Assembly XMM registers 8 to 15

Asked Dec 30 '19 at 13:00

Active Dec 30 '19 at 14:09

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For some more optimized memory functions I am looking to utilize the XMM8 to XMM15 registers as that would mean I could move more data per loop. However, Intel insinuates different behaviors with these registers. I failed to find any weird bugs in my code to that regard but I am unsure if there might be any dangers laying around the corner. Intel states the following in their documentation:

XMM8 through XMM15 are available using REX.R in 64-bit mode.

To which I only have one questions, what is REX.R?

edited Dec 30 '19 at 13:08

Peter Cordes

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asked Dec 30 '19 at 13:00

Harm Smits

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Keep reading the vol.2 manual about REX prefixes. – Peter Cordes Dec 30 '19 at 13:08
So its just a 64 bit prefix required for certain operations? – Harm Smits Dec 30 '19 at 13:30
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REX is a 1-byte prefix (that exists only in 64-bit mode), and yes it's required for accessing XMM8..15 and/or r8..r15. And/or 64-bit operand-size. – Peter Cordes Dec 30 '19 at 13:34
Ah I see. That does make sense. I suppose NASM already handles this but good to know nonetheless. Thanks! – Harm Smits Dec 30 '19 at 13:39

Assembly XMM registers 8 to 15

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