[gp2000]

The links to Don Hodges site are great if you want the explicit low-level details. For instance, he has a patch to fix the kill screen:

http://donhodges.com/how_high_can_you_get2.htm

I made a much less serious patch -- a backwards-facing Pac-Man that places dots instead of eating them:

http://members.shaw.ca/gp2000/bacman.html