A.5.212 PFACC: Packed Single-Precision FP Accumulate
PFACC mm1,mm2/m64 ; 0F 0F /r AE [PENT,3DNOW]
PFACC adds the two single-precision FP values from the destination
operand together, then adds the two single-precision FP values from the
source operand, and places the results in the low and high doublewords
of the destination operand.
The operation is:
dst[0-31] := dst[0-31] + dst[32-63],
dst[32-63] := src[0-31] + src[32-63].