NASM 2.05 based x86 Instruction Reference[ch225]
A.5.201 PADDUSB, PADDUSW: Add Packed Unsigned Integers With Saturation PADDUSB mm1,mm2/m64 ; 0F DC /r [PENT,MMX] PADDUSW mm1,mm2/m64 ; 0F DD /r [PENT,MMX] PADDUSB xmm1,xmm2/m128 ; 66 0F DC /r [WILLAMETTE,SSE2] PADDUSW xmm1,xmm2/m128 ; 66 0F DD /r [WILLAMETTE,SSE2] PADDUSx performs packed addition of the two operands, storing the result in the destination (first) operand. PADDUSB treats the operands as packed bytes, and adds each byte individually; and PADDUSW treats the operands as packed words. When an individual result is too large to fit in its destination, a saturated value is stored. The resulting value is the maximum value that will fit in the available space.