At 09:54 PM 6/25/01 -0400, Drew Vassallo wrote:
>>  org     4
>>int
>>   movwf  W_TEMP              ;  "W_TEMP" in Banks 0 and 1
>>   movf   STATUS, w
>
>Ouch, Myke.  I just realized that this is wrong.
>
>You can't use "movf STATUS, W" because it can (and will) change the Zero bit
>and affect the initial STATUS value.

Actually, the above works just fine.  What happens is that the original
value contained in STATUS gets moved to W.  Only after it is in W does
STATUS get updated, by which time you don't care anymore because you have a
good copy sitting there in W.

My normal sequence is: (14 bit core devices)

     movwf  W_SAVE    ;save W on whatever page we happen to be in
     movfw  STATUS    ;good copy of status in W, status reg now trashed
     clrf   STATUS    ;now select ram page 0
     movwf  S_SAVE    ;save status

Exiting the ISR is just the reverse:

     movfw  S_SAVE    ;get save copy of status
     movwf  STATUS    ;status now restored, including current ram page
     swapf  W_SAVE,F  ;pre-swap saved copy of W
     swapf  W_SAVE,W  ;swap saved copy again, now putting it into W

Again, note that you have to reserve W_SAVE on each ram page, at the same
offset as page 0.  I use W_SAVE1, W_SAVE2, W_SAVE3 to avoid assembler errrors.

dwayne



Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

Celebrating 17 years of Engineering Innovation (1984 - 2001)

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