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From Intel's SDM, section 2.2.1 specifies that the REX prefix is used to:

  • Specify GPRs and SSE registers.
  • Specify 64-bit operand size.
  • Specify extended control registers

But what do the letters REX stand for in the acronym?

Omar Darwish
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    Based on other names, I'd give a guess of register extensions. – Thomas Jager Dec 03 '19 at 20:46
  • probably similar to the R in RAX: [What does the 'R' in x64 register names stand for?](https://softwareengineering.stackexchange.com/q/127668/98103), [What does the R stand for in RAX, RBX, RCX, RDX, RSI, RDI, RBP, RSP?](https://stackoverflow.com/q/10995349/995714) – phuclv Dec 04 '19 at 01:45

1 Answers1

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This 2002 Hot Chips presentation by AMD expands the acronym on slide 10: "REX (Register Extension)".

Kevin McGrath and Dave Christie, "The AMD x86-64 Architecture: Extending the x86 to 64 bits", Hot Chips 14, August 2002.

njuffa
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