> Van: John Considine <jconsi@MAGICNET.NET>
> Aan: PICLIST@MITVMA.MIT.EDU
> Onderwerp: Re: Thanks Dan Larson subtract smaller # from Larger #
> Datum: woensdag 3 november 1999 19:21

This "solution" only works for numbers upto 127.  Subtracting 2 from 200
would give a *valid* number, but with the sign-flag set.  So the "solution"
would mangle it into something else :-(   Where the Sign-flag is the
Over/underflow for small numbers (0 thru 127), the Carry is the Overflow
for large numbers (0 thru 255) ...

You can still use the Two-complement trick though, just replace the check
for the Sign-flag with a check for the Borrow-flag ( NOT Carry-flag).

Regards,
  Rudy Wieser

> Very ingenious,  I never would have though of the two's complement after
> already having done the original subtraction.
>
> Thanks.
>
> Dan Larson wrote:
>
> > On Wed, 3 Nov 1999 10:08:20 -0500, John Considine wrote:
> >
> > >I am trying to make sure that I subtract a smaller number from a
larger
> > >number.  However, when the subwf does the two's complement apparently
it
> > >sets the C flag.  For  example, Value1 = H'5C' and Value2 = H'72'
causes
> > >C to be set.  Even though 72 is greater than 5C.
> > >
> > >How does everybody else check that a smaller number is being
subtracted
> > >from a larger number?
> >
> > How about just doing the subtract, regardless of operand order.
> > Then, check the sign bit (bit seven).  If bit 7 is set, complement
> > and increment the result (two's complement).  If bit 7 is not set,
> > then leave the result alone.  The result will be the absolute value
> > of the difference.  Is this what you want?
> >
> > ; Using three RAM location and preserving Value1 and value2
> >
> >      movf     Value1, W
> >      movwf    Result
> >      movf     Value2, W
> >      subwf    Result, F
> >      btfss    Result, 7
> >      goto     Done
> >      comf     Result, F
> >      incf     Result, F
> > Done:
> >
> > ; Using two RAM locations and destroying Value2
> >
> >      movf     Value1, W
> >      subwf    Value2, F
> >      btfss    Value2, 7
> >      goto     Done
> >      comf     Value2, F
> >      incf     Value2, F
> > Done:
> >
> > I hope this helps you...
> >
> > Dan