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(nasm.info)Section B.4.238

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B.4.238. `PMULHRWC', `PMULHRIW': Multiply Packed 16-bit Integers With Rounding, and Store High Word
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     PMULHRWC mm1,mm2/m64         ; 0F 59 /r              [CYRIX,MMX]
     PMULHRIW mm1,mm2/m64         ; 0F 5D /r              [CYRIX,MMX]

   These instructions take two packed 16-bit integer inputs, multiply
the values in the inputs, round on bit 15 of each result, then store
bits 15-30 of each result to the corresponding position of the
destination register.

   * For `PMULHRWC', the destination is the first source operand.

   * For `PMULHRIW', the destination is an implied register (worked out
     as described for `PADDSIW' (*Note Section B.4.200::)).

   The operation of this instruction is:

        dst[0-15]  := (src1[0-15] *src2[0-15]  + 0x00004000)[15-30]
        dst[16-31] := (src1[16-31]*src2[16-31] + 0x00004000)[15-30]
        dst[32-47] := (src1[32-47]*src2[32-47] + 0x00004000)[15-30]
        dst[48-63] := (src1[48-63]*src2[48-63] + 0x00004000)[15-30]

   See also `PMULHRWA' (*Note Section B.4.239::) for a 3DNow! version of
this instruction.