If Correction by Replace or Error Classification with any correction method (repair, enhanced repair, or replace is used, BitGen uses the "-g essentialbits:yes" option to generate an EBC and EBD file.
For more information on these files, see (Xilinx Answer 41197).
The BitGen report which has a ".bgn" extension tells the user how many essential bits are in the design.
Look at the bottom of the report for something similar to the following:
"This design has 206786 essential bits out of 33128352 total (0.62%)."
The numbers vary depending on the design and device size.
The calculation of essential bits should be enabled as part of your SEM IP generation.
Please refer to the SEM IP Product Guide (PG036) for instructions of how to enable essential bits generation.
13/12/2012 - Update with reference to SEM Documentation.
03/10/2011 - Initial release