Modifying opb_bram under EDK

The opb_bram is for use with the block ram inside the FPGA. Are you really planning to add redundancy and checkbits to that? If so, I would wonder why. If the BRAM can be corrupted, then so can the rest of the FPGA, so are you also adding error detection and correction to the rest of the FPGA?

My EDK does come with source for the opb_bram, assuming you are referring to EDK6.3/hw/XilinxProcessorIPLib/pcores/opb_bram_if_cntlr_v1_00_a/hdl/vhdl So I cannot imagine why you wouldn't have it.

The code for this is generated on the fly in an EDK project. It should end up being located in project_dir/hdl/elaborate/...

So I guess in your case, create a simple project with the desired block ram, then copy and modify the generated file.