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Difference between Xilinx shift_extract and shreg_extract constraints?

Can someone explain the difference between the Xilinx shift_extract and shreg_extract constraints?

Shreg_extract appears to prevent inferring SRL16 based shifters, what does shift_extract do?

Thanks!

John Providenza

Reply to
johnp
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Hi John,

The SHIFT_EXTRACT attribute/constraint prevents (or not) XST from extracting logical shift operations during synthesis. That is, if your design contains a parallel shift left/right of a bus by some dynamic distance (a "barrel shifter"), XST can try to do something clever with it (rather than just leaving it as random logic). It doesn't have anything to do with SRL16s.

Cheers,

-Ben-

Reply to
Ben Jones

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