On most of the commercial games I've tried on my devcarts the PGM ROM will work just fine with no CHR ROM in place except of course the graphics will be pretty hard to follow. On some games such as Xexyz if you don't put in the CHR chip the game either just "hangs" at the "green screen" or later or just doesn't work properly. On these games it seems the main code execution is dependent on the values in the CHR ROM. Has anybody ever seen any code that would account for this?