What does 'REX' stand for in an x86-64 REX prefix?

Question

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?

Pledge

Based on other names, I'd give a guess of register extensions. – Metrorrhagia

probably similar to the R in RAX: What does the 'R' in x64 register names stand for?, What does the R stand for in RAX, RBX, RCX, RDX, RSI, RDI, RBP, RSP? – Ferroconcrete

Answer 1

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.

Berners