Re: [avr-gcc-list] Interrupt prologue and epilogue

[Galen Seitz]

Simon Han <address@hidden> wrote:

>       Is there any documentation about how interrupt prologue and
> epilogue  are implemented?  I am mainly interested in what registers
> need to be  save and why.  Please treat me as a person who is trying
> to implement  interrupt handler by hand.  :-)

http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_reg_usage

>       I understand that we have to save call-used registers, but I
> am not  clear why r28 and r29 need to be saved.  Further, why is it
> safe to  not save other registers?  Prologue also does the following,
> which I  hope to figure out.
> 
> push __zero_reg__
> push __tmp_reg__
> in __tmp_reg__,__SREG__
> push __tmp_reg__
> clr __zero_reg__
> 

r1 (zero_reg) and r0 (tmp_reg) are scratch regs, so they need to be saved.
Called functions expect r1 to be zero.  Status reg obviously needs to be
saved.

r29:r28 is sometimes used as a frame pointer.  After reading the FAQ,
I'm not sure why it needs to be saved.


galen