Sun Oct 21 10:16:39 EDT 2012

What to save in ISR?

The main question is, is it safe to use the stacks?  I think so,
because none of the ASM instructions leave the stacks in an
inconsistent state.  So the recipe is:

- dup is MOVWF which does not affect status flags, so WREG can be
  saved first.
- STATUS can then be copied into wreg.

The above are just STATUS @.  To restore, care needs to be taken that
the drop doesn't mess up the flags after restoring it.  So STATUS !
can't be used.  There is a "nfdrop" that uses MOVFF to not affect
flags.  This should work:

  dup STATUS !

- save STATUS before doing any dup, since dup affects flags.

ther eis an