Date : Wed, 15 Apr 2009 01:29:39 +0100
From : jgh@... (Jonathan Graham Harston)
Subject: MDFS as Local Filing System
"Ian Wolstenholme" wrote:
> I'm not sure if this has been tried before, but I wonder if it would be
> possible to write a filing system ROM based on the MDFS file server
HADFS was written with the intention of it being as close to
looking like connecting to an MDFS server as possible. If I'd had
adequate documentation or even access to an MDFS when first writing
HADFS I'd have implemented some of the lower levels more in line
with the way the MDFS actually does. One thing I particulalrly like
is that there's a pseudo-directory with entries in it for "$",
"%PrintQ", "%Passwords", etc., so those objects don't have to be
treated as special cases, they are just searched for normally in
effectively the parent of "$".
> - logging on with *I AM to a specific directory with password protection
*I AM logs into a specific directory, passwords ignored, but can be
checked via a veneer layer
> - larger directories, up to the Econet limit of 250 entries
HADFS directories can contain as many entries as there is disk
space.
> - space accounting
File/directory accounting exists, space accounting can be
implemented via a veneer layer.
> - date-stamping of files
HADFS implements date-stamping.
> The idea would be that you would initiate it with *MDFS or similar
> and then have access to any MDFS partition on a locally connected
> floppy or hard disc, with all the usual Econet filing system commands
Any filing system that accesses natively formatted MDFS devices is
limited in certain aspects.
Floppy disks are MFM formatted with 1024-byte sectors, so twice the
problem of accessing DOS disks. You would only be able to access
floppies with a double-density disk interface, such as the 1770.
I've imaged MDFS floppies on RISC OS, and started some coding for
the Master but never finished it.
MDFS hard drives are formatted with 512-byte sectors gathered in
pairs to form 1024-byte logical sectors. The SJ documentation says
that a BBC SCSI hard drive can be plugged into the MDFS, so there's
probably milage in a BBC SCSI interface being able to access the
data on an MDFS drive. Having never had a BBC SCSI interface I've
never been able to test this.
> The only drawback I can think of at the moment would be the file
> size limit of 8MB on Econet where it is 16MB on ADFS.
Also, MDFS disks don't have free space maps, they are constructed
in memory by the server and are rather large, so saving data would
be a complicated matter. Also, directories don't have a pointer to
their parent, to access "^" the MDFS scans back down the directory
tree stored in memory from "$".
I've written SJFiler, an application that can read and extract data
from MDFS floppies and images, at http://mdfs.net/Apps/Networking
It runs on RISC OS or Windows, and on RISC OS can access floppies
directly. When running on Windows it currently can only access disk
images.
--
J.G.Harston - jgh@... - mdfs.net/User/JGH
RISC OS Internationalisation - http://mdfs.net/Software/RISCOS