instruction to pack signed words into unsigned bytes.
Takes either two MMX registers
or an MMX register (source) and a 64 bit
memory location (destination) as operands
The high 32 bits of the destination register are filled with the 4 original words of the destination register, packed down to bytes. Word values over 255 are clamped
to 255; negative word values are clamped to 0. The low 32 bits of the destination register are filled in the same fashion, but with the source words from the source operand