Vol. 3A 12-3

INTEL

®

 MMX

™

 TECHNOLOGY SYSTEM PROGRAMMING

12.2.1 

Effect of MMX, x87 FPU, FXSAVE, and FXRSTOR

Instructions on the x87 FPU Tag Word

Table 12-3 summarizes the effect of MMX and x87 FPU instructions and the FXSAVE and FXRSTOR instructions on 
the tags in the x87 FPU tag word and the corresponding tags in an image of the tag word stored in memory.
The values in the fields of the x87 FPU tag word do not affect the contents of the MMX registers or the execution of 
MMX instructions. However, the MMX instructions do modify the contents of the x87 FPU tag word, as is described 
in Section 12.2, “The MMX State and MMX Register Aliasing.” These modifications may affect the operation of the 
x87 FPU when executing x87 FPU instructions, if the x87 FPU state is not initialized or restored prior to beginning 
x87 FPU instruction execution.
Note that the FSAVE, FXSAVE, and FSTENV instructions (which save x87 FPU state information) read the x87 FPU 
tag register and contents of each of the floating-point registers, determine the actual tag values for each register 
(empty, nonzero, zero, or special), and store the updated tag word in memory. After executing these instructions, 
all the tags in the x87 FPU tag word are set to empty (11B). Likewise, the EMMS instruction clears MMX state from 
the MMX/floating-point registers by setting all the tags in the x87 FPU tag word to 11B.

12.3 SAVING 

AND 

RESTORING 

THE MMX STATE AND REGISTERS

Because the MMX registers are aliased to the x87 FPU data registers, the MMX state can be saved to memory and 
restored from memory as follows:

Table 12-2.  Effects of MMX Instructions on x87 FPU State

MMX Instruction 

Type

x87 FPU Tag 

Word

TOS Field of x87 

FPU Status Word

Other x87 FPU 

Registers

Bits 64 Through 79 

of x87 FPU Data 

Registers

Bits 0 Through 63 

of x87 FPU Data 

Registers

Read from MMX 

register

All tags set to 00B 

(Valid)

000B

Unchanged

Unchanged

Unchanged

Write to MMX 

register

All tags set to 00B 

(Valid)

000B

Unchanged

Set to all 1s

Overwritten with 

MMX data

EMMS

All fields set to 

11B (Empty)

000B

Unchanged

Unchanged

Unchanged

Table 12-3.  Effect of the MMX

,

 x87 FPU, and FXSAVE/FXRSTOR Instructions on the x87 FPU Tag Word

Instruction

Type

Instruction

x87 FPU Tag Word

Image of x87 FPU Tag Word Stored in 

Memory

MMX

All (except EMMS)

All tags are set to 00B (valid).

Not affected.

MMX

EMMS

All tags are set to 11B (empty).

Not affected.

x87 FPU

All (except FSAVE, 

FSTENV, FRSTOR, 

FLDENV)

Tag for modified floating-point register is 

set to 00B or 11B.

Not affected.

x87 FPU and 

FXSAVE

FSAVE, FSTENV, FXSAVE

Tags and register values are read and 

interpreted; then all tags are set to 11B.

Tags are set according to the actual 

values in the floating-point registers; 

that is, empty registers are marked 11B 

and valid registers are marked 00B 

(nonzero), 01B (zero), or 10B (special).

x87 FPU and 

FXRSTOR

FRSTOR, FLDENV, 

FXRSTOR

All tags marked 11B in memory are set 

to 11B; all other tags are set according 

to the value in the corresponding 

floating-point register: 00B (nonzero), 

01B (zero), or 10B (special).

Tags are read and interpreted, but not 

modified.