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:
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: