Behavior of bus address in "popl %eax"

Asked Jul 05 '20 at 22:52

Active Jul 05 '20 at 23:40

Viewed 84 times

In a scenario where I have the following assembly instruction:

popl %eax

Would there be any information flowing through the address bus?

From what I understand about the functionality of a CPU, there would be no information flowing through it, since it would only be storing the value of %esp into %eax, therefore using only the CPU bus. Nevertheless, I think I am wrong. Can anyone explain it to me?

edited Jul 05 '20 at 23:40

Peter Cordes

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asked Jul 05 '20 at 22:52

Vitor Carvalho

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`pop` does not transfer `esp` into `eax`. It reads from memory at address `esp`. – Jester Jul 05 '20 at 22:57
Ooh, okay, so it would have to look for what is inside esp address, wouldn't it? – Vitor Carvalho Jul 05 '20 at 22:58
That is correct. – Jester Jul 05 '20 at 22:58
Thank you, Jester! – Vitor Carvalho Jul 05 '20 at 22:59
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Note that depending on whether the CPU has caches or not, no access to external RAM may actually happen. Refer to the instruction set reference for the behaviour of instructions. – fuz Jul 05 '20 at 23:30
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Real x86 CPUs are not simple. [x86 registers: MBR/MDR and instruction registers](https://stackoverflow.com/q/51522368). A 386 without a data cache might put the address of every load on an actual external bus, but modern CPUs work in cache lines. – Peter Cordes Jul 05 '20 at 23:41

Behavior of bus address in "popl %eax"

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