Re: [avr-gcc-list] avr-gcc ABI question

[E. Weddington]

Trampas wrote:

> First off I am using avr-gcc 3.4.1 I have the following function and was
> wondering if the return value was correct. That is where I call the function
> it looks to be reading r24 and r25 as the 16bit return value.
>
> UWORD getMilliSeconds()
> {
>         UWORD temp;
>         TIMSK=TIMSK & ~(1<<OCIE1A); //Turn Timer1 compare 1A interrupt off
> 174:    89 b7           in      r24, 0x39       ; 57
> 176:    8f 7e           andi    r24, 0xEF       ; 239
> 178:    89 bf           out     0x39, r24       ; 57
>         temp=MilliSeconds;
> 17a:    20 91 20 01     lds     r18, 0x0120
> 17e:    30 91 21 01     lds     r19, 0x0121
>         TIMSK=TIMSK | (1<<OCIE1A); //Turn Timer1 compare 1A interrupt on
> 182:    89 b7           in      r24, 0x39       ; 57
> 184:    80 61           ori     r24, 0x10       ; 16
> 186:    89 bf           out     0x39, r24       ; 57
>         return temp;
> }
> 188:    c9 01           movw    r24, r18
> 18a:    08 95           ret
>
>  
I'm not quite sure what you are asking.

Yes, the return value is in r25:r24.
Yes, the return value was copied to r25:r24 from r19:r18 (by this: movw 
r24,r18).
Yes r18 was loaded with 0x0120 and r19 was loaded with 0x0121, which 
correspondes to the "temp = MilliSeconds;" statement. It can only be 
assumed that MilliSeconds has the correct value that you want (since you 
don't show how MilliSeconds got that value).

Here is the info on what registers are used:
<http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_reg_usage>
Perhaps this will help with what you are looking for?

Eric