Date   : Sat, 27 May 2006 23:02:32 +0100
From   : Andrew Benham <adsb@...>
Subject: Re: Back in circulation

Jules Richardson wrote:

> I think there's also an issue in that surely the formatting code is 
> going to expect to be able to send a "define drive geometry" command to 
> an Adaptec bridge board - that command's not going to make any sense 
> when sent straight to a SCSI disk and will presumably fail.

Only if one uses Acorn's SuperForm formatter.  Other formatters (e.g.
the one from the 'Tubby utils') don't assume one is using an Adaptec
board - they assume the drive geometry has been set by another
mechanism.

> There may be a similar issue with ADFS on subsequent starts of the 
> machine - some bridge boards allow persistent storing of drive geometry 
> on the board itself, but I don't recall if the Adaptec does that.

The Adaptec board stores the drive geometry at the start of the disk
(and pretends for read and writes that sector 0 is offset).

> ADFS is going to read the drive geometry that the formatter has 
> stored at block 0 and try to issue its own Adaptec-formatted "define 
> drive geometry" command at each boot. Who knows how it'll cope when that 
> fails if issued to a SCSI disk.

Er, I don't think so.  ADFS only talks SCSI to the drive.  As far as
SCSI is concerned, the hard disk is just an array of sectors.  The
ST506 drive has to worry about Cylinders/Heads/Sectors, and its the job
of the Adaptec bridge board to handle the translation.  At reset I guess
the Adaptec board reads the disk geometry stored on the disk - but this
is for its own info, and nothing to do with ADFS.

There are ways of setting the drive geometry for other bridge boards.
I modified the ROM on my Xebec board to hard-code the drive geometry
(before I got an Adaptec board).  I've also heard of people storing a
tiny !BOOT program to set the drive geometry - this file must be
entirely within the first 32 (or 33) sectors on the disk to ensure that
it can always be executed irrespective of what geometry the bridge board
believes it has after a reset.

> I suspect what you're seeing is the formatter essentially issuing a 
> "format unit with default b/s" command - I can't remember if b/s would 
> be set within the format unit command itself, or in a previous geometry 
> definition.

The "Format Unit" command (opcode 04H) doesn't specify the sector size,
so it has to be set elsewhere.

-- 
Andrew Benham         adsb@...       
Southgate, London N14, United Kingdom

The gates in my computer are AND OR and NOT, not "Bill"