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(nasm.info)Section B.4.208

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B.4.208. `PDISTIB': MMX Packed Distance and Accumulate with Implied Register
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     PDISTIB mm,m64                ; 0F 54 /r             [CYRIX,MMX]

   `PDISTIB', specific to the Cyrix MMX extensions, treats its two input
operands as vectors of eight unsigned bytes. For each byte position, it
finds the absolute difference between the bytes in that position in the
two input operands, and adds that value to the byte in the same
position in the implied output register. The addition is saturated to
an unsigned byte in the same way as `PADDUSB'.

   To work out the implied register, invert the lowest bit in the
register number. So `PDISTIB MM0,M64' would put the result in `MM1', but
`PDISTIB MM1,M64' would put the result in `MM0'.

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

   Operation:

        dstI[0-7]     := dstI[0-7]   + ABS(src0[0-7] - src1[0-7]),
        dstI[8-15]    := dstI[8-15]  + ABS(src0[8-15] - src1[8-15]),
        .......
        .......
        dstI[56-63]   := dstI[56-63] + ABS(src0[56-63] - src1[56-63]).