3 ms·

I'm pretty sure that x87 is not microcode emulated (at least the basic arithmetic ops). I'm sure they use the same FPUs as SSE of course.

• dragontamer 4y ago
  x87 is 80-bit floating point. They literally don't fit inside of 64-bit doubles of SSE. The extra bits need to be emulated. EDIT: And I'm sure there's some program out there that actually relies on those extra 16-bits of precision, and they'd be pissed if their least-significant bit had a fraction-of-a-bit more error per operation.
  • gpderetta 4y ago
    They are not emulated, they run at optimal latency (in fact on Ice Lake FADD has better latency than ADDSD!), although at a lower throughput as there are less dedicated execution units.
    • dragontamer 4y ago
      That's a strong point. I guess they really aren't emulated then. That really makes me wonder how the 80-bits are stored then. I guess the "stack" is just part of the register-renaming mechanism? Huh... AVX registers are 256-bits, so I guess 80-bits fits in each one.
      • gpderetta 4y ago
        Yes, x86 stack per se doesn't exist anymore and it is mapped to the general register file. I have no idea how the 80 bits are handled. I thought that the AVX registers mapped to multiple entries in the file, but maybe I'm wrong.
      • monocasa 4y ago
        Its own register file (that is shared with the AVX-512 K mask registers). https://travisdowns.github.io/blog/2020/05/26/kreg2.html https://travisdowns.github.io/blog/2020/05/26/kreg2.html
        • gpderetta 4y ago
          Nice find! In fact I think I must have read this article when it made the rounds. Then I must have promptly forgotten it :)