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(nasm.info)Section B.4.196

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B.4.196. `PACKSSDW', `PACKSSWB', `PACKUSWB': Pack Data
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     PACKSSDW mm1,mm2/m64          ; 0F 6B /r             [PENT,MMX]
     PACKSSWB mm1,mm2/m64          ; 0F 63 /r             [PENT,MMX]
     PACKUSWB mm1,mm2/m64          ; 0F 67 /r             [PENT,MMX]

     PACKSSDW xmm1,xmm2/m128       ; 66 0F 6B /r     [WILLAMETTE,SSE2]
     PACKSSWB xmm1,xmm2/m128       ; 66 0F 63 /r     [WILLAMETTE,SSE2]
     PACKUSWB xmm1,xmm2/m128       ; 66 0F 67 /r     [WILLAMETTE,SSE2]

   All these instructions start by combining the source and destination
operands, and then splitting the result in smaller sections which it
then packs into the destination register. The `MMX' versions pack two
64- bit operands into one 64-bit register, while the `SSE' versions pack
two 128-bit operands into one 128-bit register.

   * `PACKSSWB' splits the combined value into words, and then reduces
     the words to bytes, using signed saturation. It then packs the
     bytes into the destination register in the same order the words
     were in.

   * `PACKSSDW' performs the same operation as `PACKSSWB', except that
     it reduces doublewords to words, then packs them into the
     destination register.

   * `PACKUSWB' performs the same operation as `PACKSSWB', except that
     it uses unsigned saturation when reducing the size of the elements.

   To perform signed saturation on a number, it is replaced by the
largest signed number (`7FFFh' or `7Fh') that _will_ fit, and if it is
too small it is replaced by the smallest signed number (`8000h' or
`80h') that will fit. To perform unsigned saturation, the input is
treated as unsigned, and the input is replaced by the largest unsigned
number that will fit.