> >You only need to save/restore the registers that your interrupt
> >service routine uses... You don't even need to store W and STATUS,
> >if your ISR doesn't affect them.
>
> For an example of this:
>
>         org     004h                    ; Interrupt Service Routine
>         bcf     T0IF                    ; Clear interrupt source
>         btfsc   TimerRunning            ; Timer active?
>         decfsz  ms_Timer,F              ;       Doesn't touch STATUS register
>         nop                             ;       Needed because of DECFSZ
>         retfie                          ;       Back to caller

Another useful variation:
        ...
        decfsz  ticks,f
         retfie
        movwf   save_w          ; If we had enough ticks...
        movf    STATUS,w        ; we do the "main" part of our ISR...
        movwf   save_stat       ; and now have to save status/W

In many cases, you'll have parts of the ISR that need to use W/status, but
if 90% of interrupts don't need them you can save time by only saving when
necessary.