source= http://www.piclist.com/postbot.asp?id=piclist\2002\11\13\182704a

Each sector (typically 512 bytes) of a floppy disk has a CRC (Cyclic
Redundancy Check) code stored with it. CRC is a fancy checksum - an
algorithm calculates a 16-bit (IIRC) number that is based on the contents of
the 512 bytes of data and writes it to the disk at the same time as the data
is written. When the drive reads data from a sector, the *drive controller*
(no CPU involved) calculates the CRC from the data read and compares it with
the CRC that is stored on the disk. I believe the algorithm used is designed
to be such that typical errors in the data (or the CRC itself) will mean
that the two CRCs will be different.

An algorithm that ensures a statistically fair spread of calculated CRCs
will read a faulty disk and calculate a CRC equal to the stored value only 1
in 2^16 times - on which occasion, the error will go undetected. The use a
of an algorithm that anticipates the typical type of error will do better
than that.

Steve.


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