Disk II Checksum

["Bryan Parkoff" <nospam@nospam.com>]

    Do you trust checksum of Disk II?  If magnetic field on the disk is 
degraded or damaged, luckily, you can be able to recover 256 bytes per 
sector.  It may not be exact byte what you expected.  It may be two bytes or 
more which they are wrong.  For example below.

41 42 43 44 45 46 47 48 49 is the data.  After EOR is done, different data 
with checksum below.

Original: 41 03 01 07 01 03 01 0F 01 -- 49 (Checksum)

    You EORed to match checksum without any problem.  What happen next?

Damaged: 41 03 01 03 01 07 01 0F 01 -- 49 (Checksum)

    Notice 4th byte and 6th byte are damaged.  4th byte's 3rd bit vanished. 
6th byte's 3rd bit appeared.  Try EOR and see what happen like below.

41 42 43 40 41 46 47 48 49 -- 49

    After you finished EORing and byte value: 49 matched, but you see 4th 
byte and 5th byte have wrong data.  It should be 44 and 45.

    It is very rare.  Did it happen to you?

Bryan Parkoff