Date   : Fri, 18 Mar 2011 02:03:35 -0000
From   : dm.hunt@... (David Hunt)
Subject: How much sideways ROM is possible ?

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 ?

>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. 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.

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!) :)

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 ?

Cheers

Dave