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(nasm.info)Section B.4.223

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B.4.223. `PFRSQIT1': Packed Single-Precision FP Reciprocal Square Root, First Iteration Step
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     PFRSQIT1 mm1,mm2/m64          ; 0F 0F /r A7          [PENT,3DNOW]

   `PFRSQIT1' performs the first intermediate step in the calculation of
the reciprocal square root of a single-precision FP value. The first
source value (`mm1' is the square of the result of a `PFRSQRT'
instruction, and the second source value (`mm2/m64' is the original
value.

   For the final step in a calculation, returning the full 24-bit
accuracy of a single-precision FP value, see `PFRCPIT2' (*Note Section
B.4.222::).  For more details, see the AMD 3DNow! technology manual.