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A comment on Intel x86 documentation has more pages than the 6502 has transistors. Parent comment

Re:Intel x86 documentation has more pages than the 6502 has ... by flamedoge on Wednesday December 07, 2016 @11:38PM (#13128435)
What was the main reason to add REX prefix for 64bit? Why not create longer register bits to hold 16 registers? Was it for easier binary to binary transformation?
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Re:Intel x86 documentation has more pages than the 6502 has ... by pcwalton on Thursday December 08, 2016 @12:33AM (#13128622)
Because AMD was deathly afraid of AMD64 going the way of Itanium. So they went out of their way to make their architecture as similar to 32-bit x86 as possible, right down to reusing the encoding.

They also probably figured that they could reuse decoding logic.

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