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3.1i XST - Limitation of the IOB constraint with a shift register (SRL) extraction turned on.
When a shift register LUT (SRL) is inferred, the IOB=TRUE attribute does not take effect.
To use the IOB registers in this case, shift register extraction must be completely turned off. This can be done by using the attribute "shreg_extract."
For more details about the shreg_extract attribute, please see the XST User Guide at:
NOTE: This problem is fixed in the 4.1i software release.