Adrian Kennard <adrian@rhanna.demon.co.uk> wrote:

> Seeing as I have spent half a day chasing this bug, I thought I
> would share it with you all.
>
> If you write an interrupt handler on a 16C84, then you need to
> manually store the W register and STATUS register to ensure they
> main program doesn't go wrong. .... I forgot that re-loading W would
> of course change the Z flag !!! So you must restore STATUS, then
> restore W, and then set or clear Z depending on the stored value of
> STATUS. What a mess...

Adrian:

Sorry you wasted half a day on this.  An easier way is the
following, which is on mmy web page and has been in the Microchip
data books for a couple of years now:

    INTW    EQU     [any page-0 register]
    INTS    EQU     [any page-0 register]

            ....

    ; INTERRUPT SERVICE ROUTINE:

    PUSH    MOVWF   INTW                    ;SAVE THE W-REGISTER.

            SWAPF   STATUS,W                ;SAVE THE STATUS
            MOVWF   INTS                    ;REGISTER.

            BCF     STATUS,RP0              ;MAKE SURE WE'RE ON
                                            ;REGISTER PAGE 0
                                            ;[optional].

            ....

    POP     SWAPF   INTS,W                  ;RESTORE THE STATUS
            MOVWF   STATUS                  ;REGISTER

            SWAPF   INTW                    ;RESTORE THE
            SWAPF   INTW,W                  ;W-REGISTER.

            RETFIE                          ;RETURN AND RE-ENABLE
                                            ;INTERRUPTS.

Note that the STATUS register doesn't HAVE to be SWAPFed into and
out of INTS; doing so just keeps the Z flag from being disturbed on
ENTRY to the interrupt handler.  I've never needed this feature, but
it doesn't take any more code or time than using MOVFs, so there's
no reason not to do it.

-Andy

Andrew Warren - fastfwd@ix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geocities.com/SiliconValley/2499