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Fanael's random ruminations

Archives for the second quarter of 2020

The legend of "x86 CPUs decode instructions into RISC form internally"

Published on the

Topics: microarchitectural-musings, mythbusting

There is a widespread idea that modern high-performance x86 processors work by decoding the "complex" x86 instructions into "simple" RISC-like instructions that the rest of the pipeline then operates on. But how close is this idea to how the processors actually work internally?

To answer this question, let's analyze how different x86 processors, ranging from the first "modern" Intel microarchitecture, P6, to their current designs, handle the following simple loop (the code is 32-bit just to allow us to discuss very old x86 processors):

x86 assembly
    add [edx], eax
    add edx, 4
    sub eax, 1
    jnz .loop
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