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(nasm.info)Section B.4.231

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B.4.231. `PMADDWD': MMX Packed Multiply and Add
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     PMADDWD mm1,mm2/m64           ; 0F F5 /r             [PENT,MMX]
     PMADDWD xmm1,xmm2/m128        ; 66 0F F5 /r     [WILLAMETTE,SSE2]

   `PMADDWD' treats its two inputs as vectors of signed words. It
multiplies corresponding elements of the two operands, giving doubleword
results. These are then added together in pairs and stored in the
destination operand.

   The operation of this instruction is:

        dst[0-31]   := (dst[0-15] * src[0-15])
                                    + (dst[16-31] * src[16-31]);
        dst[32-63]  := (dst[32-47] * src[32-47])
                                    + (dst[48-63] * src[48-63]);

   The following apply to the `SSE' version of the instruction:

        dst[64-95]  := (dst[64-79] * src[64-79])
                                    + (dst[80-95] * src[80-95]);
        dst[96-127] := (dst[96-111] * src[96-111])
                                    + (dst[112-127] * src[112-127]).