Date   : Sat, 31 Jul 2010 09:00:21 +0000 (GMT)
From   : tommowalker@... (Tom Walker)
Subject: Why did Acorn ADFS only allow 640KB on a floppy?

> > Again, like with DDFS, I've seen the latter done (on a LOT
> > of Atari ST discs), but it was never officially supported
> > outside of custom formatters.
> 
> If we really want to throw the cat among the pidgeons, we can look at 
> the Amiga's "forget the FDC, we'll do it ourselves" approach which 
> squeezed something like 880K onto a conventional disc, with special 
> custom formatters that pushed it into the 900s - though you did end up 
> with situations where a computer could only read its own discs...

Well if we're talking Amiga, it was entirely possible to forget the concept
of sectors altogether, and just put 6.5k on a track ending up with 1040k
per disc - which a number of games did.

But my point was the ST could achieve 10x512 on a normal WD1772 controller,
so there's no reason the Beeb couldn't achieve similar over-formats (whether
10x512 or 18x256).

This is largely due to the flexibility of the 177x controllers though...
the 8271 (as well as the uPD765/8272 found in PCs and later Arcs) have the
gap lengths more fixed.

Tom