Date   : Tue, 16 Mar 1999 10:57:49 +0000
From   : jgh@... (Jonathan Graham Harston)
Subject: Differences between model A and model B

>Message-Id: <199903121124.LAA26999@...>
 
Adam Hamilton x8712 <Adam_Hamilton@...> wrote:
> Please can someone tell me ALL the differences between the BBC model A
> and model B computers.
> Including hardware and firmware - was the OS and Basic different
> versions, was the processor clock the same speed etc.
 
The versions of Basic and the OS are irrelevant to the difference between
a model A and a model B.  That's like the difference between, say, an
application with German-language Messages and one with English-language
messages.
 
All early BBCs had OS 0.10 (in EPROM) and Basic 1.  After a short while
the full OS 1.20 (in mask ROM) became available and all BBCs were shipped
with it and an upgrade option for about a tenner was offered.
 
Early BBCs had a linear power supply instead of a switched-mode supply,
whether model A or B.  You needed a switched mode supply if you wanted to
use a parasitic disk drive (one without it's own PSU), 
 
Model A machines had 16k of ram instead of 32k, and the following items
were missing: all external sockets except TV, Serial and Tape, PCB
circuitry for ADC, User VIA (printer port + user port), Tube buffer.
 
Serial circuitry was present, as it is used for the cassette interface,
but the actual RS423 socket may have been absent, so saving 15p.  RGB
circuitry was present, as it is used for the TV interface, but the actual
RGB socket may have been absent, saving another 15p.
 
At that time the only really expensive difference between a model A and B
was the ram, so many people added the extra components themselves.  It was
common to come across 32k model As (added extra memory) and 16k model Bs
(added extra hardware).
 
All OSs interrogated the ram at power-on to see how much was there and
operated within the memory found.  Eg with only 16k you couldn't select
MODE 0/1/2 (20k needed) or MODE 3 (16k needed).  If you forced a large
memory MODE selection, you got MODE 4/5/6 instead (10k/10k/8k).
 
In 1982, Basic 2 became available.  This was mainly a bug-fix of Basic 1,
and included a couple of ommitted features, such as OSCLI and the
OPENUP/OPENOUT token discrepancy corrected, and all BBCs were shipped with
it.  Again, the version of Basic is irrelavant to what hardware model you
plugged it into.
 
All models had sideways rom selection circuitry, but only OS 1.20 onwards
used it.  So, if you added a disk interface, you needed to ensure you had
OS 1.20 as well as the FDC hardware and the DFS rom.
 
Whether a BBC had a disk interface is irrelevant to the actual model, but
it made sense to ensure the BBC was a model B when putting a disk
interface in.  I have come across some BBCs that are essentially a 32k
model A with a disk interface.
 
-- 
J.G.Harston (JGH BBC PD Library) 70 Camm Street, Walkley, SHEFFIELD S6 3TR
jgh@...                - Running on BBCs & Masters with SJ MDFS FileServer
Z80+6502/CoPro+Tubes/Econet+SJ -- http://www.li [Website temporarily down]
   On the way home my operating system crashed, until a passer-by
      pressed my Ctrl-Alt-Delete keys...              [Futurama, Groening]