It gets worse with x86_64 though. Normally, an instruction that isn't 64-bit will clear the upper 32 bits of a 64-bit register. The NOP is special, because it s

It gets worse with x86_64 though. Normally, an instruction that isn't 64-bit will clear the upper 32 bits of a 64-bit register. The NOP is special, because it shouldn't do anything. All the other 32-bit XCHG instructions still clear the upper 32 bits of a 64-bit register. So the NOP really is different, and a normal XCHG EAX,EAX is not possible with the 0x90 encoding. You can get a normal XCHG EAX,EAX via the 2-byte ModRM form of the instruction.