In fact, once the 486 hit, 0x90 was an explicit NOP (1 cycle instead of the 3 that an XCHG usually takes, won't stall the pipeline even if other stuff touches A

In fact, once the 486 hit, 0x90 was an explicit NOP (1 cycle instead of the 3 that an XCHG usually takes, won't stall the pipeline even if other stuff touches AX, etc.). Similarly, on x64 0x90 is a true no-op, while stuff like XCHG EBX, EBX clears the upper 32 bits of the register.