On Sun, 2006-01-01 at 22:00 -0800, William Chops Westfield wrote:
> On Jan 1, 2006, at 3:26 PM, Herbert Graf wrote:
> 
> >> Try enabling "Integer Promotions" in the C18
> >> preferences and see if that helps.
> >
> > BINGO!
> 
> Good.  If this is an ip checksum routing, you were missing
> the code that does the end-around carry folding in between
> the loop at the complement...

The code I included was only relavent to the problem I had. It is indeed
an IP checksum routine, and after a little more debugging my packet
sniffers show the correct checksum on the wire!

Of course, then the next step was a UDP checksum routine. Spent some
time trying to figure out why it wasn't working right, and then I
discovered if you set it to 0x0000 machines recognize that as "no
checksum"! :)

> Also, while this is a sort of standard unix ip checksum
> algorithm, it's a REALLY BAD algorithm to use on a CPU
> that doesn't natively support 32bit integers.  It dates
> back to vaxen and 68000s, and is not good for PICs...
> (alas, C is not so good for implementing the IP checksum
> algorithm, because it doesn't give you access to the 'extra'
> carry bit..)

I was trying to think of a better way to do it, but in C alone I don't
really see anything immensely better. I suppose I could inline an
assembly routine, but I'd rather not spend the extra time for something
I don't need.

It's no matter, the app is more of a test bench then anything, and code
size, speed and efficiency don't matter in the least, it just has to
work.

TTYL

-----------------------------
Herbert's PIC Stuff:
http://repatch.dyndns.org:8383/pic_stuff/

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