Date   : Fri, 18 Mar 2011 07:22:33 +0000
From   : robert@... (Rob)
Subject: How much sideways ROM is possible ?

On 18 March 2011 02:03, David Hunt <dm.hunt@...> wrote:
> Hi Folks,
>
> I have a collection of 8M bit (1M x 8) EPROMs from a long defunct system and
> wondered if there was a way of shoehorning a chip of that size into a BBC B
> or a M128 ?

It ought to be possible, but you it would need some extra work, as you surmised.

>
> >From the circuit diagram I can see ROMSEL and the quad latch, if I were to
> use all 16 slots then I would need to replicate the often needed ROMs
> throughout the 1M byte address space. A simple pair of toggle switches could
> then select which bank of 16 ROMs you wanted to use, 0 - 3.

Yes, you would need a language and probably a filing system in each.

> I am using a
> GoMMC in both the B and the M128 so I won't get away with 16 ROMs, perhaps 8
> banks of 8 ROMs and use a counter IC and push button to select the bank.

Probably better to use banks of 8, yes, as you can then use every slot
for useful stuff.  If you can find 64 ROMS you want, that is !

You used to be able to get little push-button counter switches
(displayed a number, with a button to increment or decrement) that
output in binary.  Those would be easy to use too, if you can find
one.

Remember, though, at a minimum you would need to reset immediately
after swapping the ROMs over, and probably be better to switch with
the power off..!

>
> A bit of a long shot, but can the OS use all 8 bits of the value of ROMSEL
> or would the OS need patching, that would give you 256 ROMs (4M bytes!) :)

Quick answer: No.  Far too many things rely on that number being <16,
lots of 16 to 0 counts, and there are various tables that are only 16
bytes in length that are indexed by ROM number that you would have
trouble extending.   Not to mention having to do the same again for
every ROM you want to include.

>
> I did have a Solidisk FourMeg 256 many moons ago, apart from reliability,
> its functionality was very useful - has anyone thought of re-making
> something like it ?

I think the Master reproduced most of it's functionality, didn't it?

Rob