A custom board need a specific map file?

For a custom board, is it necessary to write a specific map file like 'rpxlite.c' and 'mbx860.c'? I'm really new to MTD stuff. I looked into these two files as examples and found that they are almost similar. BTW, what's differece between '__raw_readb()' and 'readb()'? Thanks.

- Shawn.