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(nasm.info)Section B.4.224

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B.4.224. `PFRSQRT': Packed Single-Precision FP Reciprocal Square Root Approximation
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     PFRSQRT mm1,mm2/m64           ; 0F 0F /r 97          [PENT,3DNOW]

   `PFRSQRT' performs a low precision estimate of the reciprocal square
root of the low-order single-precision FP value in the source operand,
storing the result in both halves of the destination register. The
result is accurate to 15 bits.

   For higher precision reciprocals, this instruction should be
followed by two more instructions: `PFRSQIT1' (*Note Section B.4.223::)
and `PFRCPIT2' (*Note Section B.4.221::). This will result in a 24-bit
accuracy. For more details, see the AMD 3DNow! technology manual.