PCjs Machines
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The MS-DOS Encyclopedia
Appendix O: IBM PC ROM BIOS Calls
To invoke an IBM PC ROM BIOS routine, set register AH to the desired
function and execute the software interrupt (INT) for the desired
routine.
Graphics pixel coordinates and cursor row and column coordinates are
always zero based.
Interrupt 10H: Video Services
Function 00H: Set Video Mode
To call:
AH = 00H
AL = mode:
= 00H 16-shade gray text 40 by 25 B000:8000H
EGA: 64-color
= 01H 16/8-color text 40 by 25 B000:8000H
EGA: 64-color
= 02H 16-shade gray text 80 by 25 B000:8000H
EGA: 64-color
= 03H 16/8-color text 80 by 25 B000:8000H
EGA: 64-color
= 04H 4-color graphics 320 by 200 B000:8000H
= 05H 4-shade gray graphics 320 by 200 B000:8000H
= 06H 2-shade gray graphics 640 by 200 B000:8000H
= 07H monochrome text 80 by 25 B000:0000H
= 08H 16-color graphics 160 by 200 B000:0000H
= 09H 16-color graphics 320 by 200 B000:0000H
= 0AH 4-color graphics 640 by 200 B000:0000H
= 0BH Reserved
= 0CH Reserved
= 0DH 16-color graphics 320 by 200 A000:0000H
= 0EH 16-color graphics 640 by 200 A000:0000H
= 0FH monochrome graphics 640 by 350 A000:0000H
= 10H 16/64-color graphics 640 by 350 A000:0000H
Returns:
Nothing
Function 01H: Set Cursor Size and Shape
To call:
AH = 01H
CH = starting scan line
CL = ending scan line
Note: CH < CL gives normal one-part cursor; CH > CL gives two-part
cursor; CH = 20H gives no cursor.
Returns:
Nothing
Function 02H: Set Cursor Position
To call:
AH = 02H
BH = display page (0 in graphics)
DH = row number
DL = line number
Returns:
Nothing
Function 03H: Read Cursor Position, Size, and Shape
To call:
AH = 03H
BH = display page
Returns:
CH = starting scan line
CL = ending scan line
DH = row number
DL = column number
Function 04H: Read Light-Pen Position
To call:
AH = 04H
Returns:
AH = status:
= 01H pen triggered
= 00H not triggered
BX = pixel column number
CH = pixel line number
CX = pixel line number for some EGA modes
DH = character row number
DL = character column number
Function 05H: Select Active Page
To call:
AH = 05H
AL = page number:
= 00-07H 40-column text modes
= 00-03H 80-column text modes
= varies EGA graphics modes
Note: Each page = 2 KB in 40-column text mode, 4 KB in 80-column text
mode.
Returns:
Nothing
Function 06H: Scroll Window Up
Function 07H: Scroll Window Down
To call:
AH = 06H scroll up
= 07H scroll down
AL = number of lines to scroll (00H blanks screen)
BH = display attributes for blank lines
CH = row number of upper left corner
CL = column number of upper left corner
DH = row number of lower right corner
DL = column number of lower right corner
Returns:
Nothing
Function 08H: Read Character and Attribute at Cursor
To call:
AH = 08H
BH = display page (for text mode only)
Returns:
If text mode:
AH = color attributes of character
AL = ASCII character from current location
If graphics mode:
AL = ASCII character (00H if unmatched)
Function 09H: Write Character and Attribute
To call:
AH = 09H
AL = ASCII character to write
BH = display page
BL = text attribute or graphics foreground color
CX = number of times to write character (must be > 0)
Returns:
Nothing
Note: Cursor position unchanged.
Function 0AH: Write Character Only
To call:
AH = 0AH
AL = ASCII character to write
BH = display page
BL = graphics foreground color (unused in text modes)
CX = number of times to write character (must be > 0)
Returns:
Nothing
Note: Cursor position unchanged.
Function 0BH: Select Color Palette
To call:
AH = 0BH
BH = palette color ID
BL = color or palette value
Returns:
Nothing
Function 0CH: Write Pixel Dot
To call:
AH = 0CH
AL = color attribute of pixel
CX = pixel column number
DX = pixel raster line number
Returns:
Nothing
Function 0DH: Read Pixel Dot
To call:
AH = 0DH
CX = pixel column number (0-based)
DX = pixel raster line number (0-based)
Returns:
AL = pixel color attribute
Function 0EH: Write Character as TTY
To call:
AH = 0EH
AL = ASCII character
BH = display page
BL = foreground color of character (unused in text mode)
Returns:
Nothing
Note: Cursor position advanced; beep, backspace, linefeed, and
carriage return active; all other characters displayed.
Function 0FH: Get Current Video Mode
To call:
AH = 0FH
Returns:
AH = characters per line (20, 40, or 80)
AL = current video mode (see Interrupt 10H Function 00H)
BH = active display page
Function 13H: Write Character String
To call:
AH = 13H
AL = subfunction number:
= 00H string shares attribute in BL, cursor unchanged
= 01H string shares attribute in BL, cursor advanced
= 02H each character has attribute, cursor unchanged
= 03H each character has attribute, cursor advanced
BH = active display page
BL = string attribute (for AL = 00H or 01H only)
CX = length of character string
DH = starting row number
DL = starting column number
ES:BP = address of string to be displayed
Note: For AL = 00H or 01H, string = (char, char, char, ...). For AL =
02H or 03H, string = (char, attr, char, attr, ...).
Returns:
Nothing
Note: For AL = 01H or 03H, cursor position set to location following
last character output.
Interrupt 11H: Get Peripheral Equipment List
Returns:
AX = equipment list code word (bit settings
PPMURRRUFFVVUUCI):
= PP number of printers installed
= M 1 if internal modem installed
= RRR number of RS-232 ports installed
= U unused
= FF number of floppy-disk drives minus 1 (0 = one drive)
= VV initial video mode:
00 = reserved
01 = 40-by-25 color
10 = 80-by-25 color
11 = 80-by-25 monochrome
= U unused
= C 1 if math coprocessor installed
= I 1 if IPL (Initial Program Load) diskette installed
Interrupt 12H: Get Usable Memory Size (KB)
Returns:
AX = available memory size in KB
Interrupt 13H: Disk Services
Function 00H: Reset Disk System
To call:
AH = 00H
AL = drive number:
= 00-7FH floppy disk
= 80-FFH fixed disk
Returns:
CF = 0 no error
= 1 error
AH = error code (see Interrupt 13H Function 01H below)
Function 01H: Get Disk Status
To call:
AH = 01H
Returns:
AL = disk status of previous disk operation:
= 00H no error
= 01H invalid command
= 02H address mark not found
= 03H write attempt on write-protected disk (F)
= 04H sector not found
= 05H reset failed (H)
= 06H floppy disk removed (F)
= 07H bad parameter table (H)
= 08H DMA overflow (F)
= 09H DMA crossed 64 KB boundary
= 0AH bad sector flag (H)
= 10H data error
= 11H ECC data error (H)
= 20H controller failed
= 40H seek failed
= 80H time out
= AAH drive not ready (H)
= BBH undefined error (H)
= CCH write fault (H)
= E0H status error (H)
Note: H = fixed disk only, F = floppy disk only.
Function 02H: Read Disk Sectors
Function 03H: Write Disk Sectors
Function 04H: Verify Disk Sectors
Function 05H: Format Disk Tracks
To call:
AH = 02H read disk sectors
= 03H write disk sectors
= 04H verify disk sectors
= 05H format disk track
AL = number of sectors
CH = cylinder number
CL = sector number (unused if AH = 05H)
DH = head number
DL = drive number
ES:BX = buffer address (unused if AH = 04H)
Returns:
CF = 0 no error
= 1 error
AH = error code (see Interrupt 13H Function 01H)
If AH was 05H on call:
ES:BX = 4-byte address field entries, 1 per sector:
= byte 0 cylinder number
= byte 1 head number
= byte 2 sector number
= byte 3 sector-size code:
00H 128 bytes per sector
01H 256 bytes per sector
02H 512 bytes per sector (standard)
03H 1024 bytes per sector
Function 08H: Get Current Drive Parameters
To call:
AH = 08H
DL = drive number
Returns:
AX = 00H
BH = 00H
BL = drive type
CH = low-order 8 bits of 10-bit maximum number of cylinders
CL = bits 7 and 6 high-order 2 bits of 10-bit maximum
number of cylinders
= bits 5-0 maximum number of sectors/track
DH = maximum head number
DL = number of drives installed
ES:DI = address of floppy-disk-drive parameter table
Function 09H: Initialize Hard-Disk Parameter Table
To call:
AH = 09H
Returns:
Nothing
Function 0AH: Read Long
Reads 512-byte sector plus 4-byte ECC code.
To call:
See Interrupt 13H Function 02H.
Returns:
See Interrupt 13H Function 02H.
Function 0BH: Write Long
Writes 512-byte sector plus 4-byte ECC code.
To call:
See Interrupt 13H Function 03H.
Returns:
See Interrupt 13H Function 03H.
Function 0CH: Seek to Head
Positions head but does not transfer data.
To call:
See Interrupt 13H Functions 02H and 03H.
Returns:
See Interrupt 13H Functions 02H and 03H.
Function 0DH: Alternate Disk Reset
To call:
AH = 0DH
DL = drive number
Returns:
Nothing
Function 10H: Test for Drive Ready
To call:
AH = 10H
DL = drive number
Returns:
AH = status
Function 11H: Recalibrate Drive
To call:
AH = 11H
DL = drive number
Returns:
AH = status
Function 14H: Controller Diagnostic
To call:
AH = 14H
Returns:
AH = status
Function 15H: Get Disk Type
To call:
AH = 15H
DL = drive number
Returns:
AH = drive type code:
= 00H no drive present
= 01H cannot sense when floppy disk is changed
Function 16H: Get Disk Type
To call:
AH = 16H
DL = drive number to check
Returns:
AH = 00H no change
= 06H floppy-disk change
Function 17H: Set Disk Type
To call:
AH = 17H
DL = drive number
AL = floppy-disk type code
Returns:
Nothing
Interrupt 14H: Serial Port Services
Function 00H: Initialize Port Parameters
To call:
AH = 00H
AL = serial port parameters (bit settings BBBPPSCC):
= BBB baud rate:
000 110 baud
001 150 baud
010 300 baud
011 600 baud
100 1200 baud
101 2400 baud
110 4800 baud
111 9600 baud
= PP parity code:
00 none
01 odd
10 none
11 even
= S number of stop bits code:
0 one stop bit
1 two stop bits
= CC character size:
00 unused
01 unused
10 7-bit character size
11 8-bit character size
DX = serial port number (0 = first port)
Returns:
Nothing
Function 01H: Send One Character
To call:
AH = 01H
AL = character to send
DX = serial port number (0 = first port)
Returns:
AH = error status (see Interrupt 14H Function 03H below):
= 00H no error
Function 02H: Receive One Character
To call:
AH = 02H
DX = serial port number (0 = first port)
Returns:
AL = character received
AH = error status (see Interrupt 14H Function 03H below):
= 00H no error
Function 03H: Get Port Status
To call:
AH = 03H
DX = serial port number (0 = first port)
Returns:
AX = serial port status:
= 8000H time out
= 4000H transfer shift register empty
= 2000H transfer holding register empty
= 1000H break detect
= 0800H framing error
= 0400H parity error
= 0200H overrun error
= 0100H data ready
= 0080H received line signal detect
= 0040H ring indicator
= 0020H data set ready
= 0010H clear to send
= 0008H delta receive line signal detect
= 0004H trailing edge ring detector
= 0002H delta data set ready
= 0001H delta clear to send
Note: Multiple conditions can be active simultaneously.
Interrupt 15H: Miscellaneous System Services
Function 00H: Turn On Cassette Motor
Function 01H: Turn Off Cassette Motor
To call:
AH = 00H turn on cassette motor
= 01H turn off cassette motor
Returns:
Nothing
Function 02H: Read Data from Cassette
To call:
AH = 02H
CX = number of bytes to read
ES:BX = buffer address
Returns:
CF = 0 no error
= 1 error
AH = error status (if needed):
= 01H CRC error
= 02H bit signals scrambled
= 03H no data found
DX = number of bytes read
ES:BX = location following last byte read
Function 03H: Write Data to Cassette
To call:
AH = 03H
CX = number of bytes to write
ES:BX = buffer address
Note: Blocking factor = 256 bytes/block.
Returns:
CX = 00H
ES:BX = location following last byte written
Interrupt 16H: Keyboard Services
Function 00H: Read Next Character
To call:
AH = 00H
Returns:
If ASCII characters:
AH = standard PC keyboard scan code
AL = ASCII character
If extended ASCII codes:
AH = extended ASCII code
AL = 00H
Note: Does not return until character is read; removes character from
keyboard buffer.
Function 01H: Report If Character Ready
To call:
AH = 01H
Returns:
ZF = 0 character ready
= 1 character not ready
AH = see Interrupt 16H Function 00H
AL = see Interrupt 16H Function 00H
Note: Returns immediately; does not remove character from keyboard
buffer.
Function 02H: Get Shift Status
To call:
AH = 02H
Returns:
AL = shift status:
= 01H right shift active
= 02H left shift active
= 04H Ctrl active
= 08H Alt active
= 10H Scroll Lock active
= 20H Num Lock active
= 40H Caps Lock active
= 80H insert state active
Note: Multiple states can be active simultaneously.
Interrupt 17H: Printer Services
Function 00H: Send Byte to Printer
To call:
AH = 00H
AL = character to be printed
DX = printer number
Returns:
AH = status (see Interrupt 17H Function 02H
Function 01H: Initialize Printer
To call:
AH = 01H
DX = printer number
Returns:
AH = status (see Interrupt 17H Function 02H below)
)
Function 02H: Get Printer Status
To call:
AH = 02H
DX = printer number
Returns:
AH = status:
= 01H time out
= 02H unused
= 04H unused
= 08H I/O error
= 10H printer selected
= 20H out of paper
= 40H printer acknowledgment
= 80H printer not busy (bit off, 0, = busy)
Note: Multiple states can be active simultaneously.
Interrupt 18H: Transfer Control to ROM-BASIC
Interrupt 19H: Reboot Computer (Warm Start)
Interrupt 1AH: Get/Set Time/Date
Function 00H: Read Current Clock Count
To call:
AH = 00H
Returns:
AL = midnight signal
CX = high-order word of tick count
DX = low-order word of tick count
Function 01H: Set Current Clock Count
To call:
AH = 01H
CX = high-order word of tick count
DX = low-order word of tick count
Returns:
Nothing
Function 02H: Read Real-Time Clock
To call:
AH = 02H
Returns:
CF = 0 clock running
= 1 clock stopped
CH = hours in BCD
CL = minutes in BCD
DH = seconds in BCD
Function 03H: Set Real-Time Clock
To call:
AH = 03H
CH = hours in BCD
CL = minutes in BCD
DH = seconds in BCD
DL = 00H standard time
= 01H daylight saving time
Returns:
Nothing
Function 04H: Read Date from Real-Time Clock
To call:
AH = 04H
Returns:
CF = 0 clock running
= 1 clock stopped
CH = century in BCD (19 or 20)
CL = year in BCD
DH = month in BCD
DL = day in BCD
Function 05H: Set Date in Real-Time Clock
To call:
AH = 05H
CH = century in BCD (19 or 20)
CL = year in BCD
DH = month in BCD
DL = day in BCD
Returns:
Nothing
Function 06H: Set Alarm
To call:
AH = 06H
CH = hours in BCD
CL = minutes in BCD
DH = seconds in BCD
Returns:
CF = status:
= 0 operation successful
= 1 alarm already set or clock stopped
Function 07H: Reset Alarm (Turn Alarm Off)
To call:
AH = 07H
Returns:
Nothing
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