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3-280 Vol. 2A EMMS—Empty MMX Technology State
INSTRUCTION SET REFERENCE, A-M
EMMS—Empty MMX Technology State
Description
Sets the values of all the tags in the x87 FPU tag word to empty (all 1s). This opera-
tion marks the x87 FPU data registers (which are aliased to the MMX technology
registers) as available for use by x87 FPU floating-point instructions. (See Figure 8-7
in the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, for
the format of the x87 FPU tag word.) All other MMX instructions (other than the
EMMS instruction) set all the tags in x87 FPU tag word to valid (all 0s).
The EMMS instruction must be used to clear the MMX technology state at the end of
all MMX technology procedures or subroutines and before calling other procedures or
subroutines that may execute x87 floating-point instructions. If a floating-point
instruction loads one of the registers in the x87 FPU data register stack before the
x87 FPU tag word has been reset by the EMMS instruction, an x87 floating-point
register stack overflow can occur that will result in an x87 floating-point exception or
incorrect result.
EMMS operation is the same in non-64-bit modes and 64-bit mode.
Operation
x87FPUTagWord ← FFFFH;
Intel C/C++ Compiler Intrinsic Equivalent
void _mm_empty()
Flags Affected
None.
Protected Mode Exceptions
#UD If CR0.EM[bit 2] = 1.
#NM If CR0.TS[bit 3] = 1.
#MF If there is a pending FPU exception.
#UD If the LOCK prefix is used.
Real-Address Mode Exceptions
Same exceptions as in protected mode.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
0F 77 EMMS Valid Valid Set the x87 FPU tag word to empty.