> From: Scott Dattalo <sdattalo@UNIX.SRI.COM>
SNIP
> Uhh, yeah. Sorry. But what about this case: 95 + 90
>
> BCD_ADD:                        ;Y = Y + X
>         MOVLW   66               W = 66
>         ADDWF   X,W              W = 95 + 66 = FB
>         ADDWF   Y,F              Y = 90 + FB = 8B, C=1, DC=0
>         MOVLW   -60              W = -60 = 0xa0
>         SKPDC                    NOT SKIPPED
>          MOVLW  -66               W = -66 = 0x9a
>         SKPC                     SKIPPED
>          ADDWF  Y,F               NO ADD, SO RESULT = 8B, C=1
>         RETLW   0
>
> If C=1 AND the sum of the least significant nibbles is less than 10
> then the result will be incorrect.
Very sorry! I guess I should test my routines before posting :).

This sould do it:
BCD_ADD:                        ;Y = Y + X
        MOVLW   66
        ADDWF   X,W
        ADDWF   Y,F
        MOVLW   0
        SKPDC
         IORLW  6
        SKPC
         IORLW  60
        SUBWF   Y,F
        ANDLW   60              ;What you suggested.
        RETLW   0

BCD_MINUS:                      ;Y = Y - X
        MOVF    X,W
        SUBWF   Y,F
        MOVLW   0
        SKPDC
         IORLW  6
        SKPC
         IORLW  60
        SUBWF   Y,F
        ANDLW   60
        RETLW   0

> Excellent! However from my simulation results, Z is set only when
> the result is 0 (and not for an underflow). If you macro-ize this
> routine then the upper nibble of W will have the carry information.
> Or perhaps, you can add :
>
>         SUBWF   Y,F
>         ANDLW    0x60   ;Set Z if underflow occurred
>         RETLW    0
Thanks for the suggestion, I implemented it above.

Reggie