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Learning the PDP-10

Since I’ve more or less achieved all my goals for the PCjs PDP-11 Emulator, I’ve decided to turn my attention to an even older historically important DEC computer: the PDP-10.

Having never used a real DEC PDP-10, I’ve had a steep learning curve creating a PDP-10 Emulator. So far, the only PDP-10 Hardware Documentation I’ve read (or rather, skimmed) is the PDP-10 System Reference Manual (1968).

To make it easier to create and run some simple PDP-10 Opcode Tests, most of my initial development efforts have centered on the built-in PDP-10 Debugger. In addition to the usual disassembler capabilities, I’ve also included an assembler, so that I can easily enter instruction sequences like this:

a 100;
hrli 1,111111;
hrri 1,444444;
hlr 2,1;
hrl 2,1;
end

and immediately see the corresponding opcodes:

>> u 100 l4
000100: 505040 111111  HRLI    1,111111
000101: 541040 444444  HRRI    1,444444
000102: 544100 000001  HLR     2,1
000103: 504100 000001  HRL     2,1

It’s not a full-featured assembler; it’s little more than a brute-force reverse look-up through the disassembler’s opcode-to-mnemonic tables. But it gets the job done.

Next, I’ve written an initial set of PDP-10 Opcode Tests and compared the results to those produced by SIMH, and so far, so good. More than anything else, that’s just a testament to DEC’s excellent documentation.

One of the challenges in creating a JavaScript-based PDP-10 Emulator involves 36-bit arithmetic, because JavaScript bit-wise operators maintain only 32 bits. Extreme care must be taken to use only arithmetic operators whenever accessing and/or preserving the 4 most-significant bits in any 36-bit value.

So, before I started this project, I wrote a proof-of-concept Int36 Class that supports 36-bit addition, subtraction, multiplication, and division, as well as a number of helper functions (e.g., toString()). Further adaptation will be required, however, because Int36 multiplication produces 72-bit results and Int36 division uses 72-bit dividends, whereas the PDP-10 uses “double length fixed point” values that contain only 70 bits of magnitude with two matching sign bits.

I just finished writing opcode handlers for all the PDP-10 “Half Word” instructions (i.e., HLL, HRL, HRR, HLR, with all mode variations and bit extension options), but that’s just the tip of the iceberg. Hopefully within another month or so I’ll be able to find the time to finish the rest of the PDP-10 opcode handlers.

And then maybe we’ll be able to run some real PDP-10 software in our browsers.

[GitHub Source]

Jeff Parsons
Feb 28, 2017