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by Jiří {x2} Činčura

Two little nuggets about NOP instruction

2 Jul 2018 Assembly

I’m researching some spin locks stuff last few days. That led to checking also NOP instruction and I’ve found two interesting little nuggets.

#1

If you look at the opcode for NOP, you’ll find it’s 0x90. So far so good. But XCHG EAX, EAX is 0x90 as well on Intel x86. Interesting! XCHG EAX, EAX is doing “nothing”, so, I think architects just reused it. Certainly, I’m not the first one to realize this, it’s described in the documentation (i.e. here), but who reads it, right?

#2

All the above is on Intel x86. On x64 (AMD64) if the NOP would be XCHG EAX, EAX, the upper 32 bits of RAX would be cleared (because of how the 64-bit extension of x86 works) on NOP. Not good. Thus on x64 the XCHG EAX, EAX was changed to be 0x87, 0xC0 (more info).