"NO_CHANGE" operating mode is not supported when 32kx1 is used.
This issue applies to Virtex-4 only. If the 32kx1 primitive is used, when a read is performed on the top 16kx1 RAMB16 followed by a write operation on the bottom 16kx1 RAMB16, the output will change although the operating mode is "NO_CHANGE". This occurs in both behavioral and structural simulations.
This issue will be addressed in future release.