Even "true" noop instructions then ones generated by writing corresponding opcode in assembly often decode to "no side effect instruction". Something like excha

Even "true" noop instructions then ones generated by writing corresponding opcode in assembly often decode to "no side effect instruction". Something like exchanging register with itself, or adding 0 to register. Modern high performance cpu designs will probably optimize those instructions and not preform the useless no side effect operation, but on simpler cpus extra logic for detecting it and not performing anything might not be worth it. So those noops will actually try to execute the no side effect operation which matches with decoded instruction. Wikipedia has table of them for different architectures https://en.wikipedia.org/wiki/NOP_(code) https://en.wikipedia.org/wiki/NOP_(code)