EMMS

Empty MMX Technology State

Opcodes

Hex Mnemonic Encoding Long Mode Legacy Mode Description
0F 77 EMMS A Valid Valid Set the x87 FPU tag word to empty.

Instruction Operand Encoding

Op/En Operand 0 Operand 1 Operand 2 Operand 3
A NA NA NA NA

Description

Sets the values of all the tags in the x87 FPU tag word to empty (all 1s). This operation 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 IntelĀ®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.

Pseudo Code

x87FPUTagWord = FFFFH;

Flags Affected

None.

Exceptions

64-Bit Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Real-Address Mode Exceptions

Same exceptions as in protected mode.

Protected Mode Exceptions

Exception Description
#UD If the LOCK prefix is used.
#MF If there is a pending FPU exception.
#NM If CR0.TS[bit 3] = 1.
#UD If CR0.EM[bit 2] = 1.