John Payson has asked me to forward this message to the list.

-Andy

------- Forwarded Message From John Payson Follows -------

Subject:       Re: How to decrement W
To:            fastfwd@ix.netcom.com
Date:          Sat, 13 Jul 1996 16:15:02 -0500 (CDT)
From:          "John Payson" <supercat@mcs.com>


> > I new to PIC programming.  I'm having trouble figuring out how to
> > decrement W.  Anyone care to help me out?
>
> If you're using a 12-bit PIC (16C5x), which doesn't have the "ADDLW"
> instruction, you can do this:
>
>     MOVWF   TEMP        ;Store W temporarily in a file register.
>
>     MOVLW   1           ;Subtract 1 from that register, leaving the
>     SUBWF   TEMP,W      ;result in W.

Hmm... this seems a little long and complicated.  If there is no need
to preserve TEMP or flags, simply use:

 movwf  temp
 decf   temp,w

If you need to preserve flags, use

 movwf  temp
 decfsz temp,w
 nop            ; Or figure something useful to put here

If you need to preserve temp, you could use either:

 ;      ;       ;       W       temp
 addwf  temp    ;       W       W+t
 subwf  temp,w  ;       t       W+t
 decf   temp    ;       t       W-1+t
 subwf  temp,w  ;       W-1     W-1+t
 subwf  temp    ;       W-1     t

[which I came up with] or you could use one of these:

 ;      ;       ;       W       temp
 subwf  temp,w  ;       t-W     t
 decf   temp    ;       t-W     t-1
 subwf  temp,w  ;       W-1     t-1
 incf   temp    ;       W-1     t

or the similar

 ;      ;       ;       W       temp
 incf   temp    ;       W       t+1
 subwf  temp,w  ;       t-W+1   t+1
 decf   temp    ;       t-W+1   t
 subwf  temp,w  ;       W-1     t

both of which were produced [in addition to my solution] by a little
QBASIC program I wrote [at bottom of message].  Note that if you for
some reason wanted to subtract one from both the W and some other
register, you could use:

 ;      ;       ;       W       temp
 subwf  temp,w  ;       t-W     t
 decf   temp    ;       t-W     t-1
 subwf  temp,w  ;       W-1     t-1

Anyway, for the QBASIC program y'all have been waiting for... here it
is. It's pretty kludged, but I wrote it just for some little dabbling,
not for any real workhorse stuff...




proglen = 5
numops = 6

DIM inst(proglen)

PRINT
PRINT "Scanning:"

Foo:

i = 0
WHILE inst(i) >= numops
  inst(i) = 1
  i = i + 1
WEND
inst(i) = inst(i) + 1

ww = 1: wt = 0: wc = 0: tw = 0: tt = 1: tc = 0

FOR i = 0 TO proglen - 1
  ON inst(i) GOSUB aww, awf, sww, swf, incf, decf
NEXT

IF ww = 1 AND wt = 0 AND wc = 1 AND tw = 0 AND tt = 1 AND tc = 0 THEN
'IF ww = -1 AND wt = 0 AND wc = 0 AND tw = 0 AND tt = 1 AND tc = 0
THEN
  FOR i = 0 TO proglen - 1
    PRINT inst(i);
  NEXT
  PRINT
END IF

IF inst(proglen) = 0 THEN GOTO Foo

END

aww: ww = tw + ww: wt = tt + wt: wc = tc + wc: RETURN
awf: tw = tw + ww: tt = tt + wt: tc = tc + wc: RETURN
sww: ww = tw - ww: wt = tt - wt: wc = tc - wc: RETURN
swf: tw = tw - ww: tt = tt - wt: tc = tc - wc: RETURN
incf: tc = tc + 1: RETURN
decf: tc = tc - 1: RETURN

------ End of Forwarded Message from John Payson ---------


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