alt Hacker NewsUnfortunately, 6502 can't XOR the accumulator with itself. I don't recall if the Z80 can, and loading an immediate 0 would be most efficient on those anyway.
Replies
bonzini • today at 1:20 PM
The Z80 can do either LD A,0 or SUB A or XOR A, but the LD is slower due to the extra memory cycle to load the second byte of the instruction.
repelsteeltje • today at 2:28 PM
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
XOR A absolutely works on Z80 and it's of course faster and shorter than loading a zero value with LD A,0. LD A,0 is encoded to 2 bytes while XOR A is encoded as a single opcode. XOR A has the additional benefit to also clear all the flags to 0. Sub A will clear the accumulator, but it will always set the N flag on Z80.