Date   : Wed, 12 May 1993 17:24:28 BST
From   : "M.J.Ebourne" <M.J.Ebourne@...>
Subject: Re: A new thing...

> Excellent!
> I figure that this works by switching the banks between the interlace, so
lines  
> from each bank get interleaved(?). I had wondered if this was possible, but
I  
> don't have shadow RAM to try it with (If this works well enough I might get 

> some!). Once selected how well does it sit with the rest of the system (ie
is  
> it transparent?), or are special drivers needed? Will it work with any shadow 

> board?

Some bad news for you... This is master only. Beeb shadow ram boards
(at leaset, the ones I've seen) are completely different in operation. The
problem is that the 6845 is wired into the main RAM, and needs to be because
it is that which carries out the refresh. Hence the screen is always displayed
from main RAM. If you put shadow mode in, the main RAM is paged out & the
shadow RAM paged in, so all your data is stored in their. Then OSWRCH is
trapped & the main RAM paged in before & out again after. There is no
physical way of displaying the screen from the shadow RAM.

I know this because I tried to do it, & then had to disassemble the
controlling software to find out why it didn't work. Obvious really.
What confused me is the manual stated clearly that the screen was displayed
from shadow ram, but they are definately lying. :-(

On the bright side, masters are quite cheap now... Ask Richard Whitehand
(who should be listening in to this...), he's got half a dozen & they
are currently in his bad books, so he's bound to be keen to get rid of
them. :-)

-- 
Martin Ebourne                             Email: mje@...        
Electronics, Southampton University