Intel 253668-032US Benutzerhandbuch

12-4 Vol. 3

INTEL

MMX

™

TECHNOLOGY SYSTEM PROGRAMMING

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:

•

Execute an FSAVE, FNSAVE, or FXSAVE instruction to save the MMX state to
memory. (The FXSAVE instruction also saves the state of the XMM and MXCSR
registers.)

•

Execute an FRSTOR or FXRSTOR instruction to restore the MMX state from
memory. (The FXRSTOR instruction also restores the state of the XMM and
MXCSR registers.)

The save and restore methods described above are required for operating systems
(see Section 12.4, “Saving MMX State on Task or Context Switches”). Applications
can in some cases save and restore only the MMX registers in the following way:

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

(zero), or 10B (special).

Tags are read and

interpreted, but not modified.