instruction to pack signed words into signed 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 127 are clamped
to 127; word values under -128 are clamped to -128. The low 32 bits of the destination register are filled in the same fashion, but with the source words from the source operand