[PIC] Double length decrement

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[PIC] Double length decrement

David C Brown
I am ashamed to admit that I am struggling to do this in a PIC18F88.
Subtract 1 from a sixteen bit number and indicate when it goes negative
Can anyone spare my blushes?
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak                           Phone: 01663 733236
Derbyshire                eMail: [hidden email]
SK23 7ND          web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



*Sent from my etch-a-sketch*
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Re: [PIC] Double length decrement

Neil
Quick first thought...
Subtract 1 from lower byte.
If it goes negative, then you need to subtract 1 from upper byte (else
you're done and result is not negative).
If decrementing upper byte goes negative, then there's your negative flag.

Cheers,
-Neil.



On 2/22/2021 2:21 PM, David C Brown wrote:

> I am ashamed to admit that I am struggling to do this in a PIC18F88.
> Subtract 1 from a sixteen bit number and indicate when it goes negative
> Can anyone spare my blushes?
> __________________________________________
> David C Brown
> 43 Bings Road
> Whaley Bridge
> High Peak                           Phone: 01663 733236
> Derbyshire                eMail: [hidden email]
> SK23 7ND          web: www.bings-knowle.co.uk/dcb
> <http://www.jb.man.ac.uk/~dcb>
>
>
>
> *Sent from my etch-a-sketch*

--
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Re: [PIC] Double length decrement

David C Brown
Yeh.  I was getting confused about knowing when it was negative.
Carry being clear when it goes negative always confuses me.
But I am OK now, thanks
__________________________________________
David C Brown
43 Bings Road
Whaley Bridge
High Peak                           Phone: 01663 733236
Derbyshire                eMail: [hidden email]
SK23 7ND          web: www.bings-knowle.co.uk/dcb
<http://www.jb.man.ac.uk/~dcb>



*Sent from my etch-a-sketch*


On Mon, 22 Feb 2021 at 19:34, Neil <[hidden email]> wrote:

> Quick first thought...
> Subtract 1 from lower byte.
> If it goes negative, then you need to subtract 1 from upper byte (else
> you're done and result is not negative).
> If decrementing upper byte goes negative, then there's your negative flag.
>
> Cheers,
> -Neil.
>
>
>
> On 2/22/2021 2:21 PM, David C Brown wrote:
> > I am ashamed to admit that I am struggling to do this in a PIC18F88.
> > Subtract 1 from a sixteen bit number and indicate when it goes negative
> > Can anyone spare my blushes?
> > __________________________________________
> > David C Brown
> > 43 Bings Road
> > Whaley Bridge
> > High Peak                           Phone: 01663 733236
> > Derbyshire                eMail: [hidden email]
> > SK23 7ND          web: www.bings-knowle.co.uk/dcb
> > <http://www.jb.man.ac.uk/~dcb>
> >
> >
> >
> > *Sent from my etch-a-sketch*
>
> --
> http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
--
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View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist