Not only bootloaders, but also monitors often make use of function calls from the app to the monitors. I can remember many years ago the 8051 monitor chips published entry points to things like putchar, etc.

I don't want to get that inflexible, but it might be nice to reuse some of the functions in the bootloader (instead of having two copies of them ... one in the bootloader and one in the appl). Any bright ideas how to do it? Just have a table in a fixed part of memory that contains the addresses of the various functions?