NASM 2.05 based x86 Instruction Reference[ch221]
A.5.197 PADDB, PADDW, PADDD: Add Packed Integers PADDB mm1,mm2/m64 ; 0F FC /r [PENT,MMX] PADDW mm1,mm2/m64 ; 0F FD /r [PENT,MMX] PADDD mm1,mm2/m64 ; 0F FE /r [PENT,MMX] PADDB xmm1,xmm2/m128 ; 66 0F FC /r [WILLAMETTE,SSE2] PADDW xmm1,xmm2/m128 ; 66 0F FD /r [WILLAMETTE,SSE2] PADDD xmm1,xmm2/m128 ; 66 0F FE /r [WILLAMETTE,SSE2] PADDx performs packed addition of the two operands, storing the result in the destination (first) operand. - PADDB treats the operands as packed bytes, and adds each byte individually; - PADDW treats the operands as packed words; - PADDD treats its operands as packed doublewords. When an individual result is too large to fit in its destination, it is wrapped around and the low bits are stored, with the carry bit discarded.