Comment Re:typo in CPU (Score 1) 27
And it gets more confusing as some people put 65C02 processors in or a 65816 (which includes 65C02 instructions). Even in the Atari community, people will get it backwards.
The summary mentions added instructions, so I assume those are 65C02 instructions, most notably the STZ instruction (store zero), allowing you to write zero to memory without altering one of the real registers. The downside is that there were undocumented instructions that people had figured out on the original 6502, and some people used them. Apparently it was done in some cases to obfuscate code to make removing copy protection more difficult. A few of the instructions were actually useful in some cases. So adding in new instructions from the 65C02 or 65816 reduces compatibility. The only exception would be the original opcodes that locked up the CPU.
As a side note, the undocumented instructions were caused by not having entries in the instruction decoding table for them, so they got a mix of entries for other instructions, enabling various parts of the CPU in ways that behaved strangely.