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They were also discussed at length in the German magazine "64er". fe here: https://www.64er-magazin.de/8503/opcodes.html

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I remember the knowledge/idea of illegal opcodes for the C64 slowly making their way through the hacker/cracker community. Like secret knowledge being distilled back then via BBS

they were quite useful. I distinctly remember there were undocumented NOPs consuming a different number of cycles than the default one EA. These were fe used to pad raster line interrupt routines with cycle precision. (damn, I'm old)

IIRC the "normal" NOP took 2 cycles, so we used the illegal ones to waste an odd number of cycles for accurate raster line timing.

It's not like it's hard to find a harmless 3 cycle instruction.

For example Store A/X/Y to any unused/scratch Zero Page location.

Or condition branch to the next instruction using a condition known to be true .. BCC/BCS is often the most convenient. Or just JMP to the next instruction, though it's 3 bytes of code instead of 2.

INC/DEC of a scratch Zero Page location is useful for a 5 cycle delay in 2 bytes of code if you don't mind changing NZ flags, but the store/branch/jump alternatives don't even do that.

What do you mean by "fe"? "For example "?

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