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(nasm.info)Section B.4.230

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B.4.230. `PMACHRIW': Packed Multiply and Accumulate with Rounding
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     PMACHRIW mm,m64               ; 0F 5E /r             [CYRIX,MMX]

   `PMACHRIW' takes two packed 16-bit integer inputs, multiplies the
values in the inputs, rounds on bit 15 of each result, then adds bits
15-30 of each result to the corresponding position of the _implied_
destination register.

   The operation of this instruction is:

        dstI[0-15]  := dstI[0-15]  + (mm[0-15] *m64[0-15]
                                               + 0x00004000)[15-30],
        dstI[16-31] := dstI[16-31] + (mm[16-31]*m64[16-31]
                                               + 0x00004000)[15-30],
        dstI[32-47] := dstI[32-47] + (mm[32-47]*m64[32-47]
                                               + 0x00004000)[15-30],
        dstI[48-63] := dstI[48-63] + (mm[48-63]*m64[48-63]
                                               + 0x00004000)[15-30].

   Note that `PMACHRIW' cannot take a register as its second source
operand.