A.5.224 PFRSQRT: Packed Single-Precision FP Reciprocal Square Root
Approximation
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 (section A.5.223) and PFRCPIT2 (section
A.5.221). This will result in a 24-bit accuracy. For more details, see
the AMD 3DNow! technology manual.