It didn't understand castling, en passant and promotions. It would also only play white.
I did once try to pit it against a modern engine. It seemed to do reasonably well until the modern engine sneakily castled, and I was unable to tell 1k Chess about this, thus forcing it to forfeit the game.
Of course, since posting that, I since found this:
Chess for the PC; 392 bytes. You put it in your boot sector. Still no castling, en passant and promotions, though, although there's an 831 byte version which has those.
