Date   : Fri, 09 Jul 2004 11:20:25 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: Acorn Winchester unit

Johan Heuseveldt <johan@...> wrote:
> IDE still supports the standard figures for number of heads, tracks
> per platter/disc, sectors per track and bytes per sector. The bytes
> per sector remain the same, but the other three can be changed in
> whatever scheme you like. It is called a logical model, and the
> default is just the model as expected (with most BIOSes)
> And, of course, there is LBA too.
 
Talking of IDE drives, I've been working on an IDE interface card for the
Beeb/Master. It should be do-able in two chips, two sockets and a bit of
wiring. See http://www.mdfs.net/Info/Comp/BBC/Hardware/IDE/
 
Using an emulated IDE interface I have successfully patched ADFS 1.50 to
access IDE drives, see http://www.mdfs.net/Info/Comp/Hardware/IDE/ADFS/
 
With IDE 512byte sectors are arranged as 256 word of 2 bytes. The patched
ADFS throws away half the drive capacity by ignoring the high byte of each
word.  With some IDE drives it is possible to program their geometry to
have 256byte sectors. I need to get the hardware working to investigate
this further.
 
Within the zip file is a program that lays out a blank ADFS filesystem for
any size drive as specified by the user, as well as one that creates a
blank ADFS disk image. These can be adapted to talk to another device to
create a blank filesystem.
 
> There is a limit on the drive's maximum size imposed by ADFS. Free
> space Map is using three bytes for block(=sector)numbers. This would mean:
> 
>  2^24 sectors/blocks = 2^24 * 256 bytes = 2^32 bytes = 4 GBytes.
> 
> Or is ADFS using other measurements when dealing with hard drives?
 
The ADFS filesystem structure uses 3-byte addresses, so can fit on a
device up to 4G in size.  However, 8-bit ADFS talks to devices using
21-bit SCSI addressing:
  byte 0: sector b0-b7
  byte 1: sector b8-b15
  byte 2: sector b16-b20 + drive b0-b2
 
so that largest device 8bit ADFS can *access*, as opposed to what the
filestructure can use, is 2^21 sectors, or 512M.
 
> So any modern drive is a waste of space, as 4 Gigs is pretty low
> these days. So:
> 
>  1 Is a not-too-new drive of 4 GB - or just a little more - still young
>     enough to support the /logical/ model of 256 bytes per sector?
 
I can pick up 540M IDE drives for a fiver, that's what I've been using for
developing the IDE interface.  I've created an ADFS disk image with half
of the JGH Software Library on it, and it's just over 3M!
 
> In my SJ Research's MDFS there is such a drive too.
> Perhaps also using 256 bytes per sector?
 
MDFS floppies used 1024byte sectors, the filesystem uses 1024 blocks, I'm
inclined to beleive that the MDFS hard drives use 1024byte sectors -
either that or 256byte sectors used in groups of 4.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
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