MMX 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. Compare PACKSSWB, PACKUSDW.

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