vfmaddrnd231pd is a unique instruction from the FMA instruction set. Most instructions in that set have mnemonics like vfmadd231pd, but here rnd means something very specific. The Intel Software Development Manual documents it as: VFMADDRND231PD – Fused Multiply-Add of Packed Double-Precision Floating-Point Values with rounding control […] The immediate byte defines several bit fields that control rounding, DAZ, FTZ, and exception suppression (See Table 5-3).