59 lines
3 KiB
Markdown
59 lines
3 KiB
Markdown
---
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layout: post
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title: Learning the PDP-10
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date: 2017-02-28 22:00:00
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permalink: /blog/2017/02/28/
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---
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Since I've more or less achieved all my goals for the PCjs [PDP-11 Emulator](/devices/pdp11/machine/),
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I've decided to turn my attention to an even older historically important DEC computer:
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the [PDP-10](https://en.wikipedia.org/wiki/PDP-10#Cancellation_and_influence).
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Having never used a real DEC PDP-10, I've had a steep learning curve creating a [PDP-10 Emulator](/modules/pdp10/).
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So far, the only [PDP-10 Hardware Documentation](/pubs/dec/pdp10/) I've read (or rather, skimmed) is the
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[PDP-10 System Reference Manual (1968)](http://archive.pcjs.org/pubs/dec/pdp10/ka10/DEC-10-HGAA-D_PDP-10_System_Reference_Manual_May1968.pdf).
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To make it easier to create and run some simple [PDP-10 Opcode Tests](/apps/pdp10/tests/opcodes/), most of my initial development
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efforts have centered on the built-in PDP-10 Debugger. In addition to the usual disassembler capabilities, I've also included
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an assembler, so that I can easily enter instruction sequences like this:
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a 100;
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hrli 1,111111;
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hrri 1,444444;
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hlr 2,1;
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hrl 2,1;
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end
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and immediately see the corresponding opcodes:
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>> u 100 l4
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000100: 505040 111111 HRLI 1,111111
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000101: 541040 444444 HRRI 1,444444
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000102: 544100 000001 HLR 2,1
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000103: 504100 000001 HRL 2,1
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It's not a full-featured assembler; it's little more than a brute-force reverse look-up through the disassembler's
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opcode-to-mnemonic tables. But it gets the job done.
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Next, I've written an initial set of [PDP-10 Opcode Tests](/apps/pdp10/tests/opcodes/) and compared the results to those produced
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by [SIMH](https://github.com/simh/simh), and so far, so good. More than anything else, that's just a testament to DEC's
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excellent documentation.
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One of the challenges in creating a JavaScript-based [PDP-10 Emulator](/modules/pdp10/) involves 36-bit arithmetic, because
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JavaScript bit-wise operators maintain only 32 bits. Extreme care must be taken to use only *arithmetic* operators whenever
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accessing and/or preserving the 4 most-significant bits in any 36-bit value.
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So, before I started this project, I wrote a proof-of-concept [Int36 Class](/modules/shared/lib/int36.js) that supports 36-bit
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addition, subtraction, multiplication, and division, as well as a number of helper functions (e.g., *toString()*). Further
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adaptation will be required, however, because **Int36** multiplication produces 72-bit results and **Int36** division uses 72-bit
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dividends, whereas the PDP-10 uses "double length fixed point" values that contain only 70 bits of magnitude with two matching
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sign bits.
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I just finished writing opcode handlers for all the PDP-10 "Half Word" instructions (i.e., *HLL*, *HRL*, *HRR*, *HLR*, with all
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mode variations and bit extension options), but that's just the tip of the iceberg. Hopefully within another month or
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so I'll be able to find the time to finish the rest of the PDP-10 opcode handlers.
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And then maybe we'll be able to run some *real* PDP-10 software in our browsers.
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*[@jeffpar](http://twitter.com/jeffpar)*
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*Feb 28, 2017*
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