>Peter Baines <PICLIST@MITVMA.MIT.EDU> wrote:
>
>> does anybody have some code to subtract one 16 bit number from
>> another NOT using 2s compliment so that it returns an ABS result
>> with a flag (AKA register) to say if it is negative.
>
>Peter:
>
>This routine computes DEST = DEST - SOURCE.  After it executes, the
>Carry flag is valid (i.e., Carry = 0 if the result is negative,
>Carry = 1 if the result is zero or positive).  Note that the Zero
>flag is NOT necessarily valid after execution.
>
>    MOVF   SOURCELO,W
>    SUBWF  DESTLO
>    MOVF   SOURCEHI,W
>    SKPC
>    INCFSZ SOURCEHI,W
>    SUBWF  DESTHI
>
>-Andy
>

Peter,
Try this... returns ABS(DEST-SOURCE) in DEST

        MOVF   SOURCELO,W
        SUBWF  DESTLO
        MOVF   SOURCEHI,W
        SKPC
        INCFSZ SOURCEHI,W
        SUBWF  DESTHI
;
        SKPNC
        GOTO DONE        ; DEST > SOURCE
                         ;
        COMF    DESTLO   ; NEGATE if SOURCE > DEST
        INCF    DESTLO
        SKPNZ
        DECF    DESTHI
        COMF    DESTHI

DONE    ; DEST = ABS( DEST - SOURCE )

I don't know if there is any simpler way, seem like there
should be some cleverer way of doing the subtract.


Ray Gardiner (DSP Systems) ray@dsp-systems.com http://www.dsp-systems.com
private email to:- ray@netspace.net.au