Where applicable, use a UCF to assign the constraints. In the case of NOREDUCE, please see (Xilinx Solution 9912).
The PIN_ASSIGN attribute will not propagate the constraint properly on a non-standard bus such as (15 downto 8) instead of (7 downto 0). Please see (Xilinx Solution 10363) for more information.
NOTE: This problem is fixed in the 4.1i software release.