[avr-gcc-list] Hard coded asm in an interrupt routine

[Grant Stockly]

I have a time critical routine written in assembler that has to be ran 
exactly the same length regardless of the outcome of various condition 
loops (+-650ns).  I've used nops and jmps to acheive +-2 instruction cycle 
runtime for a ~500 instruction routine.

Now I run into the problem of integrating this into a gcc C project.  Every 
time there is an external interrupt I want to run this piece of code.  Can 
I assume that during a 'C' Interrupt that the general purpose registers 
such as r16, r17  and r20, r21 are safe to destroy?  I need to hard code 
this to work exactly this way.

I also need to be able to initialize the stack about 30 bytes below the end 
of external ram.  Can anyone point me to the file that has this info?

I'm coming from the IAR 1.4 era, and avr-gcc is looking good.  $2460 is too 
much for IAR's new compilier.  Agree?  : )  I have experience programming 
linux kernel device drivers and user space tcp programs on bsd.

Thanks,
Grant

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