While working on some graphics code for the 65816 processor, I realized that I needed an 8-bit rotation of a binary value. However, neither the 6502 nor 65816 provide such a rotation — the best you can get is 9-bit (or 17-bit if you're in 16-bit register mode).
The code snips below ought to work both for the 6502 and the 65816, and can be written to even be m/x-bit independent as well.
8/16-bit Rotate Left
Here's a dirty little trick for performing a fairly rapid 8/16-bit rotate-left operation.
asl
adc #0
The idea here is that the ASL instruction will shift bit 7 (or 15) into the carry flag, and a 0 into bit 0. Since the addition can only add one or zero (depending on the state of the carry flag), the result is that bit 0 becomes the former value of the carry bit.
8/16-bit Rotate Right
This is the fastest I could come up with.
lsr
bcc *+2 ; or *+3 for 16-bit mode
ora #$80 ; or #$8000 for 16-bit mode
