Date   : Wed, 26 Oct 2005 16:07:22 +0100
From   : Richard Gellman <splodge@...>
Subject: Re: BBC using 3.5 high density format

Paul J wrote:

> Has anyone had a go at getting a 3.5 high density drive working 
> at high density capacity ?
>  
> I found some interesting notes on an atari site, the atari st uses an 
> 'ajax' floppy controller that supports double density, high density, 
> and extended density floppys. The controller is a direct pin-pin 
> replacement for the WD 1772 controller. To switch to high density 
> formats Pin 18 needs a 16mhz clock frequency, instead of the normal 
> 8mhz for double density disks.
>  
> Could either hard wire a manual switch to a clock generator, to change 
> densities; or wire the hd detect microswitch in the drive to the 
> controller through the drive cable.
>  
> Would need a customised ADFS rom capable to driving the high density 
> format.
>  
> Just wonderred if anyone has had a go at this,
>  
> Regards,
> Paul.

*comes out from behind the pete-turnbull-proof wall*

Two things spring to mind:

1) The Master 128 (possibly also the BBC B?) has a 16Mhz clock generator
already. VIDPROC downsteps it to 8, 4, 2, and 1Mhz clocks (as this is
VIDPROC, it suggests the B has it too).

2) You wouldn't necessarily need a modified ADFS ROM. Since the disks
are HD, the PC floppy drive will read them happily (at least in terms of
density). You only need a PC program to re-format them to 256-byte
sectors (PCs use 512-byte sectors), and "inject" the root directory.

 From working with the 1770 emulation in BeebEm, ADFS only uses the read
sector(s)/write sector(s) commands for data access, which don't take
into account the layout data either side of the sector (sector ID, etc)
- they simply read in n sectors x b bytes-per-sector bytes on a req/ack
basis.

Once such a disk is made therefore, it should "just work" (tm). Of
course, if you want the fully independent mode of operation, then yes, a
patched ADFS is required ;)

-- Richard