Date   : Sat, 21 Sep 2002 16:31:52 +0100
From   : "Richard Gellman" <r.gellman@...>
Subject: Re: BBC B with edge-connectors instead of IDC connectors?

To summarise everything:

I concur on the Model A thing. As soon as you said "No analogue port" I
thought "upgraded Model A".

There is a book called "Programming the BBC Micro" which has near the back a
list of components that have to be fitted to upgrade an A to a B.
They're all "off-the-shelf" components, so back then the average electronics
hobbyist could do it easily and cheaply (in fact, for probably less than the
price difference between an A and a B at the time). Another thing to keep in
mind, is that the Model B is overly versatile, that is to say, a lot of
people will never use *every* interface on their computer, and as such, if
one is upgrading, will never bother to add on the ports they know they'll
never use.

By the looks of your board, I'd say its just had the disc interface fitted,
and the extra RAM to make it into a Model B (OS upgrade as well?).

As for the whole filing system issue..

Well, first up, yes the ADFS in a Master 128/Compact is part of what Acorn
likes to call "the 1Mbit ROM". I have counted up, and yes there is in fact
1024x1024 bits of information in it :). The ADFS is Paged ROM 13 in this
chip, and can be read from the Master as a ROM image using a standard method
of disabling interrupts, switching banks, copy the data, switch banks back,
enable interrupts, and save the data. This *could* then be used, in theory,
if blown into an EPROM, in a model B.

There *are* problems with that though. First off, the one you missed, is
that the ADFS in the master is Master optimised. i.e. its been written with
a BBC Master in mind, and so expects a 1770/1772 at &FE28, the drive control
register at &FE2B, and it also has some 65C12 opcodes, meaning the best
you'll get out of it in a Model B, is a very efficient, ROM-based method of
crashing your computer :(.

All this however, can be partially cirumvented (WHAT?!). There were numerous
"daughterboards" available with 1770 chpis on them, and the ADFS ROMs to go
with them, compatible with the Acorn ADFS. If you want a real challenege
though, All is needed is to pick up on the memory addresses coming from the
CPU bus, map the 1770 chip into &FE28, build a quick drive control register
at &FE2B, and plug a 65C12 CPU into your CPU socket. Far easier to get a
Model B 1770 upgrade though :)

The disk interface you have though is 8271. This is the single density disk
interface made by intel, and will not handle ADFS disks at all. (IMO, it
barely handles single density :) ).

The DNFS ROM was indeed sold/sent out as a cunning upgrade to solve many
problems with one chip. The chip simulatenously upgraded any existing DFS or
NFS you had, consequently saving on a ROM space, as well as providing the
essential Tube IO code to allow a second processor. For interest, this
system was not used in the Master 128, as everything but NFS was part of the
1Mbit ROM (except the DFS was made for the 1770 FDC).

Hope the info's been useful :)

-- Richard