This repository contains a chess playing program for the historic ENIAC computer, and the tooling used to create it. It implements 4 ply alpha-beta search with a simple materiel evaluation function, and would generate one move in about 5 hours of ENIAC machine time (much faster in simulation, of course).

The goal is to demonstrate that a reasonably powerful chess playing program was already possible when ENIAC became the first operational general purpose digital computer in early 1946, perhaps a few months after Alan Turing set down (what we believe to be) the first reference to the possibility of a digital computer playing chess. Or perhaps the goal is to demonstrate that, even though chess has been a measure of human intelligence for centuries and then a measure of machine intelligence, it's not actually that difficult if the very first computer could play it at an amateur level.

ENIAC was not originally a stored program computer. Instead, it was a bunch of function units that could be wired together with patch cables. People later figured out how to rig it up to execute opcodes stored in the 3,600 digits of the machine's "function tables." These were originally intented to be lookup tables for things like cosines but ended up being used as the machine's ROM.

To make ENIAC play chess, first we set up the machine to implement a custom CPU with 51 opcodes, essentially a "virtual machine." The VM appears to the programmer to have 10 registers and 75 words of linear addressable memory, where each word is two decimal digits, and would execute about 500 virtual instructions per second on the real machine. On the bright side, we have space for somewhat less than 1,800 instructions in ROM, and plenty of chess programs have been smaller than this. Then we wrote a tiny chess program for this VM, based on alpha-beta search with some extensions.

Interaction with the final program is via virtual punched cards. The human sets up the board via punched cards, each of which writes a one word to a given address. After the search completes, the machine prints its best move as from and to squares on a similar card. Because ENIAC is so slow, reasonable play takes nearly a day per move, so we envision the machine used to play correspondence chess.

Creating this program involves multiple layers of tools and cross-validation.

• Programming an ENIAC virtual machine - how we built chessvm on top of ENIAC's ill-suited hardware.
• ChessVM Reference - chessvm instruction set, architecture, and microcode reference
• Putting the Chess in ENIAC Chess - How our tiny chess program runs on our tiny VM

This project relies on a pulse-level ENIAC simulator derived from Briain Stuart's original simulator. Assuming that's installed, and the starting position is in board.fen,

make chess
python fen2deck.py < board.fen > asm/chess.deck
eniacsim chess.e

This will read in the deck, and (eventually) output a single card with four digits FFTT, where FF=from square, TT=tosquare. The squares are numbered 11 to 88 in rank,file notation. Currently ENIAC always plays white, though setting memory location fromp to 10 (adding a card punched 3510 to the intial deck) should make it play black.

You can also run the chess program much faster (~1000x vs ~100x realtime) by executing on the VM emulator chsim, which skips pulse-level simulation. Note that eniacsim also executes chsim in parallel to validate the ENIAC VM implementation chessvm.easm against the VM emulator chsim.cc.

chsim/chsim -f asm/chess.deck chess.e

You can simply use the script runchess.py to play a game against ENIAC. Just run

python runchess.py

The script will build the chess program and invoke eniacsim in a mode that loads chsim as a dynamic library to get faster simulation. It also starts an eniacsim debug GUI for inspecting machine state at https://localhost:8000/vm.html and another for visualizing ENIAC hardware state at https://localhost:8000/blinkenlights

We implemented these simpler games during VM development. Python wrappers provide nice interfaces so you don't have to punch cards manually (create .deck files) to enter your move. Try any of

python runtic.py
python runc4.py
make life; eniacsim chess.e

Do make test to test easm, chasm, chsim, and chess.

To test the VM implementation, do make vmtest to assemble chessvm.easm into an ENIAC patch, assemble asm/vmtest.asm into switch settings, and then concatenate the two into the simulator to run a self test. A successful test will print out TTSS where TT=incrementing test numbers and SS=test status code, where success is 00 and anything else is failure. The ENIAC may also hang or loop, of course. This also tests chsim by running the VM emulator in parallel and comparing the results to the simulator state.

To test the chess engine, do python asm_test.py. This will assemble asm/movegen_test.asm,asm/move_test.asm and asm/chess.asm to test move generation, move execution, and move search respectively.

This work is licensed under a Creative Commons Attribution 4.0 International License