Date   : Thu, 16 Oct 2008 00:04:30 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: WD179x in an FPGA

Mark McDougall wrote:
> I haven't even looked into the BBC emulated disk formats, but I'm assuming
> either that there's a similar format specified, or another format which
> contains only sector data. Anyway I'll check it out sometime soon...
 
BBC disk images tend to just contain the disk sector contents, not
the raw low level disk track structure. They are mostly sequential
data, that is, the data on the disk just continuous through the
image file, so the 256 bytes of the 0th logical sector are followed
by the 256 bytes of the 1th logical sector and so on.
 
If an image file has an extension, then:
 
 .ssd - is a sequential image, most commonly used for images of
 single density disks such as DFS, AcornCPM, HADFS, TorchCPM
 
 .dsd - is an interleaved image, most commonly used for storing two
 sides of a DFS disk. Bytes 0*256 to 10*256-1 are the first 256*10
 bytes of the first disk (ie, DFS drive 0 or 2), followed by bytes
 10*256 to 20*256-1 being the first 256*10 bytes of the second disk
 (ie, DFS drive 1 or 3), followed by the next 256*10 bytes of the
 first disk again
 
 .adf - is a sequential image, most commonly used for images of
 double density disks such as ADFS disks.
 
However, whatever the extension, a disk image file is just a
sequential sequence of the data bytes in the disk sectors.
 
A suitable raw disk image format is described in relation to BBC
disk formats at http://mdfs.net/Docs/Comp/BBC/FileFormat/RawDisk
and http://mdfs.net/Docs/Comp/BBC/FileFormat/RawDFS
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
05:10:36, 18-Nov-2008 - RISC OS time rolls over to &5000000000
05:10:36, 18-Nov-2008 - RISC OS time rolls over to &5000000000