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U. Glaeser

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FIGURE 39.17 Packed multiply high instruction.<br />

FIGURE 39.18 Packed multiply low instruction.<br />

Figure 39.18 shows a packed multiply low instruction, which places only the less significant lower<br />

halves of the products into the target register.<br />

IA-64 generalizes this with its packed multiply and shift right instruction (see Fig. 39.19),<br />

which does a parallel multiplication followed by a right shift. Instead of being able to choose either the<br />

upper or the lower half of the products to be put into the target register, it allows multiple 4 different 16-bit<br />

fields from each of the 32-bit products to be chosen and placed in the target register. Ideally, saturation<br />

4<br />

In IA-64 the right-shift amounts are limited to 0, 7, 15, or 16 bits, so that only 2 bits in the packed multiply<br />

and shift right instruction are needed to encode the four shift amounts.<br />

© 2002 by CRC Press LLC

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