XOR'ing a register with itself is the idiom for zeroing it out. Why not sub?

Re:XOR'ing a register with itself is the idiom for zeroing i... by repelsteeltje on Wednesday April 22, 2026 @10:28AM (#47864165)

You're absolutely right, I stand corrected.

The 6502 gets by doing immediate load: 2 clock cycles, 2 bytes (frequently followed by single byte register transfer instruction). Out of curiosity I did a quick scan of the MOS 1.20 rom of the BBC micro:

  LDY #0 (a0 00): 38 hits
  LDX #0 (a2 00): 28 hits
  LDA #0 (a9 00): 48 hits

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Re:XOR'ing a register with itself is the idiom for zeroing i... by tom_ on Wednesday April 22, 2026 @09:47PM (#47871457)

Are you sure you're not an LLM? There is no way anybody writing 6502 would do anything else, because there's no other way to do it.

(You can squeeze in a cheeky Txx instruction afterwards to get a 2-or-more-for-1, if that would be what you need - but this only saves bytes. Every instruction on the 6502 takes 2+ cycles! You could have done repeated immediate loads. The cycle count would be the same and the code would be more general.)

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Re:XOR'ing a register with itself is the idiom for zeroing i... by repelsteeltje on Thursday April 23, 2026 @09:19AM (#47875445)

> Are you sure you're not an LLM?

Hard to tell, but I don't think so ;-)

I suppose using Txx instructions rather than LDx is more of an idiom than intended to conserve space. Also, could an LDx #0 potentially be 3 cycles in the edge case where the PC crosses a page boundary? (I'm probably confused? Red herring?)

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Re:XOR'ing a register with itself is the idiom for zeroing i... by tom_ on Thursday April 23, 2026 @08:45PM (#47884156)

I don't know how the 6502's PC increment actually worked, but it was an exception to the general rule of page crossings (or the possibility thereof) incurring a penalty, or, as was also sometimes the case, just ignored entirely. (One big advantage of the latter approach: doing nothing does take 0 cycles.)

The full 16 bits would be incremented after each instruction byte fetched, and it didn't cost any extra if there was a carry out of the MSB.

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