Re: [avr-gcc-list] Avoid register saving

[Christian Vogel]

Hi,

On Thu, Apr 01, 2004 at 09:42:35AM +0100, Filipe Campos wrote:
> But You give a good idea, How the ISR can be writen in ASM and than compiled
> by the AVR-GCC ?

You can use __attribute__((naked)), then the compiler will not add any
additional code to a function, you have to do all the housekeeping
yourself.

I would propose that you write the routine in C first, then look
at the generated assembly code and work from there...

I did this once; if I remember correctly, the gcc I used for this
code made something ugly out of the simple isr...

        register unsigned char tick asm("r3");

        #if NAKED_IRQ
        __attribute__ ((naked))
        SIGNAL(SIG_OUTPUT_COMPARE1A){
            asm("push r0");
            asm("in r0,0x3f"); /* saves status register... */
            tick++;            /* inc r3, may modify status reg. */
            asm("out 0x3f,r0");
            asm("pop r0");
            asm("reti");
        }
        #else
        SIGNAL(SIG_OUTPUT_COMPARE1A){
            tick++;
        }
        #endif

  Chris

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