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U. Glaeser

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FIGURE 39.28 Pack instruction converts larger subwords to smaller ones.<br />

FIGURE 39.29 Unpack high instruction.<br />

FIGURE 39.30 Unpack low instruction.<br />

unpack high or unpack low. The high/low unpack instructions select and unpack the high or low<br />

order subwords of a source register, when used with register zero as the second source register. 5<br />

Subword Permutation Instructions<br />

Ideally, it is desirable to be able to perform all possible permutations on packed data. This is only possible<br />

for small numbers of subwords. When the number of subwords increases, the number of control bits<br />

required to specify arbitrary permutations becomes too large to be encoded in an instruction. For the case<br />

of n subwords, the number of control bits used to specify a particular permutation of these n subwords<br />

5 Register zero gives a constant value of “zero” when used as a source register.<br />

© 2002 by CRC Press LLC

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