Date   : Mon, 27 Apr 2009 20:52:20 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: A thought for Domesday

Joel Rowbottom wrote:
> I'm having some issues doing this tbh, mostly in the file DATA1 where it
> dies at an as-yet-unknown place and I get Disc Error 03. The only way to
[snip]
> My next step on this is to write something (probably utilising the ARM7)
> which will chunk the data off - then at least I'm not starting from the
 
The disk I took to Wakefield with networking software on for Joel
had a handful of disk error on it. I could only copy it with a
short program that manually copied short chunks of file and stepped
past errors. At home I have a "RdDodgy" program for doing this sort
of data extraction which should work copying from VFS. I've put a
copy at http://mdfs.net/Software/FileTools/RdDodgy
 
> Jonathan: You might be able to answer this one, what's the maximum file
> size on ADFS then?
 
512M.
 
Though, strictly speaking, it's 511.9M.
 
See http://mdfs.net/Docs/Comp/Disk/Format/ADFS
 
The ADFS file system structure stores file lengths as 4-byte
numbers, so the file system can store 4G-1byte file lengths. The
ADFS file system structure stores sector start addresses as 3-byte
numbers, so the maximum disk size the ADFS file system structure
can access is 4G-1byte.
 
However, the ADFS sector access API uses 21-bit sector numbers (the
drive number is in the top three bits), so the largest sector
number accessible if &1FFFFF, giving a maximum disk size of 512M.
As the first seven sectors contain the free space map and the root
directory, the maximum file size is &1FFFF900 bytes.
 
The VFS sector access API (OSWORD &62) uses exactly the same
control block with exactly the same sector address format - 21 bits
for sector number and 3 bits for disk number - so that explies that
the largest data objects and disks on VFS is also 512M.
 
I shall set my object-to-source converter chugging away at the VFS
ROM image ;) Initial draft at http://mdfs.net/roms/Filing/Video
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
A Review of Sheffield City Council's Members' Allowances Scheme
                                  See http://mdfs.net/payreform