Acorn 8-bit Computers

One of the early “home computers” was hardly known outside of the United Kingdom, but in its home country, Acorn owned the education market. Launched by Cambridge Processor Unit Ltd. (CPU) in January 1979, a little over a year after the Apple II, Commodore PET, and Radio Shack TRS-80 came to market, the Acorn System 75 computer kit debuted.

Acorn computer line

Some members of the Acorn and BBC Micro computer family.

Acorn System 1 personal computerCPU’s next project was the Acorn Micro-computer, later dubbed the Acorn System 1, and marketed primarily to the laboratory market. This was a two-board computer – one with a keypad, LEDs, and a cassette interface, the other with the CPU, memory, and other components. The system was designed to work with the Eurocard connector.

The System 2 put the CPU card from the System 1 in a Eurocard rack. Standard features of the System 2 included a keyboard controller, an external keyboard, a text display interface, and support for a cassette tape drive. System 3 added a floppy disk controller, and System 4 used a larger case with room for a second floppy drive. The System 5 moved from a 1 MHz 6502 CPU to a newer 2 MHz version.

The Acorn Atom

Acorn AtomThe Acorn Atom was the company’s first home computer, essentially a System 3 without a floppy drive. And like many computers of the era, it had the keyboard built into the chassis instead of external. The base 2 KB version was available as a kit or fully assembled, and power users could buy an 8 KB version. It was possible to expand RAM to 12 KB, and a ROM upgrade from 8 KB to 12 KB was also available.

The Atom used an MC6847 video chip to output text and graphics to a TV or video monitor, much as the Apple II line did, but at much lower resolution – 64 x 64 with 4 colors or 256 x 192 in monochrome mode.

Acorn had developed its own in-house networking technology known as Econet, which was available for the Atom, making it the first home computer that made it easy to add networking. Econet supports up to 254 devices per network segment and can bridge up to 127 segments to support over 32,000 machines!

The BBC Micro

BBC MicroThe British Broadcasting Corporation (BBC) planned to do a series, the BBC Computer Literacy Project, with an emphasis on education. The BBC published its specifications and put computer production out for bids. As you may have already guessed, Acorn won the bid – in fact, it was the only company to meet all of the BBC’s specifications, in fact, it exceeded them in nearly every area – and the BBC Micro went on sale in 1981.

Thanks to the BBC contract, Acorn’s computer ended up in about 80% of schools in the United Kingdom, and due to that, some families also made it their home computer. The BBC Microcomputer included the faster 2 MHz 6502 found in the Model 5 and also included Econet for networking.

Graphics took a big step forward with the BBC Micro, offering 640 x 256 resolution with 2 colors, 320 x 256 with up to 4, and 160 x 256 with up to 16.

Acorn’s biggest problem with the BBC Micro was the same one the Commodore 64 faced – keeping up with demand. Acorn also entered the US and West German education markets. Over time, Acorn sold over 1.5 million BBC Micros.

The Acorn Electron

Acorn ElectronThe Acorn Electron was designed as a more affordable version of the BBC Micro, but it also cut some corners. For instance, it used an odd 4-bit memory bus that reduced expenses at the cost of memory access being half as fast as on the BBC Micro, although it could access ROM at full speed. (Macintosh users suffered the same kind of indignity with the Macintosh LC: It’s CPU had a 32-bit memory bus but the memory was accessed 16 bits at a time.)

The Acorn Communicator

The Communicator was a business computer with a built-in modem. Introduced in 1985, the Communicator used a 16-bit Western Design Center 65816 CPU instead of the 6502 used in previous Acorn models. (The 65816 was also used in the Apple IIGS.) The Communicator sold in low numbers and primarily to travel agents who used Prestel and similar networks for online bookings.

Acorn’s RISC Processor

Except for its earliest models, Acorn had built its computers around the 6502 microprocessor, which was also used by Apple, Atari, Commodore, and others. Seeing the end of the 8-bit era approaching, Acorn knew that it was time to move to a new architecture.

Unable to find an existing solution, Acorn decided to heed the Berkeley RISC concept in a processor of its own design. The RISC concept is a Reduced Instruction Set Computer, where the CPU would be optimized for a limited set of instructions that it could perform very efficiently. Acorn also wanted a low-latency CPU, which is what they had in the 6502.

The Acorn RISC Machine (ARM) project officially began in October 1983, although advance work had already been done on the RISC concept. The first ARM chip (a.k.a. ARM1) was produced on April 26, 1985, and it worked perfectly the first time it was powered up. The 32-bit ARM1 was used as a co-processor in the BBC Micro to run simulations and CAD software so the RISC team could design support chips for the new CPU as well as the ARM2 CPU.

1983 and 1984

Especially in the US, 1983 had been a very trying year for the home computer industry. IBM had taken over the business world, and Commodore was dropping the price of the Commodore 64 from US$595 to an eventual US$99 in big steps. Commodore’s goal was to drive Texas Instruments out of the home computer market as revenge for TI destroying Commodore’s calculator business.

Commodore dropped the price of its entry-level VIC-20 so low that TI could only respond by selling its TI-99/4A at a loss – or give up on selling its inventory of home computers. Texas Instruments was the first casualty of the 1983 Home Computer Price War, and Atari became the second. In the end, Commodore hurt itself as much as anyone, and despite selling 10 million Commodore 64s, the company soon went bankrupt.

In an interesting and unexpected twist, Commodore got rid of its founder, Jack Tramiel, who went on to buy Atari from Warner Brothers in 1984. That was also the year that Apple introduced the Macintosh and nearly went bankrupt due to poor sales. Atari would go on to become a player in the next phase of the home computer market, primarily marketing its Atari ST series against the Macintosh and Commodore’s Amiga.

Acorn was also going through dire financial difficulties and became a subsidiary of Olivetti in February 1985.

The BBC Master

Acorn replaced the BBC Micro with the BBC Master in early 1986. Available with 128 KB of system memory, the Master used the MOS Technology 65SC12 CPU, an offshoot of the 6502 used in so many home computers with two new addressing modes.

The Master 512 included a 10 MHz Intel 80816 CPU co-processor with 512 MB of memory; it ran DOS Plus and Digital Research GEM (also used on the Atari ST series) via the 80186.

The BBC Master remained in production into 1994. It helped keep Acorn going while it worked on its RISC computer.

If you used an Acorn 8-bit computer or BBC Micro, consider joining the Acorn Computer and BBC Micro Enthusiasts group on Facebook.

Photos (except for Acorn System 1) by Simon Inns, Creative Commons license, used with his kind permission.


keywords: #acorncomputers #acornatom #bbcmicro #acornelectron #bbcmaster

short link: