A Quick History...

Acorn started life as the Cambridge Processor Unit in December 1978, a company formed by Chris Curry and Herman Hauser with the intention of making fruit machines for Ace Coin Equipment. However, after a meeting with Roger Wilson who had developed an 8-bit system but was struggling to produce a chip for the operating system, Acorn Computing was born...

System 1
In March 1979, Acorn began selling its first computer - the System 1.  It was a self assembly hobbyist kit based around the 6502 processor with LED display, a 512 byte ROM, 256 bytes of RAM, keypad and cassette interface.
Development was rapid and over the year, more advanced systems were produced for the home market.  SYSTEM 5 was the final version which had to a maximum of 32k of RAM.
Already these were seen as “practical” machines and add-ons started to appear to make the most of this practicality.

Acorn Atom
Whilst working with Sinclair, Chris Curry had seen the massive growth in computer enthusiast market and believed Acorn could tap into this by essentially fitting a System-1 computer into a friendly consumer-styled case.

As both a kit and a ready assembled computer, very well built and professional looking in appearance, the Atom was launched in March 1980 and came equipped with several expansion ports, 2k of RAM, multi channel sound and a large 8k ROM containing an integer-only BASIC. From their earlier experience with the Atom, Acorn saw expansion as the key to the market and with all the expansion ports already there it was only a matter of time before customers would start to buy their add-on hardware.

Over the next few years Acorn continued their policy of professional and practicality in the development of the Atom and quickly incorporated a (limited) colour graphics array to produce graphs and simple images, designed disk drives rather than mess around with tapes and concentrated on future expandability of memory in its board designs to be ready for the anticipated fall in price of RAM.

In late 1980, Acorn had a prototype computer called the Proton, which was in the design stages of becoming a replacement to the Acorn Atom.

This coincided with the BBC launching a national computer literacy campaign and their invitation to UK manufacturers to submit a design for a computer to be BBC branded. The specification required was very similar to the prototype Proton.

After a frenzied week of activity to get the computer to a demonstration stage, Acorn invited the BBC in to view it. The team were so impressed that they immediately placed an initial order for 12,000 machines (which was their estimate of the potential market size at the time) and the BBC Micro was born.

After this initial order, many more were to follow and overall the Beeb sold over a million units.

Two versions of the BEEB were made:
The Model A had a 2MHz 6502 processor, 16k of RAM and a massive 32k ROM containing the Acorn BASIC system (probably the best dialect of BASIC used in 8-Bit micros of the time) which was quickly renamed BBC BASIC.  The Model A also had both a UHF TV socket and an RGB monitor outlet (so users had an affordable upgrade route), a cassette tape connection and most importantly, industry standard analog and RS423 serial ports. 
Two special expansion ports were also built in - one designed for the Robotics market and the other a 1MHz system for direct systems access making it probably the most versatile and expandable micro computer on the market at the time.

The Model B had 32k of RAM and a Centronics printer interface, and later models quickly followed with a disk drive interface and an Econet
With the might of the BBC to market the models, the computer became the standard for education and 70% of schools and colleges chose the machine for their rapidly growing computer departments.  Over 1 million Beebs were sold making it an important part of computer history.

Acorn Electron
With all this expansion capability and multiple interfaces the BBC was not a cheap computer. Sinclair’s Spectrum and the Commodore computers started to impact on sales and in 1983 Acorn decided to create a cheaper model to compete.Using the BBC Micro’s architecture but with simplified and reduced circuitry, a computer was created which was much cheaper to produce and had a ready made back catalogue of software - the Acorn Electron was born.  It had the excellent BBC BASIC II built in, but had virtually none of the expansion ports that were already built into the BBC. It did still however have the TV and monitor ports.
Clever marketing again though, provided an upgrade path for those wanting to add peripherals via the “Plus 1” and “Plus 3” expansion units.Unfortunately for Acorn, delays in the production of single ULA design (the BBC had two) meant that the Electron missed the critical pre-Christmas sales period in 1984 which hurt the company severely in financial terms and they had to be rescued by Olivetti.
BBC Model B+

Olivetti felt that the business market was being missed and so in 1985, production was switched to the BBC Model B+ which came in two forms, the 64K machine and the 128K machine.

The B+ looked almost identical to the earlier BBC micro from the outside but contained a redesigned circuit board layout. The 6502 processor was replaced with the newer 65C12 and there was provision for both 8271 and 1770 disc interfaces to be fitted (although not at the same time).

The sideways ROM sockets were sensibly moved on the new main board to make them accessible without removing the keyboard. BASIC II was supplied as before along with a new edition, version 2.00, of the operating system.
A combined ROM provided both BASIC and OS in a single chip.
Never again would "BBC Computer" appear in the start-up message for an Acorn machine - instead the user was now greeted with the message "Acorn OS" after switch-on.

A further 32K of additional RAM was provided for the BBC Model B+. This took the form of 20K shadow screen RAM and 12K of extra workspace memory. Using the shadow RAM to hold the screen memory map meant that all the memory previously only available in Teletext mode 7 was available to the high resolution graphics modes.

In its 128K form, the Model B+ had a 64K sideways RAM daughter board fitted which allowed the contents of ROMs to be stored on disc and loaded into memory when required, just as if they were present as ROMs inside the computer. This would reduce the need for opening up the computer to change ROMs and having to store languages and applications on EPROM chips.
Sideways RAM became invaluable to ROM developers who could now try out their efforts without having to blow an EPROM every time a test was needed.

BBC Master
In January 1986 Acorn launched the BBC Master 128 - backwardly compatible with the earlier BBC models the machine had a slight processor change (65C02) to be able to address 128k RAM. 
It also sported a numeric keypad and cartridge slots.

The bulging ROM now contained a word processor and spreadsheet as well as BBC BASIC. The Master also had ADFS (advanced disk filing system) built in to use the new 3.5” floppy disks.

In September 1986 the BBC Master Compact was launched for the Christmas market with built-in disk drive and a sleek new styling with would be carried forward into the RISC based processor Archimedes range.

BBC Master Compact
In September 1986 the BBC Master Compact was launched for the Christmas market with built-in disk drive and a sleek new styling which would be carried forward into the RISC based processor Archimedes range.

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