Does it run on a ZX81? If not I'd guess it might be using short cuts not available on the original machine. Easy way to get them smaller and be legitimate ches

Does it run on a ZX81? If not I'd guess it might be using short cuts not available on the original machine. Easy way to get them smaller and be legitimate chess (This seems to be missing half the rules) would be let white move, then resign. You might have to allow white to resign / offer a draw as a move as well.