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No, x87/MMX and SSE XMM were not aliased. They were two separate register files.

(You may be confusing it with XMM vs YMM vs ZMM, i.e. SSE vs AVX vs AVX512.)

It’s more likely they’re confusing it with 3DNow! (AMD’s own media extension focused on floating point math). That was strictly aliased and therefore an either/or situation on chips that had both.

Sure, 3DNow! operated on MMX registers.

But the OP was talking about Intel's P3... so 3DNow! wasn't a factor...

I think we’re agreeing. I think they’re mixing up the 3DNow introduction with the SSE one and conflating them into one.

SSE never had aliasing, so OP’s comment is strictly incorrect either way.

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