Re: Disk II Checksum

[russotto@grace.speakeasy.net (Matthew T. Russotto)]

In article <Gt-dnVKgR7i7jFDYnZ2dnUVZ_u2mnZ2d@comcast.com>,
Alex Freed <alexf@mirrow.com> wrote:
>"Michael J. Mahon" <mjmahon@aol.com> wrote in message 
>SLydnXx975xZfVHYnZ2dnUVZ_uPinZ2d@comcast.com">news:SLydnXx975xZfVHYnZ2dnUVZ_uPinZ2d@comcast.com...
>> There is one chance in 256 of an incorrect read producing
>> a correct checksum, but this level of integrity checking
>> seems to be sufficient.
>
>Assuming all errors are equaly probable.

A bytewise checksum catches all errors confined to a single byte.

>-What are the odds of meeting a dinosaur on the street?
>- Fifty-fifty. Either you meet it or not!
>
>I'm not an expert at all, but as I recall it CRC was invented
>because the odds of finding an error are much better than
>using a simple 16 bit checksum.

Yes.  A CRC finds all burst errors with a length less than or equal to
that of the CRC polynomial, all one bit errors, and most two-bit
errors.  Typically they are constructed to catch all parity errors
(errors in an odd number of bits) as well.
-- 
  There's no such thing as a free lunch, but certain accounting practices can
  result in a fully-depreciated one.