CLASH: a new chess engine for the 1965 PDP-8

[Dream.Build.Create]

I've written a chess program for the DEC PDP-8, to see how much of what this community has learned over the last fifty years still works on a 1965 architecture at its original speed.

The machine: a 12-bit word, no hardware stack (JMS stores the return address in the subroutine's first word), no multiply, memory in 4K-word fields, and about 416,000 instructions a second on a PDP-8/e. CLASH is written in PDP-8 assembly, occupies 16K words, and runs standalone. At PDP-8/e speed it searches roughly 175 nodes a second, which means three or four ply in a typical middlegame and deeper as the position simplifies.

What went in: iterative deepening, alpha-beta with PVS, killer and piece-square move ordering, quiescence search of captures and promotions with a check extension, null-move pruning, pondering, repetition detection without a hash table, and a small book. Scores are 12-bit with pawn = 32. There is no transposition table; at a few thousand nodes a move the hit rate didn't justify the memory or the key maintenance. Time control is by node count, so the program behaves identically on a simulator, a PiDP-8 or real hardware.

The search and evaluation owe their shape to H.G. Muller's micro-Max 4.8, and the move-ordering key came from Ed Schroder's Rebel write-up. What was taken and what was changed is set out here:
https://github.com/dream-build-create/p ... VENANCE.md

The PDP-8 already had a chess program: John Comeau's CHEKMO-II (DECUS, 1974), a complete program in 4K words with alpha-beta, iterative deepening, SEE and check extensions. Its console conventions are the model for CLASH's, and both programs are in the release.

Against the lichess computer at level 5, rapid 15+10, 20 games at PDP-8/e speed, CLASH scored 12.5/20.

To try it:
- lichess: challenge PDP8-CLASH (5 minutes and slower).
- UCI: the Windows zip runs the real binary under SIMH throttled to PDP-8/e speed, and works in Arena, Cute Chess or cutechess-cli.
https://github.com/dream-build-create/p ... -Chess.zip
- README: https://github.com/dream-build-create/pdp8-chess

Chris Peters