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(nasm.info)Section B.4.78

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B.4.78. `FEMMS': Faster Enter/Exit of the MMX or floating-point state
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     FEMMS                         ; 0F 0E           [PENT,3DNOW]

   `FEMMS' can be used in place of the `EMMS' instruction on processors
which support the 3DNow! instruction set. Following execution of
`FEMMS', the state of the `MMX/FP' registers is undefined, and this
allows a faster context switch between `FP' and `MMX' instructions. The
`FEMMS' instruction can also be used _before_ executing `MMX'
instructions