This is pretty cool. What is your chess playing background?
Casual/amateur. I learned how to play at age 5 and soon could beat everyone in my family and school. It's one of my lifelong hobbies but I never joined a chess club or played in any tournaments. I'm a professional computer programmer. In fact, back in 1983 when I played this chess game I was also attending high school and writing video games in 6502 and 6809 assembly language as a freelance developer.
It always amazed me that Atari could write a chess program with only 128 bytes of RAM. The gaming magazines at the time went on and on about the VCS limit of 3 (or 6) sprites per row and how this was overcome with the "venetian blind" trick. As if the hardest part of a chess program is the graphics.
Recently I Googled the subject again and mostly found the same garbage about venetian blinds. One comment here on AtariAge had a good clue. Usually a chess program is recursive and stores a complete board at each node in the call tree. Instead, you could just have a global board in RAM (say 32 bytes, one for each piece) and remember "diffs" so you can undo a move after considering it. If a diff was 4 bytes, you could recurse 5 ply for 30 bytes including the return address on the stack. This design might also explain the multiple move bug at levels 6 and 7. Perhaps moves didn't always get properly undone at deeper plies.
Oh wow, I forgot I'd even made this thread.
I joined this site just to reply! The Google search above led me here and this thread was crying out for someone with Video Chess level 7 experience.