For ECC enabled designs, the app_wdf_mask signal can be used along with the byte write command (WR_BYTE_COMMAND) to perform a read modified write operation. However, in MIG v3.7 and previous releases the app_wdf_mask does not exist in the top-level of the design and is declared as an internal wire and tied to all zeros for ECC enabled designs.
Starting in the ISE 13.2 softwarerelease in MIG v3.8, app_wdf_mask is a top-level port for ECC enabled designs and can now be driven along with the byte write command (WR_BYTE_COMMAND) in order to drive partial mask enabled data, if read modified write operations with ECC are required.
If read modified write operations are not used, then app_wdf_mask can be driven to all zeros along with normal byte commands.