Date   : Sat, 31 Jul 2010 14:14:11 GMT
From   : jgh@... (J.G.Harston)
Subject: Why did Acorn ADFS only allow 640KB on a floppy?

Steven Flintham wrote:
> floppy. So why did Acorn not do the same with ADFS? Is there some
...
> filing system? The Master FDC at least must have been capable of it
> because the Master 512 used an 800KB format.)

Because ADFS S/M/L is an 8-bit filing system. Not that it runs on
an 8-bit CPU, but that it uses 8-bit sectors - ie, 256-byte
sectors. You can get 800K on a floppy with the same controller, but
with 5 x 1024-byte sectors. ADFS S/M/L is intrinsically based on
256-byte sectors.

Five open channels with 256-byte sectors means &500 bytes of buffer
space. Five open channels with 1024-byte sectors means &1400 bytes
of buffer space.

Admittedly, you could use blocking/deblocking between logical
256-byte sectors and physical 1024-byte sectors, but do you want to
find the space in the ADFS ROM to do it?

DOSFS for the Master lets you use 720K disks, but that's because
the disks use 512-byte sectors, and the filing system and the
on-disk file-system are written and designed for 9-bit sectors.

-- 
J.G.Harston - jgh@...      - mdfs.net/jgh
Whitby Yards Gazetteer - http://mdfs.net/Docs/Books/YofWhitby/Gazetteer