Date   : Fri, 30 Jul 2010 22:58:53 +0200
From   : rick@... (Rick Murray)
Subject: Why did Acorn ADFS only allow 640KB on a floppy?

On 30/07/2010 21:44, Steven Flintham wrote:

> lot of the third party double density DFSs let you put 720KB on a
> floppy. So why did Acorn not do the same with ADFS?

I think the ADFS problem is related to containing 256 bytes in a sector, 
thus requiring 16 sectors per track for 80 tracks on 2 sides.

   256 * 16 * 80 * 2 = 655360

It's a maths problem. The ADFS discs that provide 800K have a different 
geometry (5 * 1024 per track = 5120; as opposed to 16 * 256 = 4096). The 
larger formats, and possibly even DOS 512 byte sectoring, would not 
really be feasible on BBC hardware as it would be a fair bit of 
workspace, plus the complications of being larger than a page. I think 
the 256 byte sectors fit in well with how the system works, and for that 
we have to put up with 640K discs.

The most I'm aware of on a generic single sided DFS disc is 320K, using 
the same geometry as above (256/16/80) on one side. Flip the disc 
over... 640K. That said, I'm not aware of a floppy drive with only one 
head, so either by the time I started seeing them they had dual heads 
and DFS was behind the times, or one head was a dummy.

My dual drives both provide heads on both sides, I know as it worked 
something like 800K/disc on the MDFS. One of the few parts I was able to 
rescue from it when it went bang.


> For that matter, why did no one push it to 800KB in a 'normal' BBC
> filing system?

I would imagine it is totally possible, but do you want to lose 1024 
bytes for disc workspace on a system with already restricted memory?


> 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.


Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...