Date   : Fri, 30 Jul 2010 23:55:50 +0100
From   : tom@... (Tom Seddon)
Subject: Why did Acorn ADFS only allow 640KB on a floppy?

On Fri, 30 Jul 2010 22:19:52 +0100, you wrote:

>On 30/07/10 21:58, Rick Murray wrote:
>> 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 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...

No funny business required. Opus DDOS (and the related Challenger)
offer(ed) 720K discs, using a normal 1770 FDC. 2 sides, 80 tracks, 18
sectors per track, 256 bytes per sector. Don't know why this layout
wasn't more popular, but perhaps there's a good reason.

(DDOS puts PAGE at &1900; Challenger, at &E00.)

DDOS splits each side into multiple drives (0A, 0B, etc.) with each
drive being up to 256K. And each drive has the usual catalogue limit
of 31. (From memory, the catalogue layout was exactly the same as
Acorn DFS, which is where the 256K limit comes from -- the sector
field is 10 bits. There's just a volume table somewhere, giving a
per-drive offset for the sector indexes in that drive's catalogue. You
can reuse the single-density catalogue management code that way, I
suppose.)

So it's not entirely seamless, but I'd argue it's close enough to
boggo DFS to count as "conventional"...

--Tom

P.S. This is emulated in model-b, which emulates Opus 1770 and
Challenger interfaces. The emulation is not particularly brilliant,
but it never seemed to bother anybody, so once I'd got some stuff off
my old Challenger discs I left it alone.