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.

I'm as puzzled as you are about the comf thing. I'm trying to find the book
I learned binary math from.

Thanks for your time, I know it's in great demand.

---
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 14:35
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: PIC Math FAQ page
Importance: Low


On Fri, 11 Feb 2000, James Newton wrote:

> 4. I'm worried that the comments I've added (of the form W= and A2=) on
each
> line of John Payson's 16 bit Binary to BCD conversion code may be
incorrect.
> Specifically: Am I getting the subtraction by addition idea correct? How
can
> comf do a negate when it does a ones complement vice a twos complement?
Math
> is not my strongest point. see:
> http://techref.massmind.org/microchip/math/radix/b2bu-16b5d.htm

Hi James,

after you sent me the (private) message the other day, I did spend a few
minutes running through the comments.

        movf    NumH,w  ;w  = A3*16+A2
        andlw   $0F     ;w  = A2
        addwf   Hund,f  ;B2 = A3-46 + A2 = A3+A2-46
>>>     addwf   Hund,f  ;B2 = 2*(A3+A2-46) = 2A3+2A2-92
        addwf   Ones,f  ;B0 = A3+4 + A2 = A3+A2+4

should be:
        addwf   Hund,f  ;B2 = 2*A2+A3-46


And

>>>     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

 -92 << 1  does not equal -184

-92 = 0xa4
 and 0xa4 << 1 is 0x48

Then the next line is going to pick up the carry. So these two lines
should read instead:

>>>     rlf     Tens,f  ;B1 = 2*(4A2+A1+0xa4) = 8A2+2A1+0x48
>>>     rlf     Ones,f  ;B0 = 2*(A3+A2+A1+4)+1 = 2A3+2A2+2A1+9

These are the only two errors that immediately popped out. However, since
the results of these calculations are used further down in the code, the
errors are propogated. There may be a bug silently lurking in this code -
I hadn't noticed the C bit thing before...


Scott