[avr-gcc-list] Interrupt prologue and epilogue

[Simon Han]

Hi all,

	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.  :-)

	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__

Thank you for any pointers,
Simon