Ok, so I was reading through the AMD64 manuels and knowing that nop is really an xchg eax, eax, I looked at the xchg and found something interesting, that it seems a byte can be encoded into the instruction for specifying the registers (apologies I'm on my iPod): picture.

So what I am wondering is how does the processor know if there is a byte after to work with or is it that that extra register has to be of type rAX causing it to actually still be the one byte 0x90