instruction to pack signed double words into signed words.
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 2 original double words of the destination register, packed down to words. Dword
values over 32767 are clamped
to 32767; dword values under -32768 are clamped to -32768. The low 32 bits of the destination register are filled in the same fashion, but with the source words from the source operand