3 ms·

Sure. That "If" kind of makes it moot. That's all I was trying to point out - that it's a documentation thing rather than something in the design of the chip an

• creshal 11y ago
  > Edit: What do you mean "still treat it as xchg"? On those chips, isn't there no distinction between xchg and what we might retrospectively call "nop"? Perhaps this is something I'm missing. XCHG EAX, EAX in its dumbest interpretation loads EAX into a temporary register, replaces it with the contents of EAX and restores the temporary data to… EAX. So, it is an operation that does nothing, but it does nothing in an elaborate way. You can skip it instead of executing it, but only if your other code doesn't depend on 0x90 taking exactly three clock cycles. The "treat 0x90 as NOP and skip it instead of wasting three cycles" optimization was only done with the 486 and up, and wasn't retroactively applied to the 386 embedded versions. Doing so would have messed up their timings, and would have needed a small design change, both not interesting to that customer base. (The 386 and derivatives were still produced for embedded use for a long, long time, past 2001 – and thus, after the introduction of AMD64. When its instruction set was drafted, new 386-based embedded devices were still designed.)