Date   : Mon, 09 Feb 2004 22:46:47 GMT
From   : Pete Turnbull <pete@...>
Subject: Re: BBC floppy / drive formats]

On Feb 9, 21:15, Colin wrote:
> Ooh heck, this is all a bit much for me!
> I'm not into the inner workings yet and this is my first encounter
with a
> disc drive (it's a TEAC FD-55AV-10-U part number 19307200-10
according to
> the label inside it).
> If I have this right, one can still buy the correct size discs from
PC shops
> (PC World for instance) but they won't work with the BBC Micro if
they have
> been formatted as you need to format them on the Beeb?,but you can't
format
> them on the Beeb as it won't recognise them. Sounds a bit catch 22.

It's not quite that bad.  Ignore the table Jules posted, it's not
helpful if you don't understand what's going on.  It's essentially a
table of (nearly) all possible permutations (not just the standard
ones).  If I wanted to be really picky, I'd point out that in fact it's
not even complete, as it misses out some once-common drive sizes.
 Here's the simple (I hope) explanation:

Acorn produced two versions of disk filing systems for the Beeb and its
successors.  Other people produced copies and variants, but the two to
begin with are called DFS (Disk Filing System), which came first, and
ADFS (Advanced Disk Filing System), which came later.

DFS came with a couple of utilities on disk, to format and verify,
blank (or pre-used-elsewhere) disks.  Some later versions, and
third-party versions, of DFS had the capability built in. ADFS also
comes with a utility on disk.

DFS is a pretty simple system, which uses only one side of a disk at a
time.  If you happen to have a double-sided drive, DFS simply thinks of
the two sides as separate drives.  The first side of the first drive is
Drive 0, the first side of the next drive is Drive 1, the second sides
are Drive 2 and Drive 3 respectively.

It also has a "flat" directory structure; that is, there's only one
directory per disk (or side), the top level directory, and it contains
only files, not subdirectories.  However, you can get close to the
effect of subdirectories one level deep, by using single-character
prefixes.

DFS is a single-density system, so you need disks which are suitable
for single (or double) density (not "high" density).  DFS will work
with 40-track or 80-track drives.

Technically speaking, DFS uses Frequency Modulation (FM) to store the
data in single density, using a data rate of 125kbps.  It formats disks
as 10 sectors per track, 256 bytes per sector, and either 40 or 80
tracks per side, depending on your drive.  That gives a capacity of
100KB or 200KB, depending on the tracks.

To format a disk you need a utility called FORM40 for 40-track disks,
or FORM80 for 80-track ones.  Or some equivalent, often just called
FORMAT or similar.  Or copy of the Disc Doctor ROM, or any of various
others, that have a formatter utility included.

ADFS was designed to give higher capacities using the same drives, and
also to give a hierarchical directory structure (multiple levels of
subdirectories).  It has some other advantages (free space map,
non-contiguous files, etc), but that's the main idea.

ADFS gets the extra capacity by using double density instead of single
density.  That's what an IBM PC uses for its 360K and 720K formats.
 You need disks rated for double density; any labelled for single
density should also work perfectly[1].

If you have a 40-track single-sided, 80-track single-sided, or 80-track
double-sided disk, ADFS can format it to a standard layout.  It can
actually handle 40-track double sided as well, but that's not quite as
standard in the Acorn world.

ADFS also comes with a formatting utility on disk, just one this time,
called AFORM, but it understands three formats called S (small), M
(medium), and L (large).  These correspond to single-sided 40-track
with a capacity of 160KB, SS 80 with 320KB, DS 80 with 640KB.

Technically speaking, it uses Modified FM (MFM) to store the data in
double density, using a data rate of 250kbps. It formats disks as 16
sectors per track, 256 bytes per sector, and 40 or 80 tracks on one or
two sides according to your drive (you tell it what kind by specifiying
S, M, or L when formatting).

Does that help?

I don't know if your Teac is SS or DS, 40-track or 80-track.  The AV
designation is an OEM one, not listed on Teac's website.  Try it and
see.  You won't do any real harm if you try to format 80 tracks and it
only has 40; the worst will be the clattering noise it makes after it
gets halfway.  If its single-sided and you try to format the second
side, it will either fail (and do nothing, for a long time), or succeed
in reformatting ithe first side.

[1] I once got into a "discussion" with someone about this.  I will
merely point out to the technically interested that although the data
rates are different between FM and MFM, the bit rate and flux
transition densities are the same.   As for 48tpi/40-track versus
96tpi/80-track, I will point out that the bit density around a track is
an order of magnitude greater than the track-to-track distance, and I
will laugh at anyone who suggests you shouldn't use "48tpi" disks in an
80-track drive.  Now, double-sided vs. single-sided, that's a different
story...

Oh, and a little footnote.  In my previous message I mentioned 600kb/s
in connection with HD disks.  That's only for 3.5" disks; 5.25" HD
disks use 500kb/s, which is why they get 16 sectors per track instead
of 18.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York