Date   : Sun, 17 Oct 2010 14:33:32 +0100
From   : afra@... (Phill Harvey-Smith)
Subject: Fw: DD & HD Floppies

J.G.Harston wrote:
> Dave Curran wrote:
>> The ideal would be adapting one of the DFS's to work with 1.44M disks -
> 
> See all the work Beardo has done with HD disk access. It is *very*
> difficult on a BBC due to the incredibly tight access loops to
> get the data transfered fast enough, and needs customised hardware
> and rewritten firmware. See
>    http://beebwiki.jonripley.com/High_density_floppy_disc_access
>    http://regregex.bbcmicro.net/#prog.edospat
> and the thread starting here back in 2005:
>    http://mdfs.net/Archive/BBCMicro/2005/10/30/210658.htm

Part of the problem though with the Opus board is that the WD2791 that 
it uses has an inverted databus, replacing it with a WD2793 would solve 
this as the chips are otherwise identical.

This would make the timing not quite so tight as all the EOR 
instructions would not be needed to flip the bytes going between the CPU 
and the disk controler.

What is puzzling is why Opus decided to use this controler when the 
WD2793, would not have had the same limitation.

Cheers.

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.