RE: [avr-gcc-list] ATmega2560 and r30/31 save in function prologue

[Stu Bell]

Preston Wilson wrote:

> "Stu Bell" wrote:
> 
> > Hi all,
> > 
> > I'm chasing down a problem with WinAVR 20080430 and my ATmega2560
code.
> > In generated code comparisons of an ISR between 20071221 and
20080430, 
> > I noticed that r30 and r31 are not saved on the stack.  Normally,
this 
> > wouldn't be a problem as the ISR itself does not use r30/r31.
> > 
> > However, this ISR calls a function that does use r30/r31.  Again, it
> > *should* be no problem, since the function has the responsibility to

> > store r30/r31 if it uses it.
> 
> It is the callers responsibility to save r30 and r31 registers if they

> are needed across a function call.  At least that is my reading of the

> following FAQ.
> 
> http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_reg_usage
> 
> I admit I do not know what is supposed to happen if an ISR is using a 
> call-used register.
> 
> -Preston

Up until WinAVR 2008*, the ISR took the monkey to save those registers.
This appears to be a breakdown of the ISR prologue, although I have not
confirmed that yet with a test case.

Best regards, 

Stu Bell 
DataPlay (DPHI, Inc.)