Unless I'm missing something, the encodings are different at bits 6 and 7. Looking at xlogicx's post, MOV has them set to 0 while ROR has them at 1. On a highe

Unless I'm missing something, the encodings are different at bits 6 and 7. Looking at xlogicx's post, MOV has them set to 0 while ROR has them at 1. On a higher level, this post got me to realize that there could be ops (in this case nops) that have equivalent effects but are encoded as different instructions. Even though registers might state-change equivalently, I can see that the internal processor state might mutate differently. Maybe one nop encoding is faster, and maybe the caches get hit differently, etc. As an assembler, what reasons might there be to prefer one encoding over another? When designing an architecture, I can imagine putting a "fast nop" in the instruction encoder that essentially just short circuits around it. Is this something that's done in practice?