> From: Scott Dattalo <sdattalo@UNIX.SRI.COM>
SNIP
> > I'll try:
> >
> > BCD_ADD:                        ;Y = Y + X
> >         MOVLW   66                  W = 66
> >         ADDWF   X,W                 W = 95 + 66 = FB
> >         ADDWF   Y,F                 Y = 95 + FB = 90, C=1
> >         MOVLW   -60                 W = -60
> >         SKPDC                               NOT SKIPPED
> >          MOVLW  -66                 W = -66
> >         SKPC                                SKIPPED
> >          ADDWF  Y,F                  NO ADD, SO RESULT = 90, C=1
> >         RETLW   0
>
> Close, but try X = 0x95 and Y = 0x95. The correct answer should
> be Y=0x80 and C=1. However, this routine will produce Y=0x2A and
> C=1. You still need that annoying normalization step.

This code will produce 90 with C=1.
Should be the right answer 95+95 = 190 not _180_.
The code must be correct!


>
> > BCD_MINUS:                      ;Y = Y - X
> >         MOVF    X,W
> >         SUBWF   Y,F
> >         MOVLW -6
> >         SKPC
> >          MOVLW  -66
> >         SKPDC
> >          ADDWF  Y,F
> >         RETLW   0
>
> Try X=0x25, Y=0x16. This produces 0xf1 instead of 0x91.
I goofed on this one :(.

How about this:
BCD_MINUS:              ;Y = Y - X
        MOVF    X,W
        SUBWF   Y,F
        ANDLW   0       ;Set Zero flag
        SKPDC
         IORLW  6
        SKPC
         IORLW  60
        SUBWF   Y,F     ;Zero flag is cleared
        RETLW   0
Test ZERO flag for underflow.

Reggie