David Horne's 1K Chess on the ZX81 (2001)
Impressive, as is the recent smaller program from a French programmer.Note, however, that both have sacrificed some rules. I believe they have both left out en passant, which is rare enough that it won't be missed most of the time. It's rare enough that if you use it against a casual player, there is a good chance they will accuse you of making it up.Castling, however, which is also omitted, is much more serious. Getting castled is one of the top goals of the opening. It happens in nearly every game. Taking that out is huge.Pawn promotion is missing from the older program but included in the smaller French program (making the French achievement even more impressive). I'd say that is not as big an omission as castling, because many games end before it gets to that point. Still, the threat of promotion has a major influence on endgame play.So what we actually have are very impressive implementations of chess-like games. That raises the question of what is the smallest implementation of full chess?I suspect that both these programs could be expanded to the full rules without making them a lot bigger, by taking the short cut of allowing the human to make those moves, but not the computer (especially if the computer trusts the human and does not check for legality of the human's move). I think you could defend a claim that you then have full chess, and it is just a limitation of the AI that it does not consider those moves.