DeiMOS – A Superoptimizer for the MOS 6502

reminds me a bit of https://pubby.games/codegen.html just that its approach seems way more refined and useful.

If you assume that A * 10 isn't going to overflow, so that ASL A moves 0 into the carry flag (so no need for CLC), then instead of using the undocumented RRA opcode, you can just do:

sta $00

asl a

asl a

adc $00

asl a

This is also 7 bytes, but is faster since adc $00 is 3 cycles, vs rra $00 being 5 cycles.

The A = max(A, X) example is certainly interesting, but not very useful since it loops through the code twice (very slow) and assumes that $8a is available. The much faster obvious version only adds one byte:

stx $00

cmp $00

bcs done

txa

done:

Interesting and fun read - we are well into the terrain of what was completely impossible to do back then. Now I can't wait to see a faster AppleSoft ROM ;-)

That’s incredibly clever and a fun read. Well done!

I imagine lots of demo coders glancing back and forth between that writeup and their own carefully hand-tuned assembly.

Great