A very simple example of this is the accumulator form of XCHG (exchange values). The accumulator form (AX/EAX/RAX is the accumulator) is encoded as: 10010

A very simple example of this is the accumulator form of XCHG (exchange values). The accumulator form (AX/EAX/RAX is the accumulator) is encoded as: 10010rrr rrr = register 000 = AX/EAX/RAX 001 = CX/ECX/RCX 010 = DX/EDX/RDX 011 = BX/EBX/RBX 100 = SP/ESP/RSP 101 = BP/EBP/RBP 110 = SI/ESI/RSI 111 = DI/EDI/RDI More inportantly, the XCHG instruction does NOT affect the flags. So the upshot is that the instruction `XCHG accumulator,accumulator` is effectively a no-operation, and yes, Intel does document the encoding for NOP as being 10010000 or `XCHG accumulator,accumulator`. Early chips in the x86 line (8086, 80286) actually did the physical exchange, but as the architecture grew with OoO operations, the chips now detect 0x90 as a NOP instruction and deal with it differently (most likely to prevent pipeline stalls waiting for the results of `XCHG accumulator,accumulator').