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I have looked -- no luck so far.

I will keep looking...

Another fun issue with the split 64/64 support: lo-hi order.

You see, when two orders are possible, of course Intel had to ship both.

Yeeep, P3 vs P4. Which used which, is left as an exercise to you, dear reader.

Sure, 3DNow! operated on MMX registers.

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

Please refrain from making things up -- Long Mode has always supported x87.

You may be mis-remembering LAHF/SAHF.