Osborne-1 SD format

From: Dave Dunfield <dave04a_at_dunfield.com>
Date: Sun Feb 20 10:08:09 2005

>IIRC, the Osborne Double density was 5 sectors with 1024 bytes per sector.
>> Why do they "appear" to read at FM 1024 byte sectors (with correct
>> 1-5 sector numbers/ordering)?
>I'm not familiar with the quirks of Teledisk. Could you be looking at a
>double density Osborne disk? (and seeing MFM) Or is Teledisk giving
>"random" results when it doen't have an answer?

>It would appear that your "single density" Osborne diskette is
>really a "double density" Osborne diskette.
>But there are a few other possibilities. I have seen quite a few
>single sided format diskettes that were reformatted from other
>double sided formats, leaving a readily readable different format
>on the back side.

Hi Fred,

Thanks for the info - It's interesting that a DD O1 disk would match
what I am seeing - I grabbed this disk from the faceplate bin of my
single-density O1, and I'm pretty sure that it was last formatted on
that machine - however evidence would suggest that I may be seeing
a DD disk - so I'll double check that I got it out of the right
Osborne (my other ones have the DD option), and that it does read on
that machine... Perhaps I have been engaging in the persuit of an un-
domesticated ornithoid!

In the meantime, I have been looking at a Cromemco CDOS disk (5.25"),
another disk that Teledisk cannot copy, and it is equally perplexing...
This disk has track-0 at single-density (both sides) and the remainder
of the disk in double-density.

It reads fine in my System-3, and I *CAN* read all of the sectors
from it on various PC's, however not reliably at all. They do always
show correctly that they are single-density on track 0 and double-
density on the remaining tracks (some PC's can't detect the single-
density sectors at all), however repeated read-id's on a track fail
to turn up all of the sector numbers, unless I let it go for a long
time. Even 50 revolutions of the disk will not always turn up all
the sector numbers (but they are there and will sometimes reveal
themselves - often different sectors are better on different PC's).

With a PC format disk, and most other DD disks, I can simply issue
a "read id" with the wrong data type - this times out at the index
hole after two revolutions, at which point I set the right data type
and perform repeated "read id"s until I see the same id again - on
most disks, this quite reliably gives me the sector interleave pattern
on the track.

With the Cromemco disk, this does not work, as often I won't see all
of the sector id's until I have reissued the "read id" many times,
often seeing some sectors 30-40 times before seeing one occurance of
the "tough" sectors. Clearly even if I do eventually get all the
sector Id's, I cannot determine what the interleave is at all..., not
to mention that this makes the "analysis stage" a but lengthy.

Repeated attempts to read all id's on a track will often/usually
turn up different sectors (but not all of them in one go).

Attempt to read these sectors will usually fail, but will sometimes
work - again, very unreliable.

Read-track appears to work, however I have not yet determined if it
is simply "missing" some sectors or not - since I don't know the
sector interleave, and read-track does not provide any information
about the order in which it ecountered the sectors (read-track seems
pretty useless on the 765 series).

This most interesting about all this (at least to me) is that the
symptoms occur on the double-density portion of the disk, as well
as the fact that it is intermittant (ie: not a complete
incompatibility) - Again, it looks like there might be something
slightly non-standard about the CDOS disk format... (anyone know?)

dave04a (at)    Dave Dunfield
dunfield (dot)  Firmware development services & tools: www.dunfield.com
com             Collector of vintage computing equipment:
Received on Sun Feb 20 2005 - 10:08:09 GMT

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