Date   : Fri, 24 Jun 2005 10:22:21 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: Watford Electronics DFS

> In article <050622145502@...>, Jonathan Graham Harston
> <jgh@...> writes
> 
> >No, there is Watford DFS for the 8271 and Watford DFS for the
> >1770.
> 
> You'll find the Watford 1770 DFSes are actually called DDFS.
> 
> >Looking through the Watford DFS ROM images I have, I can confirm:
> >
> >1.10  - 8271
> >1.30  - 8271
> >1.43  - 8271
> >1.44  - 8271
> 
> Agreed.
> 
> >1.54T - 1770 - Tube Host
> 
> 1.54T actually declares itself as DDFS 1.54T, if you'd care to try it.
> The extra D is "D"ouble density, which the 1770 is capable of.
Yep. 16 sectors per track, 320K per side - which neccessitated
Watford extending the catalogue information to have a DFS disk
structure larger than 256K.
 
Acorn DFS uses 18-bit addresses, so the largest addressable unit
is 256*2^18-1 bytes -> 256K-1. Watford uses 'spare' bits scattered
around the catalogue to get 19-bit addresses and access up to
512K. I've got the documents somewhere - I need to find them and
type them up.
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
RISC OS Choices System - http://mdfs.net/Software/RISCOS