NASM 2.05 based x86 Instruction Reference[ch239]

A.5.215 PFMAX: Packed Single-Precision FP Maximum PFMAX mm1,mm2/m64 ; 0F 0F /r A4 [PENT,3DNOW] PFMAX returns the higher of each pair of single-precision FP values. If the higher value is zero, it is returned as positive zero.