Opinions on 16-bit checksums.

From: Kermit (Kermit333_at_hotmail.com)
Date: 08/25/04 

Date: Tue, 24 Aug 2004 22:17:49 GMT

Hello all,

Since I see that Endianess is a current topic, I'd like to ask a question
concerning it and calculating 16-bit checksums. I've never seen a standard
anywhere that describes exactly what a checksum is.

However, I have mainly seen them calculated in one of three ways.

1) Simply calculate a 32-bit value which is the sum of each byte added to a
running sum. At the end, cast this 32-bit value to a 16-bit value, and call
it the checksum.

2) Calculate the same way as above, but pull two bytes of the file out at a
time such that you are adding them as 16-bit values in one of the following
ways.

        a) Little-endian
        b) Big-endian.

For example, lets say I have a 6-byte file consisting of:

0x00 0x01 0x02 0x03 0x04 0x05

Algorithm 1 would yield:

0x00 + 0x01 + 0x02 + 0x03 + 0x04 + 0x05 = 0x000f

Algorithm 2-a would yield:

0x0100 + 0x0302 + 0x0504 = 0x0906

Algorithm 2-b would yield:

0x0001 + 0x0203 + 0x0405 = 0x0609

I've always preferred to calculate it 16-bits at a time, because that way,
you can detect byte-reversal issues. And, apparently, I've always calculated
them assuming a little endian format. However, I recently ran into a problem
with a distributor trying to program a big-endian file into a flash part.
They switched the bytes, then ignored it, which is their fault, but my
problem. But, since it was the first time I'd ever worked with a big-endian
binary file, I guess it threw me and I wanted to double check my thinking.

How does everyone else on the list calculate a 16-bit checksum.

--kermit 

-- 
It wasn't easy being Greazy ....but it was interesting.

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