Linux has a good range of extremely strong chess engines such as Stockfish, Critter, Togo II, Crafty, GNU Chess, and Komodo. The chess engines featured in this article offer no match to a good chess engine, but they show how much can be achieved with a minuscule codebase.
My favourite ZX81 games were Flight Simulation, 3D Monster Maze, Galaxians, and above all 1K ZX Chess. Only the latter was written for the unexpanded ZX81. In fact, David Horne’s 1K ZX Chess was coded in a mere 672 bytes of RAM. However, the game managed to implement most chess rules, and offer a computer opponent. While some important rules were omitted (castling, pawn promotion, and en passant capture), it was still amazing to be able to play against artificial intelligence. The game took up a fair chunk of my misspent youth.
1K ZX Chess remained the smallest implementation of chess on any computer for 33 years until the record was broken by BootChess this year, and subsequently by Toledo AtomChess. These three games do not implement all of the chess rules, so for completeness I have included my favourite small implementation of chess that implements a complete set of chess rules.