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The C64 used the 6510 CPU which is a slightly different version of the 6502. One difference was that you had a few extra instructions. For example, I believe you could transfer the index registers so you wouldn't have to go to memory. TXY, TYX. These could be done in 1 clock cycle versus 4 clock cycles if you had to get a memory.

I imagine emulators need to consider this.

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The only difference you can rely on between a 6502 and 6510 is the IO ports on the 6510.

Beyond that, the handling of undocumented opcodes can be different between different models.

TXY/TYX is only present on the 16-bit 65C816

Well, I had a C64 and a C128 and I do remember using them. Am I misremembering?

[Update] I guess there was an entire set of illegal opcodes on the 6510. Unfortunately, I longer have my source code; that shoebox is long gone.

The one thing I do remember is that it had something to do with registers because it gave my code a computer significant performance boost because I didn't have to go to memory so often.

References:

http://codebase64.org/doku.php?id=illegalmnemonicslist

http://www.ffd2.com/fridge/docs/6502-NMOS.extra.opcodes

>I guess there was an entire set of illegal opcodes on the 6510

Yes, but they were the same set of illegal/undocumented opcodes as the 6502.

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