Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual page 1702

EMMS—Empty MMX State
Opcode
0F 77
Description
Sets the values of all the tags in the FPU tag word to empty (all ones). This operation
marks the MMX technology registers as available, so they can subsequently be used by
floating-point instructions. (See Figure 7-11 in the Intel Architecture Software
Developer's Manual, Volume 1, for the format of the FPU tag word.) All other MMX
technology instructions (other than the EMMS instruction) set all the tags in FPU tag
word to valid (all zeros).
The EMMS instruction must be used to clear the MMX technology state at the end of all
MMX technology routines and before calling other procedures or subroutines that may
execute floating-point instructions. If a floating-point instruction loads one of the
registers in the FPU register stack before the FPU tag word has been reset by the EMMS
instruction, a floating-point stack overflow can occur that will result in a floating-point
exception or incorrect result.
Operation
FPUTagWord  FFFFH;
Flags Affected
None.
Additional Itanium System Environment Exceptions
Itanium Reg Faults Disabled FP Register Fault if PSR.dfl is 1.
Protected Mode Exceptions
#UD
#NM
#MF
Real-Address Mode Exceptions
#UD
#NM
#MF
Virtual-8086 Mode Exceptions
#UD
#NM
#MF
4:400
Instruction Description
EMMS Set the FP tag word to empty.
If EM in CR0 is set.
If TS in CR0 is set.
If there is a pending FPU exception.
If EM in CR0 is set.
If TS in CR0 is set.
If there is a pending FPU exception.
If EM in CR0 is set.
If TS in CR0 is set.
If there is a pending FPU exception.
Volume 4: IA-32 Intel
®
MMX™ Technology Instruction Reference