x87 Tag unexpected value

PMatos — Wed, 18 Sep 2024 08:35:35 GMT

I was playing with x87 part of the ISA. According to the combined ISA manual, section 9.6.2 it says:

> When an MMX instruction (other than the EMMS instruction) is executed, the processor changes the x87 FPU state as follows:
>
> * The entire x87 FPU tag word is set to the valid state (00B in all tag fields).

among other things but the FPU tag word value was the one I couldn't reproduce. I wrote the following:

section .data
 quadword_data dq 0x1234567890abcdef

section .text
global _start

_start:
finit
fld1
fldpi
movq mm0, [quadword_data]
hlt

Before movq, the tag word is 0xfff as expected since top is at r6 and r6 and r7 are valid. But after movq I was expecting the tag word to be 0x0 but instead it's 0x556. Why is this and why does it differ with what's specified in the manual?

Thanks.

topic x87 Tag unexpected value in Software Tuning, Performance Optimization & Platform Monitoring

x87 Tag unexpected value