[avr-gcc-list] Writing code to multiple targets

[Larry Barello]

I have a bunch of code written for the classic AVR on
avr-gcc/gas.  I want to port it to the IAR compiler series
and I want to accommodate the various classes of AVR chips.
The latter is not so big a deal since the differences are
largely covered in the ioxxxx.h files. Yet, there are a few.
For example the clock divisor bits for TC0 change between
classic and mega103, the "names" of UCR and USR change
between classic and mega163.

1. Is there a preferred approach to dealing with these
differences beyond "#ifdef ATmega163 ..."

2. Does anyone have experience with writing code for both
IAR and Avr-gcc and is willing to share any macros/headers
they have developed to simplify the process?