Scott Dattalo <sdattalo@unix.sri.com> wrote:

> >    movf    (bbb), w
> >    skpnc
> >    incfsz  (bbb), w
> >    addwf   (aaa), f
>
> And as Bob F. and I were recently discussing, this comes from
> Microchip's AN617. The author is Frank Testa.

Scott & Bob:

I'd also be inclined to believe that Frank Testa was the first to use
that implementation... His use is the earliest reference I've seen.

Of course, others among us have discovered it independently.

Testa also pointed out -- and I only learned about this in 1994, so
he knew about it earlier -- that, with a minor modification, the
code works for valid-carry two-byte SUBTRACTION, too:

    MOVF   SOURCELO,W
    SUBWF  DESTLO
    MOVF   SOURCEHI,W
    SKPC
    INCFSZ SOURCEHI,W
    SUBWF  DESTHI

Note that the Z flag is invalid at the conclusion of the suhtraction.

-Andy

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