Re: [avr-gcc-list] Interrupt prologues

[E. Weddington]

Curtis Maloney wrote:

> All looks sane enough.  Saving the zero, temp, and status regs, and 
> the two regs the ISR uses.
>
> However, why save the zero and tmp regs, when they are not used at all 
> in the function?  Is this because they're "hidden" from GCC, and saved 
> as a precaution?
Well it's not "hidden" from GCC. GCC knows about them. But it's part of 
the ABI that the zero and tmp regs can be used by any function. So, an 
interrupt happens, and the ISR needs to save and restore it as your 
generated code in the ISR could use those registers, for example 
assigning zero to a variable.

If you're absolutely sure that your ISR won't use them and you need 
faster timing, I think the method you can use is to declare your ISR 
with a naked attribute, and then do the prologue and epilogue yourself. 
I haven't used that method, but search this list. IIRC, it was talked 
about recently.

HTH
Eric