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From: adrie@philica.ica.philips.nl (Adrie Koolen)
Newsgroups: comp.os.minix
Subject: Re: Larger / non-std fd formats (ST).
Message-ID: <813@philica.ica.philips.nl>
Date: 16 May 91 08:43:46 GMT
References: <3115@krafla.rhi.hi.is> <3143@krafla.rhi.hi.is>
Reply-To: adrie@beitel.ica.philips.nl (Adrie Koolen)
Organization: Philips TDS, Innovation Centre Aachen
Lines: 50

In article <3143@krafla.rhi.hi.is> adamd@rhi.hi.is (Adam David) writes:
>I have heavily modified format.c and it now succesfully formats DD disks to
>924k and HD disks to 1848k using 512-byte sectors. Changes to support ED disks
>up to 3696k are trivial. An ED format of 3864k is possible using 1024-byte
>sectors but will require further modification to the system, because the
>512-byte sector size seems to be fairly extensively used as a constant.

Although it's technically marvelous that you can squeeze that much data on
a floppy diskette, I advise NOT to do it. Three important points in diskette
data storage come to my mind now:

  1. High capacity (as high as possible)
  2. Compatibility (media level as well as data level)
  3. Reliability

With respect to the first item, you've gained a substantial improvement in the
storage capacity, but obviously you've lost compatibility with many computer
systems and users. While this may not seem important to you now, this
incompatibility will get to you later.

The most important problem however, is the reliability of the data storage.
In high density, you format 84 cylinders with 22 sectors of 512 bytes per track.
You cannot guarantee that every drive can read/write 84 cylinders and a drive
that cannot read past the 80th cylinder is not implicitly faulted. Furthermore,
the coating of the magnetic media need not extend to the 84th cylinder and when
it does, the coating on the last cylinder might not be as thick as on the first
80 certified cylinders.

The gaps are needed to allow fluctuations in the spindle motor (and to allow
easier synchronization of the data seperators). If a sector is rewritten after
a long time and the spindle motor runs faster than the last time the sector was
written, the angle the sector is written over, becomes larger. The gap after the
sector provides some extra space for the sector. In the IBM system 34 diskette
format definition (MFM), minimum gap sizes are defined to allow for 3% speed
differences (+1.5% / -1.5%). I bet that you dropped below those gap sizes.
That's all right, I hear you say, because modern diskette drive have much better
motors. But don't forget that your drive may be running at 303 rpm and the drive
of someone else at 297 rpm, both stable at within the specs! Also, your drive
might wear out and drop speed after a few years.

I can imagine that you only want to use your squeezed diskettes for your own
archiving purposes, but even that I advise against. I've wanted to restore some
old programs I wrote on my Commodore PET some 10 years ago on a PC. The PET used
a 5.25" double sided floppy diskette with 10 512 bytes sectors per track. The PC
constantly had problems with the short gap before the first sector of each track
and therefore could not synchronize its data separators. Sectors 2-10, I could
read but not the first sector.

Adrie Koolen (adrie@ica.philips.nl)
Philips Innovation Centre Aachen