> Just for a start, DON'T re-enable GIE inside the ISR.  The RETFIE
> instruction does this automagically, and, if you have had an int whilst
> processing the isr, you'll blow your stack really quickly this way.  This
> is the sort of thing a simulator will probably never find, BTW, as its very
> time-sensitive.

As a slight clarification, never enable GIE within an ISR unless you are
absolutely sure you know what you're doing *AND* you have counted up the
stack overhead.  I have on a couple of programs used a short ISR which ran,
essentially, this:

        [ save W and PSW ]
        [ do a little bit of work]
        [ restore W and PSW ]
        decfsz  counter,f
         retfie
        bsf     gie
        [ save W and PSW to another pair of locations]
        [ set up counter again]
        [ do 'a lot' more work (more than a timer-ticks' worth ]
        [ restore PSW and W]
        retfie

My smaller interrupt routine does not make any sub-calls.  My larger one
does make one level of calls, but they are "guarded" with disable-timer
interrupt and enable-timer interrupt instructions.

For applications where an ISR has to do periodic "thinks" but mostly just
keeps a counter or two, I think this approach is an excellent one.  Has
anyone else used it?