I would like to create my own custom flash loader. I studied the apps of ATMEL but somehow, I can't find a step by step procedure to implement my own loader.

What I have in mind:
write a loader/flash programming Function, locating it in the upper part of the flash.
From there erase the user program, then reprogram it.
What confuses me in the documentation is how to call erase / program functions of the built in boot loader. Do I have to know their absolute addresses? What about different versions (I have Uart versions 1.1.2 and 1.4.0 on different chips we bought)

I appreciate any hints

Werner