Date   : Sat, 21 Jan 2006 20:14:47 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: eBay What is this?

Andrew Benham wrote:
> Ian Wolstenholme wrote:
> 
> The Adaptec ACB4000 stores the geometry of the drive on (I believe)
> (part of) physical track 0

I can't remember if that's an ACB4000 'feature' or an ADFS SCSI code one...

I seem to recall that some BBC-based Torch systems use a Xebec board with 
modified firmware EPROM so that the default drive geometry matched whatever 
drive the system was talking to (they only used a couple of different models 
of hard disk, so this wasn't an unreasonable thing to do)

> The Xebec board has the drive geometry hard-coded in the EPROM -
> I believe it's for a 5MB Shugart drive.

Probably. Default values are 153 cylinders, 4 heads, reduced write current at 
cylinder 128, write precomp at cylinder 64, and 11 bit ECC data burst length.

I think ST506 was fixed at 17 sectors per track for a 512 byte sector size, so 
that's 153 * 4 * 17 * 512 = 5.1MB ...

>  I changed the drive geometry
> in the EPROM to match my disk's parameters.  Or you could run a tiny
> program to issue the appropriate SASI/SCSI command to set the geometry -

Yep.

> if the program is on the hard disk, it'll need to be at the very start
> of the disk so it can be found even though the controller has the
> wrong geometry.

Yep - except that I'm pretty sure that OS-9 defines sector 0 as being used for 
something else, so there's nowhere to store such a program (or disk geometry) 
when using an OS-9 filesystem as this drive possibly had at one time...


cheers

Jules