Re: [avr-gcc-list] Using a register throughout a program

[Peter N Lewis]

> Everything's working alright, but looking at the generated assembler
> source, I noticed that my variable 'outcount' is stored in the AVR's SRAM
> and loaded each time the signal handler gets called. I would have preferred
> to store it in a register, because it's used fairly often.

This should be a FAQ.

the short answer seems to be "there is no easy way"..  A more 
complicated answer appeared recently:

At 11:57 +0800 6/9/01, Lai Kang Tin wrote:
> I think you may try to change config/avr/avr.h in gcc source directory,
> there are some defines for the usage of register allocation, change
> what register(s) you wanted to FIXed purpose, but it seemded that you
> must not change some of them such as R24 R25, X,Y,Z,...
> Then recompile the GCC.

But in this case, I question the wisdom.  What speed is your 
processor running at and what speed is your serial running at?

Assuming 4MHz processor and 115,200 baud (pretty close to the worst 
case I can imagine), your interrupt will get executed at roughly 
11kHz, or once ever 350 cycles.  Is the extra couple cycles to load 
and store the variable (and extra couple cycles of interrupt latency) 
really worth worrying about?

Enjoy,
   Peter.

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