NASM 2.05 based x86 Instruction Reference[ch245]
A.5.221 PFRCPIT1: Packed Single-Precision FP Reciprocal, First Iteration Step PFRCPIT1 mm1,mm2/m64 ; 0F 0F /r A6 [PENT,3DNOW] PFRCPIT1 performs the first intermediate step in the calculation of the reciprocal of a single-precision FP value. The first source value ("mm1" is the original value, and the second source value ("mm2/m64" is the result of a PFRCP instruction. For the final step in a reciprocal, returning the full 24-bit accuracy of a single-precision FP value, see PFRCPIT2 (section A.5.222). For more details, see the AMD 3DNow! technology manual.