Date : Tue, 08 Jun 1999 00:37:06 +0100
From : Stuart William McConnachie <stuart@...>
Subject: ADFS Disk Images
A while ago Steven Flintham posted a request asking for details of the
various disk image formats. I replied to him in person, but not to the
list, as I considered my reply to be a little in depth. However, Steven
has suggested that what I had to say my be of use to some of you, so my
reply to him is included below:
________________________________________________________________________
Ummm. As far as pcBBC is concerned all disk drives are 80T double sided
floppies. pcBBC uses the file extension to identify the disk format type
(DFS/ADFS) and layout (interleaved/non-interleaved), but not the number of
tracks or sides of the drive. It assumes that the BBC file system is
intelligent enough not to attempt to read or write to sectors past the end of
the disk as indicated by the disk size as stored in the DFS/ADFS catalogue.
Therefore a disk that is formatted as 40T single sided, should stay 40T single
sided!
The following rules are applied when it comes to deciphering the file
extension. Essentially there are only 4 types of file:
.ADS, .ADM, .ADF extensions
==========================
File is assumed to be a 80 track, 16 sector per track double sided
***NON-INTERLEAVED*** ADFS disk.
N.B. the sectors for the second side are not interleaved, so you get the first
80 tracks from one side and then the 80 tracks from the other (if indeed there
is another side). This makes sense because that was the way the BBC wrote to
the disk, filling first the "top" surface and then the "bottom". You also save
50% (or more) of your PC hard drive space if you use ADS or ADM disks, or your
L formatted disks are only half full.
The reason for the 3 extensions is that I started out by calling all ADFS disks
.ADF, and then someone (Chris Richardson actually) pointed out that he had a
load of interleaved ADFS disks (which I don't really approve of) and would I
mind supporting them in pcBBC. Therefore I created the .ADL extension, and at
the same time added the .ADS and .ADM for coherency (but at the moment pcBBC
does nothing different with these than it would do with a .ADF file).
.ADL extension
=============
File is assumed to be a 80 track, 16 sector per track double sided
***INTERLEAVED*** ADFS disk.
I don't really approve of this format as I've said before, but I added it for
Chris as he especially requested it. As with any of the other ADFS image
formats you are of course free to format it as either a S, M or L. Of course
you will be wasting all those interleaved tracks if you use S or M formats!
.DSD extension
=============
File is assumed to be a 80 track, 10 sector per track double sided
***INTERLEAVED*** DFS disk.
Tracks from alternate sides are interleaved, BBC drive 0 first then BBC drive
2. Again I don't really like this format very much, but it is included for
compatibility.
.IMG, .SSD
=========
This is the default image format for pcBBC or 80 track, 10 sectors per track
double sided ***NON-INTERLEAVED*** DFS disk.
Note also that all other unknown extensions are also assumed to be this format.
Basically I'd stick to the .IMG and .ADF extensions. If you want to be able to
identify the format type of your ADFS disks, then use the .ADS and .ADM
extensions. Personally I'd steer clear of the interleaved .ADL and .DSD
formats for the following reasons:
1) This was **NOT** how a BBC regarded the two disk surfaces
2) It wastes space for S and M format disks and L format ADFS disks that are
half full.
3) If you confuse a non-interleaved/interleaved format disk (by renaming it to
the wrong extension for example) then writing to it in an emulator will totally
muck up the disk. You will also get corrupt files when you read from the disk!
4) With the non-interleaved formats BBC files appear as contiguous chunks in
the image file. This makes them easy to view in a hex or disk editor.
5) There are enough confusing image formats out there already.
About the only thing going for it is the .DSD format is useful if you want to
keep two sides of a DFS disk together.
My responses have been mainly from a pcBBC point of view. However I don't
think I've said or done anything which contradicts with anyone else's
definition of these formats.
Regards, S.
--
Stuart McConnachie (stuart@... )
43 The Hollows, Long Eaton, Nottingham, NG10 2ES, UK
Mobile: 0966 224307