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:
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.