Date   : Mon, 09 Feb 2004 09:44:59 -0800
From   : Angus Duggan <angus.duggan@...>
Subject: Re: Discs

Jules Richardson writes:
>Caveat - I'm not sure about the 40/80 track switching for reading a 40
>track disk in an 80 track drive. For a couple of the Watford drives I
>have, it seems extra circuitry was added to stock drives to (presumably)
>double-step the heads in hardware when the drive was selected as a 40
>track unit. I guess the floppy controller IC in the BBC just doesn't
>have the ability to do this under software control?

Yes, it can. There are several 8271 DFSes that do this. The trick is to fool
the internal 8271 track register by doubling it before any step, and halving
it afterwards. Auto-detection is done for Acorn DFS discs by checking the
number of sectors in the catalogue. My own HDFS uses this technique:

http://knackered.petestack.com/beeb/hdfs.html

a.