EDN8 loads mappers from .RBF files. Basically, RBF files is a quartus binary output, without any changes.
OS load mapper files from /EDFC/MAPS/ 
.RBF file size recommended to be divisible by 512, with 0xff bytes in the end.
.RBF file name must be in upper case, otherwise OS may not find the file, if file was copied from linux machine.
.RBF file can be loaded via USB for testing, in this case size and file name does not matter.
During loading via USB, .RBF file will not be saved on SD card, it will be loaded directly to fpga config ram and will be active till reset. After reset OS will use default files from SD card.
Each .RBF file may contain multiple mappers, or just one

OS use mappers association table, to link some mapper number to specific .RBF file. This table stored in MAPROUT.BIN
MAPROUT.BIN contains 2 tables, 256 bytes each. Upper table is for normal mappers (begins at offset 0), lower is for save state mappers (begins at offset 0x100).
Each record in MAPROUT.BIN one byte. Offset equal to mapper number, value equal to .RBF name (decimal number)
How to link mapper number to .RBF file:
Let say 000.RBF contains implementation for mapper 3 and for mapper 7. Need to write 0x00 to the MAPROUT.BIN at offset 0x03 and 0x07 (MAPROUT.BIN[3] = 0, MAPROUT.BIN[7] = 0)
Value 0xFF means that mapper is not supported.
If save stated option turned on, OS will check value from lower table first, if it contains 0xff, OS will try to load non save state mapper from upper table

Last reccord in both tables reserved for OS, always should be 0xff
record 0xfe used for FDS