Are you saying that the
     rlf     Tens,f  ;B1 = 2*(4A2+A1-92) = 8A2+2A1-184
is guarrenteed to generate a carry and that this is the +1 in the
     rlf     Ones,f  ;B0 = 2*(A3+A2+A1+4) = 2A3+2A2+2A1+9
and if so how can you be sure of the carry? Wow... I'm brain fried... I'll
have to think about this more later.

Thanks again.

---
James Newton mailto:jamesnewton@geocities.com 1-619-652-0593
http://techref.massmind.org NEW! FINALLY A REAL NAME!
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-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Scott Dattalo
Sent: Friday, February 11, 2000 15:19
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: PIC Math FAQ page
Importance: Low


On Fri, 11 Feb 2000, James Newton wrote:

> Doh! adding A2 twice does not multiply by 2! Thanks for spotting that one.
>
> >-92 = 0xa4
> > and 0xa4 << 1 is 0x48
> yes, but 0x48 or (72 + x) mod 256 is (x-184) mod 256 or in other words,
> adding 72 and wrapping around 256 is the same as subtracting 184. I think
> I'm right on that one.

True but, there's also that carry bit. A two's complement can be written
in terms of a one's complement:

two's complement of x = 1 + one's complement of x.
So, taking this snippet from your web page:

> >>>     rlf     Tens,f  ;B1 = 2*(4A2+A1-92) = 8A2+2A1-184
> >>>     rlf     Ones,f  ;B0 = 2*(A3+A2+A1+4) = 2A3+2A2+2A1+8
>         comf    Ones,f  ;B0 = -2A3-2A2-2A1-8
>         rlf     Ones,f  ;B0 = 2*(-2A3-2A2-2A1-8) = -4A3-4A2-4A1-16


     rlf     Tens,f  ;B1 = 2*(4A2+A1-92) = 8A2+2A1-184 (I'll leave this)
     rlf     Ones,f  ;B0 = 2*(A3+A2+A1+4) = 2A3+2A2+2A1+9
     comf    Ones,f  ;B0 = ~(2A3+2A2+2A1+9) = -2A3-2A2-2A1-8
     rlf     Ones,f  ;B0 = 2*(-2A3-2A2-2A1-8) = -4A3-4A2-4A1-16

The comf's comment is still the same...

So now that I think about it, Payson new what he was doing (ah, should I
have ever doubted...)

Scott