Developer Guide and Reference


The EMMS Instruction: Why You Need It

Using EMMS is like emptying a container to accommodate new content. The EMMS instruction clears the MMX™ registers and sets the value of the floating-point tag word to empty.
You should clear the MMX™ registers before issuing a floating-point instruction because floating-point convention specifies that the floating-point stack be cleared after use. Insert the EMMS instruction at the end of all MMX™ code segments to avoid a floating-point overflow exception.

Why You Need EMMS to Reset After an MMX™ Instruction

Failure to empty the multimedia state after using an MMX™ technology instruction and before using a floating-point instruction can result in unexpected execution or poor performance.

Product and Performance Information


Intel's compilers may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include SSE2, SSE3, and SSSE3 instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors. Certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors. Please refer to the applicable product User and Reference Guides for more information regarding the specific instruction sets covered by this notice.

Notice revision #20110804