Block RAM used in the 9K mode (RAMB8BWER) can fail to initialize user specified data or default values (all zeros) during configuration in all Spartan-6devices.
Based on the selected IP Parameters, the cores listed below might use a block RAM in 9K mode and need to use the default or pre-initialized values.
If you are using one of the IP listed below, you should review the MAP report to see if your design is using the RAMB8BWER.
As of ISE Design Suite 13.1, designs containing 9K block RAM will see a physical DRC warning in MAP and in BitGen.
If you see this warning and are using one of the IP listed below then your design does contain a RAMB8BWER, and you should see the Spartan-6 Errata (including EN148) and (Xilinx Answer 39999) for more details on the Spartan-6 9k block RAM initialization issue.