> Bob,
>
> I was taught that you use it as a carry/borrow (read carry not borrow) bit
depending upon the previous operation. All of this is as you have said a
"natural consequence" of two's complement arithmetic, which allows a value
to be added (or subtracted) across multiple bytes (words, etc.) by adding
the carry (or subtracting the borrow) on each higher order byte ...
>
> David W. Gulley
> Destiny Designs

David, that is an excellent way to look at it.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

>
> On Thu, 17 May 2001, Bob Ammerman wrote:
> > David:
> >
> > The PIC microcontrollers use the convention that after a subtract, the
carry
> > flag is set if there was _no_ borrow. The carry flag if clear if there
was a
> > borrow.
> >
> > This may seem backwards, but it turns that it is a natural consequence
of
> > implementing subtraction as the addition of the twos complement.
> >
> > Here is an example. Say you want to compute 10-4:
> >
> > In binary:
> >
> > 1010 - 0100
> >
> > Converting to addition of the twos complement:
> >
> > 1010 + 1100 = Carry + 0110
> >
> > Note that there was no borrow, but carry was set.
> >
> > Now on the other hand, say you want to compute 4-10:
> >
> > In binary:
> >
> > 0100 - 1010
> >
> > Converting to addition of the twos complement:
> >
> > 0100 + 0110 = 1010
> >
> > Note that there is a borrow, but carry was not set.
> >
> > Some processors contain special logic to reverse the meaning of carry
for a
> > subtract instruction, but the PICs do not. IMHO such logic is a waste of
> > silicon. Dealing with the inversion is a simple matter for the compiler
or
> > the assembly language programmer.
> >
> > Bob Ammerman
> > RAm Systems
> > (contract development of high performance, high function, low-level
> > software)
> >
> >
> > ----- Original Message -----
> > From: "David Cary" <dcary@BRUNSWICKOUTDOOR.COM>
> > To: <PICLIST@MITVMA.MIT.EDU>
> > Sent: Thursday, May 17, 2001 2:53 PM
> > Subject: [EE]: carry result: Robin Abbott
> >
> >
> > > Dear PICers,
> > >
> > > I've just taken over maintenance of page
> > >   http://www.piclist.com/techref/microchip/compcon.htm
> > > .
> > > It claims that Robin Abbott once said
> > >
> > > <BLOCKQUOTE>
> > > Mind you it would have been easier if Microchip had simply
> > > put the logic in Hardware to "correct" the carry flag after a
subtraction!
> > > </blockquote>
> > >
> > > What is the "correct" Carry result after a subtraction ?
> > > I have some friends that are making tiny CPUs inside FPGAs,
> > > and I'm sure they want to pick whatever Carry flag result
> > > makes the software guys happy. Which would be ... ?
> > >
> > > --
> > > David Cary
> > >
> > > MAILER-DAEMON@eagle.mail.pipex.net on 2001-05-17 01:32:40 PM tells me
that
> > > <robin.abbott@dial.pipex.com>:
> > > This account does not exist
> > >
> > > --
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> > > [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads
> > >
> > >
> >
> > --
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>

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