Re:How many registers does an x86-64 CPU have? (2020)
by Joker_vD on Saturday February 14, 2026 @10:51AM
(#47015419)
The same way AMD added 8 new GPRs, I imagine: by introducing a new instruction prefix.
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Re:How many registers does an x86-64 CPU have? (2020)
by bonzini on Saturday February 14, 2026 @04:01PM
(#47018341)
Yes, two in fact. One is the same prefix (with subtle differences of course, it's x86) that was introduced for AVX512's 32 registers. The other is new and it's a two byte extension (0xd6 0x??) of the REX prefix (0x40-0x4f).
The longer prefix has extra functionality such as adding a third operand (i.e. add r8, r15, r16), blocking flags update, and accessing a few new instructions (push2, pop2, ccmp, ctest, cfcmov).