Date   : Wed, 15 Oct 2008 21:02:56 +0100
From   : mfirth@... (Michael Firth)
Subject: WD179x in an FPGA

----- Original Message ----- 
From: "Mike Tomlinson" <mike@...>

<snip>

> The usual format is (raw) sector data, though I think you have to check
> to see whether the tracks are interleaved or not on double-sided disk
> images.  I've just Googled for "bbc micro disk image specification" for
> you but not found anything useful.  It's been discussed here before,
> perhaps a search of the archive will turn something up.
>
> Generally, .ssd is single sided disk, .dsd is double-sided, .adf is
> ADFS.  You have to examine the image to see whether it's 40 or 80 track.
>
The 'dsd' format is indeed interleaved, you get all the sectors (0-9) of the 
first track of the first side, followed by the sectors from the first track 
of the second side, then the second track of the first side, and so on.

For the 'ssd' format, it is just a straight sector dump of the first 
(usually) side of the disk. The are applications (e.g. the HADFS backup 
program) that will create an 'extended ssd' file for both sides of a dual 
sided disk, as not interleaving the sides is actually more useful for most 
'in BBC' applications. There are utility programs for converting between a 
'dsd' and an 'extended ssd' file.

I'm not sure if the 'adl' ADFS format (adl is the 640k version, which is 
more typical of the BBC ADFS) is interleaved or not.

Michael