NASM 2.05 based x86 Instruction Reference[ch246]
A.5.222 PFRCPIT2: Packed Single-Precision FP Reciprocal/ Reciprocal Square Root, Second Iteration Step PFRCPIT2 mm1,mm2/m64 ; 0F 0F /r B6 [PENT,3DNOW] PFRCPIT2 performs the second and final intermediate step in the calculation of a reciprocal or reciprocal square root, refining the values returned by the PFRCP and PFRSQRT instructions, respectively. The first source value ("mm1") is the output of either a PFRCPIT1 or a PFRSQIT1 instruction, and the second source is the output of either the PFRCP or the PFRSQRT instruction. For more details, see the AMD 3DNow! technology manual.