How many registers does an x86-64 CPU have?

There's definitely several ways to ask the question, but IMHO the best answer to an unqualified question is to frame it in terms of what the necessary state to store in a thread context. To that end, the registers boil down to: * 16 general-purpose registers. * 16 or 32 vector registers. * 7 vector mask registers * 8 x87 floating-point registers, with 8 MMX registers aliased. (To separate or not separate x87/mmx is definitely a challenging question) * 3…

This is how many registers the ISA exposes, but not the number of registers actually in the CPU. Typical CPUs have hundreds of registers. For example, Zen 4 's integer register file has 224 registers, and the FP/vector register file has 192 registers (per Wikipedia). This is useful to know because it can effect behavior. E.g. I've seen results where doing a register allocation pass with a large number of registers, followed by a pass with the number of registers…

I dislike weird semantic hoop-jumping articles like this. It purposefully obfuscates a simple question to make it seem more complex. Just in the GPR section, the answer is simple...in long mode there are 16 that can be subdivided. There aren't 58 additional registers because of that. Or, to get more specific, how many pieces of data are there in this union: union Registers { struct { unsigned short int AF; }; struct { unsigned char F, A; }; } regs…