Date   : Fri, 30 Jul 2010 22:19:52 +0100
From   : mlist@... (Steven Flintham)
Subject: Why did Acorn ADFS only allow 640KB on a floppy?

On 30/07/10 21:58, Rick Murray wrote:
> On 30/07/2010 21:44, Steven Flintham wrote:
>> P.S. I tried to find a manual or something to confirm the existence of a
>> DDFS offering 720KB, but surprisingly failed
>
> You won't find it. Maths again. You'll need nine sectors per track for
> eighty tracks on two sides. Oh, and 512 byte sectors. I've just
> described a DOS disc. :-) Needs 512*9 for that to work. You'll be able
> to write software to access these discs, but you won't find a
> conventional (D)DFS offering them, they're all 256/sector.

I was so sure I had not made this up :-) I just went and looked in a 
copy of The Micro User (July 1987, FWIW). I have a Watford Electronics 
ad in front of me saying their DDFS "gives 80% more storage per disc in 
double density mode, P.S. - Please note that not all DDFSs are capable 
of providing the full 80% storage increase", and I make that 720K for a 
double-sided 80 track disc (400K with standard DFS, 400*1.8=720). Am I 
being dense (no pun intended!) or is the ad misleading?

I like your reasoning that 1024 byte sectors are bad news on a machine 
like the BBC, except for claims like Watford's. Maybe their DDFS raised 
PAGE higher than &1900, I have no idea...

Steve