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Re: Disk II Checksum
Matthew T. Russotto wrote:
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.
And it catches many multibyte errors, as well.
The bytewise EOR is a longitudinal parity bit, so any odd
number of errors in any "corresponding bit" will be caught.
The complication for the Disk ][ is that the EOR is applied to
the 6-bit chunks of data prior to nibble encoding, so there is no
straightforward correspondence to the bits recorded on the disk.
As I noted elsewhere, this also means that the maximal strength
of the check is one random bad block read as correct in 64, if
the data bits are "noise-like", which, in practice, they aren't.
-michael
NadaNet networking for Apple II computers!
Home page: http://members.aol.com/MJMahon/
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