Re: 6502 illegal opcodes questions

["sicklittlemonkey" <Nick.Westgate@gmail.com>]

> Since there were several versions of the 6502, isn't it likely that
> some had *differing* illegal ops and results?

The surprising answer is yes, but not many. ;-)

The earliest documentation I can find is Apple Assembly Line's
"So-Called Unused Opcodes" from 1981 (
http://bobsc5.home.comcast.net/aal/1981/aal8103.html#a2 ).

In 1995 I unknowingly duplicated this work (in less detail) and my
results on a Rockwell 6502 were almost identical to Bob
Sander-Cederlof's Synertek 6502:
http://www.apple2.org.za/mirrors/ground.icaen.uiowa.edu/Mirrors/uni-kl/hardware/undocumented_6502_opcodes

The Apple-based research doesn't differ much from the more detailed
research done on the C64, Atari and even BBC systems. It seems that
this is because the same MOS 6502 mask was used for the 6510 and other
6502 implementations.

In any case, some Apple games use these opcodes, and many more C64
games do. Although a multibyte NOP is often sufficient, in other cases
the expected operation is required, not hard to implement, and improves
emulation authenticity (e.g. crash behaviour).

Cheers,
Nick.