Re: EDD + CARD

[mjmahon@aol.com (Michael J. Mahon)]

Matthew Russotto replied:

>In article <20020726152105.06824.00001292@mb-fc.aol.com>,
>Michael J. Mahon <mjmahon@aol.com> wrote:
>>
>>Interesting.  The authors have apparently developed their own
>>lexicon for the Disk ][ formats, quite different from the conventional
>>one (perhaps invented for "Beneath Apple DOS"?).  For example,
>>their "timing bytes" are usually referred to as "self-sync bytes" (or
>>"sync bytes", or even "sync nibbles", since they are not data bytes).
>
>Beneath Apple DOS gets the lowest-level totally wrong, I think --
>they talk of clock bits between each data bit, which if I understand
>correctly simply aren't there; that would be an FM scheme.  
>
>Not that this has anything to do with self-sync bytes, which they do get
>right.

Yes, earlier today I pulled out "Beneath Apple DOS" to check what
they called things, and was amazed to find that they described the
low-level as FM encoding, with clock 1's between each data bit!
This had the surprising effect of doubling the read pulse rate to
500KHz--which it seems would have tipped them off to the error.

They must also have noticed a certain illogic in the GCR pattern
constraints (required for accurate bit timing recovery), when FM
would completely remove any constraints.  (In fact, the "4+4"
coding used for addresses is exactly FM, itself.)

I had an old note in there correcting my copy, and I checked that
by "Beneath Apple ProDOS" they had it right, though their read
waveform drawings are still a little peculiar, with little circles on the
baseline to mark the "edges" of the bit cells--right where their clock
pulses used to be.  ;-)

There was apparently an interaction between Jim Sather and Worth
and Lechner around the time of his writing "Understanding the Apple
//e", and this may have contributed to the correction.  I note that
Sather refers readers to "Beneath" for info on file system structures
and calling higher-level API's, and Worth and Lechner refer readers
to "Understanding" for more detail on the encoding and controller
operation.

It was fun to look back into these works and find a connection!
(I've re-read Sather on the Disk ][ controller every couple of years--
it's one of Woz's digital engineering sonatas in my book.  ;-)

-michael

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