MMX 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.
Compare
PACKSSDW,
PACKUSWB.