Intel® 64 and IA-32 Architectures Optimization Reference Manual

OPTIMIZING FOR SIMD INTEGER APPLICATIONSTwo signed doublewords are used as source operands and the result is interleavedsigned words. The sequence in Example 5-6 can be extended in SSE2 to interleaveeight signed words using XMM registers.MM/M64mmD C B AD 1B 1C 1A 1mmOM15160Figure 5-2. Interleaved Pack with SaturationExample 5-6. Interleaved Pack with Saturation Code; Input:MM0 signed source1 value; MM1 signed source2 value; Output:MM0 the first and third words contain the; signed-saturated doublewords from MM0,; the second and fourth words contain; signed-saturated doublewords from MM1;packssdw mm0, mm0 ; pack and sign saturatepackssdw mm1, mm1 ; pack and sign saturatepunpcklwd mm0, mm1 ; interleave the low-end 16-bit; values of the operandsPack instructions always assume that source operands are signed numbers. Theresult in the destination register is always defined by the pack instruction thatperforms the operation. For example, PACKSSDW packs each of two signed 32-bitvalues of two sources into four saturated 16-bit signed values in a destinationregister. PACKUSWB, on the other hand, packs the four signed 16-bit values of twosources into eight saturated eight-bit unsigned values in the destination.5-9