Date   : Fri, 24 Feb 2006 11:12:06 +0000
From   : Andrew Benham <adsb@...>
Subject: Re: Beeb ROMS

Thomas Horsten wrote:

> I have a few Beeb ROMS in my collection, one of them is an INTER WORD
> original, which has a blue label and a funny construction. It consists
> of a socket with a small PCB, on the PCB is mounted a smallish IC and
> some of the pins from the socket is connected to it, then the "real"
> ROM is sitting on another socket on top of that.
> 
> I'm curious about it - is it some kind of copy protection, and if so
> how does it work? Or is it an adapter they made so a larger ROM can be
> used in a 16k slot, with some sort of bank switching?

Computer Concepts' InterWord is in a 32k ROM (27256).  The code is
effectively written in two 'banks', each of 16k.  The carrier board
includes a PAL which switches between the 2 banks.  If you like,
it's paged ROMs above a single ROM socket.

The bank switching is done by the presence of certain addresses on
the address bus.  I've got the details somewhere - I'd actually
reverse engineered it *before* I took over maintenance of Beebug's
"Master ROM" (which uses the same carrier board) and they gave me
the documentation.

Computer Concepts also had a spell-checker 'ROM' which used a different
carrier board and a ROM with 8 16k banks in it.

-- 
Andrew Benham           adsb@...       
Southgate, London N14, United Kingdom

The gates in my computer are AND OR and NOT, not "Bill".