On Wed, Sep 17, 1997 at 04:19:31PM +0930, Mike Smith wrote:
> On 16 Sep 97 at 21:56, Pioneer Microsystems wrote:

> > into a windowed C554. All will fit properly, but I have a problem.
> > This part has only one bank of user RAM.  Upon interrupting, I might
> > be in the upper page, and then my W save action will go zinging off
>
> How about
> save RP0 to RP1 (which is unused, but available)

The problem with this solution is that RP1 is marked by Microchip
as not to be used - they advise that it should be left at zero.
While in the chip itself RP1 does work, and has no bad side effects,
if you run the MPLAB simulator for certain chips (not sure if it affects
the '554, but it does affect the '62x chips) then setting RP1 does
Bad Things. Specifically, all registers become inaccessible, including
the STATUS register, so the only way to recover is to reset the simulator.

Here's a better solution (or at least one sure to work): this code is
what our compiler produces for the '554 interrupts (I've added comments):


_isr
        btfsc   3,5             ;test if RP0 is set
        goto    l30001
        movwf   saved_w         ;if not, bank0 is selected, save w
        movf    3,w             ;load status into w
        goto    l30002          ;rejoin common code
l30001
        bcf     3,5             ;clear RP0 so we can
        movwf   saved_w         ;    save W
        movf    3,w             ;get STATUS (with cleared RP0)
        iorlw   32              ;re-set RP0 in the saved copy
l30002
        movwf   saved_status    ;store saved status in memory

;x.c: 6: i++;                   ; here's your real work
        incf    (_i& (0+127))& (0+127)
        btfsc   3,2
        incf    ((_i+1)& (0+127))& (0+127)

        movf    saved_status,w  ;now recover saved status
        andlw   223             ;reset RP0
        movwf   3               ;store into STATUS
        swapf   saved_w         ;restore W
        swapf   saved_w,w       ;  using swapf's to preserve STATUS
        btfsc   saved_status,5  ;now test the saved RP0
        bsf     3,5             ;set the real RP0 if required
        retfie                  ;and continue our regular program...


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