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AR# 19716

XST - XST now has limited support for null arrays


With the release of 6.3i XST, XST now has limited support of null arrays. In VHDL, a null array is any range that has no value. For example: 


std_logic_vector(1 to 0) 




for index in 1 to 0 loop 


or any other type of construct where the direction goes contrary to the range values given. XST does not support all uses of null arrays.


The following table lists supported and unsupported uses of null arrays: 


Null Array Support
Null Array Support


In the instances where null arrays are not supported, XST will still pass the design, but in rare circumstances there could be some undesirable results such as multi-source errors, internal/fatal errors, or incorrect logic. Avoid using null array constructs in cases where they are not properly supported.

AR# 19716
Date Created 09/03/2007
Last Updated 05/16/2014
Status Archive
Type General Article