Really simple question. Reading this it seems that branch instructions index off of the next instruction. For instance:



When assembled, would BRA LABEL be BRA -1 or BRA -3 (80 FF or 80 FD)?

The document I mentioned shows the case of:


Now, if this were to assemble to BRA 0 we'd be on top of the next instruction, as it seems to imply, but if it assembles to BRA -2 since the label is 2 addresses behind the next instruction, we'd be infinite looping on our current instruction. Which way is correct?

It would be BRA -3 or BRA $FD. The 6502 uses two's complement for negative numbers.

BRA 0 is correct (-2+2=0). You could see it as a 3 cycle 2 byte NOP.

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6502 sources on GitHub: https://github.com/Klaus2m5

Thank you for the quick response.

I knew about two's complement, I messed up my numbering, I corrected it but it seems you caught it before I got to it

This is what I had thought, and why I didn't use the code that I got out of my assembler, as I figured this would be a stumbling block. Many thanks, now I can make this change and get on with some more features that are badly needed.