Date   : Fri, 18 Mar 2011 08:21:51 +0100
From   : kortink@... (John Kortink)
Subject: How much sideways ROM is possible ?

On Fri, 18 Mar 2011 02:03:35 -0000, "David Hunt"
<dm.hunt@...> wrote:

>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 ?

With enough physical force, yes. ;-)

>>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.

Only if you left the top two address lines floating. Which
is never a good idea, really. Simply tie them to ground,
and you won't have to replicate.

>A simple pair of toggle switches could
>then select which bank of 16 ROMs you wanted to use, 0 - 3.

Are you sure you can find that many useful ROMS ? ;->

>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!) :)

The OS would need patching, but it wouldn't be difficult
to do, and it would probably work without patching anything
else. But note that you can't easily get 256 on the Master,
because it uses the top bit of ROMSEL for something else.

>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 suppose not many people find that having many ROMs in
the machine is useful. I use only one (and then only for
development purposes, really).


John Kortink

-- 

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