User's Manual

12-2 Vol. 3

INTEL

MMX

™

TECHNOLOGY SYSTEM PROGRAMMING

result, the MMX register mapping is fixed and is not affected by value in the Top Of

Stack (TOS) field in the floating-point status word (bits 11 through 13).

When a value is written into an MMX register using an MMX instruction, the value also

appears in the corresponding floating-point register in bits 0 through 63. Likewise,

when a floating-point value written into a floating-point register by a x87 FPU, the

low 64 bits of that value also appears in a the corresponding MMX register.

The execution of MMX instructions have several side effects on the x87 FPU state

contained in the floating-point registers, the x87 FPU tag word, and the x87 FPU

status word. These side effects are as follows:

• When an MMX instruction writes a value into an MMX register, at the same time,

bits 64 through 79 of the corresponding floating-point register are set to all 1s.

• When an MMX instruction (other than the EMMS instruction) is executed, each of

the tag fields in the x87 FPU tag word is set to 00B (valid). (See also

Section

12.2.1, “Effect of MMX, x87 FPU, FXSAVE, and FXRSTOR Instructions on the x87

FPU Tag Word.”)

Figure 12-1. Mapping of MMX Registers to Floating-Point Registers

Floating-Point Registers

x87 FPU Status Register

x87 FPU Tag

MMX Registers

TOS

MM7

MM6

MM5

MM4

MM3

MM2

MM1

MM0

TOS = 0

000