BH
and information placed in caller's buffer (see Notes)
Notes:
■ The caller's buffer is filled in with information that depends on the
current video display mode:
╓┌───┌─────────┌─────────────────────────────────────────────────────────────╖
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
00H─03H pointer to functionality information (see next Note)
04H current video mode
05H─06H number of character columns
07H─08H length of video refresh buffer (bytes)
09H─0AH starting address in buffer of upper left corner of display
0BH─1AH cursor position for video pages 0─7 as eight 2-byte entries;
first byte of each pair is y coordinate, second byte is x
coordinate
1BH cursor starting line
1CH cursor ending line
1DH active display page
1EH─1FH adapter base port address (3BXH monochrome, 3DXH color)
20H current setting of register 3B8H or 3D8H
21H current setting of register 3B9H or 3D9H
22H number of character rows
23H─24H character height in scan lines
25H active display code (see Int 10H Function 1AH)
26H inactive display code (see Int 10H Function 1AH)
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
26H inactive display code (see Int 10H Function 1AH)
27H─28H number of displayable colors (0 for monochrome)
29H number of display pages
2AH number of scan lines
00H = 200 scan lines
01H = 350 scan lines
02H = 400 scan lines
03H = 480 scan lines
04H─FFH = reserved
2BH primary character block (see Int 10H Function 11H Subfunction
03H)
2CH secondary character block
2DH miscellaneous state information
Bit(s) Significance
0 = 1 if all modes on all displays active
(always 0 on MCGA)
1 = 1 if gray-scale summing active
2 = 1 if monochrome display attached
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
2 = 1 if monochrome display attached
3 = 1 if mode set default palette loading
disabled
4 = 1 if cursor emulation active (always 0 on
MCGA)
5 = state of I/B toggle (0 = intensity, 1 =
blink)
6─7 = reserved
2EH─30H reserved
31H video memory available
00H = 64 KB
01H = 128 KB
02H = 192 KB
03H = 256 KB
32H save pointer state information
Bit(s) Significance
0 = 1 if 512-character set active
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
0 = 1 if 512-character set active
1 = 1 if dynamic save area active
2 = 1 if alpha font override active
3 = 1 if graphics font override active
4 = 1 if palette override active
5 = 1 if display combination code (DCC)
extension active
6─7 = reserved
33H─3FH reserved
────────────────────────────────────────────────────────────────────────
■ Bytes 0─3 of the caller's buffer contain a DWORD pointer (offset in
lower word, segment in upper word) to the following information about
the display adapter and monitor:
╓┌───┌─────────┌─────────────────────────────────────────────────────────────╖
Byte(s) Contents
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
00H video modes supported
Bit Significance
0 = 1 if mode 00H supported
1 = 1 if mode 01H supported
2 = 1 if mode 02H supported
3 = 1 if mode 03H supported
4 = 1 if mode 04H supported
5 = 1 if mode 05H supported
6 = 1 if mode 06H supported
7 = 1 if mode 07H supported
01H video modes supported
Bit Significance
0 = 1 if mode 08H supported
1 = 1 if mode 09H supported
2 = 1 if mode 0AH supported
3 = 1 if mode 0BH supported
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
3 = 1 if mode 0BH supported
4 = 1 if mode 0CH supported
5 = 1 if mode 0DH supported
6 = 1 if mode 0EH supported
7 = 1 if mode 0FH supported
02H video modes supported
Bit(s) Significance
0 = 1 if mode 10H supported
1 = 1 if mode 11H supported
2 = 1 if mode 12H supported
3 = 1 if mode 13H supported
4─7 = reserved
03H─06H reserved
07H scan lines available in text modes
Bit(s) Significance
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
Bit(s) Significance
0 = 1 if 200 scan lines
1 = 1 if 350 scan lines
2 = 1 if 400 scan lines
3─7 = reserved
08H character blocks available in text modes (see Int 10H Function
11H)
09H maximum number of active character blocks in text modes
0AH miscellaneous BIOS capabilities
Bit Significance
0 = 1 if all modes active on all displays
(always 0 for MCGA)
1 = 1 if gray-scale summing available
2 = 1 if character font loading available
3 = 1 if mode set default palette loading
available
4 = 1 if cursor emulation available
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
4 = 1 if cursor emulation available
5 = 1 if EGA (64-color) palette available
6 = 1 if color register loading available
7 = 1 if color register paging mode select
available
0BH miscellaneous BIOS capabilities
Bit(s) Significance
0 = 1 if light pen available
1 = 1 if save/restore video state available
(always 0 on MCGA)
2 = 1 if background intensity/blinking
control available
3 = 1 if get/set display combination code
available
4─7 = reserved
0CH─0DH reserved
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
0CH─0DH reserved
0EH save area capabilities
Bit(s) Significance
0 = 1 if supports 512-character sets
1 = 1 if dynamic save area available
2 = 1 if alpha font override available
3 = 1 if graphics font override available
4 = 1 if palette override available
5 = 1 if display combination code extension
available
6─7 = reserved
0FH reserved
────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 10H [PS/2]
Function 1CH (28)
Save or restore video state
────────────────────────────────────────────────────────────────────────────
Saves or restores the digital-to-analog converter (DAC) state and color
registers, ROM BIOS video driver data area, or video hardware state.
Call with:
AH = 1CH
AL = subfunction
00H to get state buffer size
01H to save state
02H to restore state
CX = requested states
Bit(s) Significance (if set)
0 save/restore video hardware state
1 save/restore video BIOS data area
2 save/restore video DAC state and color registers
3─15 reserved
ES:BX = segment:offset of buffer
Returns:
If function supported
AL = 1CH
and, if called with AL = 00H
BX = buffer block count (64 bytes per block)
or, if called with AL = 01H
State information placed in caller's buffer
or, if called with AL = 02H
Requested state restored according to contents of caller's buffer
Notes:
■ Subfunction 00H is used to determine the size of buffer that will be
necessary to contain the specified state information. The caller must
supply the buffer.
■ The current video state is altered during a save state operation (AL =
01H). If the requesting program needs to continue in the same video
state, it can follow the save state request with an immediate call to
restore the video state.
■ This function is supported on the VGA only.
────────────────────────────────────────────────────────────────────────────
Int 11H [PC] [AT] [PS/2]
Get equipment configuration
────────────────────────────────────────────────────────────────────────────
Obtains the equipment list code word from the ROM BIOS.
Call with:
Nothing
Returns:
AX = equipment list code word
Bit(s) Significance
0 = 1 if floppy disk drive(s) installed
1 = 1 if math coprocessor installed
2 = 1 if pointing device installed (PS/2)
2─3 system board ram size (PC, see Note)
00 = 16 KB
01 = 32 KB
10 = 48 KB
11 = 64 KB
4─5 initial video mode
00 reserved
01 40-by-25 color text
10 80-by-25 color text
11 80-by-25 monochrome
6─7 number of floppy disk drives (if bit 0 =
1)
00 = 1
01 = 2
10 = 3
11 = 4
8 reserved
9─11 number of RS-232 ports installed
12 = 1 if game adapter installed
13 = 1 if internal modem installed (PC and XT
only)
= 1 if serial printer attached (PCjr)
14─15 number of printers installed
Note:
■ Bits 2─3 of the returned value are used only in the ROM BIOS for the
original IBM PC with the 64 KB system board and on the PCjr.
────────────────────────────────────────────────────────────────────────────
Int 12H [PC] [AT] [PS/2]
Get conventional memory size
────────────────────────────────────────────────────────────────────────────
Returns the amount of conventional memory available for use by MS-DOS and
application programs.
Call with:
Nothing
Returns:
AX = memory size (in KB)
Notes:
■ On some early PC models, the amount of memory returned by this function
is controlled by the settings of the dip switches on the system board
and may not reflect all the memory that is physically present.
■ On the PC/AT, the value returned is the amount of functional memory
found during the power-on self-test, regardless of the memory size
configuration information stored in CMOS RAM.
■ The value returned does not reflect any extended memory (above the 1 MB
boundary) that may be installed on 80286 or 80386 machines such as the
PC/AT or PS/2 (Models 50 and above).
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 00H
Reset disk system
────────────────────────────────────────────────────────────────────────────
Resets the disk controller, recalibrates its attached drives (the
read/write arm is moved to cylinder 0), and prepares for disk I/O.
Call with:
AH = 00H
DL = drive
00H─7FH floppy disk
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function should be called after a failed floppy disk Read, Write,
Verify, or Format request before retrying the operation.
■ If called with DL >= 80H (i.e., selecting a fixed disk drive), the
floppy disk controller and then the fixed disk controller are reset. See
also Int 13H Function 0DH, which allows the fixed disk controller to be
reset without affecting the floppy disk controller.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 01H
Get disk system status
────────────────────────────────────────────────────────────────────────────
Returns the status of the most recent disk operation.
Call with:
AH = 01H
DL = drive
00H─7FH floppy disk
80H─FFH fixed disk
Returns:
AH = 00H
AL = status of previous disk operation
00H no error
01H invalid command
02H address mark not found
03H disk write-protected (F)
04H sector not found
05H reset failed (H)
06H floppy disk removed (F)
07H bad parameter table (H)
08H DMA overrun (F)
09H DMA crossed 64 KB boundary
0AH bad sector flag (H)
0BH bad track flag (H)
0CH media type not found (F)
0DH invalid number of sectors on format (H)
0EH control data address mark detected (H)
0FH DMA arbitration level out of range (H)
10H uncorrectable CRC☼ or ECC☼ data error
11H ECC corrected data error (H)
20H controller failed
40H seek failed
80H disk timed-out (failed to respond)
AAH drive not ready (H)
BBH undefined error (H)
CCH write fault (H)
E0H status register error (H)
FFH sense operation failed (H)
H = fixed disk only, F = floppy disk only
Note:
■ On fixed disks, error code 11H (ECC data error) indicates that a
recoverable error was detected during a preceding Read Sector (Int 13H
Function 02H) function.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 02H
Read sector
────────────────────────────────────────────────────────────────────────────
Reads one or more sectors from disk into memory.
Call with:
AH = 02H
AL = number of sectors
CH = cylinder
CL = sector
DH = head
DL = drive
00H─7FH floppy disk
80H─FFH fixed disk
ES:BX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AH = 00H
AL = number of sectors transferred
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ On fixed disks, the upper 2 bits of the 10-bit cylinder number are
placed in the upper 2 bits of register CL.
■ On fixed disks, error code 11H indicates that a read error occurred that
was corrected by the ECC algorithm; in this event, register AL contains
the burst length. The data returned is probably good, although there is
a small chance that the data was not corrected properly. If a
multi-sector transfer was requested, the operation was terminated after
the sector containing the read error.
■ On floppy disk drives, an error may result from the drive motor being
off at the time of the request. The ROM BIOS does not automatically wait
for the drive to come up to speed before attempting the read operation.
The requesting program should reset the floppy disk system (Int 13H
Function 00H) and retry the operation three times before assuming that
the error results from some other cause.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 03H
Write sector
────────────────────────────────────────────────────────────────────────────
Writes one or more sectors from memory to disk.
Call with:
AH = 03H
AL = number of sectors
CH = cylinder
CL = sector
DH = head
DL = drive
00H─7FH floppy disk
80H─FFH fixed disk
ES:BX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AH = 00H
AL = number of sectors transferred
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ On fixed disks, the upper 2 bits of the 10-bit cylinder number are
placed in the upper 2 bits of register CL.
■ On floppy disk drives, an error may result from the drive motor being
off at the time of the request. The ROM BIOS does not automatically wait
for the drive to come up to speed before attempting the write operation.
The requesting program should reset the floppy disk system (Int 13H
Function 00H) and retry the operation three times before assuming that
the error results from some other cause.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 04H
Verify sector
────────────────────────────────────────────────────────────────────────────
Verifies the address fields of one or more sectors. No data is transferred
to or from memory by this operation.
Call with:
AH = 04H
AL = number of sectors
CH = cylinder
CL = sector
DH = head
DL = drive
00H─7FH floppy disk
80H─FFH fixed disk
ES:BX = segment:offset of buffer (see Notes)
Returns:
If function successful
Carry flag = clear
AH = 00H
AL = number of sectors verified
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ On PCs, PC/XTs, and PC/ATs with ROM BIOS dated earlier than 11/15/85,
ES:BX should point to a valid buffer.
■ On fixed disks, the upper 2 bits of the 10-bit cylinder number are
placed in the upper 2 bits of register CL.
■ This function can be used to test whether a readable media is in a
floppy disk drive. An error may result from the drive motor being off at
the time of the request, because the ROM BIOS does not automatically
wait for the drive to come up to speed before attempting the verify
operation. The requesting program should reset the floppy disk system
(Int 13H Function 00H) and retry the operation three times before
assuming that a readable floppy disk is not present.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 05H
Format track
────────────────────────────────────────────────────────────────────────────
Initializes disk sector and track address fields on the specified track.
Call with:
AH = 05H
AL = interleave (PC/XT fixed disks)
CH = cylinder
DH = head
DL = drive
00H─7FH floppy disk
80H─FFH fixed disk
ES:BX = segment:offset of address field list (except PC/XT fixed
disk, see Note)
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ On floppy disks, the address field list consists of a series of 4-byte
entries, one entry per sector, in the following format:
Byte(s) Contents
0 cylinder
1 head
2 sector
3 sector-size code
00H if 128 bytes per sector
01H if 256 bytes per sector
02H if 512 bytes per sector (standard)
03H if 1024 bytes per sector
■ On floppy disks, the number of sectors per track is taken from the BIOS
floppy disk parameter table whose address is stored in the vector for
Int 1EH.
■ When this function is used for floppy disks on the PC/AT or PS/2, it
should be preceded by a call to Int 13H Function 17H to select the type
of medium to be formatted.
■ On fixed disks, the upper 2 bits of the 10-bit cylinder number are
placed in the upper 2 bits of register CL.
■ On PC/XT-286, PC/AT, and PS/2 fixed disks, ES:BX points to a 512-byte
buffer containing byte pairs for each physical disk sector, as follows:
Byte(s) Contents
0 00H for good sector
80H for bad sector
1 sector number
For example, to format a track with 17 sectors and an interleave of two,
ES:BX would point to the following 34-byte array at the beginning of a
512-byte buffer:
db 00h,01h,00h,0ah,00h,02h,00h,0bh,00h,03h,00h,0ch
db 00h,04h,00h,0dh,00h,05h,00h,0eh,00h,06h,00h,0fh
db 00h,07h,00h,10h,00h,08h,00h,11h,00h,09h
────────────────────────────────────────────────────────────────────────────
Int 13H [PC]
Function 06H
Format bad track
────────────────────────────────────────────────────────────────────────────
Initializes a track, writing disk address fields and data sectors and
setting bad sector flags.
Call with:
AH = 06H
AL = interleave
CH = cylinder
DH = head
DL = drive
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function is defined for PC/XT fixed disk drives only.
■ For additional information, see Notes for Int 13H Function 05H.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC]
Function 07H
Format drive
────────────────────────────────────────────────────────────────────────────
Formats the entire drive, writing disk address fields and data sectors,
starting at the specified cylinder.
Call with:
AH = 07H
AL = interleave
CH = cylinder
DL = drive
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function is defined for PC/XT fixed disk drives only.
■ For additional information, see Notes for Int 13H Function 05H.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 08H
Get drive parameters
────────────────────────────────────────────────────────────────────────────
Returns various parameters for the specified drive.
Call with:
AH = 08H
DL = drive
00H─7FH floppy disk
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
BL = drive type (PC/AT and PS/2 floppy disks)
01H if 360 KB, 40 track, 5.25"
02H if 1.2 MB, 80 track, 5.25"
03H if 720 KB, 80 track, 3.5"
04H if 1.44 MB, 80 track, 3.5"
CH = low 8 bits of maximum cylinder number
CL = bits 6─7 high-order 2 bits of maximum cylinder number
bits 0─5 maximum sector number
DH = maximum head number
DL = number of drives
ES:DI = segment:offset of disk drive parameter table
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ On the PC and PC/XT, this function is supported on fixed disks only.
■ The value returned in register DL reflects the true number of physical
drives attached to the adapter for the requested drive.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 09H
Initialize fixed disk characteristics
────────────────────────────────────────────────────────────────────────────
Initializes the fixed disk controller for subsequent I/O operations, using
the values found in the ROM BIOS disk parameter block(s).
Call with:
AH = 09H
DL = drive
80H─FFH fixed disk
and, on the PC/XT
Vector for Int 41H must point to disk parameter block
or, on the PC/AT and PS/2
Vector for Int 41H must point to disk parameter block for drive 0
Vector for Int 46H must point to disk parameter block for drive 1
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function is supported on fixed disks only.
■ For PC and PC/XT fixed disks, the parameter block format is as follows:
Byte(s) Contents
00H─01H maximum number of cylinders
02H maximum number of heads
03H─04H starting reduced write current cylinder
05H─06H starting write precompensation cylinder
07H maximum ECC burst length
08H drive options
Bit(s) Significance (if set)
0─2 drive option
3─5 reserved (0)
6 disable ECC retries
7 disable disk-access retries
09H standard time-out value
0AH time-out value for format drive
0BH time-out value for check drive
0CH─0FH reserved
■ For PC/AT and PS/2 fixed disks, the parameter block format is as
follows:
Byte(s) Contents
00H─01H maximum number of cylinders
02H maximum number of heads
03H─04H reserved
05H─06H starting write precompensation cylinder
07H maximum ECC burst length
08H drive options
Bit(s) Significance (if set)
0─2 not used
3 more than 8 heads
4 not used
5 manufacturer's defect map present at
maximum
cylinder + 1
6─7 nonzero (10, 01, or 11) if retries
disabled
09H─0BH reserved
0CH─0DH landing zone cylinder
0EH sectors per track
0FH reserved
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 0AH (10)
Read sector long
────────────────────────────────────────────────────────────────────────────
Reads a sector or sectors from disk into memory, along with a 4-byte ECC
code for each sector.
Call with:
AH = 0AH
AL = number of sectors
CH = cylinder
CL = sector (see Notes)
DH = head
DL = drive
80H─FFH fixed disk
ES:BX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AH = 00H
AL = number of sectors transferred
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function is supported on fixed disks only.
■ The upper 2 bits of the 10-bit cylinder number are placed in the upper 2
bits of register CL.
■ Unlike the normal Read Sector function (Int 13H Function 02H), ECC
errors are not automatically corrected. Multisector transfers are
terminated after any sector with a read error.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 0BH (11)
Write sector long
────────────────────────────────────────────────────────────────────────────
Writes a sector or sectors from memory to disk. Each sector's worth of
data must be followed by its 4-byte ECC code.
Call with:
AH = 0BH
AL = number of sectors
CH = cylinder
CL = sector (see Notes)
DH = head
DL = drive
80H─FFH fixed disk
ES:BX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
AH = 00H
AL = number of sectors transferred
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function is supported on fixed disks only.
■ The upper 2 bits of the 10-bit cylinder number are placed in the upper 2
bits of register CL.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 0CH (12)
Seek
────────────────────────────────────────────────────────────────────────────
Positions the disk read/write heads to the specified cylinder, but does
not transfer any data.
Call with:
AH = 0CH
CH = lower 8 bits of cylinder
CL = upper 2 bits of cylinder in bits 6─7
DH = head
DL = drive
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function is supported on fixed disks only.
■ The upper 2 bits of the 10-bit cylinder number are placed in the upper 2
bits of register CL.
■ The Read Sector, Read Sector Long, Write Sector, and Write Sector Long
functions include an implied seek operation and need not be preceded by
an explicit call to this function.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 0DH (13)
Reset fixed disk system
────────────────────────────────────────────────────────────────────────────
Resets the fixed disk controller, recalibrates attached drives (moves the
read/write arm to cylinder 0), and prepares for subsequent disk I/O.
Call with:
AH = 0DH
DL = drive
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is supported on fixed disks only. It differs from Int 13H
Function 00H in that the floppy disk controller is not reset.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC]
Function 0EH (14)
Read sector buffer
────────────────────────────────────────────────────────────────────────────
Transfers the contents of the fixed disk adapter's internal sector buffer
to system memory. No data is read from the physical disk drive.
Call with:
AH = 0EH
ES:BX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is supported by the PC/XT's fixed disk adapter only. It is
not defined for fixed disk adapters on the PC/AT or PS/2.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC]
Function 0FH (15)
Write sector buffer
────────────────────────────────────────────────────────────────────────────
Transfers data from system memory to the fixed disk adapter's internal
sector buffer. No data is written to the physical disk drive.
Call with:
AH = 0FH
ES:BX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function is supported by the PC/XT's fixed disk adapter only. It is
not defined for fixed disk adapters on the PC/AT or PS/2.
■ This function should be called to initialize the contents of the sector
buffer before formatting the drive with Int 13H Function 05H.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 10H (16)
Get drive status
────────────────────────────────────────────────────────────────────────────
Tests whether the specified fixed disk drive is operational and returns
the drive's status.
Call with:
AH = 10H
DL = drive
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is supported on fixed disks only.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 11H (17)
Recalibrate drive
────────────────────────────────────────────────────────────────────────────
Causes the fixed disk adapter to recalibrate itself for the specified
drive, positioning the read/write arm to cylinder 0, and returns the
drive's status.
Call with:
AH = 11H
DL = drive
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is supported on fixed disks only.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC]
Function 12H (18)
Controller RAM diagnostic
────────────────────────────────────────────────────────────────────────────
Causes the fixed disk adapter to carry out a built-in diagnostic test on
its internal sector buffer, indicating whether the test was passed by the
returned status.
Call with:
AH = 12H
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is supported on PC/XT fixed disks only.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC]
Function 13H (19)
Controller drive diagnostic
────────────────────────────────────────────────────────────────────────────
Causes the fixed disk adapter to run internal diagnostic tests of the
attached drive, indicating whether the test was passed by the returned
status.
Call with:
AH = 13H
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is supported on PC/XT fixed disks only.
────────────────────────────────────────────────────────────────────────────
Int 13H [PC] [AT] [PS/2]
Function 14H (20)
Controller internal diagnostic
────────────────────────────────────────────────────────────────────────────
Causes the fixed disk adapter to carry out a built-in diagnostic
self-test, indicating whether the test was passed by the returned status.
Call with:
AH = 14H
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is supported on fixed disks only.
────────────────────────────────────────────────────────────────────────────
Int 13H [AT] [PS/2]
Function 15H (21)
Get disk type
────────────────────────────────────────────────────────────────────────────
Returns a code indicating the type of floppy or fixed disk referenced by
the specified drive code.
Call with:
AH = 15H
DL = drive
00H─7FH floppy disk
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = drive type code
00H if no drive present
01H if floppy disk drive without change-line support
02H if floppy disk drive with change-line support
03H if fixed disk
and, if fixed disk (AH = 03H)
CX:DX = number of 512-byte sectors
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is not supported on the PC or PC/XT.
────────────────────────────────────────────────────────────────────────────
Int 13H [AT] [PS/2]
Function 16H (22)
Get disk change status
────────────────────────────────────────────────────────────────────────────
Returns the status of the change line, indicating whether the disk in the
drive may have been replaced since the last disk access.
Call with:
AH = 16H
DL = drive
00H─7FH floppy disk
Returns:
If change line inactive (disk has not been changed)
Carry flag = clear
AH = 00H
If change line active (disk may have been changed)
Carry flag = set
AH = 06H
Notes:
■ If this function returns with the carry flag set, the disk has not
necessarily been changed; the change line can be activated by simply
unlocking and locking the disk drive door without removing the floppy
disk.
■ This function is not supported for floppy disks on the PC or PC/XT.
────────────────────────────────────────────────────────────────────────────
Int 13H [AT] [PS/2]
Function 17H (23)
Set disk type
────────────────────────────────────────────────────────────────────────────
Selects a floppy disk type for the specified drive.
Call with:
AH = 17H
AL = floppy disk type code
00H not used
01H 320/360 KB floppy disk in 360 KB drive
02H 320/360 KB floppy disk in 1.2 MB drive
03H 1.2 MB floppy disk in 1.2 MB drive
04H 720 KB floppy disk in 720 KB drive
SL = drive
00H─7FH floppy disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This function is not supported for floppy disks on the PC or PC/XT.
■ If the change line is active for the specified drive, it is reset. The
ROM BIOS then sets the data rate for the specified drive and media type.
────────────────────────────────────────────────────────────────────────────
Int 13H [AT] [PS/2]
Function 18H (24)
Set media type for format
────────────────────────────────────────────────────────────────────────────
Selects media characteristics for the specified drive.
Call with:
AH = 18H
CH = number of cylinders
CL = sectors per track
DL = drive
00H─7FH floppy disk
Returns:
If function successful
Carry flag = clear
AH = 00H
ES:DI = segment:offset of disk parameter table for media type
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ A floppy disk must be present in the drive.
■ This function should be called prior to formatting a disk with Int 13H
Function 05H so that the ROM BIOS can set the correct data rate for the
media.
■ If the change line is active for the specified drive, it is reset.
────────────────────────────────────────────────────────────────────────────
Int 13H [PS/2]
Function 19H (25)
Park heads
────────────────────────────────────────────────────────────────────────────
Moves the read/write arm to a track that is not used for data storage, so
that data will not be damaged when the drive is turned off.
Call with:
AH = 19H
DL = drive
80H─FFH fixed disk
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Note:
■ This function is defined for PS/2 fixed disks only.
────────────────────────────────────────────────────────────────────────────
Int 13H [PS/2]
Function 1AH (26)
Format ESDI drive
────────────────────────────────────────────────────────────────────────────
Initializes disk sector and track address fields on a drive attached to
the ESDI Fixed Disk Drive Adapter/A.
Call with:
AH = 1AH
AL = relative block address (RBA) defect table count
0 if no RBA table
>0 if RBA table used
CL = format modifier bits
Bit(s) Significance (if set)
0 ignore primary defect map
1 ignore secondary defect map
2 update secondary defect map (see Notes)
3 perform extended surface analysis
4 generate periodic interrupt (see Notes)
5─7 reserved (must be 0)
DL = drive
80H─FFH fixed disk
ES:BX = segment:offset of RBA table
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 13H Function 01H)
Notes:
■ This operation is sometimes called a "low level format" and prepares the
disk for physical read/write operations at the sector level. The drive
must be subsequently partitioned with the FDISK command and then given a
"high level format" with the FORMAT command to install a file system.
■ If bit 4 of register CL is set, Int 15H is called with AH = 0FH and AL
= phase code after each cylinder is formatted or analyzed. The phase
code is defined as:
0 = reserved
1 = surface analysis
2 = formatting
See also Int 15H Function 0FH.
■ If bit 2 of register CL is set, the drive's secondary defect map is
updated to reflect errors found during surface analysis. If both bit 2
and bit 1 are set, the secondary defect map is replaced.
■ For an extended surface analysis, the disk should first be formatted by
calling this function with bit 3 cleared, then analyzed by calling this
function with bit 3 set.
────────────────────────────────────────────────────────────────────────────
Int 14H [PC] [AT] [PS/2]
Function 00H
Initialize communications port
────────────────────────────────────────────────────────────────────────────
Initializes a serial communications port to a desired baud rate, parity,
word length, and number of stop bits.
Call with:
AH = 00H
AL = initialization parameter (see Notes)
DX = communications port number (0 = COM1, 1 = COM2, etc.)
Returns:
AH = port status
Bit Significance (if set)
0 receive data ready
1 overrun error detected
2 parity error detected
3 framing error detected
4 break detected
5 transmit holding register empty
6 transmit shift register empty
7 timed-out
AL = modem status
Bit Significance (if set)
0 change in clear-to-send status
1 change in data-set-ready status
2 trailing edge ring indicator
3 change in receive line signal detect
4 clear-to-send
5 data-set-ready
6 ring indicator
7 receive line signal detect
Notes:
■ The initialization parameter byte is defined as follows:
7 6 5 4 3 2 1 0
Baud rate Parity Stop bits Word length
────────────────────────────────────────────────────────────────────────
000 = 110 X0 = none 0 = 1 bit 10 = 7 bits
001 = 150 01 = odd 1 = 2 bits 11 = 8 bits
010 = 300 11 = even
011 = 600
100 = 1200
101 = 2400
110 = 4800
111 = 9600
────────────────────────────────────────────────────────────────────────
■ To initialize the serial port for data rates greater than 9600 baud on
PS/2 machines, see Int 14H Functions 04H and 05H.
────────────────────────────────────────────────────────────────────────────
Int 14H [PC] [AT] [PS/2]
Function 01H
Write character to communications port
────────────────────────────────────────────────────────────────────────────
Writes a character to the specified serial communications port, returning
the current status of the port.
Call with:
AH = 01H
AL = character
DX = communications port number (0 = COM1, 1 = COM2, etc.)
Returns:
If function successful
AH bit 7 = 0
AH bits = port status
0─6
Bit Significance (if set)
0 receive data ready
1 overrun error detected
2 parity error detected
3 framing error detected
4 break detected
5 transmit holding register empty
6 transmit shift register empty
AL = character (unchanged)
If function unsuccessful (timed-out)
AH bit 7 = 1
AL = character (unchanged)
────────────────────────────────────────────────────────────────────────────
Int 14H [PC] [AT] [PS/2]
Function 02H
Read character from communications port
────────────────────────────────────────────────────────────────────────────
Reads a character from the specified serial communications port, also
returning the port's status.
Call with:
AH = 02H
DX = communications port number (0 = COM1, 1 = COM2, etc.)
Returns:
If function successful
AH bit 7 = 0
AH bits 0─6 = status
Bit Significance (if set)
1 overrun error detected
2 parity error detected
3 framing error detected
4 break detected
AL = character
If function unsuccessful (timed-out)
AH bit 7 = 1
────────────────────────────────────────────────────────────────────────────
Int 14H [PC] [AT] [PS/2]
Function 03H
Get communications port status
────────────────────────────────────────────────────────────────────────────
Returns the status of the specified serial communications port.
Call with:
AH = 03H
DX = communications port number (0 = COM1, 1 = COM2, etc.)
Returns:
AH = port status (see Int 14H Function 00H)
AL = modem status (see Int 14H Function 00H)
────────────────────────────────────────────────────────────────────────────
Int 14H [PS/2]
Function 04H
Extended initialize communications port
────────────────────────────────────────────────────────────────────────────
Initializes a serial communications port to a desired baud rate, parity,
word length, and number of stop bits. Provides a superset of Int 14H
Function 00H capabilities for PS/2 machines.
Call with:
AH = 04H
AL = break flag
00H no break
01H break
BH = parity
00H none
01H odd
02H even
03H stick parity odd
04H stick parity even
BL = stop bits
00H 1 stop bit
01H 2 stop bits if word length = 6─8 bits
01H 1.5 stop bits if word length = 5 bits
CH = word length
00H 5 bits
01H 6 bits
02H 7 bits
03H 8 bits
CL = baud rate
00H 110 baud
01H 150 baud
02H 300 baud
03H 600 baud
04H 1200 baud
05H 2400 baud
06H 4800 baud
07H 9600 baud
08H 19,200 baud
DX = communications port number (0 = COM1, 1 = COM2, etc.)
Returns:
AH = port status (see Int 14H Function 00H)
AL = modem status (see Int 14H Function 00H)
────────────────────────────────────────────────────────────────────────────
Int 14H [PS/2]
Function 05H
Extended communications port control
────────────────────────────────────────────────────────────────────────────
Reads or sets the modem control register (MCR) for the specified serial
communications port.
Call with:
AH = 05H
AL = subfunction
00H to read modem control register
01H to write modem control register
BL = modem control register contents (if AL = 01H)
Bit(s) Significance
0 data-terminal ready
1 request-to-send
2 Out1
3 Out2
4 loop (for testing)
5─7 reserved
DX = communications port number (0 = COM1, 1 = COM2, etc.)
Returns:
If called with AL = 00H
BL = modem control register contents (see above)
If called with AL = 01H
AH = port status (see Int 14H Function 00H)
AL = modem status (see Int 14H Function 00H)
────────────────────────────────────────────────────────────────────────────
Int 15H [PC]
Function 00H
Turn on cassette motor
────────────────────────────────────────────────────────────────────────────
Turns on the motor of the cassette tape drive.
Call with:
AH = 00H
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AH = status
86H if cassette not present
Note:
■ This function is available only on the PC and the PCjr. It is not
supported on the PC/XT and all subsequent models.
────────────────────────────────────────────────────────────────────────────
Int 15H [PC]
Function 01H
Turn off cassette motor
────────────────────────────────────────────────────────────────────────────
Turns off the motor of the cassette tape drive.
Call with:
AH = 01H
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AH = status
86H if cassette not present
Note:
■ This function is available only on the PC and the PCjr. It is not
supported on the PC/XT and all subsequent models.
────────────────────────────────────────────────────────────────────────────
Int 15H [PC]
Function 02H
Read cassette
────────────────────────────────────────────────────────────────────────────
Reads one or more 256-byte blocks of data from the cassette tape drive to
memory.
Call with:
AH = 02H
CX = number of bytes to read
ES:BX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
DX = number of bytes actually read
ES:BX = segment:offset + 1 of last byte read
If function unsuccessful
Carry flag = set
AH = status
01H if CRC error
02H if bit signals scrambled
04H if no data found
80H if invalid command
86H if cassette not present
Note:
■ This function is available only on the PC and on the PCjr. It is not
supported on the PC/XT and all subsequent models.
────────────────────────────────────────────────────────────────────────────
Int 15H [PC]
Function 03H
Write cassette
────────────────────────────────────────────────────────────────────────────
Writes one or more 256-byte blocks of data from memory to the cassette
tape drive.
Call with:
AH = 03H
CX = number of bytes to write
ES:BX = segment:offset of buffer
Returns:
If function successful
Carry flag = clear
CX = 00H
ES:BX = segment:offset + 1 of last byte written
If function unsuccessful
Carry flag = set
AH = status
80H if invalid command
86H if cassette not present
Note:
■ This function is available only on the PC and on the PCjr. It is not
supported on the PC/XT and all subsequent models.
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function 0FH (15)
Format ESDI drive periodic interrupt
────────────────────────────────────────────────────────────────────────────
Invoked by the ROM BIOS on the ESDI Fixed Disk Drive Adapter/A during a
format or surface analysis operation after each cylinder is completed.
Call with:
AH = 0FH
AL = phase code
0 = reserved
1 = surface analysis
2 = formatting
Returns:
If formatting or analysis should continue
Carry flag = clear
If formatting or analysis should be terminated
Carry flag = set
Notes:
■ This function call can be captured by a program so that it will be
notified as each cylinder is formatted or analyzed. The program can
count interrupts for each phase to determine the current cylinder
number.
■ The default ROM BIOS handler for this function returns with the carry
flag set.
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function 21H (33) Subfunction 00H
Read POST error log
────────────────────────────────────────────────────────────────────────────
Returns error information that was accumulated during the most recent
power-on self-test (POST).
Call with:
AH = 21H
AL = 00H
Returns:
If function successful
Carry flag = clear
AH = 00H
BX = number of POST error codes stored
ES:DI = segment:offset of POST error log
If function unsuccessful
Carry flag = set
AH = status
80H = invalid command
86H = function not supported
Notes:
■ The error log consists of single-word entries. The first byte of an
entry is the device error code, and the second is the device identifier.
■ This function is not available on the PS/2 Models 25 and 30.
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function 21H (33) Subfunction 01H
Write POST error log
────────────────────────────────────────────────────────────────────────────
Adds an entry to the power-on self-test (POST) error log.
Call with:
AH = 21H
AL = 01H
BH = device identifier
BL = device error code
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status
01H = error list full
80H = invalid command
86H = function not supported
Note:
■ This function is not available on the PS/2 Models 25 and 30.
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function 4FH (79)
Keyboard intercept
────────────────────────────────────────────────────────────────────────────
Invoked for each keystroke by the ROM BIOS's Int 09H keyboard interrupt
handler.
Call with:
AH = 4FH
AL = scan code
Returns:
If scan code consumed
Carry flag = clear
If scan code not consumed
Carry flag = set
AL = unchanged or new scan code
Notes:
■ An operating system or a resident utility can capture this function to
filter the raw keyboard data stream. The new handler can substitute a
new scan code, return the same scan code, or return the carry flag clear
causing the keystroke to be discarded. The default ROM BIOS routine
simply returns the scan code unchanged.
■ A program can call Int 15H Function C0H to determine whether the host
machine's ROM BIOS supports this keyboard intercept.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 80H (128)
Device open
────────────────────────────────────────────────────────────────────────────
Acquires ownership of a logical device for a process.
Call with:
AH = 80H
BX = device ID
CX = process ID
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status
Note:
■ This function call, along with Int 15H Functions 81H and 82H, defines
a simple protocol that can be used to arbitrate usage of devices by
multiple processes. A multitasking program manager would be expected to
capture Int 15H and provide the appropriate service. The default BIOS
routine for this function simply returns with the carry flag clear and
AH = 00H.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 81H (129)
Device close
────────────────────────────────────────────────────────────────────────────
Releases ownership of a logical device for a process.
Call with:
AH = 81H
BX = device ID
CX = process ID
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status
Note:
■ A multitasking program manager would be expected to capture Int 15H and
provide the appropriate service. The default BIOS routine for this
function simply returns with the carry flag clear and AH = 00H. See also
Int 15H Functions 80H and 82H.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 82H (130)
Process termination
────────────────────────────────────────────────────────────────────────────
Releases ownership of all logical devices for a process that is about to
terminate.
Call with:
AH = 82H
BX = process ID
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status
Note:
■ A multitasking program manager would be expected to capture Int 15H and
provide the appropriate service. The default BIOS routine for this
function simply returns with the carry flag clear and AH = 00H. See also
Int 15H Functions 80H and 81H.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 83H (131)
Event wait
────────────────────────────────────────────────────────────────────────────
Requests setting of a semaphore after a specified interval or cancels a
previous request.
Call with:
If requesting event wait
AH = 83H
AL = 00H
CX:DX = microseconds
ES:BX = segment:offset of semaphore byte
If canceling event wait
AH = 83H
AL = 01H
Returns:
If called with AL = 00H, and function successful
Carry flag = clear
If called with AL = 00H, and function unsuccessful (Event Wait already
active)
Carry flag = set
If called with AL = 01H
Nothing
Notes:
■ The function call returns immediately. If the function is successful,
bit 7 of the semaphore byte is set when the specified interval has
elapsed. The calling program is responsible for clearing the semaphore
before requesting this function.
■ The actual duration of an event wait is always an integral multiple of
976 microseconds. The CMOS date/clock chip interrupts are used to
implement this function.
■ Use of this function allows programmed, hardware-independent delays at a
finer resolution than can be obtained through use of the MS-DOS Get Time
function (Int 21H Function 2CH, which returns time in hundredths of a
second).
■ See also Int 15H Function 86H, which suspends the calling program for
the specified interval in milliseconds.
■ This function is not supported on the PS/2 Models 25 and 30.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 84H (132)
Read joystick
────────────────────────────────────────────────────────────────────────────
Returns the joystick switch settings and potentiometer values.
Call with:
AH = 84H
DX = subfunction
00H to read switch settings
01H to read resistive inputs
Returns:
If function successful
Carry flag = clear
and, if called with DX = 00H
AL = switch settings (bits 4─7)
or, if called with DX = 01H
AX = A(x) value
BX = A(y) value
CX = B(x) value
DX = B(y) value
If function unsuccessful
Carry flag = set
Notes:
■ An error condition is returned if DX does not contain a valid
subfunction number.
■ If no game adapter is installed, AL is returned as 00H for Subfunction
00H (i.e., all switches open); AX, BX, CX, and DX are returned
containing 00H for Subfunction 01H.
■ Using a 250 KOhm joystick, the potentiometer values usually lie within
the srange 0─416 (0000─01A0H).
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 85H (133)
SysReq key
────────────────────────────────────────────────────────────────────────────
Invoked by the ROM BIOS keyboard driver when the SysReq key is detected.
Call with:
AH = 85H
AL = key status
00H if key make (depression)
01H if key break (release)
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status
Note:
■ The ROM BIOS handler for this function call is a dummy routine that
always returns a success status unless called with an invalid
subfunction number in AL. A multitasking program manager would be
expected to capture Int 15H so that it can be notified when the user
strikes the SysReq key.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 86H (134)
Delay
────────────────────────────────────────────────────────────────────────────
Suspends the calling program for a specified interval in microseconds.
Call with:
AH = 86H
CX:DX = microseconds to wait
Returns:
If function successful (wait was performed)
Carry flag = clear
If function unsuccessful (wait was not performed)
Carry flag = set
Notes:
■ The actual duration of the wait is always an integral multiple of 976
microseconds.
■ Use of this function allows programmed, hardware-independent delays at a
finer resolution than can be obtained through use of the MS-DOS Get Time
function (Int 21H Function 2CH, which returns time in hundredths of a
second).
■ See also Int 15H Function 83H, which triggers a semaphore after a
specified interval but does not suspend the calling program.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 87H (135)
Move extended memory block
────────────────────────────────────────────────────────────────────────────
Transfers data between conventional memory and extended memory.
Call with:
AH = 87H
CX = number of words to move
ES:SI = segment:offset of Global Descriptor Table (see Notes)
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status
01H if RAM parity error
02H if exception interrupt error
03H if gate address line 20 failed
Notes:
■ Conventional memory lies at addresses below the 640 KB boundary, and is
used for the execution of MS-DOS and its application programs. Extended
memory lies at addresses above 1 MB, and can only be accessed by an
80286 or 80386 CPU running in protected mode. As much as 15 MB of
extended memory can be installed in an IBM PC/AT or compatible.
■ The Global Descriptor Table (GDT) used by this function must be set up
as follows:
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
00H─0FH reserved (should be 0)
10H─11H segment length in bytes (2*CX - 1 or greater)
12H─14H 24-bit source address
15H access rights byte (always 93H)
16H─17H reserved (should be 0)
18H─19H segment length in bytes (2*CX - 1 or greater)
1AH─1CH 24-bit destination address
1DH access rights byte (always 93H)
1EH─2FH reserved (should be 0)
────────────────────────────────────────────────────────────────────────
The table is composed of six 8-byte descriptors to be used by the CPU in
protected mode. The four descriptors in offsets 00H─0FH and 20H─2FH are
filled in by the ROM BIOS before the CPU mode switch.
■ The addresses used in the descriptor table are linear (physical) 24-bit
addresses in the range 000000H─FFFFFFH──not segments and offsets──with
the least significant byte at the lowest address and the most
significant byte at the highest address.
■ The block move is performed with interrupts disabled; thus, use of this
function may interfere with the operation of communications programs,
network drivers, or other software that relies on prompt servicing of
hardware interrupts.
■ Programs and drivers that access extended memory with this function
cannot be executed in the Compatibility Environment of OS/2.
■ This function is not supported on the PS/2 Models 25 and 30.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 88H (136)
Get extended memory size
────────────────────────────────────────────────────────────────────────────
Returns the amount of extended memory installed in the system.
Call with:
AH = 88H
Returns:
AX = amount of extended memory (in KB)
Notes:
■ Extended memory is memory at addresses above 1 MB, which can only be
accessed by an 80286 or 80386 CPU running in protected mode. Because
MS-DOS is a real-mode operating system, extended memory can be used for
storage of volatile data but cannot be used for execution of programs.
■ Programs and drivers that use this function cannot be executed in the
Compatibility Environment of OS/2.
■ This function is not supported on the PS/2 Models 25 and 30.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 89H (137)
Enter protected mode
────────────────────────────────────────────────────────────────────────────
Switches the CPU from real mode into protected mode.
Call with:
AH = 89H
BH = interrupt number for IRQ0, written to ICW2 of 8259 PIC #1
(must be evenly divisible by 8, determines IRQ0─IRQ7)
BL = interrupt number for IRQ8, written to ICW2 of 8259 PIC #2
(must be evenly divisible by 8, determines IRQ8─IRQ15)
ES:SI = segment:offset of Global Descriptor Table (GDT)
Returns:
If function successful (CPU is in protected mode)
Carry flag = clear
AH = 00H
CS = user-defined selector
DS = user-defined selector
ES = user-defined selector
SS = user-defined selector
If function unsuccessful (CPU is in real mode)
Carry flag = set
AH = FFH
Notes:
■ The Global Descriptor Table must contain eight descriptors set up as
follows:
Offset Descriptor usage
────────────────────────────────────────────────────────────────────────
00H dummy descriptor (initialized to 0)
08H Global Descriptor Table (GDT)
10H Interrupt Descriptor Table (IDT)
18H user's data segment (DS)
20H user's extra segment (ES)
28H user's stack segment (SS)
30H user's code segment (CS)
38H BIOS code segment
────────────────────────────────────────────────────────────────────────
The user must initialize the first seven descriptors; the eighth is
filled in by the ROM BIOS to provide addressability for its own
execution. The calling program may modify and use the eighth descriptor
for any purpose after return from this function call.
■ This function is not supported on the PS/2 Models 25 and 30.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 90H (144)
Device wait
────────────────────────────────────────────────────────────────────────────
Invoked by the ROM BIOS fixed disk, floppy disk, printer, network, and
keyboard drivers prior to performing a programmed wait for I/O completion.
Call with:
AH = 90H
AL = device type
00H─7FH serially reusable devices
80H─BFH reentrant devices
C0H─FFH wait-only calls, no corresponding Post function
ES:BX = segment:offset of request block for device types 80H─FFH
Returns:
If no wait (driver must perform its own time-out)
Carry flag = clear
AH = 00H
If wait was performed
Carry flag = set
Notes:
■ Predefined device types are:
00H disk (may time-out)
01H floppy disk (may time-out)
02H keyboard (no time-out)
03H pointing device (PS/2, may time-out)
80H network (no time-out)
FCH fixed disk reset (PS/2, may time-out)
FDH floppy disk drive motor start (may time-out)
FEH printer (may time-out)
■ For network adapters, ES:BX points to a network control block (NCB).
■ A multitasking program manager would be expected to capture Int 15H
Function 90H so that it can dispatch other tasks while I/O is in
progress. The default BIOS routine for this function simply returns with
the carry flag clear and AH = 00H.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function 91H (145)
Device post
────────────────────────────────────────────────────────────────────────────
Invoked by the ROM BIOS fixed disk, floppy disk, network, and keyboard
drivers to signal that I/O is complete and/or the device is ready.
Call with:
AH = 91H
AL = device type
00H─7FH serially reusable devices
80H─BFH reentrant devices
ES:BX = segment:offset of request block for device types 80H─BFH
Returns:
AH = 00H
Notes:
■ Predefined device types that may use Device Post are:
00H disk (may time-out)
01H floppy disk (may time-out)
02H keyboard (no time-out)
03H pointing device (PS/2, may time-out)
80H network (no time-out)
■ The ROM BIOS printer routine does not invoke this function because
printer output is not interrupt driven.
■ A multitasking program manager would be expected to capture Int 15H
Function 91H so that it can be notified when I/O is completed and
awaken the requesting task. The default BIOS routine for this function
simply returns with the carry flag clear and AH = 00H.
────────────────────────────────────────────────────────────────────────────
Int 15H [AT] [PS/2]
Function C0H (192)
Get system environment
────────────────────────────────────────────────────────────────────────────
Returns a pointer to a table containing various information about the
system configuration.
Call with:
AH = C0H
Returns:
ES:BX = segment:offset of configuration table (see Notes)
Notes:
■ The format of the system configuration table is as follows:
Byte(s) Contents
────────────────────────────────────────────────────────────────────────
00H─01H length of table in bytes
02H system model (see following Note)
03H system submodel (see following Note)
04H BIOS revision level
05H configuration flags
Bit Significance (if set)
0 reserved
1 Micro Channel implemented
2 extended BIOS data area allocated
3 Wait for External Event is available
4 keyboard intercept (Int 15H Function
4FH) available
5 real-time clock available
6 slave 8259 present (cascaded IRQ2)
7 DMA channel 3 used
06H─09H reserved
────────────────────────────────────────────────────────────────────────
■ The system model and type bytes are assigned as follows:
Machine Model byte Submodel byte
────────────────────────────────────────────────────────────────────────
PC FFH
PC/XT FEH
PC/XT FBH 00H or 01H
PCjr FDH
PC/AT FCH 00H or 01H
PC/XT-286 FCH 02H
PC Convertible F9H
PS/2 Model 30 FAH 00H
PS/2 Model 50 FCH 04H
PS/2 Model 60 FCH 05H
PS/2 Model 70 F8H 04H or 09H
PS/2 Model 80 F8H 00H or 01H
────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C1H (193)
Get address of extended BIOS data area
────────────────────────────────────────────────────────────────────────────
Returns the segment address of the base of the extended BIOS data area.
Call with:
AH = C1H
Returns:
If function successful
Carry flag = clear
ES = segment of extended BIOS data area
If function unsuccessful
Carry flag = set
Notes:
■ The extended BIOS data area is allocated at the high end of conventional
memory during the POST (Power-On-Self-Test) sequence. The word at
0040:0013H (memory size) is updated to reflect the reduced amount of
memory available for MS-DOS and application programs. The first byte in
the extended BIOS data area is initialized to its length in KB.
■ A program can determine whether the extended BIOS data area exists with
Int 15H Function C0H.
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C2H (194) Subfunction 00H
Enable/disable pointing device
────────────────────────────────────────────────────────────────────────────
Enables or disables the system's mouse or other pointing device.
Call with:
AH = C2H
AL = 00H
BH = enable/disable flag
00H = disable
01H = enable
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status
01H if invalid function call
02H if invalid input
03H if interface error
04H if resend
05H if no far call installed
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C2H (194) Subfunction 01H
Reset pointing device
────────────────────────────────────────────────────────────────────────────
Resets the system's mouse or other pointing device, setting the sample
rate, resolution, and other characteristics to their default values.
Call with:
AH = C2H
AL = 01H
Returns:
If function successful
Carry flag = clear
AH = 00H
BH = device ID
If function unsuccessful
Carry flag = set
AH = status (see Int 15H Function C2H Subfunction 00H)
Notes:
■ After a reset operation, the state of the pointing device is as follows:
∙ disabled;
∙ sample rate at 100 reports per second;
∙ resolution at 4 counts per millimeter;
∙ and scaling at 1 to 1.
The data package size is unchanged by this function.
■ The application can use the other Int 15H Function C2H subfunctions to
initialize the pointing device to other sample rates, resolution, and
scaling, and then enable the device with Int 15H Function C2H
Subfunction 00H.
■ See also Int 15H Function C2H Subfunction 05H, which incidentally
resets the pointing device in a similar manner.
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C2H (194) Subfunction 02H
Set sample rate
────────────────────────────────────────────────────────────────────────────
Sets the sampling rate of the system's mouse or other pointing device.
Call with:
AH = C2H
AL = 02H
BH = sample rate value
00H = 10 reports per second
01H = 20 reports per second
02H = 40 reports per second
03H = 60 reports per second
04H = 80 reports per second
05H = 100 reports per second
06H = 200 reports per second
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 15H Function C2H Subfunction 00H)
Note:
■ The default sample rate is 100 reports per second after a reset
operation (Int 15H Function C2H Subfunction 01H).
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C2H (194) Subfunction 03H
Set resolution
────────────────────────────────────────────────────────────────────────────
Sets the resolution of the system's mouse or other pointing device.
Call with:
AH = C2H
AL = 03H
BH = resolution value
00H = 1 count per millimeter
01H = 2 counts per millimeter
02H = 4 counts per millimeter
03H = 8 counts per millimeter
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 15H Function C2H Subfunction 00H)
Note:
■ The default resolution is 4 counts per millimeter after a reset
operation (Int 15H Function C2H Subfunction 01H).
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C2H (194) Subfunction 04H
Get pointing device type
────────────────────────────────────────────────────────────────────────────
Returns the identification code for the system's mouse or other pointing
device.
Call with:
AH = C2H
AL = 04H
Returns:
If function successful
Carry flag = clear
AH = 00H
BH = device ID
If function unsuccessful
Carry flag = set
AH = status (see Int 15H Function C2H Subfunction 00H)
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C2H (194) Subfunction 05H
Initialize pointing device interface
────────────────────────────────────────────────────────────────────────────
Sets the data package size for the system's mouse or other pointing
device, and initializes the resolution, sampling rate, and scaling to
their default values.
Call with:
AH = C2H
AL = 05H
BH = data package size in bytes (1─8)
Returns:
If function successful
Carry flag = clear
AH = 00H
If function unsuccessful
Carry flag = set
AH = status (see Int 15H Function C2H Subfunction 00H)
Note:
■ After this operation, the state of the pointing device is as follows:
∙ disabled;
∙ sample rate at 100 reports per second;
∙ resolution at 4 counts per millimeter;
∙ and scaling at 1 to 1.
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C2H (194) Subfunction 06H
Set scaling or get status
────────────────────────────────────────────────────────────────────────────
Returns the current status of the system's mouse or other pointing device
or sets the device's scaling factor.
Call with:
AH = C2H
AL = 06H
BH = extended command
00H = return device status
01H = set scaling at 1:1
02H = set scaling at 2:1
Returns:
If function successful
Carry flag = clear
AH = 00H
and, if called with BH = 00H
BL = status byte
Bit Significance
0 = 1 if right button pressed
1 = reserved
2 = 1 if left button pressed
3 = reserved
4 = 0 if 1:1 scaling
1 if 2:1 scaling
5 = 0 if device disabled
1 if device enabled
6 = 0 if stream mode
1 if remote mode
7 = reserved
CL = resolution
00H = 1 count per millimeter
01H = 2 counts per millimeter
02H = 4 counts per millimeter
03H = 8 counts per millimeter
DL = sample rate
0AH = 10 reports per second
14H = 20 reports per second
28H = 40 reports per second
3CH = 60 reports per second
50H = 80 reports per second
64H = 100 reports per second
C8H = 200 reports per second
If function unsuccessful
Carry flag = set
AH = status (see Int 15H Function C2H Subfunction 00H)
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C2H (194) Subfunction 07H
Set pointing device handler address
────────────────────────────────────────────────────────────────────────────
Notifies the ROM BIOS pointing device driver of the address for a routine
to be called each time pointing device data is available.
Call with:
AH = C2H
AL = 07H
ES:BX = segment:offset of user routine
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AH = status (see Int 15H Function C2H Subfunction 00H)
Notes:
■ The user's handler for pointing device data is entered via a far call
with four parameters on the stack:
SS:SP+0AH status
SS:SP+08H x coordinate
SS:SP+06H y coordinate
SS:SP+04H z coordinate (always 0)
The handler must exit via a far return without removing the parameters
from the stack.
■ The status parameter passed to the user's handler is interpreted as
follows:
Bit(s) Significance (if set)
────────────────────────────────────────────────────────────────────────
0 left button pressed
1 right button pressed
2─3 reserved
4 sign of x data is negative
5 sign of y data is negative
6 x data has overflowed
7 y data has overflowed
8─15 reserved
────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C3H (195)
Set watchdog time-out
────────────────────────────────────────────────────────────────────────────
Enables or disables a watchdog timer.
Call with:
AH = C3H
AL = subfunction
00H to disable watchdog time-out
01H to enable watchdog time-out
BX = watchdog timer counter (if AL = 01H)
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
Notes:
■ The watchdog timer generates an NMI interrupt.
■ This function is not available on the PS/2 Models 25 and 30.
────────────────────────────────────────────────────────────────────────────
Int 15H [PS/2]
Function C4H (196)
Programmable option select
────────────────────────────────────────────────────────────────────────────
Returns the base Programmable Option Select register address, enables a
slot for setup, or enables an adapter.
Call with:
AH = C4H
AL = subfunction
00H to return base POS adapter register address
01H to enable slot
02H to enable adapter
BL = slot number (if AL = 01H)
Returns:
If function successful
Carry flag = clear
and, if called with AL = 00H
DX = base POS adapter register address
If function unsuccessful
Carry flag = set
Notes:
■ This function is available only on machines using the Micro Channel
Architecture (MCA) bus.
■ After a slot is enabled with Subfunction 01H, specific information can
be obtained for the adapter in that slot by performing port input
operations:
Port Function
100H MCA ID (low byte)
101H MCA ID (high byte)
102H Option Select Byte 1
bit 0 = 1 if enabled, = 0 if disabled
103H Option Select Byte 2
104H Option Select Byte 3
105H Option Select Byte 4
bits 6─7 = channel check indicators
106H Subaddress Extension (low byte)
107H Subaddress Extension (high byte)
────────────────────────────────────────────────────────────────────────────
Int 16H [PC] [AT] [PS/2]
Function 00H
Read character from keyboard
────────────────────────────────────────────────────────────────────────────
Reads a character from the keyboard, also returning the keyboard scan
code.
Call with:
AH = 00H
Returns:
AH = keyboard scan code
AL = ASCII character
────────────────────────────────────────────────────────────────────────────
Int 16H [PC] [AT] [PS/2]
Function 01H
Get keyboard status
────────────────────────────────────────────────────────────────────────────
Determines whether a character is ready for input, returning a flag and
also the character itself, if one is waiting.
Call with:
AH = 01H
Returns:
If key waiting to be input
Zero flag = clear
AH = keyboard scan code
AL = character
If no key waiting
Zero flag = set
Note:
■ The character returned by this function when the zero flag is clear is
not removed from the type-ahead buffer. The same character and scan code
will be returned by the next call to Int 16H Function 00H.
────────────────────────────────────────────────────────────────────────────
Int 16H [PC] [AT] [PS/2]
Function 02H
Get keyboard flags
────────────────────────────────────────────────────────────────────────────
Returns the ROM BIOS flags byte that describes the state of the various
keyboard toggles and shift keys.
Call with:
AH = 02H
Returns:
AL = flags
Bit Significance (if set)
0 right Shift key is down
1 left Shift key is down
2 Ctrl key is down
3 Alt key is down
4 Scroll Lock on
5 Num Lock on
6 Caps Lock on
7 Insert on
Note:
■ The keyboard flags byte is stored in the ROM BIOS data area at
0000:0417H.
────────────────────────────────────────────────────────────────────────────
Int 16H [PC] [AT] [PS/2]
Function 03H
Set repeat rate
────────────────────────────────────────────────────────────────────────────
Sets the ROM BIOS key repeat ("typematic") rate and delay.
Call with:
On the PC/AT and PS/2
AH = 03H
AL = 05H
BH = repeat delay (see Notes)
BL = repeat rate (see Notes)
On the PCjr
AH = 03H
AL = subfunction
00H to restore default rate and delay
01H to increase initial delay
02H to decrease repeat rate by one-half
03H to increase delay and decrease repeat rate by
one-half
04H to turn off keyboard repeat
Returns:
Nothing
Notes:
■ Subfunctions 00H─04H are available on the PCjr but are not supported by
the PC or PC/XT ROM BIOS. Subfunction 05H is available on PC/ATs with
ROM BIOS's dated 11/15/85 and later, and on the PS/2.
■ On the PC/AT and PS/2, the value in BH controls the amount of delay
before the first repeat key is generated. The delay is always a multiple
of 250 milliseconds:
Value Delay (msec.)
00H 250
01H 500
02H 750
03H 1000
■ On the PC/AT and PS/2, the value for the repeat rate in characters per
second can be chosen from the following table:
Value Repeat rate (characters per second)
00H 30.0
01H 26.7
02H 24.0
03H 21.8
04H 20.0
05H 18.5
06H 17.1
07H 16.0
08H 15.0
09H 13.3
0AH 12.0
0BH 10.9
0CH 10.0
0DH 9.2
0EH 8.6
0FH 8.0
10H 7.5
11H 6.7
12H 6.0
13H 5.5
14H 5.0
15H 4.6
16H 4.3
17H 4.0
18H 3.7
19H 3.3
1AH 3.0
1BH 2.7
1CH 2.5
1DH 2.3
1EH 2.1
1FH 2.0
────────────────────────────────────────────────────────────────────────────
Int 16H [PC]
Function 04H
Set keyclick
────────────────────────────────────────────────────────────────────────────
Turns the keyboard click on or off.
Call with:
AH = 04H
AL = subfunction
00H to turn off keyboard click
01H to turn on keyboard click
Returns:
Nothing
Note:
■ This function is supported by the PCjr BIOS only.
────────────────────────────────────────────────────────────────────────────
Int 16H [AT] [PS/2]
Function 05H
Push character and scan code
────────────────────────────────────────────────────────────────────────────
Places a character and scan code in the keyboard type-ahead buffer.
Call with:
AH = 05H
CH = scan code
CL = character
Returns:
If function successful
Carry flag = clear
AL = 00H
If function unsuccessful (type-ahead buffer is full)
Carry flag = set
AL = 01H
Note:
■ This function can be used by keyboard enhancers and other utilities to
interpolate keys into the data stream seen by application programs.
────────────────────────────────────────────────────────────────────────────
Int 16H [AT] [PS/2]
Function 10H (16)
Read character from enhanced keyboard
────────────────────────────────────────────────────────────────────────────
Reads a character and scan code from the keyboard type-ahead buffer.
Call with:
AH = 10H
Returns:
AH = keyboard scan code
AL = ASCII character
Note:
■ Use this function for the enhanced keyboard instead of Int 16H Function
00H. It allows applications to obtain the scan codes for the additional
F11, F12, and cursor control keys.
────────────────────────────────────────────────────────────────────────────
Int 16H [AT] [PS/2]
Function 11H (17)
Get enhanced keyboard status
────────────────────────────────────────────────────────────────────────────
Determines whether a character is ready for input, returning a flag and
also the character itself, if one is waiting.
Call with:
AH = 11H
Returns:
If key waiting to be input
Zero flag = clear
AH = keyboard scan code
AL = character
If no key waiting
Zero flag = set
Notes:
■ Use this function for the enhanced keyboard instead of Int 16H Function
00H. It allows applications to test for the additional F11, F12, and
cursor control keys.
■ The character returned by this function when the zero flag is clear is
not removed from the type-ahead buffer. The same character and scan code
will be returned by the next call to Int 16H Function 10H.
────────────────────────────────────────────────────────────────────────────
Int 16H [AT] [PS/2]
Function 12H (18)
Get enhanced keyboard flags
────────────────────────────────────────────────────────────────────────────
Obtains the status of various enhanced keyboard special keys and keyboard
driver states.
Call with:
AH = 12H
Returns:
AX = flags
Bit Significance (if set)
0 right Shift key is down
1 left Shift key is down
2 either Ctrl key is down
3 either Alt key is down
4 Scroll Lock toggle is on
5 Num Lock toggle is on
6 Caps Lock toggle is on
7 Insert toggle is on
8 left Ctrl key is down
9 left Alt key is down
10 right Ctrl key is down
11 right Alt key is down
12 Scroll key is down
13 Num Lock key is down
14 Caps Lock key is down
15 SysReq key is down
Note:
■ Use this function for the enhanced keyboard instead of Int 16H Function
02H.
────────────────────────────────────────────────────────────────────────────
Int 17H [PC] [AT] [PS/2]
Function 00H
Write character to printer
────────────────────────────────────────────────────────────────────────────
Sends a character to the specified parallel printer interface port and
returns the current status of the port.
Call with:
AH = 00H
AL = character
DX = printer number (0 = LPT1, 1 = LPT2, 2 = LPT3)
Returns:
AH = status
Bit Significance (if set)
0 printer timed-out
1 unused
2 unused
3 I/O error
4 printer selected
5 out of paper
6 printer acknowledge
7 printer not busy
────────────────────────────────────────────────────────────────────────────
Int 17H [PC] [AT] [PS/2]
Function 01H
Initialize printer port
────────────────────────────────────────────────────────────────────────────
Initializes the specified parallel printer interface port and returns its
status.
Call with:
AH = 01H
DX = printer number (0 = LPT1, 1 = LPT2, 2 = LPT3)
Returns:
AH = status (see Int 17H Function 00H)
────────────────────────────────────────────────────────────────────────────
Int 17H [PC] [AT] [PS/2]
Function 02H
Get printer status
────────────────────────────────────────────────────────────────────────────
Returns the current status of the specified parallel printer interface
port.
Call with:
AH = 02H
DX = printer number (0 = LPT1, 1 = LPT2, 2 = LPT3)
Returns:
AH = status (see Int 17H Function 00H)
────────────────────────────────────────────────────────────────────────────
Int 18H [PC] [AT] [PS/2]
ROM BASIC
────────────────────────────────────────────────────────────────────────────
Transfers control to ROM BASIC.
Call with:
Nothing
Returns:
Nothing
Note:
■ This function is invoked when the system is turned on or restarted if
attempts to read a boot sector from the fixed disk or floppy disk drives
are unsuccessful.
────────────────────────────────────────────────────────────────────────────
Int 19H [PC] [AT] [PS/2]
Reboot system
────────────────────────────────────────────────────────────────────────────
Reboots the operating system from the floppy disk or fixed disk drive.
Call with:
Nothing
Returns:
Nothing
Notes:
■ The bootstrap routine reads Sector 1, Track 0 into memory at location
0000:7C00H and transfers control to the same address. If attempts to
read a boot sector from the floppy disk or fixed disk are unsuccessful,
control is transferred to ROM BASIC by execution of an Int 18H.
■ If location 0000:0472H does not contain the value 1234H, a memory test
will be performed before reading the boot sector.
────────────────────────────────────────────────────────────────────────────
Int 1AH [AT] [PS/2]
Function 00H
Get tick count
────────────────────────────────────────────────────────────────────────────
Returns the contents of the clock tick counter.
Call with:
AH = 00H
Returns:
AL = rolled-over flag
00H if midnight not passed since last read
<>00H if midnight was passed since last read
CX:DX = tick count (high 16 bits in CX)
Notes:
■ This function is supported by the PC/XT and PCjr ROM BIOS, but is not
present in the ROM BIOS for the original PC.
■ The returned value is the cumulative number of clock ticks since
midnight. There are 18.2 clock ticks per second. When the counter
reaches 1,573,040, it is cleared to zero, and the rolled-over flag is
set.
■ The rolled-over flag is cleared by this function call, so the flag will
only be returned nonzero once per day.
■ Int 1AH Function 01H can be used to set the clock tick counter to an
arbitrary 32-bit value.
────────────────────────────────────────────────────────────────────────────
Int 1AH [AT] [PS/2]
Function 01H
Set tick count
────────────────────────────────────────────────────────────────────────────
Stores a 32-bit value in the clock tick counter.
Call with:
AH = 01H
CX:DX = tick count (high 16 bits in CX)
Returns:
Nothing
Notes:
■ This function is supported by the PC/XT and PCjr ROM BIOS, but is not
present in the ROM BIOS for the original PC.
■ Int 1AH Function 00H is used to read the value of the clock tick
counter.
■ The rolled-over flag is cleared by this function call.
────────────────────────────────────────────────────────────────────────────
Int 1AH [AT] [PS/2]
Function 02H
Get time
────────────────────────────────────────────────────────────────────────────
Reads the current time from the CMOS time/date chip.
Call with:
AH = 02H
Returns:
CH = hours in binary coded decimal (BCD)
CL = minutes in BCD
DH = seconds in BCD
DL = daylight-saving-time code
00H if standard time
01H if daylight saving time
and, if clock running
Carry flag = clear
or, if clock stopped
Carry flag = set
────────────────────────────────────────────────────────────────────────────
Int 1AH [AT] [PS/2]
Function 03H
Set time
────────────────────────────────────────────────────────────────────────────
Sets the time in the CMOS time/date chip.
Call with:
AH = 03H
CH = hours in binary coded decimal (BCD)
CL = minutes in BCD
DH = seconds in BCD
DL = daylight-saving-time code
00H if standard time
01H if daylight saving time
Returns:
Nothing
────────────────────────────────────────────────────────────────────────────
Int 1AH [AT] [PS/2]
Function 04H
Get date
────────────────────────────────────────────────────────────────────────────
Reads the current date from the CMOS time/date chip.
Call with:
AH = 04H
Returns:
CH = century (19 or 20) in binary coded decimal (BCD)
CL = year in BCD
DH = month in BCD
DL = day in BCD
and, if clock running
Carry flag = clear
or, if clock stopped
Carry flag = set
────────────────────────────────────────────────────────────────────────────
Int 1AH [AT] [PS/2]
Function 05H
Set date
────────────────────────────────────────────────────────────────────────────
Sets the date in the CMOS time/date chip.
Call with:
AH = 05H
CH = century (19 or 20) in binary coded decimal (BCD)
CL = year in BCD
DH = month in BCD
DL = day in BCD
Returns:
Nothing
────────────────────────────────────────────────────────────────────────────
Int 1AH [AT] [PS/2]
Function 06H
Set alarm
────────────────────────────────────────────────────────────────────────────
Sets an alarm in the CMOS date/time chip.
Call with:
AH = 06H
CH = hours in binary coded decimal (BCD)
CL = minutes in BCD
DH = seconds in BCD
Returns:
If function successful
Carry flag = clear
If function unsuccessful (alarm already set, or clock stopped)
Carry flag = set
Notes:
■ A side effect of this function is that the clock chip's interrupt level
(IRQ8) is enabled.
■ Only one alarm may be active at any given time. The alarm occurs every
24 hours at the specified time until it is reset with Int 1AH Function
07H.
■ The program using this function must place the address of its interrupt
handler for the alarm in the vector for Int 4AH.
────────────────────────────────────────────────────────────────────────────
Int 1AH [AT] [PS/2]
Function 07H
Reset alarm
────────────────────────────────────────────────────────────────────────────
Cancels any pending alarm request on the CMOS date/time chip.
Call with:
AH = 07H
Returns:
Nothing
Note:
■ This function does not disable the clock chip's interrupt level (IRQ8).
────────────────────────────────────────────────────────────────────────────
Int 1AH [PS/2]
Function 0AH (10)
Get day count
────────────────────────────────────────────────────────────────────────────
Returns the contents of the system's day counter.
Call with:
AH = 0AH
Returns:
If function successful
Carry flag = clear
CX = count of days since January 1, 1980
If function unsuccessful
Carry flag = set
────────────────────────────────────────────────────────────────────────────
Int 1AH [PS/2]
Function 0BH (11)
Set day count
────────────────────────────────────────────────────────────────────────────
Stores an arbitrary value in the system's day counter.
Call with:
AH = 0BH
CX = count of days since January 1, 1980
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
────────────────────────────────────────────────────────────────────────────
Int 1AH [PC]
Function 80H (128)
Set sound source
────────────────────────────────────────────────────────────────────────────
Sets up the source for tones that will appear on the PCjr's "Audio Out" or
RF modulator.
Call with:
AH = 80H
AL = sound source
00H if 8253 programmable timer, channel 2
01H if cassette input
02H if "Audio In" line on I/O channel
03H if sound generator chip
Returns:
Nothing
Note:
■ This function is supported on the PCjr only.
────────────────────────────────────────────────────────────────────────────
Int 33H
Microsoft Mouse driver
────────────────────────────────────────────────────────────────────────────
The Microsoft Mouse driver makes its functions available to application
programs via Int 33H. These functions have become a de facto standard for
pointer device drivers of all varieties. Unlike the other function calls
described in this section, the Microsoft Mouse driver is not part of the
ROM BIOS but is loaded by a DEVICE= directive in the CONFIG.SYS file. All
mouse-function information applies to the Microsoft Mouse driver version
6. Earlier versions of the driver may not support all of these functions.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 00H
Reset mouse and get status
────────────────────────────────────────────────────────────────────────────
Initializes the mouse driver and returns the driver status. If the mouse
pointer was previously visible, it is removed from the screen, and any
previously installed user handlers for mouse events are disabled.
Call with:
AX = 0000H
Returns:
If mouse support is available
AX = FFFFH
BX = number of mouse buttons
If mouse support is not available
AX = 0000H
Note:
■ After a call to this function, the mouse driver is initialized to the
following state:
∙ Mouse pointer at screen center (see Int 33H Functions 03H and 04H)
∙ Display page for mouse pointer set to zero (see Int 33H Functions
1DH and 1EH)
∙ Mouse pointer hidden (see Int 33H Functions 01H, 02H, and 10H)
∙ Mouse pointer set to default arrow shape in graphics modes, or reverse
block in text modes (see Int 33H Functions 09H and 0AH)
∙ User mouse event handler disabled (see Int 33H Functions 0CH and
14H)
∙ Light pen emulation enabled (see Int 33H Functions 0DH and 0EH)
∙ Horizontal mickeys to pixels ratio at 8 to 8, vertical ratio at 16 to
8 (see Int 33H Function 0FH)
∙ Double speed threshold set to 64 mickeys/second (see Int 33H Function
19H)
∙ Minimum and maximum horizontal and vertical pointer position limits
set to include the entire screen in the current display mode (see Int
33H Functions 07H and 08H)
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 01H
Show mouse pointer
────────────────────────────────────────────────────────────────────────────
Displays the mouse pointer, and cancels any mouse pointer exclusion area
previously defined with Int 33H Function 10H.
Call with:
AX = 0001H
Returns:
Nothing
Note:
■ A counter is maintained which is decremented by calls to Int 33H
Function 02H (Hide Mouse Pointer) and incremented (if nonzero) by this
function. When the counter is zero or becomes zero, the mouse pointer is
displayed. When the mouse driver is reset with Int 33H Function 00H,
the counter is forced to -1.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 02H
Hide mouse pointer
────────────────────────────────────────────────────────────────────────────
Removes the mouse pointer from the display. The driver continues to track
the mouse position.
Call with:
AX = 0002H
Returns:
Nothing
Note:
■ A counter is maintained which is decremented by calls to this function
and incremented (if nonzero) by Int 33H Function 01H (Show Mouse
Pointer). When the counter is zero, the mouse pointer is displayed. When
the mouse driver is reset with Int 33H Function 00H, the counter is
forced to -1.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 03H
Get mouse position and button status
────────────────────────────────────────────────────────────────────────────
Returns the current mouse button status and pointer position.
Call with:
AX = 0003H
Returns:
BX = mouse button status
Bit(s) Significance (if set)
0 left button is down
1 right button is down
2 center button is down
3─15 reserved (0)
CX = horizontal (X) coordinate
DX = vertical (Y) coordinate
Note:
■ Coordinates are returned in pixels regardless of the current display
mode. Position (x,y) = (0,0) is the upper left corner of the screen.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 04H
Set mouse pointer position
────────────────────────────────────────────────────────────────────────────
Sets the position of the mouse pointer. The pointer is displayed at the
new position unless it has been hidden with Int 33H Function 02H, or the
new position lies within an exclusion area defined with Int 33H Function
10H.
Call with:
AX = 0004H
CX = horizontal (X) coordinate
DX = vertical (Y) coordinate
Returns:
Nothing
Notes:
■ Coordinates are specified in pixels regardless of the current display
mode. Position (x,y) = (0,0) is the upper left corner of the screen.
■ The position is adjusted if necessary to lie within the horizontal and
vertical limits specified with a previous call to Int 33H Functions
07H and 08H.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 05H
Get button press information
────────────────────────────────────────────────────────────────────────────
Returns the current status of all mouse buttons, and the number of presses
and position of the last press for a specified mouse button since the last
call to this function for that button. The press counter for the button is
reset to zero.
Call with:
AX = 0005H
BX = button identifier
0 = left button
1 = right button
2 = center button
Returns:
AX = button status
Bit(s) Significance (if set)
0 left button is down
1 right button is down
2 center button is down
3─15 reserved (0)
BX = button press counter
CX = horizontal (X) coordinate of last button press
DX = vertical (Y) coordinate of last button press
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 06H
Get button release information
────────────────────────────────────────────────────────────────────────────
Returns the current status of all mouse buttons, and the number of
releases and position of the last release for a specified mouse button
since the last call to this function for that button. The release counter
for the button is reset to zero.
Call with:
AX = 0006H
BX = button identifier
0 = left button
1 = right button
2 = center button
Returns:
AX = button status
Bit(s) Significance (if set)
0 left button is down
1 right button is down
2 center button is down
3─15 reserved (0)
BX = button release counter
CX = horizontal (X) coordinate of last button release
DX = vertical (Y) coordinate of last button release
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 07H
Set horizontal limits for pointer
────────────────────────────────────────────────────────────────────────────
Limits the mouse pointer display area by assigning minimum and maximum
horizontal (X) coordinates for the mouse pointer.
Call with:
AX = 0007H
CX = minimum horizontal (X) coordinate
DX = maximum horizontal (X) coordinate
Returns:
Nothing
Notes:
■ If the minimum value is greater than the maximum value, the two values
are swapped.
■ The mouse pointer will be moved if necessary so that it lies within the
specified horizontal coordinates.
■ See also Int 33H Function 10H, which defines an exclusion area for the
mouse pointer.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 08H
Set vertical limits for pointer
────────────────────────────────────────────────────────────────────────────
Limits the mouse pointer display area by assigning minimum and maximum
vertical (Y) coordinates for the mouse pointer.
Call with:
AX = 0008H
CX = minimum vertical (Y) coordinate
DX = maximum vertical (Y) coordinate
Returns:
Nothing
Notes:
■ If the minimum value is greater than the maximum value, the two values
are swapped.
■ The mouse pointer will be moved if necessary so that it lies within the
specified vertical coordinates.
■ See also Int 33H Function 10H, which defines an exclusion area for the
mouse pointer.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 09H
Set graphics pointer shape
────────────────────────────────────────────────────────────────────────────
Defines the shape, color, and hot spot of the mouse pointer in graphics
modes.
Call with:
AX = 0009H
BX = hot spot offset from left
CX = hot spot offset from top
ES:DX = segment:offset of pointer image buffer
Returns:
Nothing
Notes:
■ The pointer image buffer is 64 bytes long. The first 32 bytes contain a
bit mask which is ANDed with the screen image, and the second 32 bytes
contain a bit mask which is XORed with the screen image.
■ The hot spot is relative to the upper left corner of the pointer image,
and each pixel offset must be in the range -16 through 16. In display
modes 4 and 5, the horizontal offset must be an even number.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 0AH (10)
Set text pointer type
────────────────────────────────────────────────────────────────────────────
Defines the shape and attributes of the mouse pointer in text modes.
Call with:
AX = 000AH
BX = pointer type
0 = software cursor
1 = hardware cursor
CX = AND mask value (if BX = 0) or
starting line for cursor (if BX = 1)
DX = XOR mask value (if BX = 0) or
ending line for cursor (if BX = 1)
Returns:
Nothing
Notes:
■ If the software text cursor is selected (BX = 0), the masks in CX and DX
are mapped as follows:
Bit(s) Significance
────────────────────────────────────────────────────────────────────────
0─7 character code
8─10 foreground color
11 intensity
12─14 background color
15 blink
────────────────────────────────────────────────────────────────────────
For example, the following values would yield a software mouse cursor
that inverts the foreground and background colors:
AX = 000AH
BX = 0000H
CX = 77FFH
DX = 7700H
■ When the hardware text cursor is selected (BX = 1), the values in CX and
DX are the starting and ending scan lines for the blinking cursor
generated by the video adapter. The maximum scan line which may be used
depends on the type of adapter and the current display mode.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 0BH (11)
Read mouse motion counters
────────────────────────────────────────────────────────────────────────────
Returns the net mouse displacement since the last call to this function.
The returned value is in mickeys; a positive number indicates travel to
the right or downwards, a negative number indicates travel to the left or
upwards. One mickey represents approximately 1/200 of an inch of mouse
movement.
Call with:
AX = 000BH
Returns:
CX = horizontal (X) mickey count
DX = vertical (Y) mickey count
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 0CH (12)
Set user-defined mouse event handler
────────────────────────────────────────────────────────────────────────────
Sets the address and event mask for an application program's mouse event
handler. The handler is called by the mouse driver whenever the specified
mouse events occur.
Call with:
AX = 000CH
CX = event mask
Bit(s) Significance (if set)
0 mouse movement
1 left button pressed
2 left button released
3 right button pressed
4 right button released
5 center button pressed
6 center button released
7─15 reserved (0)
ES:DX = segment:offset of handler
Returns:
Nothing
Notes:
■ The user-defined event handler is entered from the mouse driver by a far
call with registers set up as follows:
AX mouse event flags (see event mask)
BX button state
Bit(s) Significance (if set)
0 left button is down
1 right button is down
2 center button is down
3─15 reserved (0)
CX horizontal (X) pointer coordinate
DX vertical (Y) pointer coordinate
SI last raw vertical mickey count
DI last raw horizontal mickey count
DS mouse driver data segment
■ If an event does not generate a call to the user-defined handler because
its bit is not set in the event mask, it is still reported in the event
flags during calls to the handler for events which are enabled.
■ Calls to the handler are disabled with Int 33H Function 00H or by
calling this function with an event mask of zero.
■ See also Int 33H Functions 14H and 18H.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 0DH (13)
Turn on light pen emulation
────────────────────────────────────────────────────────────────────────────
Enables light pen emulation by the mouse driver for IBM BASIC. A "pen
down" condition is created by pressing the left and right mouse buttons
simultaneously.
Call with:
AX = 000DH
Returns:
Nothing
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 0EH (14)
Turn off light pen emulation
────────────────────────────────────────────────────────────────────────────
Disables light pen emulation by the mouse driver for IBM BASIC.
Call with:
AX = 000EH
Returns:
Nothing
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 0FH (15)
Set mickeys to pixels ratio
────────────────────────────────────────────────────────────────────────────
Sets the number of mickeys per 8 pixels for horizontal and vertical mouse
motion. One mickey represents approximately 1/200 of an inch of mouse
travel.
Call with:
AX = 000FH
CX = horizontal mickeys (1─32,767, default = 8)
DX = vertical mickeys (1─32,767, default = 16)
Returns:
Nothing
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 10H (16)
Set mouse pointer exclusion area
────────────────────────────────────────────────────────────────────────────
Defines an exclusion area for the mouse pointer. When the mouse pointer
lies within the specified area, it is not displayed.
Call with:
AX = 0010H
CX = upper left X coordinate
DX = upper left Y coordinate
SI = lower right X coordinate
DI = lower right Y coordinate
Returns:
Nothing
Note:
■ The exclusion area is replaced by another call to this function or
cancelled by Int 33H Functions 00H or 01H.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 13H (19)
Set double speed threshold
────────────────────────────────────────────────────────────────────────────
Sets the threshold speed for doubling pointer motion on the screen. The
default threshold speed is 64 mickeys/second.
Call with:
AX = 0013H
DX = threshold speed in mickeys/second
Returns:
Nothing
Note:
■ Doubling of pointer motion can be effectively disabled by setting the
threshold to a very large value (such as 10,000).
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 14H (20)
Swap user-defined mouse event handlers
────────────────────────────────────────────────────────────────────────────
Sets the address and event mask for an application program's mouse event
handler and returns the address and event mask for the previous handler.
The newly installed handler is called by the mouse driver whenever the
specified mouse events occur.
Call with:
AX = 0014H
CX = event mask
Bit(s) Significance (if set)
0 mouse movement
1 left button pressed
2 left button released
3 right button pressed
4 right button released
5 center button pressed
6 center button released
7─15 reserved (0)
ES:DX = segment:offset of event handler
Returns:
CX = previous event mask
ES:DX = segment:offset of previous handler
Notes:
■ The Notes for Int 33H Function 0CH describe the information passed to
the user-defined event handler. See also Int 33H Function 18H.
■ Calls to the event handler are disabled with Int 33H Function 00H or by
setting an event mask of zero.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 15H (21)
Get mouse save state buffer size
────────────────────────────────────────────────────────────────────────────
Gets the size of the buffer required to store the current state of the
mouse driver.
Call with:
AX = 0015H
Returns:
BX = buffer size (bytes)
Note:
■ See also Int 33H Functions 16H and 17H.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 16H (22)
Save mouse driver state
────────────────────────────────────────────────────────────────────────────
Saves the mouse driver state in a user buffer. The minimum size for the
buffer must be determined by a previous call to Int 33H Function 15H.
Call with:
AX = 0016H
ES:DX = segment:offset of buffer
Returns:
Nothing
Note:
■ Call this function before executing a child program with Int 21H
Function 4BH (EXEC), in case the child also uses the mouse. After the
EXEC call, restore the previous mouse driver state with Int 33H Function
17H.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 17H (23)
Restore mouse driver state
────────────────────────────────────────────────────────────────────────────
Restores the mouse driver state from a user buffer.
Call with:
AX = 0017H
ES:DX = segment:offset of buffer
Returns:
Nothing
Note:
■ The mouse driver state must have been previously saved into the same
buffer with Int 33H Function 16H. The format of the data in the buffer
is undocumented and subject to change.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 18H (24)
Set alternate mouse event handler
────────────────────────────────────────────────────────────────────────────
Sets the address and event mask for a an application program mouse event
handler. As many as three handlers with distinct event masks can be
registered with this function. When an event occurs that matches one of
the masks, the corresponding handler is called by the mouse driver.
Call with:
AX = 0018H
CX = event mask
Bit(s) Significance (if set)
0 mouse movement
1 left button pressed
2 left button released
3 right button pressed
4 right button released
5 Shift key pressed during button press or release
6 Ctrl key pressed during button press or release
7 Alt key pressed during button press or release
8─15 reserved (0)
ES:DX = segment:offset of handler
Returns:
If function successful
AX = 0018H
If function unsuccessful
AX = FFFFH
Notes:
■ When this function is called, at least one of the bits 5, 6, and 7 must
be set in register CX.
■ The user-defined event handler is entered from the mouse driver by a far
call with registers set up as follows:
AX mouse event flags (see event mask)
BX button state
Bit(s) Significance (if set)
0 left button is down
1 right button is down
2 center button is down
3─15 reserved (0)
CX horizontal (X) pointer coordinate
DX vertical (Y) pointer coordinate
SI last raw vertical mickey count
DI last raw horizontal mickey count
DS mouse driver data segment
■ If an event does not generate a call to the user-defined handler because
its bit is not set in the event mask, it can still be reported in the
event flags during calls to the handler for events that are enabled.
■ Calls to the handler are disabled with Int 33H Function 00H.
■ See also Int 33H Functions 0CH and 14H.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 19H (25)
Get address of alternate mouse event handler
────────────────────────────────────────────────────────────────────────────
Returns the address for the mouse event handler matching the specified
event mask.
Call with:
AX = 0019H
CX = event mask (see Int 33H Function 18H)
Returns:
If function successful
CX = event mask
ES:DX = segment:offset of alternate event handler
If function unsuccessful (no handler installed or event mask does not
match any installed handler)
CX = 0000H
Note:
■ Int 33H Function 18H allows as many as three event handlers with
distinct event masks to be installed. This function can be called to
search for a handler that matches a specific event, so that it can be
replaced or disabled.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 1AH (26)
Set mouse sensitivity
────────────────────────────────────────────────────────────────────────────
Sets the number of mickeys per 8 pixels for horizontal and vertical mouse
motion and the threshold speed for doubling pointer motion on the screen.
One mickey represents approximately 1/200 of an inch of mouse travel.
Call with:
AX = 001AH
BX = horizontal mickeys (1─32,767, default = 8)
CX = vertical mickeys (1─32,767, default = 16)
DX = double speed threshold in mickeys/second (default = 64)
Returns:
Nothing
Note:
■ See also Int 33H Functions 0FH and 13H, which allow the mickeys to
pixels ratio and threshold speed to be set separately, and Int 33H
Function 1BH, which returns the current sensitivity values.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 1BH (27)
Get mouse sensitivity
────────────────────────────────────────────────────────────────────────────
Returns the current mickeys to pixels ratios for vertical and horizontal
screen movement and the threshold speed for doubling of pointer motion.
Call with:
AX = 001BH
Returns:
BX = horizontal mickeys (1─32,767, default = 8)
CX = vertical mickeys (1─32,767, default = 16)
DX = double speed threshold in mickeys/second (default = 64)
Note:
■ See also Int 33H Functions 0FH, 13H, and 1AH.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 1CH (28)
Set mouse interrupt rate
────────────────────────────────────────────────────────────────────────────
Sets the rate at which the mouse driver polls the status of the mouse.
Faster rates provide better resolution in graphics mode but may degrade
the performance of application programs.
Call with:
AX = 001CH
BX = interrupt rate flags
Bit(s) Significance
0 no interrupts allowed
1 30 interrupts/second
2 50 interrupts/second
3 100 interrupts/second
4 200 interrupts/second
5─15 reserved (0)
Returns:
Nothing
Notes:
■ This function is applicable for the InPort Mouse only.
■ If more than one bit is set in register BX, the lowest order bit
prevails.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 1DH (29)
Select pointer page
────────────────────────────────────────────────────────────────────────────
Selects the display page for the mouse pointer.
Call with:
AX = 001DH
BX = page
Returns:
Nothing
Note:
■ The valid page numbers depend on the current display mode. See Int 10H
Function 05H.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 1EH (30)
Get pointer page
────────────────────────────────────────────────────────────────────────────
Returns the current display page for the mouse pointer.
Call with:
AX = 001EH
Returns:
BX = page
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 1FH (31)
Disable mouse driver
────────────────────────────────────────────────────────────────────────────
Disables the mouse driver and returns the address of the previous Int 33H
handler.
Call with:
AX = 001FH
Returns:
If function successful
AX = 001FH
ES:BX = segment:offset of previous Int 33H handler
If function unsuccessful
AX = FFFFH
Notes:
■ When this function is called, the mouse driver releases any interrupt
vectors it has captured other than Int 33H (which may include Int 10H,
Int 71H, and/or Int 74H). The application program can complete the
process of logically removing the mouse driver by restoring the original
contents of the Int 33H vector with Int 21H Function 25H, using the
address returned by this function in ES:BX.
■ See also Int 33H Function 20H.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 20H (32)
Enable mouse driver
────────────────────────────────────────────────────────────────────────────
Enables the mouse driver and the servicing of mouse interrupts.
Call with:
AX = 0020H
Returns:
Nothing
Note:
■ See also Int 33H Function 1FH.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 21H (33)
Reset mouse driver
────────────────────────────────────────────────────────────────────────────
Resets the mouse driver and returns driver status. If the mouse pointer
was previously visible, it is removed from the screen, and any previously
installed user handlers for mouse events are disabled.
Call with:
AX = 0021H
Returns:
If mouse support is available
AX = FFFFH
BX = number of mouse buttons
If mouse support is not available
AX = 0021H
Note:
■ This function differs from Int 33H Function 00H in that there is no
initialization of the mouse hardware.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 22H (34)
Set language for mouse driver messages
────────────────────────────────────────────────────────────────────────────
Selects the language that will be used by the mouse driver for prompts and
error messages.
Call with:
AX = 0022H
BX = language number
0 = English
1 = French
2 = Dutch
3 = German
4 = Swedish
5 = Finnish
6 = Spanish
7 = Portuguese
8 = Italian
Returns:
Nothing
Note:
■ This function is only available in international versions of the
Microsoft Mouse driver.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 23H (35)
Get language number
────────────────────────────────────────────────────────────────────────────
Returns the number of the language that is used by the mouse driver for
prompts and error messages.
Call with:
AX = 0023H
Returns:
BX = language number (see Int 33H Function 22H)
Note:
■ This function is only available in international versions of the
Microsoft Mouse driver.
────────────────────────────────────────────────────────────────────────────
Int 33H
Function 24H (36)
Get mouse information
────────────────────────────────────────────────────────────────────────────
Returns the mouse driver version number, mouse type, and the IRQ number of
the interrupt used by the mouse adapter.
Call with:
AX = 0024H
Returns:
BH = major version number (6 for version 6.10, etc.)
BL = minor version number (0AH for version 6.10, etc.)
CH = mouse type
1 = bus mouse
2 = serial mouse
3 = InPort mouse
4 = PS/2 mouse
5 = HP mouse
CL = IRQ number
0 = PS/2
2, 3, 4, 5, or 7 = IRQ number
────────────────────────────────────────────────────────────────────────────
SECTION 4 LOTUS/INTEL/MICROSOFT EMS FUNCTIONS REFERENCE
────────────────────────────────────────────────────────────────────────────
Notes to the Reader
The Lotus/Intel/Microsoft Expanded Memory Specification (EMS) defines a
hardware/software subsystem, compatible with 80x86-based microcomputers
running MS-DOS, that allows applications to access as much as 32 MB of
bank-switched random-access memory. The software component, called the
Expanded Memory Manager (EMM), is installed during system initialization
by a DEVICE= directive in the CONFIG.SYS file in the root directory on the
boot disk.
After ensuring that the EMM is present (see Chapter 11), an application
program communicates directly with the EMM using software interrupt 67H. A
particular EMM function is selected by the value in register AH and a
success or error status is returned in register AH (error codes are listed
on pages 207─209). Other parameters and results are passed or returned in
registers or buffers.
An icon in each function heading indicates the EMS version in which that
function was first supported. You can assume that the function is
available in all subsequent EMS versions unless explicitly noted
otherwise.
Version icons used in the synopsis, parameters, results, or Notes section
refer to specific minor or major EMS versions, unless they include a +
sign to indicate a version and all subsequent versions.
The material in this section has been verified against the Expanded Memory
Specification version 4.0, dated October 1987, Intel part number
300275-005. This document can be obtained from Intel Corporation, 5200
N.E. Elam Young Parkway, Hillsboro, OR 97124.
Summary of EMM Functions
╓┌─┌────────────┌───────────────┌────────────────────────────────────────────╖
Function Subfunction Description
──────────────────────────────────────────────────────────────────────────
40H (64) Get Status
41H (65) Get Page Frame Address
42H (66) Get Number of Pages
43H (67) Allocate Handle and Pages
44H (68) Map Expanded Memory Page
45H (69) Release Handle and Expanded Memory
46H (70) Get Version
47H (71) Save Page Map
48H (72) Restore Page Map
49H (73) Reserved
4AH (74) Reserved
4BH (75) Get Handle Count
4CH (76) Get Handle Pages
4DH (77) Get Pages for All Handles
4EH (78) 00H Save Page Map
4EH (78) 01H Restore Page Map
4EH (78) 02H Save and Restore Page Map
4EH (78) 03H Get Size of Page Map Information
4FH (79) 00H Save Partial Page Map
Function Subfunction Description
──────────────────────────────────────────────────────────────────────────
4FH (79) 00H Save Partial Page Map
4FH (79) 01H Restore Partial Page Map
4FH (79) 02H Get Size of Partial Page Map Information
50H (80) 00H Map Multiple Pages by Number
50H (80) 01H Map Multiple Pages by Address
51H (81) Reallocate Pages for Handle
52H (82) 00H Get Handle Attribute
52H (82) 01H Set Handle Attribute
52H (82) 02H Get Attribute Capability
53H (83) 00H Get Handle Name
53H (83) 01H Set Handle Name
54H (84) 00H Get All Handle Names
54H (84) 01H Search for Handle Name
54H (84) 02H Get Total Handles
55H (85) 00H Map Pages by Number and Jump
55H (85) 01H Map Pages by Address and Jump
56H (86) 00H Map Pages by Number and Call
56H (86) 01H Map Pages by Address and Call
56H (86) 02H Get Space for Map Page and Call
Function Subfunction Description
──────────────────────────────────────────────────────────────────────────
56H (86) 02H Get Space for Map Page and Call
57H (87) 00H Move Memory Region
57H (87) 01H Exchange Memory Regions
58H (88) 00H Get Addresses of Mappable Pages
58H (88) 01H Get Number of Mappable Pages
59H (89) 00H Get Hardware Configuration
59H (89) 01H Get Number of Raw Pages
5AH (90) 00H Allocate Handle and Standard Pages
5AH (90) 01H Allocate Handle and Raw Pages
5BH (91) 00H Get Alternate Map Registers
5BH (91) 01H Set Alternate Map Registers
5BH (91) 02H Get Size of Alternate Map Register Save Area
5BH (91) 03H Allocate Alternate Map Register Set
5BH (91) 04H Deallocate Alternate Map Register Set
5BH (91) 05H Allocate DMA Register Set
5BH (91) 06H Enable DMA on Alternate Map Register Set
5BH (91) 07H Disable DMA on Alternate Map Register Set
5BH (91) 08H Deallocate DMA Register Set
5CH (92) Prepare Expanded Memory Manager for Warm Boot
Function Subfunction Description
──────────────────────────────────────────────────────────────────────────
5CH (92) Prepare Expanded Memory Manager for Warm Boot
5DH (93) 00H Enable EMM Operating-System Functions
5DH (93) 01H Disable EMM Operating-System Functions
5DH (93) 02H Release Access Key
──────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 40H (64)
Get status
────────────────────────────────────────────────────────────────────────────
Returns a status code indicating whether the expanded memory software and
hardware are present and functional.
Call with:
AH = 40H
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Note:
■ This call should be used only after an application has established that
the Expanded Memory Manager is in fact present, using one of the
techniques described in Chapter 11.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 41H (65)
Get page frame address
────────────────────────────────────────────────────────────────────────────
Returns the segment address of the page frame used by the Expanded Memory
Manager.
Call with:
AH = 41H
Returns:
If function successful
AH = 00H
BX = segment base of page frame
If function unsuccessful
AH = error code
Notes:
■ The page frame is divided into four 16 KB pages, which are used to map
logical expanded memory pages into the physical memory space of the CPU.
■ The application need not have already acquired an EMM handle to use this
function.
■ [EMS 4.0] Mapping of expanded memory pages is not necessarily limited to
the 64 KB page frame. See also Int 67H Function 58H Subfunction 00H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 42H (66)
Get number of pages
────────────────────────────────────────────────────────────────────────────
Obtains the total number of logical expanded memory pages present in the
system and the number of pages that are not already allocated.
Call with:
AH = 42H
Returns:
If function successful
AH = 00H
BX = unallocated pages
DX = total pages
If function unsuccessful
AH = error code
Notes:
■ The application need not have already acquired an EMM handle to use this
function.
■ [EMS 4.0] See also Int 67H Function 59H Subfunction 01H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 43H (67)
Allocate handle and pages
────────────────────────────────────────────────────────────────────────────
Obtains an EMM handle and allocates logical pages of expanded memory to be
controlled by that handle.
Call with:
AH = 43H
BX = number of pages to allocate (must be nonzero)
Returns:
If function successful
AH = 00H
DX = EMM handle
If function unsuccessful
AH = error code
Notes:
■ This is the equivalent of a file open function for the expanded memory
manager. The handle that is returned is analogous to a file handle and
owns a certain number of expanded memory pages. The handle must be used
with every subsequent request to map memory and must be released by a
close operation before the application terminates.
■ This function may fail because there are no handles left to allocate or
because there is an insufficient number of expanded memory pages to
satisfy the request. In the latter case, Int 67H Function 42H can be
used to determine the actual number of pages available.
■ [EMS 4.0] Int 67H Function 51H can be called to change the number of
pages allocated to an EMM handle.
■ [EMS 4.0] The pages allocated by this function are always 16 KB for
compatibility with earlier versions of EMS. See also Int 67H Function
5AH Subfunctions 00H and 01H.
■ [EMS 4.0] Handle 0000H is always available for use by the operating
system, and a prior call to this function is not required. The operating
system must call Int 67H Function 51H to assign the desired number of
pages to its reserved handle.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 44H (68)
Map expanded memory page
────────────────────────────────────────────────────────────────────────────
Maps one of the logical pages of expanded memory assigned to a handle onto
a physical memory page that can be accessed by the CPU.
Call with:
AH = 44H
AL = physical page
BX = logical page
DX = EMM handle
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The logical page number is in the range {0...n-1}, where n is the number
of pages allocated or reallocated to the handle by a previous call to
Int 67H Function 43H, 51H, or 5AH. Logical pages allocated by Int 67H
Function 43H or Function 5AH Subfunction 00H are always 16 KB long;
logical pages allocated by Int 67H Function 5AH Subfunction 01H are
referred to as raw pages and are not necessarily 16 KB.
■ [EMS 3] The physical page is in the range 0─3 and lies within the EMM
page frame, whose base address is obtained from Int 67H Function 41H.
■ [EMS 4.0] A list of the available physical pages and their addresses may
be obtained from Int 67H Function 58H Subfunction 00H.
■ [EMS 4.0] If this function is called with BX = -1, the specified
physical page is unmapped (made inaccessible for reading or writing).
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 45H (69)
Release handle and expanded memory
────────────────────────────────────────────────────────────────────────────
Deallocates the expanded memory pages assigned to a handle and then
releases the handle.
Call with:
AH = 45H
DX = EMM handle
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ If this function is not called before a program terminates, the EMS
pages it owned remain unavailable until the system is restarted.
Programs that use EMS should install their own Ctrl-C handlers and
critical-error handlers (Ints 23H and 24H) so that they cannot be
terminated unexpectedly.
■ [EMS 4.0] When a handle is released, its name is set to all ASCII nulls.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 46H (70)
Get version
────────────────────────────────────────────────────────────────────────────
Returns the EMS version supported by the expanded memory manager.
Call with:
AH = 46H
Returns:
If function successful
AH = 00H
AL = version number
If function unsuccessful
AH = error code
Notes:
■ The version number is returned in binary code decimal (BCD) format, with
the integer portion in the upper 4 bits of AL and the fractional portion
in the lower 4 bits. For example, under an EMM that supports EMS version
3.2, AL is returned as the value 32H.
■ Applications should always check the EMM version number to ensure that
all of the EMM functions they require are available.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 47H (71)
Save page map
────────────────────────────────────────────────────────────────────────────
Saves the contents of the page-mapping registers on the expanded memory
hardware, associating those contents with a particular EMM handle.
Call with:
AH = 47H
DX = EMM handle
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ This function is used by interrupt handlers or device drivers that must
access expanded memory. The EMM handle supplied to this function is the
handle that was assigned to the handler or driver during its own
initialization sequence, not to the program that was interrupted.
■ The mapping context is restored by a subsequent call to Int 67H Function
48H.
■ [EMS 4.0] This function saves only the mapping state for the 64 KB page
frame defined in EMS 3. Programs that are written to take advantage of
the additional capabilities of EMS 4.0 should use Int 67H Function 4EH
or 4FH in preference to this function.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 48H (72)
Restore page map
────────────────────────────────────────────────────────────────────────────
Restores the contents of the page-mapping registers on the expanded memory
hardware to the values associated with the specified handle by a previous
call to Int 67H Function 47H.
Call with:
AH = 48H
DX = EMM handle
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ This function is used by interrupt handlers or device drivers that must
access expanded memory. The EMM handle supplied to this function is the
handle that was assigned to the handler or driver during its own
initialization sequence, not to the program that was interrupted.
■ [EMS 4.0] This function restores only the mapping state for the 64 KB
page frame defined in EMS 3. Programs that are written to take advantage
of the additional capabilities of EMS 4.0 should use Int 67H Function
4EH or 4FH in preference to this function.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 49H (73)
Reserved
────────────────────────────────────────────────────────────────────────────
This function was defined in EMS version 3.0 but is not documented for
later EMS versions, so it should be avoided in application programs.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 4AH (74)
Reserved
────────────────────────────────────────────────────────────────────────────
This function was defined in EMS version 3.0 but is not documented for
later EMS versions, so it should be avoided in application programs.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 4BH (75)
Get handle count
────────────────────────────────────────────────────────────────────────────
Returns the number of active expanded memory handles.
Call with:
AH = 4BH
Returns:
If function successful
AH = 00H
BX = number of active EMM handles
If function unsuccessful
AH = error code
Notes:
■ If the returned number of EMM handles is zero, the expanded memory
manager is idle, and none of the expanded memory is in use.
■ The value returned by this function is not necessarily the same as the
number of programs using expanded memory because one program may own
multiple EMM handles.
■ The number of active EMM handles never exceeds 255.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 4CH (76)
Get handle pages
────────────────────────────────────────────────────────────────────────────
Returns the number of expanded memory pages allocated to a specific EMM
handle.
Call with:
AH = 4CH
DX = EMM handle
Returns:
If function successful
AH = 00H
BX = number of EMM pages
If function unsuccessful
AH = error code
Notes:
■ [EMS 3] The total number of pages allocated to a handle never exceeds
512. A handle never has zero pages allocated to it.
■ [EMS 4.0] The total number of pages allocated to a handle never exceeds
2048. A handle may have zero pages of expanded memory.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.0]
Function 4DH (77)
Get pages for all handles
────────────────────────────────────────────────────────────────────────────
Returns an array that contains all the active handles and the number of
expanded memory pages associated with each handle.
Call with:
AH = 4DH
ES:DI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
BX = number of active EMM handles
and buffer filled in as described in Notes
If function unsuccessful
AH = error code
Notes:
■ The buffer is filled in with a series of DWORD (32-bit) entries, one per
active EMM handle. The first word of an entry contains the handle, and
the second word contains the number of pages allocated to that handle.
■ The maximum number of active handles is 256 (including the operating
system handle 0), so a buffer size of 1024 bytes is adequate in all
cases.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.2]
Function 4EH (78) Subfunction 00H
Save page map
────────────────────────────────────────────────────────────────────────────
Saves the current page-mapping state of the expanded memory hardware in
the specified buffer.
Call with:
AH = 4EH
AL = 00H
ES:DI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
and buffer filled in with mapping information (see Notes)
If function unsuccessful
AH = error code
Notes:
■ The buffer receives the information necessary to restore the state of
the mapping registers using Int 67H Function 4EH Subfunction 01H. The
format of the information may vary.
■ The size of the buffer required by this function can be determined with
Int 67H Function 4EH Subfunction 03H.
■ Unlike Int 67H Function 47H, this function does not require a handle.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.2]
Function 4EH (78) Subfunction 01H
Restore page map
────────────────────────────────────────────────────────────────────────────
Restores the page-mapping state of the expanded memory hardware using the
information in the specified buffer.
Call with:
AH = 4EH
AL = 01H
DS:SI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The buffer contains information necessary to restore the state of the
mapping registers from a previous call to Int 67H Function 4EH
Subfunction 00H or 02H. The format of the information may vary.
■ Unlike Int 67H Function 48H, this function does not require a handle.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.2]
Function 4EH (78) Subfunction 02H
Save and restore page map
────────────────────────────────────────────────────────────────────────────
Saves the current page-mapping state of the expanded memory hardware in a
buffer and then sets the mapping state using the information in another
buffer.
Call with:
AH = 4EH
AL = 02H
DS:SI = segment:offset of buffer containing mapping information
(see Notes)
ES:DI = segment:offset of buffer to receive mapping information
(see Notes)
Returns:
If function successful
AH = 00H
and buffer pointed to by ES:DI filled in with mapping information (see
Notes)
If function unsuccessful
AH = error code
Notes:
■ The buffer addressed by DS:SI contains information necessary to restore
the state of the mapping registers from a previous call to Int 67H
Function 4EH Subfunction 00H or 02H. The format of the information may
vary.
■ The sizes of the buffers required by this function can be determined
with Int 67H Function 4EH Subfunction 03H.
■ Unlike Int 67H Functions 47H and 48H, this function does not require a
handle.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 3.2]
Function 4EH (78) Subfunction 03H
Get size of page map information
────────────────────────────────────────────────────────────────────────────
Returns the size of the buffer that is required to receive page-mapping
information using Int 67H Function 4EH Subfunctions 00H and 02H.
Call with:
AH = 4EH
AL = 03H
Returns:
If function successful
AH = 00H
AL = size of buffer (bytes)
If function unsuccessful
AH = error code
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 4FH (79) Subfunction 00H
Save partial page map
────────────────────────────────────────────────────────────────────────────
Saves the state of a subset of the expanded memory page-mapping registers
in the specified buffer.
Call with:
AH = 4FH
AL = 00H
DS:SI = segment:offset of map list (see Notes)
ES:DI = segment:offset of buffer to receive mapping state (see
Notes)
Returns:
If function successful
AH = 00H
and buffer filled in with mapping information (see Notes)
If function unsuccessful
AH = error code
Notes:
■ The map list contains the number of mappable segments in the first word,
followed by the segment addresses of the mappable memory regions (one
segment per word).
■ To determine the size of the buffer required for the mapping state, use
Int 67H Function 4FH Subfunction 02H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 4FH (79) Subfunction 01H
Restore partial page map
────────────────────────────────────────────────────────────────────────────
Restores the state of a subset of the expanded memory page-mapping
registers.
Call with:
AH = 4FH
AL = 01H
DS:SI = segment:offset of buffer (see Note)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Note:
■ The buffer contains mapping information and must have been prepared by a
previous call to Int 67H Function 4FH Subfunction 00H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 4FH (79) Subfunction 02H
Get size of partial page map information
────────────────────────────────────────────────────────────────────────────
Returns the size of the buffer which will be required to receive partial
page-mapping information using Int 67H Function 4FH Subfunction 00H.
Call with:
AH = 4FH
AL = 02H
BX = number of pages
Returns:
If function successful
AH = 00H
AL = size of array (bytes)
If function unsuccessful
AH = error code
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 50H (80) Subfunction 00H
Map multiple pages by number
────────────────────────────────────────────────────────────────────────────
Maps one or more of the logical expanded memory pages assigned to a handle
onto physical memory pages that can be accessed by the CPU. Physical pages
are referenced by their numbers.
Call with:
AH = 50H
AL = 00H
CX = number of pages to map
DX = EMM handle
DS:SI = segment:offset of buffer (see Note)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Note:
■ The buffer contains a series of DWORD (32-bit) entries that control the
pages to be mapped. The first word of each entry contains the logical
expanded memory page number, and the second word contains the physical
page number to which it should be mapped. If the logical page is -1, the
physical page is unmapped (made inaccessible for reading or writing).
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 50H (80) Subfunction 01H
Map multiple pages by address
────────────────────────────────────────────────────────────────────────────
Maps one or more of the logical expanded memory pages assigned to a handle
onto physical memory pages that can be accessed by the CPU. Physical pages
are referenced by their segment addresses.
Call with:
AH = 50H
AL = 01H
CX = number of pages to map
DX = EMM handle
DS:SI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The buffer contains a series of DWORD (32-bit) entries that control the
pages to be mapped. The first word of each entry contains the logical
page number, and the second word contains the physical page segment
address to which it should be mapped. If the logical page is -1, the
physical page is unmapped (made inaccessible for reading or writing).
■ The mappable segment addresses may be obtained by calling Int 67H
Function 58H Subfunction 00H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 51H (81)
Reallocate pages for handle
────────────────────────────────────────────────────────────────────────────
Modifies the number of expanded memory pages allocated to an EMM handle.
Call with:
AH = 51H
BX = new number of pages
DX = EMM handle
Returns:
If function successful
AH = 00H
BX = logical pages owned by EMM handle
If function unsuccessful
AH = error code
Note:
■ If the requested number of pages is zero, the handle is still active,
and pages can be reallocated to the handle at a later time; also, the
handle must still be released with Int 67H Function 45H before the
application terminates.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 52H (82) Subfunction 00H
Get handle attribute
────────────────────────────────────────────────────────────────────────────
Returns the attribute (volatile or nonvolatile) associated with the
specified handle. A nonvolatile memory handle and the contents of the
expanded memory pages that are allocated to it are maintained across a
warm boot operation (system restart using Ctrl-Alt-Del).
Call with:
AH = 52H
AL = 00H
DX = EMM handle
Returns:
If function successful
AH = 00H
AL = attribute
0 = volatile
1 = nonvolatile
If function unsuccessful
AH = error code
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 52H (82) Subfunction 01H
Set handle attribute
────────────────────────────────────────────────────────────────────────────
Sets the attribute (volatile or nonvolatile) associated with the specified
handle. A nonvolatile memory handle and the contents of the expanded
memory pages that are allocated to it are maintained across a warm boot
operation (system restart using Ctrl-Alt-Del).
Call with:
AH = 52H
AL = 01H
BL = attribute
0 = volatile
1 = nonvolatile
DX = EMM handle
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Note:
■ If the expanded memory hardware cannot support nonvolatile pages, this
function returns an error.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 52H (82) Subfunction 02H
Get attribute capability
────────────────────────────────────────────────────────────────────────────
Returns a code indicating whether the Expanded Memory Manager and hardware
can support the nonvolatile attribute for EMM handles.
Call with:
AH = 52H
AL = 02H
Returns:
If function successful
AH = 00H
AL = attribute capability
0 = only volatile handles supported
1 = volatile and nonvolatile handles supported
If function unsuccessful
AH = error code
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 53H (83) Subfunction 00H
Get handle name
────────────────────────────────────────────────────────────────────────────
Returns the 8-character name assigned to a handle.
Call with:
AH = 53H
AL = 00H
DX = EMM handle
ES:DI = segment:offset of 8-byte buffer
Returns:
If function successful
AH = 00H
and name for handle in specified buffer
If function unsuccessful
AH = error code
Note:
■ A handle's name is initialized to 8 zero bytes when it is allocated or
deallocated. Another name may be assigned to an active handle with Int
67H Function 53H Subfunction 01H. The bytes in a handle name need not
be ASCII characters.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 53H (83) Subfunction 01H
Set handle name
────────────────────────────────────────────────────────────────────────────
Assigns a name to an EMM handle.
Call with:
AH = 53H
AL = 01H
DX = EMM handle
DS:SI = segment:offset of 8-byte name
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The bytes in a handle name need not be ASCII characters, but the
sequence of 8 zero bytes is reserved for no name (the default after a
handle is allocated or deallocated). A handle name should be padded with
zero bytes, if necessary, to a length of 8 bytes.
■ A handle may be renamed at any time.
■ All handle names are initialized to 8 zero bytes when the system is
turned on. The name of a nonvolatile handle is preserved across a warm
boot. (See Int 67H Function 52H Subfunctions 00H and 02H.)
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 54H (84) Subfunction 00H
Get all handle names
────────────────────────────────────────────────────────────────────────────
Returns the names for all active handles.
Call with:
AH = 54H
AL = 00H
ES:DI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
AL = number of active handles
and buffer filled in with handle-name information (see Notes)
If function unsuccessful
AH = error code
Notes:
■ The function fills the buffer with a series of 10-byte entries. The
first 2 bytes of each entry contain an EMM handle, and the next 8 bytes
contain the name associated with the handle. Handles that have never
been assigned a name have 8 bytes of 0 as a name.
■ Because there is a maximum of 255 active handles, the buffer need not be
longer than 2550 bytes.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 54H (84) Subfunction 01H
Search for handle name
────────────────────────────────────────────────────────────────────────────
Returns the EMM handle associated with the specified name.
Call with:
AH = 54H
AL = 01H
DS:SI = segment:offset of 8-byte handle name
Returns:
If function successful
AH = 00H
DX = EMM handle
If function unsuccessful
AH = error code
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 54H (84) Subfunction 02H
Get total handles
────────────────────────────────────────────────────────────────────────────
Returns the total number of handles that are supported by the Expanded
Memory Manager, including the operating-system handle (0).
Call with:
AH = 54H
AL = 02H
Returns:
If function successful
AH = 00H
BX = number of handles
If function unsuccessful
AH = error code
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 55H (85) Subfunctions 00H and 01H
Map pages and jump
────────────────────────────────────────────────────────────────────────────
Alters the expanded memory mapping context and transfers control to the
specified address.
Call with:
AH = 55H
AL = subfunction
0 = map using physical page numbers
1 = map using physical page segments
DX = EMM handle
DS:SI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The buffer contains map-and-jump entries in the following format:
Offset Length Description
────────────────────────────────────────────────────────────────────────
00H 4 far pointer to jump target
04H 1 number of pages to map before jump
05H 4 far pointer to map list (see below)
────────────────────────────────────────────────────────────────────────
The map list in turn consists of DWORD (32-bit) entries, one per page.
The first word of each entry contains the logical page number, and the
second word contains the physical page number or segment (depending on
the value in register AL) to which it should be mapped.
■ A request to map zero pages and jump is not considered an error; the
effect is a simple far jump.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 56H (86) Subfunctions 00H and 01H
Map pages and call
────────────────────────────────────────────────────────────────────────────
Alters the expanded memory mapping context and performs a far call to the
specified address. When the destination routine executes a far return, the
EMM again alters the page-mapping context as instructed and then returns
control to the original caller.
Call with:
AH = 56H
AL = subfunction
0 = map using physical page numbers
1 = map using physical page segments
DX = EMM handle
DS:SI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The format of the buffer containing map and call information is:
Offset Length Description
────────────────────────────────────────────────────────────────────────
00H 4 far pointer to call target
04H 1 number of pages to map before call
05H 4 far pointer to list of pages to map before
call (see below)
09H 1 number of pages to map before return
0AH 4 far pointer to list of pages to map before
return (see below)
0EH 8 reserved (0)
────────────────────────────────────────────────────────────────────────
Both map lists have the same format and consist of a series of
double-word entries, one per page. The first word of each entry contains
the logical page number, and the second word contains the physical page
number or segment (depending on the value in register AL) to which it
should be mapped.
■ A request to map zero pages and call is not an error; the effect is a
simple far call.
■ This function uses extra stack space to save information about the
mapping context; the amount of stack space required can be determined by
calling Int 67H Function 56H Subfunction 02H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 56H (86) Subfunction 02H
Get stack space for map page and call
────────────────────────────────────────────────────────────────────────────
Returns the number of bytes of stack space required by Int 67H Function
56H Subfunction 00H or 01H.
Call with:
AH = 56H
AL = 02H
Returns:
If function successful
AH = 00H
BX = stack space required (bytes)
If function unsuccessful
AH = error code
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 57H (87) Subfunction 00H
Move memory region
────────────────────────────────────────────────────────────────────────────
Copies a memory region from any location in conventional or expanded
memory to any other location without disturbing the current expanded
memory mapping context.
Call with:
AH = 57H
AL = 00H
DS:SI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The format of the buffer controlling the move operation is:
Offset Length Description
────────────────────────────────────────────────────────────────────────
00H 4 region length in bytes
04H 1 source memory type (0 = conventional, 1 =
expanded)
05H 2 source memory handle
07H 2 source memory offset
09H 2 source memory segment or physical page
number
0BH 1 destination memory type (0 = conventional,
1 = expanded)
0CH 2 destination memory handle
0EH 2 destination memory offset
10H 2 destination memory segment or physical page
number
────────────────────────────────────────────────────────────────────────
■ A length of zero bytes is not an error. The maximum length of a move is
1 MB. If the length exceeds a single expanded memory page, consecutive
expanded memory pages (as many as are required) supply or receive the
data.
■ If the source and destination addresses overlap, the move will be
performed in such a way that the destination receives an intact copy of
the original data, and a nonzero status is returned.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 57H (87) Subfunction 01H
Exchange memory regions
────────────────────────────────────────────────────────────────────────────
Exchanges any two memory regions in conventional or expanded memory
without disturbing the current expanded memory mapping context.
Call with:
AH = 57H
AL = 01H
DS:SI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The format of the buffer controlling the exchange operation is the same
as for Int 67H Function 57H Subfunction 00H.
■ An exchange of zero bytes is not an error. The maximum length of an
exchange is 1 MB. If the length exceeds a single expanded memory page,
consecutive expanded memory pages (as many as are required) supply or
receive the data.
■ If the source and destination addresses overlap, the exchange is not
performed and an error is returned.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 58H (88) Subfunction 00H
Get addresses of mappable pages
────────────────────────────────────────────────────────────────────────────
Returns the segment base address and physical page number for each
mappable page in the system.
Call with:
AH = 58H
AL = 00H
ES:DI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
CX = number of entries in mappable physical page array
and page number/address information in buffer (see Notes)
If function unsuccessful
AH = error code
Notes:
■ Upon return from the function, the buffer contains a series of
double-word entries, one per mappable page. The first word of an entry
contains the page's segment base address, and the second contains its
physical page number. The entries are sorted in order of ascending
segment addresses.
■ The size of the buffer required can be calculated with the information
returned by Int 67H Function 58H Subfunction 01H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 58H (88) Subfunction 01H
Get number of mappable pages
────────────────────────────────────────────────────────────────────────────
Returns the number of mappable physical pages.
Call with:
AH = 58H
AL = 01H
Returns:
If function successful
AH = 00H
CX = number of mappable physical pages
If function unsuccessful
AH = error code
Note:
■ The information returned by this function can be used to calculate the
size of the buffer that will be needed by Int 67H Function 58H
Subfunction 00H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 59H (89) Subfunction 00H
Get hardware configuration
────────────────────────────────────────────────────────────────────────────
Returns information about the configuration of the expanded memory
hardware.
Call with:
AH = 59H
AL = 00H
ES:DI = segment:offset of buffer (see Notes)
Returns:
If function successful
AH = 00H
and hardware configuration information in buffer.
If function unsuccessful
AH = error code
Notes:
■ Upon return from the function, the buffer has been filled in with
hardware configuration information in the following format:
Offset Length Description
────────────────────────────────────────────────────────────────────────
00H 2 size of raw expanded memory pages (in
paragraphs)
02H 2 number of alternate register sets
04H 2 size of mapping-context save area (in
bytes)
06H 2 number of register sets that can be
assigned to DMA channels
08H 2 DMA operation type (0 = DMA may be used
with alternate register sets; 1 = only one
DMA register set available)
────────────────────────────────────────────────────────────────────────
■ The size returned for the mapping-context save area is the same as the
size returned by Int 67H Function 4EH Subfunction 03H.
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 59H (89) Subfunction 01H
Get number of raw pages
────────────────────────────────────────────────────────────────────────────
Obtains the total number of raw expanded memory pages present in the
system and the number of raw pages that are not already allocated. Raw
memory pages may have a size other than 16 KB.
Call with:
AH = 59H
AL = 01H
Returns:
If function successful
AH = 00H
BX = unallocated raw pages
DX = total raw pages
If function unsuccessful
AH = error code
Note:
■ If the Expanded Memory Manager supports only pages of standard size, the
values returned by this function are the same as those returned by Int
67H Function 42H.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5AH (90) Subfunction 00H
Allocate handle and standard pages
────────────────────────────────────────────────────────────────────────────
Allocates an EMM handle and associates standard (16 KB) expanded memory
pages with that handle.
Call with:
AH = 5AH
AL = 00H
BX = number of standard pages to allocate
Returns:
If function successful
AH = 00H
DX = EMM handle
If function unsuccessful
AH = error code
Note:
■ Unlike Int 67H Function 43H, allocating zero pages with this function
is not an error.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5AH (90) Subfunction 01H
Allocate handle and raw pages
────────────────────────────────────────────────────────────────────────────
Allocates a raw EMM handle and associates raw expanded memory pages with
that handle.
Call with:
AH = 5AH
AL = 01H
BX = number of raw pages to allocate
Returns:
If function successful
AH = 00H
DX = handle for raw EMM pages
If function unsuccessful
AH = error code
Notes:
■ Raw memory pages may have a size other than 16 KB.
■ Allocation of zero pages is not an error.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 00H
Get alternate map registers
────────────────────────────────────────────────────────────────────────────
Returns the number of the active alternate register set or, if no
alternate set is active, saves the state of the mapping registers into a
buffer and returns its address.
Call with:
AH = 5BH
AL = 00H
Returns:
If function successful and alternate map register set active
AH = 00H
BL = current active alternate map register set
If function successful and alternate map register set not active
AH = 00H
BL = 00H
ES:DI = segment:offset of alternate map register save area
(if BL = 0)
If function unsuccessful
AH = error code
Notes:
■ The address of the save area must have been specified in a previous call
to Int 67H Function 5BH Subfunction 01H, and the save area must have
been initialized by a previous call to Int 67H Function 4EH Subfunction
00H. If there was no previous call to Int 67H Function 5BH Subfunction
01H, the address returned is zero, and the registers are not saved.
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 01H
Set alternate map registers
────────────────────────────────────────────────────────────────────────────
Selects an alternate map register set or (if alternate sets are not
supported) restores the mapping context from the specified buffer.
Call with:
AH = 5BH
AL = 01H
BL = alternate register set number or 00H
ES:DI = segment:offset of map register context restore area
(if BL = 0)
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The buffer address specified in this call is returned by subsequent
calls to Int 67H Function 5BH Subfunction 00H with BL = 00H.
■ The save area must have been initialized by a previous call to Int 67H
Function 4EH Subfunction 00H.
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 02H
Get size of alternate map register save area
────────────────────────────────────────────────────────────────────────────
Returns the amount of storage needed by Int 67H Function 5BH Subfunctions
00H and 01H.
Call with:
AH = 5BH
AL = 02H
Returns:
If function successful
AH = 00H
DX = size of buffer (bytes)
If function unsuccessful
AH = error code
Note:
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 03H
Allocate alternate map register set
────────────────────────────────────────────────────────────────────────────
Allocates an alternate map register set for use with Int 67H Function 5BH
Subfunctions 00H and 01H. The contents of the currently active map
registers are copied into the newly allocated alternate map registers in
order to provide an initial context when they are selected.
Call with:
AH = 5BH
AL = 03H
Returns:
If function successful
AH = 00H
BL = alternate map register set number or zero, if no alternate
sets are available
If function unsuccessful
AH = error code
Note:
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 04H
Deallocate alternate map register set
────────────────────────────────────────────────────────────────────────────
Releases an alternate map register set that was previously allocated with
Int 67H Function 5BH Subfunction 03H.
Call with:
AH = 5BH
AL = 04H
BL = alternate register set number
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ The current alternate map register set cannot be deallocated.
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 05H
Allocate DMA register set
────────────────────────────────────────────────────────────────────────────
Allocates a DMA register set.
Call with:
AH = 5BH
AL = 05H
Returns:
If function successful
AH = 00H
BL = DMA register set number (0 = none available)
If function unsuccessful
AH = error code
Note:
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 06H
Enable DMA on alternate map register set
────────────────────────────────────────────────────────────────────────────
Associates a DMA channel with an alternate map register set.
Call with:
AH = 5BH
AL = 06H
BL = alternate map register set
DL = DMA channel number
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ If a DMA channel is not assigned to a specific register set, DMA for
that channel will be mapped through the current register set.
■ If zero is specified as the alternate map register set, no special
action is taken on DMA accesses for the specified DMA channel.
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 07H
Disable DMA on alternate map register set
────────────────────────────────────────────────────────────────────────────
Disables DMA accesses for all DMA channels associated with a specific
alternate map register set.
Call with:
AH = 5BH
AL = 07H
BL = alternate register set number
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Note:
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5BH (91) Subfunction 08H
Deallocate DMA register set
────────────────────────────────────────────────────────────────────────────
Deallocates a DMA register set that was previously allocated with Int 67H
Function 5BH Subfunction 05H.
Call with:
AH = 5BH
AL = 08H
BL = DMA register set number
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Note:
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5CH (92)
Prepare Expanded Memory Manager for warm boot
────────────────────────────────────────────────────────────────────────────
Prepares the expanded memory hardware for an impending warm boot. This
function affects the current mapping context, the alternate register set
in use, and any other expanded memory hardware dependencies that would
ordinarily be initialized at system boot time.
Call with:
AH = 5CH
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Note:
■ If an application maps expanded memory at addresses below 640 KB, the
application must trap all possible conditions that might lead to a warm
boot, so that this function can be called first.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5DH (93) Subfunction 00H
Enable EMM operating-system functions
────────────────────────────────────────────────────────────────────────────
Enables the operating-system─specific EMM functions (Int 67H Functions
59H, 5BH, and 5DH) for calls by any program or device driver. (This is
the default condition.)
Call with:
AH = 5DH
AL = 00H
BX:CX = access key (if not first call to function)
Returns:
If function successful
AH = 00H
BX:CX = access key (if first call to function)
If function unsuccessful
AH = error code
Notes:
■ An access key is returned in registers BX and CX on the first call to
Int 67H Function 5DH Subfunction 00H or 01H. The access key is
required for all subsequent calls to either function.
■ This function is intended for use by operating systems only.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5DH (93) Subfunction 01H
Disable EMM operating-system functions
────────────────────────────────────────────────────────────────────────────
Disables the operating-system─specific EMM functions (Int 67H Functions
59H, 5BH, and 5DH) for calls by application programs and device
drivers, reserving the use of these functions for the operating system.
Call with:
AH = 5DH
AL = 01H
BX:CX = access key (if not first call to function)
Returns:
If function successful
AH = 00H
BX:CX = access key (if first call to function)
If function unsuccessful
AH = error code
Notes:
■ An access key is returned in registers BX and CX on the first call to
Int 67H Function 5DH Subfunction 00H or 01H. The access key is
required for all subsequent calls to either function.
■ This function is intended for use by operating systems only.
────────────────────────────────────────────────────────────────────────────
Int 67H [EMS 4.0]
Function 5DH (93) Subfunction 02H
Release access key
────────────────────────────────────────────────────────────────────────────
Releases the access key obtained by a previous call to Int 67H Function
5DH Subfunction 00H or 01H.
Call with:
AH = 5DH
AL = 02H
BX:CX = access key
Returns:
If function successful
AH = 00H
If function unsuccessful
AH = error code
Notes:
■ With respect to the operating-system─specific expanded memory functions,
the EMM is returned to the state it had when the system was initialized.
A new access key is returned by the next call to Int 67H Function 5DH
Subfunction 00H or 01H.
■ This function is intended for use by operating systems only and can be
disabled by the operating system at any time.
────────────────────────────────────────────────────────────────────────────
Index
References to tables and illustrations are in italics.
Special Characters
| 298-99
. 187
.. 187-88
; 60
< 298-99
> 298-99
>> 298-99
@ 60
A
Absolute disk read 482-84
Absolute disk write 484-85
adapters, video display 86-87
alarm
reset 592
set 591-92
align type 38
Allocate alternate map register set (EMS) 641
Allocate DMA register set (EMS) 642
Allocate handle and pages (EMS) 617-18
Allocate handle and raw pages (EMS) 639
Allocate handle and standard pages (EMS) 638-39
Allocate memory block 438-39
ANSI.SYS device driver, screen control 91
escape sequences used with 92-93
APPEND 490-91
application program interface (API) 320
application programs. See MS-DOS application programs, porting to OS/2;
MS-DOS application programs, structure of; MS-DOS application programs,
writing compatible
arena entries 196
arena headers 196, 201
diagram example 202
ASCII escape code 92-93
ASCII mode 69
character-device drivers in 261-62
ASCII text files 56
ASCIIZ strings 24, 139, 168
.ASM files 45. See also assembly-language programs
assembly-language programs 37-42
to access file allocation table 191
BREAK.ASM 75-78
CLEAN.ASM 304-9
DRIVER.ASM 283-91
DUMP.ASM 152-61
HELLO.COM example 27-30, 33-36
program modules 37
program procedures 41-42
program segments 38-41
PROTO.ASM 301-2
SHELL.ASM program 229-38
TALK.ASM 113-26
ZERODIV.ASM 254, 255-58
ASSIGN 489
ASSUME statement 29, 33
attribute byte
color text display 98
monochrome text display 97
attribute word, device 264
Auxiliary device (AUX) 12, 106, 298. See also serial port
Auxiliary input 344-45
Auxiliary output 345-46
B
background, set 508-9
BACKUP command 15
.BAT (batch) files 15
Batch files 15
binary mode 69
character-device drivers in 261-62
output 93-94
BIOS module 12-13, 17
get address of extended, 574
BIOS parameter block (BPB) 181, 189
build 272
structure 269
bit planes 101
blink/intensity bit, toggle 513
block-device drivers 260, 262
check for remoteness 423-24
check removability of 422-23
generic I/O control of 429-32
read control data from 418-19
write control data to 419-20
Boot disk device (block device) 12
boot drive, get 392-93
boot sector, disk 179-82
map of 180
partial disassembly of 182
partial hex dump 181
bootstrap routine 16, 17
border
get color 514
get palette and 514
set 508-9
set color 512
BREAK.ASM program 75-78
break flag, get or set 392-93
Buffered keyboard input 351-52
Build BIOS Parameter Block (function 02H) 272
C
CALL instructions 41
Cancel device redirection 467-68
cassette motor
read 561
turn off 560-61
turn on 560
write 562
.C files 45. See also C language
Change sharing retry count 425-26
character blocks, set specifier 520
character-device drivers 260, 261-62
ASCII vs binary mode 261-62
generic I/O control 426-29
read control data from 415-16
write control data to 416-17
character input/output. See also keyboard input; mouse, input; pointing
device, input; printer output; serial port
Int 21H 44H IOCTL (I/O control) 411-43
Int 21H functions, summary 337-38 (table)
processing typical I/O request 281-82
Character input with echo 343
Character input without echo 349-50
Character output 344
CHDIR (CD) command 167
Check if block device is remote 423-24
Check if block device is removable 422-23
Check if handle is remote 424-25
Check input status 353, 420-21
Check output status 421-22
child programs 218
CHKDSK command 15, 174, 222
C language
CLEAN.C 309-11
compiler (see C Optimizing Compiler)
DUMP.C program 151, 161-63
linking Ctrl-C and Ctrl-Break handlers to programs in 75-80
MOUDEMO.C 82-83
polling mouse and displaying mouse coordinates 82-83
PROTO.C 303
SHELL.C 225-29
TRYBREAK 78-79
tutorials 63
class type 38
CLEAN filter 303-11
assembly source code 304-9
C source code 309-11
clock, set tick count 589-90
CLOCK driver 282
Close file 357-58, 404-5
code page, get or set 474-75
code segment 38
code segment (CS) register 30
Color/Graphics Adapter (CGA) 86, 98, 102
color page state
get 517
set 515-16
color register(s)
get 516
get block of 516-17
set 514-15
set block of 515
COM1, COM2, COM3 devices 106, 110-12, 298
combine type 38
command code routines, device-driver 267-81
function 00H, Driver initialization 268-69
function 01H, Media Check 270-71
function 02H, Build BIOS Parameter Block (BPB) 272
function 03H, I/O-Control Read 272-73
function 04H, Read 273
function 05H, Nondestructive Read 274
function 06H, Input Status 274
function 07H, Flush Input Buffers 274-75
function 08H, Write 275
function 09H, Write with Verify 276
function 0AH, Output Status 276
function 0BH, Flush Output Buffers 276
function 0CH, I/O-Control Write 276-77
function 0DH, Device Open 277
function 0EH, Device Close 277-78
function 0FH, Removable Media 278
function 10H, Output Until Busy 278-79
function 13H, Generic IOCTL 279-80
function 17H, Get Logical Device 280
function 18H, Set Logical Device 280-81
names of, and MS-DOS version support 267-68 (table)
COMMAND.COM file 14-16
load 20
replacing 13
use of EXEC function 218
COMMAND.COM PLUS 13
command processor (shell) 13. See also COMMAND.COM file
commands, types of, accepted by COMMAND.COM 14-15
command tail 24, 220-21
Commit file 476-77
compatibility and portability 313-31
degrees of compatibility 314-18
MS-DOS applications 315-17
ROM BIOS and hardware-compatible applications 317-18
OS/2 compatibility 318-31
.COM program file(s) 15, 22, 25-30, 45
assembly-language program transformed into 27-30
vs .EXE files 22, 36 (table)
memory allocation for 197-98
memory image of a typical 26
CONFIG.SYS file 12
installing device driver 293
opening 18-19
configuration
get equipment 535-36
get information 525
get system environment 573-74
console, direct input/output 347-48. See also video display
Console display and keyboard (CON) 12, 298-99
control data
read, from block-device driver 418-19
read, from character-device driver 415-16
write, to block-device driver 419-20
write, to character-device driver 416-17
Controller drive diagnostic 551
Controller internal diagnostic 551
Controller RAM diagnostic 550
cooked mode 69
C Optimizing Compiler 44, 48-50
environmental variables 48
version 5.1 switches 49-50
COPY command 14, 58
Country information
get extended 470-74
get or set 395-98
CP/M operating system 4, 5
FCB compatibility with 129, 130-31
program segment prefix compatibility with 24, 25
Create directory 398-99
Create file 364-65, 401-2
Create new file 458-59
Create new PSP 378-79
Create temporary file 457-58
CREF utility 44, 56-58
cross-reference listing for HELLO.REF 57
.CRF files 45, 56
Critical-error handler address 481-82
critical-error handlers 24, 145, 147-51
address 481-82
skeleton program example 150-51
stack at entry to 148, 149
cross-reference listing. See CREF utility
Ctrl-Break and Ctrl-C handlers 72-80
compatibility issues 317
Ctrl-C handler address 480-81
high-level languages and 75-80
cursor
addressing 97
enable/disable emulation 528
get position 502-3
read character and attribute at 506
set position 501-2
set type 501
write character and attribute at 506-7
write character at 507-8
D
data segment 38
data segment (DS) register 31, 35
Date and time device (CLOCK$) 12
day count
get 592-93
set 593
Deallocate alternate map register set (EMS) 642
Deallocate DMA register set (EMS) 644
.DEF files 45
Delay 568-69
DEL(ETE) command 14
Delete directory 399
Delete file 361-62, 40, sug>8
dependency statements 61
descriptors, memory segment 321
device
cancel redirection 467-68
close 565
get device information 412-13
open 564-65
post 572-73
read file or 405-6
redirect 466-67
set device information 414-15
wait 571
write file or 406-7
Device Close (command code function 0EH) 277-78
Device close (MS-DOS function) 565
DEVICE commands 12
device drivers, installable 12-13, 259-96
CLOCK driver 282
command-code routines 267-81
debugging 295-96
chain before/after driver installation 294
chain listing 295
device attribute word 264
error codes 267
MS-DOS type 260-63
processing of typical input/output requests 281-82
structure of MS-DOS 263-67
device header 263-64
interrupt routine 26-67
strategy routine 265
writing and installing 282-95
assembly 283-92
installation 293-95
linking 293
device drivers, resident 12-13
Device Open (command-code function 0DH) 277
Device open (MS-DOS function) 564-65
Device post 572-73
Device wait 571-72
Digital Research 4
DIR command 14, 167, 174
Direct console I/O 347-48
directory 166, 167-73
create 398-99
delete 399
format of a single entry in a disk 184, 185
functions controlling 167-68 get current 437-38
hierarchical (tree) structure 166, 167
moving files 173
root 184-86
searching 168-73
set current 400
directory operations, Int 21H functions summary 339
Disable DMA on alternate map register set (EMS) 643-44
Disable EMM operating system functions (EMS) 645-46
Disable mouse driver 608-9
disk(s) 177-94. See also drive, logical; ESDI Fixed Disk Drive Adapter
absolute read 482-84
absolute write 484-85
boot sector 179-82
controller drive diagnostic 551
controller internal diagnostic 551
controller RAM diagnostic 550
file allocation table 182-84
interpreting the 188-92
files area 186-88
fixed-disk partitions 192-94
format 543
format bad track 542
format track 541-42
get change status 552-53
get current 367
get default drive data 368-69
get drive allocation information 394-95
get drive data 370
get drive parameters 543-44
get drive status 549
get type 552
initialize fixed disk characteristics 544-45
map of typical logical volume 179
park heads 554-55
read sector 538-39
read sector buffer 548
read sector long 545-46
recalibrate 550
reserved area 182
reset 354-55
reset fixed disk system 548
root directory 184-86, 187
seek 547
select 355-56
set media type 554
set type 553
set verify flag 387-88
verify sector 540
write sector 539-40
write sector buffer 549
write sector long 546-47
disk bootstrap routine 16
memory location of 17
disk management, Int 21H functions summary 339
disk-related errors 147, 148 (table)
Disk reset 354-55
disk system
get status 537-38
reset 536-37
disk transfer area (DTA) 25, 130
get 388-89
set 368
display page, set 503-4
Display string 350-51
DOS kernel 12, 18
memory location of 19
double-byte character sets (DBCS), get lead byte table 469-70
drive, logical 166, 16,-73. See also disk(s)
get map 433
set map 434
vs volume 174
driver. See device drivers, installable; device drivers, resident
DRIVER.ASM program 283-91
Driver Initialization (function 00H) 268-69
DUMP.ASM program 151, 152-61
subroutines 163
DUMP.C program 151, 161-63
Duplicate handle 435
dynamic link library 331
dynamic memory allocation 199, 200, 201
E
echo
character input with 343
character input without 349-50
unfiltered character input without 348-49
EMS. See Expanded Memory Specification (EMS)
Enable/disable cursor emulation 528
Enable/disable default palette loading 526-27
Enable/disable gray-scale summing 527
Enable/disable pointing device 574-75
Enable/disable screen refresh 529
Enable/disable video 527
Enable DMA on alternate map register set (EMS) 643
Enable EMM operating system functions (EMS) 645
Enable mouse driver 609
encapsulation of subroutines 323, 324-25
end of interrupt (EOI) 250
ENDP command 35, 41
ENDS command 29, 38
END statement 30-31, 36, 41
Enhanced Graphics Adapter (EGA) 86, 97, 98, 102
Enter protected mode 570-71
environment block 24, 220, 224-25
dump of a typical 224
three strings contained in 225
EQU statement 33
error codes, device driver 267
error codes, MS-DOS 145-51
critical 145, 147-51
expanded memory 207-9
error information, get/set 453-56
escape sequences, ANSI 92-93
ESDI Fixed Disk Drive Adapter (EP>
format drive 555
format periodic interrupt 562-63
Event wait 566-67
Exchange memory regions (EMS) 635-36
EXE2BIN utility 44, 55-56
EXEC function 15, 217-42. See also Int 21H Function 4BH
calling convention 222
compatibility in MS-DOS applications 317
environment block 220, 224-25
example programs SHELL.C and SHELL.ASM 225-40
basic flow of both 239-40
internal commands in 239
example use of 223-24
loading overlays with 240, 241-42
making memory availinug for 218-19
reference 441-42
requesting 219-21
returning from 221-24
.EXE (executable) program file(s) 15, 22, 30-36, 45
assembly language program transformed into 33-36
vs .COM files 22, 36 (table)
converting, to .COM files (see EXE2BIN utility)
header 30
load module contents 33 load module format 32
memory allocation for 198
memory image of 31
use for compatible MS-DOS applications 315
Expanded Memory Manager (EMM) 203-4
checking for 204, 205-6
enable/disable system functions 645-46
error codes 207-9
Expanded Memory Specification (EMS) 201-11
checking for expanded memory 204-6
expanded memory defined 203-4
functions reference (see Section IV)
summary 614-15
use of expanded memory 20,-11
skeleton program illustrating 210-11
Extended communications port control 559-60
extended file control block 131
volume-label search using 175
Extended initialize communications port 558-59
extended memory 204, 212-15
moving blocks of data between conventional memory and 213-15
Extended open file 478-80
external (extrinsic) commands 15
external hardware interrupts 247
extra segment (ES) register 31
F
FAR attribute 35
vs NEAR 29
faults (internal hardware interrupts) 246, 321
file(s)
area, in disks 186-88
close 357-58, 404
commit 476-77
create 364-65, 401-2
create new 458-59
create temporary 457-58
delete 361-62, 407-8
extended open 478-80
find first 358-59, 445-46
find next 360-61, 446-47
get file size 375-76
get/set date and time 450-51
lock/unlock file region 460-61
logical drive 166
moving 123
name and location 166
open 356-57, 402-4
read 405-6
rename 366, 449-50
types 45
write 406-7
file-access skeleton program
using FCB functions 134, 135-37
using handle functions 141, 142-43
file allocation table (FAT) 16, 182-84
assembly program to access 191
contents 183
interpreting 188-92
media descriptor bytes 183
file attributes, get or set 410-11
file control blocks (FCBs) 25, 128
default 130, 221
directory searching with 169, 170-71
extended 131, 133-34, 175
file management with FCB functions 129-39
advantages/disadvantages 138-39
file-access skeleton program 134-38
functions listed 132
vs handle functions 128
normal 129, 133-34
before/after open call (Int 21H Function 0FH) 137
restricted use 316, 319
typical operation of 130
file management 127-63
example programs DUMP.ASM and DUMP.C 151-63
FCB functions 128, 129-39
handle functions 128, 139-44
MS-DOS error codes 145-51
filename
fully qualified 16
parse 382-83
requesting EXEC function 219
file operations, Int 21H functions summary 338
file pointer, set 408-9
file system 166
structure 167
filters 297-311
building 300-303
CLEAN filter 303-11
operation of 299-300
prototype 301-3
system support for 298-99
Find first file 358-59, 445-46
Find next file 360-61, 44-47
fixed-disk partitions 192-94
font functions 518-24
Format bad track 542
Format drive 543
Format ESDI drive 555-56
Format ESDI drive periodic interrupt 562-63
Format track 541-42
Flush input buffer and then input 353-54
Flush Input Buffers (function 07H) 274-75
Flush Output Buffers (function 0BH) 276
G
Generic I/O control for block devices 429-32
Generic I/O control for character devices 426-29
Generic IOCTL (function 13H) 279-80
Get addresses of mappable pages (EMS) 636
Get address of alternate mouse event handler 606
Get address of extended BIOS data area 574
Get all handle names (EMS) 631
Get alternate map registers (EMS) 639-40
Get attribute capability (EMS) 630
Get block of color registers 516-17
Get border color 514
Get button press information 596
Get button release information 597
Get color page state 517
Get color register 516
Get communications port status 558
Get configuration information 525
Get conventional memory size 536
Get current directory 437-38
Get current disk 367
Get cursor position 502-3
Get date 384-85, 591
Get day count 592
Get DBCS lead byte table 469-70
Get default drive data 368-69
Get device inform~on 412-13
Get disk change status 552-53
Get disk system status 537-38
Get disk type 552
Get drive allocation information 394-95
Get drive data 370-71
Get drive parameters 543-44
Get drive status 549
Get DTA address 388-89
Get enhanced keyboard flags 586-87
Get equipment configuration 535-36
Get extended country information 470-74
Get extended error information 453-56
Get extended memory size 570
Get file size 375-76
Get font information 524
Get functionality/state information 531-34
Get handle attribute (EMS) 629
Get handle count (EMS) 621-22
Get handle name (EMS) 630
Get handle pages (EMS) 622
Get hardware configuration (EMS) 637-38
Get interrupt vector 393-94
Get keyboard flags 582
Get keyboard status 582
Get language number 610
Get light pen position 503
Get Logical Device (command-code function) 280
Get logical drive map 433
Get machine name 461-62
Get mouse information 611
Get mouse position and button status 595
Get mouse save state buffer size 603
Get mouse sensitivity 607
Get MS-DOS version number 389-90
Get number of mappable pages (EMS) 637
Get number of pages (EMS) 617
Get number of raw pages (EMS) 638
Get or set allocation strategy 452-53
Get or set break flag, get boot drive 392-93
Get or set code page 474-75
Get or set country information 395-98
Get or set display combination code 530-31
Get or set file Attributes 410-11
Get or set file date and time 450-51
Get page frame address (EMS) 616
Get pages for all handles (EMS) 623
Get palette and border 514
Get palette register 513
Get pointer page 608
Get pointing device type 577
Get printer setup string 463-64
Get printer status 588
Get PSP address 468-69
Get redirection list entry 464-65
Get return code 444-45
Get size of alternate map register save area (EMS) 641
Get size of page map information (EMS) 625
Get size of partial page map information (EMS) 626-27
Get stack space for map page and call (EMS) 634
Get status (EMS) 616
Get system environment 573-74
Get tick count 589
Get time 386, 590
Get total handles (EMS) 632
Get verify flag 448
Get version (EMS) 619
Get video mode 511
Graphics CardPlus 87
graphics mode memory-mapped programming 101-3
gray-scale
enable/disable summing 527
get values 517
GROUP directive 39
H
handle functions
check if handle is remote 424-25
directory searching 169-70, 172-73
DUMP.ASM program 151, 152-62
DUMP.C program 151, 161-63
duplicate handle 435
file/record management with 139-44
advantages/disadvantages 144
vs FCB functions 128
file access skeleton program 141-43
functions listed 140-41
typical operation 139
keyboard input 62, 67-69
redirect handle 436-37
set handle count 475-76
use for compatible MS-DOS applications 316
volume-label search using176
hardware-compatible applications 314-15, 317-18
header(EP>
device 263, 264, 269
.EXE program files 30
Hercules Graphics Card 87, 97, 98
HELLO.COM program 27, 28-29, 30
hex dump of 33
map produced by Object Linker during generation of 51
HELLO.EXE program 33, 34-35, 36
HELLO.REF program, cross-reference listing 57
.H files 45
Hide mouse pointer 595
I
IBMBIO.COM file 16
disk location 189-92
IBM Corporation, role in MS-DOS development 4-5
IBMDOS.COM file 13, 16
IBM PC 64
PC/AT 64
PS/2 64
regen buffers in memory for various adapters 87
"ill-behaved" applications 315
.INC files 45
In-Color Card 87
Initialize communications port 556-57
Initialize fixed disk characteristics 544-45
Initialize or scroll window down 505-6
Initialize or scroll window up 505
Initialize pointing device interface 577-78
Initialize printer port 587-88
input. See character input/output; keyboard input; mouse, input; pointing
device, input; serial port
input buffer, flush 353-54
Input/Output (I/O)-Control Read (function 03H) 272-73
Input/Output (I/O)-Control Write (function 0CH) 276-77
input/output (I/O) redirection 67, 298-99
input status, check 353, 420
Input Status (command-code function 06H) 274
INS8250 Asynchronous Communications Controller 112
installable device drivers 12-13
Int 10H, ROM BIOS video driver
Function 00H, Set video mode 94, 500
Function 01H, Set cursor type 94, 501
Function 02H, Set cursor position 94, 501
Function 03H, Get cursor position 94, 502
Function 04H, Get light pen position 95, 503
Function 05H, Set display page 95, 503
Function 06H, Initialize or scroll window up 95, 505
Function 07H, Initialize or scroll window down 95, 505
Function 08H, Read character and attribute at cursor 95, 506
Function 09H, Write character and attribute at cursor 94, 506
Function 0AH, Write character at cursor 94, 507
Function 0BH, Set palette, background, or border 95, 508
Function 0CH, Write graphics pixel 95, 509
Function 0DH, Read graphics pixel 95, 510
Function 0EH, Write character in teletype mode 94, 510
Function 0FH, Get video mode 94, 511
Function 10H palette functions
Subfunction 00H, Set palette register 511
Subfunction 01H, Set border color 512
Subfunction 02H, Set palette and border 512-13
Subfunction 03H, Toggle blink/intensity bit 513
Subfunction 07H, Get palette register 513
Subfunction 08H, Get border color 514
Subfunction 09H, Get palette and border 514
Subfunction 10H, Set color register 514
Subfunction 12H, Set block of color registers 515
Subfunction 13H, Set color page state 515-16
Subfunction 15H, Get color register 516
Subfunction 17H, Get block of color registers 516
Subfunction 1AH, Get color page state 517
Subfunction 1BH, Set gray-scale values 517
Function 11H, font functions
Subfunctions 00H and 10H, Load user font 518
Subfunctions 01H and 11H, Load ROM 8-by-14 font 518
Subfunctions 02H and 12H, Load ROM 8-by-8 font 519
Subfunction 03H, Set block specifier 520
Subfunctions 04H and 14H, Load ROM 8-by-16 font 520
Subfunction 20H, Set Int 1FH font pointer 521
Subfunction 21H, Set Int 43H for user's font 522
Subfunction 22H, Set Int 43H for ROM 8-by-14 font 522
Subfunction 23H, Set Int 43H for ROM 8-by-8 font 523
Subfunction 24H, Set Int 43H for ROM 8-by-16 font 523
Subfunction 30H, Get font information 524
Function 12H
Subfunction 10H, Get configuration information 525
Subfunction 20H, Select alternate printscreen 526
Subfunction 30H, Set scan lines 526
Subfunction 31H, Enable/disable default palette loading 526-27
Subfunction 32H, Enable/disable video 527
Subfunction 33H, Enable/disable gray-scale summing 527
Subfunction 34H, Enable/disable cursor emulation 528
Subfunction 35H, Switch active display 528
Subfunction 36H, Enable/disable screen refresh 529
Function 13H, Write string in teletype mode 529
Function 1AH, Get or set display combination code 530
Function 1BH, Get functionality/state information 531
Function 1CH, Save or restore video state 534
Int 11H, Get equipment configuration 535
Int 12H, Get conventional memory size 536
Int 13H, ROM BIOS disk driver 319
Function 00H, Reset disk system 536
Function 01H, Get disk system status 537
Function 02H, Read sector 538
Function 03H, Write sector 539
Function 04H, Verify sector 540
Function 05H, Format track 541
Function 06H, Format bad track 542
Function 07H, Format drive 543
Function 08H, Get drive parameters 543
Function 09H, Initialize fixed disk characteristics 544
Function 0AH, Read sector long 545
Function 0BH, Write sector long 546
Function 0CH, Seek 547
Function 0DH, Reset fixed disk system 548
Function 0EH, Read sector buffer 548
Function 0FH, Write sector buffer 549
Function 10H, Get drive status 549
Function 11H, Recalibrate drive 550
Function 12H, Controller RAM diagnostic 550
Function 13H, Controller drive diagnostic 551
Function 14H, Controller internal diagnostic 551
Function 15H, Get disk type 552
Function 16H, Get disk change status 552
Function 17H, Set disk type 553
Function 18H, Set media type for format 554
Function 19H, Park heads 554
Function 1AH, Format ESDI drive 555
Int 14H, ROM BIOS Serial communications port driver 111
Function 00H, Initialize communications port 556
Function 01H, Write character to communications port 557
Function 02H, Read character from communications port 558
Function 03H, Get communications port status 558
Function 04H, Extended initialize communications port 558
Function 05H, Extended communications port control 559
Int 15H, ROM BIOS I/O Subsystem Extensions
Function 00H, Turn on cassette motor 560
Function 01H, Turn off cassette motor 560
Function 02H, Read cassette 561
Function 03H, Write cassette 562
Function 0FH, Format ESDI drive periodic interrupt 562
Function 21H
Subfunction 00H, Read POST error log 563
Subfunction 01H, Write POST error log 563
Function 4FH, Keyboard intercept 564
Function 80H, Device open 564
Function 81H, Device close 565
Function 82H, Process termination 566
Function 83H, Event wait 566
Function 84H, Read joystick 567
Function 85H, SysReq key 568
Function 86H, Delay 568
Function 87H, Move extended memory block 569
Function 88H, Get extended memory size 570
Function 89H, Enter protected mode 570
Function 90H, Device wait 571
Function 91H, Device post 572
Function C0H, Get system environment 317, 573
Function C1H, Get address of extended BIOS data area 574
Function C2H
Subfunction 00H, Enable/disable pointing device 574
Subfunction 01H, Reset pointing device 575
Subfunction 02H, Set sample rate 576
Subfunction 03H, Set resolution 576
Subfunction 04H, Get pointing device type 577
Subfunction 05H, Initialize pointing device interface 577
Subfunction 06H, Set scaling or get status 578
Subfunction 07H, Set pointing device handler address 579
Function C3H, Set watchdog time-out 580
Function C4H, Programmable option select 580
Int 16H, ROM BIOS keyboard driver 322
Function 00H, Read character from keyboard 581
Function 01H, Get keyboard status 582
Function 02H, Get keyboard flags 582
Function 03H, Set repeat rate 583
Function 04H, Set keyclick 584
Function 05H, Push character and scan code 585
Function 10H, Read character from enhanced keyboard 585
Function 11H, Get enhanced keyboard status 586
Function 12H, Get enhanced keyboard flags 586
Int 17H, ROM BIOS Parallel port printer driver 108-19
Function 00H, Write character to printer 587
Function 01H, Initialize printer port 587
Function 02H, Get printer status 588
Int 18H, ROM BASIC 588
Int 19H, ROM BIOS Reboot system 588
Int 1AH, Real-time (CMOS) Clock Driver
Function 00H, Get tick count 589
Function 01H, Set tick count 589
Function 02H, Get time 590
Function 03H, Set time 590
Function 04H, Get date 591
Function 05H, Set date 591
Function 06H, Set alarm 591
Function 07H, Reset alarm 592
Function 0AH, Get day count 592
Function 0BH, Set day count 593
Function 80H, Set sound source 593
Int 20H, Terminate process 341
Int 21H, MS-DOS system functions
function execution in a typical I/O request 281-82
function summary by category 337-40 (table)
function summary by number 335-37 (table)
Function 00H, Terminate process 342
Function 01H, Character input with echo 70, 148, 343
Function 02H, Character output 90, 344
Function 03H, Auxiliary input 110, 344-45
Function 04H, Auxiliary output 110, 345-46
Function 05H, Printer output 107, 346-47
Function 06H, Direct console I/O 70, 73, 90, 347-48
Function 07H, Unfiltered character input without echo 70, 73, 348-49
Function 08H, Character input without echo 70, 349-50
Function 09H, Display string 90, 350-51
Function 0AH, Buffered keyboard input 70-71, 351-52
Function 0BH, Check input status 70, 353
Function 0CH, Flush input buffer and then input 70, 353-54
Function 0DH, Disk reset 354-55
Function 0EH, Select disk 167, 355-56
Function 0FH, Open file 132, 137, 356-57
Function 10H, Close file 132, 357-58
Function 11H, Find first file 358-59
Function 12H, Find next file 360-61
Function 13H, Delete file 132, 361-62
Function 14H, Sequential read 132, 362-63
Function 15H, Sequential write 132, 363-64
Function 16H, Create file 132, 364-65
Function 17H, Rename file 132, 173, 366-67
Function 18H, Reserved 367
Function 19H, Get current disk 167, 168, 367
Function 1AH, Set DTA address 130, 132, 368
Function 1BH, Get default drive data 368-69
Function 1CH, Get drive data 370
Function 1DH, Reserved 371
Function 1EH, Reserved 371
Function 1FH, Reserved 371
Function 20H, Reserved 371
Function 21H, Random read 132, 372-73
Function 22H, Random write 132, 373-75
Function 23H, Get file size 132, 375-76
Function 24H, Set relative record number 132, 376
Function 25H, Set interrupt vector 147, 252, 253, 316, 377-78
Function 26H, Create new PSP 378-79
Function 27H, Random block read 132, 379-80
Function 28H, Random block write 132, 381-82
Function 29H, Parse filename 129, 132, 382
Function 2AH, Get date 384-85
Function 2BH, Set date 385
Function 2CH, Get time 386
Function 2DH, Set time 386-87
Function 2EH, Set verify flag 387-88
Function 2FH, Get DTA address 388-89
Function 30H, Get MS-DOS version number 148, 319, 389
Function 31H, Terminate and stay resident 252, 253, 390-91
Function 32H, Reserved 392
Function 33H, Get or set break flag, get boot drive 392-93
Function 34H, Reserved 393
Function 35H, Get interrupt vector 252, 316, 393-94
Function 36H, Get drive allocation information 394-95
Function 37H, Reserved 395
Function 38H, Get or set country information 395-98
Function 39H, Create directory 167, 398-99
Function 3AH, Delete directory 167, 399
Function 3BH, Set current directory 167, 400
Function 3CH, Create file 140, 401-2
Function 3DH, Open file 107, 110, 140, 204, 402-4
Function 3EH, Close file 140, 204, 404
Function 3FH, Read file or device 67, 69, 71, 109, 110, 141, 298,
300, 322, 405-6
Function 40H, Write file or device 35, 88, 107, 109, 110, 141,
298, 300, 322, 406-7
Function 41H, Delete file 141, 407-8
Function 42H, Set file pointer 141, 408-9
Function 43H, Get or set file attributes 141, 410-11
Function 44H, IOCTL (I/O control) 69, 93-94, 111, 204, 205, 411-34
Subfunction 00H, IOCTL: get device information 412-13
Subfunction 01H, IOCTL: set device information 414-15
Subfunction 02H, IOCTL: read control data from character device driver
415-16
Subfunction 03H, IOCTL: write control data to character device driver
416-17
Subfunction 04H, IOCTL: read control data from block device driver
418-19
Subfunction 05H, IOCTL: write control data to block device driver
419-20
Subfunction 06H, IOCTL: check input status 420-21
Subfunction 07H, IOCTL: check output status 421-22
Subfunction 08H, IOCTL: check if block device is removable 422-23
Subfunction 09H, IOCTL: check if block device is remote 423-24
Subfunction 0AH, IOCTL: check if handle is remote 424-25
Subfunction 0BH, IOCTL: change sharing retry count 425-26
Subfunction 0CH, IOCTL: generic I/O control for character devices
426-29
Subfunction 0DH, IOCTL: generic I/O control for block devices 429-32
Subfunction 0EH, IOCTL: get logical drive map 433
Subfunction 0FH, IOCTL: set logical drive map 434
Function 45H, Duplicate handle 141, 435
Function 46H, Redirect handle 141, 299, 436-37
Function 47H, Get current directory 167, 168, 437-38
Function 48H, Allocate memory block 196, 202, 323, 438-39
Function 49H, Release memory block 196, 323, 439-40
Function 4AH, Resize memory block 196, 198, 202, 219, 239, 322,
440-41
Function 4BH, Execute program (EXEC) 202, 299, 441-42 (see also EXEC
function)
Function 4CH, Terminate process with return code 26, 31, 35, 317,
443-44
Function 4DH, Get return code 221, 444-45
Function 4EH, Find first file 445-46
Function 4FH, Find next file 446-47
Function 50H, Reserved 447
Function 51H, Reserved 447
Function 52H, Reserved 447
Function 53H, Reserved 448
Function 54H, Get verify flag 448
Function 55H, Reserved 448
Function 56H, Rename file 141, 173, 449-50
Function 57H, Get or set file date and time 141, 450-51
Function 58H, Get or set allocation strategy 452-53
Function 59H, Get extended error information 130, 145, 148, 453-56
Function 5AH, Create temporary file 141, 457-58
Function 5BH, Create new file 141, 458-59
Function 5CH, Lock or unlock file region 141, 460-61
Function 5DH, Reserved 461
Function 5EH, Machine name and printer setup
Subfunction 00H, Get machine name 461-62
Subfunction 02H, Set printer setup string 462-63
Subfunction 03H, Get printer setup string 463-64
Function 5FH, Device redirection
Subfunction 02H, Get redirection list entry 464-65
Subfunction 03H, Redirect device 466-67
Subfunction 04H, Cancel device redirection 467-68
Function 60H, Reserved 468
Function 61H, Reserved 468
Function 62H, Get PSP address 468-69
Function 63H, Get DBCS lead byte table 469-70
Function 64H, Reserved 470
Function 65H, Get extended country information 470-74
Function 66H, Get or set code page 474-75
Function 67H, Set handle count 141, 475-76
Function 68H, Commit file 141, 476-77
Function 69H, Reserved 477
Function 6AH, Reserved 477
Function 6BH, Reserved 477
Function 6CH, Extended open file 141, 478-80
Int 22H, Terminate handler address 480
Int 23H, Ctrl-C handler address 317, 480-81
Int 24H, Critical-error handler address 147, 317, 481-82
Int 25H, Absolute disk read 482-84
Int 26H, Absolute disk write 319, 484-85
Int 27H, Terminate and stay resident 486-87
Int 28H, Reserved 487
Int 29H, Reserved 487
Int 2AH, Reserved 487
Int 2BH, Reserved 487
Int 2CH, Reserved 487
Int 2DH, Reserved 488
Int 2EH, Reserved 488
Int 2FH, Multiplex interrupt 488
Function 01H, Print spooler 488-89
Function 02H, ASSIGN 489
Function 10H, SHARE 490
Function B7H, APPEND 490-91
Int 33H, Microsoft Mouse driver 593
Function 00H, Reset mouse and get status 80, 594
Function 01H, Show mouse pointer 80, 594
Function 02H, Hide mouse pointer 80, 595
Function 03H, Get mouse position and button status 80, 595
Function 04H, Set mouse pointer position 80, 596
Function 05H, Get button press information 80, 596
Function 06H, Get button release information 80, 597
Function 07H, Set horizontal limits for pointer 80, 597
Function 08H, Set vertical limits for pointer 80, 598
Function 09H, Set graphics pointer shape 80, 598
Function 0AH, Set text pointer type 80, 599
Function 0BH, Read mouse motion counters 80, 599
Function 0CH, Set user-defined mouse event handler 80, 600
Function 0DH, Turn on light pen emulation 80, 601
Function 0EH, Turn off light pen emulation 80, 601
Function 0FH, Set mickeys to pixels ratio 80, 601
Function 10H, Set mouse pointer exclusion area 80, 602
Function 13H, Set double speed threshold 81, 602
Function 14H, Swap user-defined mouse event handlers 81, 603
Function 15H, Get mouse save state buffer size 81, 603
Function 16H, Save mouse driver state 81, 604
Function 17H, Restore mouse driver state 81, 604
Function 18H, Set alternate mouse event handler 81, 604
Function 19H, Get address of alternate mouse event handler 81, 606
Function 1AH, Set mouse sensitivity 81, 606
Function 1BH, Get mouse sensitivity 81, 607
Function 1CH, Set mouse interrupt rate 81, 607
Function 1DH, Select pointer page 81, 608
Function 1EH, Get pointer page 81, 608
Function 1FH, Disable mouse driver 81, 608
Function 20H, Enable mouse driver 81, 609
Function 21H, Reset mouse driver 81, 609
Function 22H, Set language for mouse driver messages 81, 610
Function 23H, Get language number 81, 610
Function 24H, Get mouse information 81, 611
Int 67H, Expanded Memory Manager functions 204, 205, 207
Function 40H, Get status 616
Function 41H, Get page frame address 616
Function 42H, Get number of pages 617
Function 43H, Allocate handle and pages 617
Function 44H, Map expanded memory page 618
Function 45H, Release handle and expanded memory 619
Function 46H, Get version 619
Function 47H, Save page map 620
Function 48H, Restore page map 620
Function 49H, Reserved 621
Function 4AH, Reserved 621
Function 4BH, Get handle count 621
Function 4CH, Get handle pages 622
Function 4DH, Get pages for all handles 623
Function 4EH
Subfunction 00H, Save page map 623
Subfunction 01H, Restore page map 624
Subfunction 02H, Save and restore page map 624
Subfunction 03H, Get size of page map information 625
Function 4FH
Subfunction 00H, Save partial page map 625
Subfunction 01H, Restore partial page map 626
Subfunction 02H, Get size of partial page map information 626
Function 50H
Subfunction 00H, Map multiple pages by number 627
Subfunction 01H, Map multiple pages by address 627
Function 51H, Reallocate pages for handle 628
Function 52H
Subfunction 00H, Get handle attribute 629
Subfunction 01H, Set handle attribute 629
Subfunction 02H, Get attribute capability 630
Function 53H
Subfunction 00H, Get handle name 630
Subfunction 01H, Set handle name 631
Function 54H
Subfunction 00H, Get all handle names 631
Subfunction 01H, Search for handle name 632
Subfunction 02H, Get total handles 632
Function 55H
Subfunctions 00H and 01H, Map pages and jump 633
Function 56H
Subfunctions 00H and 01H, Map pages and call 633
Subfunction 02H, Get stack space for map page and call 634
Function 57H
Subfunction 00H, Move memory region 635
Subfunction 01H, Exchange memory regions 635
Function 58H
Subfunction 00H, Get addresses of mappable pages 636
Subfunction 01H, Get number of mappable pages 637
Function 59H
Subfunction 00H, Get hardware configuration 637
Subfunction 01H, Get number of raw pages 638
Function 5AH
Subfunction 00H, Allocate handle and standard pages 638
Subfunction 01H, Allocate handle and raw pages 639
Function 5BH
Subfunction 00H, Get alternate map registers 639
Subfunction 01H, Set alternate map registers 640
Subfunction 02H, Get size of alternate map register save area 641
Subfunction 03H, Allocate alternate map register set 641
Subfunction 04H, Deallocate alternate map register set 642
Subfunction 05H, Allocate DMA register set
Subfunction 06H, Enable DMA on alternate map register set 643
Subfunction 07H, Disable DMA on alternate map register set 643
Subfunction 08H, Deallocate DMA register set 644
Function 5CH, Prepare expanded memory manager for warm boot 644
Function 5DH
Subfunction 00H, Enable EMM operating system functions 645
Subfunction 01H, Disable EMM operating system functions 645
Subfunction 02H, Release access key 646
Intel 80x86 microprocessor family 4, 8, 38, 64, 203
interrupts and 246-51
Intel 8259A Programmable Interrupt Controller 112, 320
internal hardware interrupts 246
internal (intrinsic) commands 14
interrupt(s) 13, 244-45. See also Int 10H through Int 67H
external hardware 247
internal hardware 246
servicing 250-51
software 247-49
types 244
interrupt handlers 16
example (ZERODIV.ASM) 254-58
MS-DOS and 252-53
servicing 250, 251
tasks 245
typical 251
interrupt (intr) routine, device-driver 266-67, 293. See also command code
routines
interrupt vector 17, 244
get 393-94
set 377-78
interrupt vector table 250
IOCTL (I/O control). See Int 21H, Function 44H
IO.SYS file 16, 17
memory location of 18
J
Japanese character set 6
joystick, read 567
K
kernel. See DOS kernel
keyboard
get enhanced flags 586-87
get enhanced status 586
get flags 582
get status 582
input with/without echo 70
intercept 564
key repeat rate and delay 583-84
push character and scan code in buffer 585
read character from 581
read character from enhanced 585
set keyclick 584
keyboard input 65-72
buffered 351-52
Ctrl-C and Ctrl-Break handlers 72-80, 317
with handles 66, 67-69
read character from keyboard 581
with ROM BIOS functions 71-72
with traditional calls 69-71
Keyboard input with echo 70
Keyboard input without echo 70
Keyboard intercept 564
Korean character set 6
L
.LIB files 44, 45, 58. See also Library Manager (LIB)
Library Manager (LIB) 44, 58-60
operations prefix characters 58
table-of-contents listing for SLIBC.LIB 59
light pen
get position 503
turn off emulation 601
turn on emulation 601
line printer (PRN) 12, 106, 298
LINK. See Object Linker (LINK)
Load ROM 8-by-8 font 519
Load ROM 8-by-14 font 518-19
Load ROM 8-by-16 font 520-21
Load user font 518
Lock or unlock file region 460-61
Lotus/Intel/Microsoft Expanded Memory (LIM EMS). See Expanded Memory
Specification (EMS)
LPT1, LPT2, LPT3 devices 106, 298
.LST files 45
M
machine name, get 461-62
Macro Assembler (MASM) 44, 45-47
command line mode 46
interactive mode 46
levels
modules 37
procedures 41-42
segments 38-41
tutorials 63
version 5.1 switches 47
make files 61
MAKE utility 60-61
switches for 61
Map expanded memory page (EMS) 618
.MAP files 45
Map multiple pages by address (EMS) 627-28
Map multiple pages by number (EMS) 627
Map pages and call (EMS) 633-34
MASM. See Macro Assembler (MASM)
master boot record 192
Media Check (function 01H) 270-71
memory
allocation
dynamic, of additional 199-201
shrinking 197-99
conventional 196
moving blocks of data between extended memory and 213-15
expanded (see Expanded Memory Specification (EMS))
image of .COM file 26
image of .EXE file 31
location of disk bootstrap program in 17
location of IO.SYS in 18
location of ROM bootstrap routine in 16
location of SYSINIT, DOS kernel, MSDOS.SYS in 19
making available, for EXEC function 218-19
map after startup 20
RAM 196
memory areas, 196. See also arena entries; arena headers; transient program
area (TPA)
memory block
allocate 438-39
get/set allocation strategy 452-53
move extended 569-60
release 439-40
resize 440-41
memory interlace 203
memory management 195-215
arena headers 201-2
expanded memory 203-11
using 207-11
extended memory 212-15
Int 21H functions summary 339
MS-DOS applications compatibility and 316
using memory-allocation functions 197-202
memory-mapped input/output 86, 96-103
graphics mode 101-3
text mode 96-101
memory models 39
segments, groups, classes for 40
memory segment 321-22
memory size
get conventional 536
get extended 570
mickeys, set to pixel ratio 601
Microsoft Mouse driver 593-611
miscellaneous system functions, Int 21H functions summary 340
MKDIR (MD) command 167
Monochrome/Printer Display Adapter (MDA) 86, 97, 98
example dump, regen buffer 98
MOUDEMO.C program 82-83
mouse. See also pointing device
disable driver 608-9
driver 593
enable driver 609
get address of alternate event handler 606
get button press information 596
get button release information 597
get information 611
get language number 610
get mouse save state buffer size 603-4
get position and button status 595
get sensitivity 607
hide pointer 595
input 80-83
read motion counters 599-600
reset and get status 594
reset driver 609
save driver state 604
set alternate event handler 604-5
set double speed threshold 602
set graphics pointer shape 598
set interrupt rate 607
set language for driver messages 610
set pointer exclusion area 602
set pointer horizontal limits 597-98
set pointer page 608
set pointer position 596
set pointer vertical limits 598
set sensitivity 606
set text pointer type 599
set user-defined event handler 600-601
show pointer 594-95
summary of function calls 494-99
swap user-defined event handlers 603
Move extended memory block 569-70
Move memory region (EMS) 635
MS-DOS. See also Operating System/2 (OS/2)
genealogy 3-9
interrupt handlers and 252-53
loading 16-20
programming tools (see programming tools)
structure 12-16
MS-DOS application programs, porting to OS/2 318-31
conversion 326-30
encapsulation 323, 324-25
MS-DOS function calls and OS/2 counterparts 328-29
optimization 330-31
rationalization 322-23
ROM BIOS functions and OS/2 equivalents used in MS-DOS applications 330
segmentation 321-22
MS-DOS application programs, structure of 21-42
assembly-language programs 27-30, 37-42
.COM programs introduced 25-30
creation of 62-63
.EXE programs introduced 30-36
program procedures 41-42
program segment prefix 23-25
MS-DOS application programs, writing compatible 314, 315-17
check host capabilities 316
exception handling 317
input and output 316
memory management 316
process management 317
program structure 315
MS-DOS error codes 145-51
MS-DOS functions 334
conversion of, to OS/2 function calls 326-27
display functions 88-94
binary output mode 93-94
screen control 91-93
EXEC (see EXEC function)
file control block (FCB) 129-39
handle 139-44
memory management/allocation 196, 197-202
OS/2 equivalents to 328-29
printer output 107-9
reference (see Section II)
serial port 109-12
typical in-line code for call to 324
MSDOS.SYS file 13, 16
memory location of 19
MS-DOS versions
1.0 4-5, 138
1.25 5
2.00 5-6, 174
error codes 145
volume-label search under 175
2.11, 2.25 6
3.0 6-7, 138, 174
error codes 145-46
volume-label search under 176
3.1, 3.2, 3.3, 4.0 7
get number 389-90
support for select command code routines by 267-68 (table)
Multi-Color Graphics Array (MCGA) 86, 102
Multiplex interrupt 488
N
NAME statement 27, 33
NEAR attribute 27
vs FAR 29
NEAR RETURN 27
network functions, Int 21H functions summary 339
Nondestructive Read (function 05H) 274
non-disk-related errors 147, 148 (table)
O
Object Linker (LINK) 37, 44, 50-55
map produced by, of HELLO.EXE program 51
switches accepted by 53-55
object modules 37
libraries (see Library Manager (LIB))
linking .COM files from 27, 37. See also Object Linker (LINK)
.OBJ files 45
Open file 356-57, 402-4
Operating System/2 (OS/2) 7
code optimization 330-31
compatibility issues 318-20
function calls equivalent to MS-DOS function calls 328-29
function calls equivalent to ROM BIOS function calls 330
porting MS-DOS applications to OS/2 320-31
ORG instruction 29
output. See character input/output; printer output; serial port
output status, check 421-22
Output Status (command-code function 0AH) 276
Output Until Busy (function 10H) 278-79
overlays, loading with EXEC 240, 241-42
P
PAGE command 27, 33
page frame 203
palette
enable/disable default 526-27
get border and 514
get register 513
set 508-9
set border and 512-13
set register 511-12
parallel ports 106
parameter block, requesting EXEC function 220-21
parent programs 218
Park heads 554-55
Parse filename 382-84
partitions, fixed-disk 192-94
Paterson, Tim 4
path 166
PC-DOS
version 1.0 4
version 1.1 5
version 2.0 5-6
version 3.0 193-94
piping parameters 24
pixel 101
formula to calculate bit position for 102-3
read graphics 510
set mickeys to pixel ratio 601-2
write graphics 509
pointing device
enable/disable 574-75
get device type 577
get scaling or get status 578-79
initialize interface 577-78
input 80-83
reset 575
set handler address 579-80
set resolution 576-77
set sample rate 576
POP instruction 35
portability. See compatibility and portability
POST (power-on self-test)
read error log 563
write error log 563-64
Prepare expanded memory manager for warm boot (EMS) 644-45
Presentation Manager, OS/2 318
printer 106, 107-9. See also line printer (PRN); standard printer (stdprn)
get setup strings 463-64
get status 588
initialize port 587
write character to 587
printer output 106, 107-9, 346-47. See also TALK.ASM program
printer setup string
get 463-64
set 462-63
printscreen, select alternate 526
Print spooler 488-89
PRN device 12, 106, 298-99
PROC command 29, 35, 41
procedure, declaring beginning/end of 29
process management
for compatibility in MS-DOS applications 317
Int 21H functions summary 339
terminate process 566
Process termination 566
Programmable Interrupt Controller (PIC) 247
Programmable option select 580-81
programming tools 43-64
C Optimizing compiler 48-50
CREF utility 56-58
example using 62-63
EXE2BIN utility 55-56
file types 45
Library Manager 58-60
MAKE utility 60-61
MASM 45-47 (see also Macro Assembler (MASM))
Object Linker 50-55 (see also Object Linker (LINK))
resources and references 63-64
program modules, assembly-language 37
program procedures 41-42
program segment prefix (PSP) 15, 23-25
create new 378-79
get address 468-69
structure of 23
program segments, assembly-language 38-41
protected mode, enter 570-71
PROTO.ASM program 301-2
PROTO.C program 303
P-system operating system 5
Push character and scan code 585
PUSH instruction 35
R
Random block read 379-80
Random block write 381-82
Random read 372-73
Random write 373-75
rationalizing code 322-23
raw mode 69
Read (function 04H) 273
Read cassette 561
Read character and attribute at cursor 506
Read character from communications port 558
Read character from enhanced keyboard 585
Read character from keyboard 581
Read control data from block-device driver 418-19
Read control data from character device driver 415-16
Read file or device 405-6
Read graphics pixel 510
Read joystick 567
Read mouse motion counters 599-600
Read POST error log 563
Read sector 538-39
Read sector buffer 548
Read sector long 545-46
Reallocate pages for handle (EMS) 628
Reboot system 588-89
Recalibrate drive 550
record(s)
set relative number 376-77
using FCB functions 129-39
using handle functions 139-44
record operations, Int 21H functions summary 338-39
Redirect device 466-67
Redirect handle 436-37
redirection, input/output 24, 67, 298-99
cancel 467-68
redirection list entry, get 464-65
.REF files 45, 56
refresh buffer 86
regen buffer 86
example dump of MDA adapter 98
formula to determine offset 102
memory diagram showing location of 87
Release access key (EMS) 646
Release handle and expanded memory (EMS) 619
Release memory block 439-40
Removable Media (function 0FH) 278
REN(AME) command 14
Rename file 366-67, 449-50
request header format 265
command codes for (see command code routines, device-driver)
reserved area, disk 182
reserved functions
EMS 621
Int 21H functions summary 340
Reset alarm 592
Reset disk system 536-37
Reset fixed disk system 548
Reset mouse and get status 594
Reset mouse driver 609
Reset pointing device 575
resident device drivers 12
Resize memory block 440-41
RESTORE command 15
Restore mouse driver state 604
Restore page map (EMS) 620-21, 624
Restore partial page map (EMS) 626
RET instruction 41
retrace interval 100
return code
get 444
terminate process with 443-44
RMDIR (RD) command 167
ROM 8-by-8 font
load 519
set Int 43H for 523
ROM 8-by-14 font
load 518-19
set Int 43H for 522-23
ROM 8-by-16 font
load 520-21
set Int 43H for 523-24
ROM BASIC 588
ROM BIOS
display functions 94-96, 330
interrupts of special importance to 247, 248-49
keyboard functions 67
input with 71-72
ROM BIOS compatibility 314-16, 317-18
avoid unstable hardware 318
check host capabilities 317-18
functions of, and OS/2 equivalents 330
ROM BIOS function calls. See also Section III
summary 494-99
ROM bootstrap routine 16
root directory 166, 184-86, 187
partial hex dump 186
RS-232 serial-interface standard 106
RS-422 serial-interface standard 106
S
Save and restore page map (EMS) 624-25
Save mouse driver state 604
Save or restore video state 534-35
Save page map (EMS) 620, 623
Save partial page map (EMS) 625-26
scan lines, set 526
screen control with MS-DOS functions 91-93
screen refresh, enable/disable 529
Search for handle name (EMS) 632
Seattle Computer Products 4
Seek 547
SEGMENT command 29, 33, 38
segment register 321
Select alternate printscreen 526
Select disk 355-56
selector 321
Select pointer page 608
Sequential read 362-63
Sequential write 363-64
serial port 106, 109-12. See also TALK.ASM program
extended initialize port 558-59
extended port control 559-60
get status 558
initialize 556-57
read character from 558
write character to 557
Set alarm 591-92
Set alternate map registers (EMS) 640
Set alternate mouse event handler 604-5
Set block of color registers 515
Set block specifier 520
Set border color 512
Set color page state 515-16
Set color register 514-15
Set current directory 400
Set cursor position 501-2
Set cursor type 501
Set date 385, 591
Set day count 593
Set device information 414-15
Set disk type 553
Set display page 503-4
Set double speed threshold 602
Set DTA address 368
Set file pointer 408-9
Set graphics pointer shape 598
Set gray-scale values 517
Set handle attribute (EMS) 629
Set handle count 475-76
Set handle name (EMS) 631
Set horizontal limits for pointer 597-98
Set Int 1FH font pointer 521
Set Int 43H for ROM 8-by-8 font 523
Set Int 43H for ROM 8-by-14 font 522-23
Set Int 43H for ROM 8-by-16 font 523-24
Set Int 43H for user's font 522
Set interrupt vector 377-78
Set keyclick 584
Set language for mouse driver messages 610
Set Logical Device (function 18H) 280-81
Set logical drive map 434
Set media type for format 554
Set mickeys to pixels ratio 601
Set mouse interrupt rate 607
Set mouse pointer exclusion area 602
Set mouse pointer position 596
Set mouse sensitivity 606
Set palette and border 512-13
Set palette, background, or border 508-9
Set palette register 511-12
Set pointing device handler address 579-80
Set printer setup string 462-63
Set relative record number 376-77
Set repeat rate 583-84
Set resolution 576-77
Set sample rate 576
Set scaling or get status 578-79
Set scan lines 526
Set sound source 593
Set text pointer type 599
Set tick count 589-90
Set time 386-87, 590
Set user-defined mouse event handler 600-601
Set verify flag 387-88
Set vertical limits for pointer 598
Set video mode 500-501
Set watchdog time-out 580
SHARE 490
shell. See COMMAND.COM file; command processor (shell)
SHELL.ASM program 229-38
SHELL.C program 225-29
Show mouse pointer 594-95
SLIBC.LIB, table-of-contents listing for 59
Softech company 5
software interrupts, 247-49
sound source, set 593
STACK attribute 31
stack pointer (SP) register 25-26, 31, 35
stack segment 38
stack segment (SS) register 31, 35
standard auxiliary device (stdaux) 20, 323
default device 298
handle 66
standard error device (stderr) 20
default device 298
handle 66
standard input device (stdin) 20
default device 298
handle 66, 67
standard list device 20, 323
standard output device (stdout) 20
default device 298
handle 66
standard printer (stdprn)
default device 298
handle 66
strategy (strat) routine, device-driver 265, 293
string(s)
display 350-51
Swap user-defined mouse event handlers 603
Switch active display 528-29
switches
C Optimizing compiler 49-50
Library Manager 60
Macro Assembler 47
Make utility 61
Object Linker 53-55
SYSINIT module 17, 18, 20
installing device drivers 293
memory location of 19
SysReq key 568
system file table 140-41
T
TALK.ASM program 113-26
teletype mode
write character in 510-11
write string in 529-30
terminal-emulator program. See TALK.ASM program
Terminate and stay resident 390-91, 486-87
Terminate handler address 480
Terminate process 341, 342
Terminate process with return code 443-44
text-mode memory-mapped programming 96-101
threads 331
time and date
day count 592, 593
get date 384-85, 591
get time 386, 590
set date 385, 591
set time 386-87, 590
TITLE command 27, 33
Toggle blink/intensity bit 513
transient program 15, 22. See also .COM program file(s);
.EXE (executable) program file(s)
transient program area (TPA) 15, 196. See also arena entries; arena
headers
TREE command 174
TRYBREAK.C program 78-79
Turn off cassette motor 560-61
Turn on cassette motor 560
Turn off light pen emulation 601
Turn on light pen emulation 601
U
Unfiltered character input without echo 348-49
UNIX/XENIX operating system 66, 128, 139
user font
load 518
set Int 1FH pointer 521
set Int 43H for 522
V
verify flag, get 448
Verify sector 540
video display 85-103
adapters 86-87
enable/disable 527
get functionality/state information 531-34
get or set combination code 530-31
memory-mapped techniques 96-103
graphics mode 101-3
text mode 96-101
MS-DOS display functions 88-94
binary output mode 93-94
screen control with 91-93
ROM BIOS display functions 94-96
save or restore video state 534-35
support considerations 88
switch active display 528-29
Video Graphics Array (VGA) 86, 97, 98, 102
video mode
get 511
set 500-501
VOL command 174
volume labels 174-76
search, using extended file control block 175
W
watchdog time-out, set 580
window
initialize or scroll down 505-6
initialize or scroll up 505
Windows 7, 318
Write (function 08H) 275
Write cassette 562
Write character and attribute at cursor 506-7
Write character at cursor 507-8
Write character in teletype mode 510-11
Write character to communications port 557
Write character to printer 587
Write control data to block-device driver 419-20
Write control data to character-device driver 416-17
Write File or Device 406-7
Write graphics pixel 509
Write POST error log 563-64
Write screen in teletype mode 529-30
Write sector 539
Write sector buffer 549
Write sector long 546-47
Write with Verify (function 09H) 276
Z
ZERODIV.ASM program 254, 255-58
Zilog Z-80 microprocessor 4