Ron Kreymborg <PICLIST@MITVMA.MIT.EDU> wrote:

> >         movf    new+1,w         ; subtract lsbs
> >         subwf   old+1,w
> >         movf    new,w           ; subtract msbs
> >         btfss   status,c        ; including any carry
> >         addlw   1
> >         subwf   old,w
> >         btfsc   status,c        ; new smaller?
> >         goto    smaller         ; yes
> >         goto    larger          ; no

I replied:

> your code works for Marcel's application (12-bit subtraction), but
> it fails on 16-bit numbers when "new" is greater than 0xFF00.
>
> A routine that works for all 16-bit numbers is:
>
>         MOVF    NEWLO,W
>         SUBWF   OLDLO,W
>         MOVF    NEWHI,W
>         SKPC
>         INCFSZ  NEWHI,W
>         SUBWF   OLDHI,W
>         BC      NEW_IS_LARGER_OR_EQUAL
>         GOTO    NEW_IS_SMALLER

and Ron responded:

> it works fine when new > 0xff00.

    No it doesn't, Ron.

    Your code works when new is less than 0xFF00:

        ; NEWHI = 00, NEWLO = 20.
        ; OLDHI = 00, OLDLO = 10.

        movf    new+1,w         ; W = 20, CARRY = unknown.
        subwf   old+1,w         ; W = F0, CARRY = 0.
        movf    new,w           ; W = 00, CARRY = 0.
        btfss   status,c        ; -- skip not taken --
        addlw   1               ; W = 01, CARRY = 0.
        subwf   old,w           ; W = FF, CARRY = 0.
        btfsc   status,c        ; -- skip is taken --
        goto    smaller         ; -- this instruction is skipped --
        goto    larger          ; -- this branch is taken --

    But it fails when new is > 0xFF00:

        ; NEWHI = FF, NEWLO = 20.
        ; OLDHI = FF, OLDLO = 10.

        movf    new+1,w         ; W = 20, CARRY = unknown.
        subwf   old+1,w         ; W = F0, CARRY = 0.
        movf    new,w           ; W = FF, CARRY = 0.
        btfss   status,c        ; -- skip not taken --
        addlw   1               ; W = 00, CARRY = 1.
        subwf   old,w           ; W = FF, CARRY = 1.
        btfsc   status,c        ; -- skip not taken --
        goto    smaller         ; -- this branch is taken --
        goto    larger          ; -- this branch is NOT taken --

> However, a quick check of your code would indicate that, for
> example, 65329 < 1104.

    A less-quick check would've shown that my code ALWAYS gave the
    wrong answer, since I erroneously swapped OLD and NEW as I
    transcribed it from a message I wrote a couple of years ago.

    Sorry about that... It should have read:

        MOVF    OLDLO,W
        SUBWF   NEWLO,W
        MOVF    OLDHI,W
        SKPC
        INCFSZ  OLDHI,W
        SUBWF   NEWHI,W
        BC      NEW_IS_LARGER_OR_EQUAL
        GOTO    NEW_IS_SMALLER

> I guess the argument here is whether we are dealing with signed
> numbers or not. Marcel didn't mention it, and I assumed they
> weren't.

    My code assumes unsigned numbers, too.

    -Andy

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