Hello John .

To correct my recent mistake I offer the same idea as your one ,
but for multibyte substraction .

>         movf    S1,w
>         btfsc   C
>          incfsz S1,w
>          addwf  D1,f

After thinking some times about this tricky code I can tell:
It's really one from finest(beautiful) PIC code construction .


My example for multibyte substraction :

    ; D1 = D1 - S1 - borrow
    ; before/after C = 0 - subtraction was with borrow
    ; before/after C = 1 - subwf was without borrow

...
    MOVFW  S1
    SKPC
    DECFSZ S1,W
    SUBWF  D1,F
...

WBR Dmitry.

P.S. I get just another confirmation that PIC code set is not bad ;)