Re: [avr-gcc-list] interrupt optimization

[David Bourgeois]

Thanks for your answer David.
Unfortunately going entirely in asm for the interrupt is not the option I  
would like to have here.

> First, can you not enable this int unless it's needed?

This was a caricatural example. Actually my prog outputs 8 bits pwm so I  
only have the prescaler to set the frequency. I need to further correct it  
by software in the interrupt.

> Second, it dosen't matter what the compiler is doing, an ASM coded int  
> needs
> to save SREG, and any regs you modify during the int, and that's all.

I can code parts of the ISR in assembly but I would like to keep at least  
the inline function call in C because it's quite complex and will probably  
need further modifications. So in this case I need to save all registers  
my C function uses before the function call.
I can check them in the lss file and manually include psuh/pop in inline  
asm but then I'm afraid the function will use other registers if I modify  
it later on. I can still be carefull and modify the inline asm accordingly  
but I'm just asking here if there's a better way of doing this.

> I've even done ints that didn't bother saving the sreg, but those were
> special cases where I knew absolutely that I wasn't doing anything that
> would change the contents of SREG.
>
> So:
>
> Push R16  ;saving whatever the compiler thought was going on.
> in R16,SREG
> push R16 ;saving sreg
>
> int code, using R16
>
> pop R16
> out SREG,R16
> pop R16
> reti
>
> When I code in assembler, I reserve a register to hold SREG, and two more
> (typically) for interrupt use, as ITEMP and ITEMP2.  This allows me to  
> do a
> lot of things without pushes and pops.
>
> In one app, on an 8515 at 8 mhz, I had to deal with 300,000 ints/sec  
> from a
> bursty source.  No problem with this approach.



--
David Bourgeois

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