Re: Disk II Checksum

["Alex Freed" <alexf@mirrow.com>]

"Michael J. Mahon" <mjmahon@aol.com> wrote in message 
UomdnRsI-69uulDYnZ2dnUVZ_tKjnZ2d@comcast.com">news:UomdnRsI-69uulDYnZ2dnUVZ_tKjnZ2d@comcast.com...
>>
>> Assuming all errors are equaly probable.
>
> Which is a reasonable assumption, since the only
> categories of errors in the Disk ][ serial channel are
> random bit flips (extra pulse or missing pulse) or
> burst errors (media blemish).

Michael, I'm not claiming that's not true but I'm not sure
that it is true either.

For example the recorded  bytes are far from random: high
bit is always set and only some values are legal.

We know that more than 2 zeros back to back make reading
not reliable. Consider a byte like 100100xx.
If the "1" in bit 4 reads as "0" this can be messy.

Again I'm not claiming this changes the odds in any particular way,
but the odds may not be exactly 1/256.

-Alex.