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. Because floating-point convention specifies that the floating-point stack be cleared after use, you should clear the MMX registers before issuing a floating-point instruction.  You should 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(TM) Instruction

Caution

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