Rahul Sharma (Editor)

BBC Micro

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Acorn Computers

Retail availability
12 years

8-bit home computer

BBC Micro

Release date
1 December 1981; 35 years ago (1981-12-01)

Introductory price
£235 Model A, £335 Model B (in 1981)

1994; 23 years ago (1994)

The BBC Microcomputer System, or BBC Micro, is a series of microcomputers and associated peripherals designed and built by the Acorn Computer company for the BBC Computer Literacy Project, operated by the British Broadcasting Corporation. Designed with an emphasis on education, it was notable for its ruggedness, expandability, and the quality of its operating system. An accompanying 1982 television series "The Computer Programme" featuring Chris Serle learning to use the machine was also broadcast on BBC 2.


After the Literacy Project's call for bids for a computer to accompany the TV programmes and literature, Acorn won the contract with the Proton, a successor of its Atom computer prototyped at short notice. Renamed the BBC Micro, the system was adopted by most schools in the United Kingdom, changing Acorn's fortunes. It was also moderately successful as a home computer in the UK despite its high cost. Acorn also employed the machine to simulate and develop the ARM architecture which, many years later, has become hugely successful for embedded systems, including tablets and cellphones. In 2013 ARM was the most widely used 32-bit instruction set architecture.

While nine models were eventually produced with the BBC brand, the phrase "BBC Micro" is usually used colloquially to refer to the first six (Model A, B, B+64, B+128, Master 128, and Master Compact), excluding the Acorn Electron; subsequent BBC models are considered as part of Acorn's Archimedes series.


During the early 1980s, the BBC started what became known as the BBC Computer Literacy Project. The project was initiated partly in response to an ITV documentary series The Mighty Micro, in which Dr Christopher Evans of the UK's National Physical Laboratory predicted the coming microcomputer revolution and its effect on the economy, industry, and lifestyle of the United Kingdom.

The BBC wanted to base its project on a microcomputer capable of performing various tasks which they could then demonstrate in the TV series The Computer Programme. The list of topics included programming, graphics, sound and music, teletext, controlling external hardware, and artificial intelligence. It developed an ambitious specification for a BBC computer, and discussed the project with several companies including Acorn Computers, Sinclair Research, Newbury Laboratories, Tangerine Computer Systems, and Dragon Data.

The Acorn team had already been working on a successor to their existing Atom microcomputer. Known as the Proton, it included better graphics and a faster 2 MHz MOS Technology 6502 central processing unit. The machine was only at the design stage at the time, and the Acorn team, including Steve Furber and Sophie Wilson, had one week to build a working prototype from the sketched designs. The team worked through the night to get a working Proton together to show the BBC. Not only was the Acorn Proton the only machine to match the BBC's specification, it also exceeded it in nearly every parameter. Based on the Proton prototype the BBC signed a contract with Acorn as early as February 1981; by June the BBC Micro's specifications and pricing were decided.

Market impact

The machine was released as the BBC Microcomputer on 1 December 1981, although production problems pushed delivery of the majority of the initial run into 1982. Nicknamed "the Beeb", it was popular in the UK, especially in the educational market; about 80% of British schools had a BBC microcomputer, as the government from 1981 to 1984 subsidized half the cost of a Research Machines 380Z or BBC Micro.

BYTE called the BBC Micro Model B "a no-compromise computer that has many uses beyond self-instruction in computer technology". It called the Tube interface "the most innovative feature" of the computer, and concluded that "although some other British microcomputers offer more features for a given price, none of them surpass the BBC ... in terms of versatility and expansion capability". As with Sinclair's ZX Spectrum and Commodore's Commodore 64, both released later in 1982, demand greatly exceeded supply. For some months, there were long delays before customers received the machines they had ordered.

Efforts were made to market the machine in the United States and West Germany. By October 1983, the US operation reported that American schools had placed orders with it totalling $21 million. In October 1984, while preparing a major expansion of its US dealer network, Acorn claimed sales of 85 per cent of the computers in British schools, and delivery of 40,000 machines per month. That December, Acorn stated its intention to become the market leader in US educational computing. The New York Times considered the inclusion of local area networking to be of prime importance to teachers. The operation resulted in advertisements by at least one dealer in Interface Age magazine, but ultimately the attempt failed. The success of the machine in the UK was due largely to its acceptance as an "educational" computer – UK schools used BBC Micros to teach computer literacy, information technology skills and a generation of games programmers. Acorn became more known for its model B computer than for its other products. Some Commonwealth countries, including India, started their own Computer Literacy programs around 1987 and used the BBC Micro, a clone of which was produced by Semiconductor Complex Limited and named the SCL Unicorn.

The Model A and the Model B were priced initially at £235 and £335 respectively, but increasing almost immediately to £299 and £399 due to increased costs. The Model B price of nearly £400 was roughly £1200 (€1393) in 2011 prices. Acorn anticipated the total sales to be around 12,000 units, but eventually more than 1.5 million BBC Micros were sold.

The cost of the BBC Models was high compared to competitors such as the ZX Spectrum and the Commodore 64, and from 1983 on Acorn attempted to counter this by producing a simplified but largely compatible version intended for game playing, the 32K Acorn Electron.

Hardware features: Models A and B

The Model A had 16 KB of user RAM, while the Model B had 32 KB. A feature that the Micro shared with other 6502 computers such as the Apple and the early Commodore models was that the RAM was clocked twice as fast as the CPU, with alternating access given to the CPU and the video display circuits. This gave the BBC Micro a fully unified memory address structure without speed penalties. To use the CPU at full speed (2 MHz) required the memory system to be capable of performing four million access cycles per second. Hitachi was the only company, at the time, that made a DRAM that went that fast. So for the prototype the only four 4816s in the country got hand-carried by the rep. Most competing microcomputers with memory-mapped display incurred CPU speed penalties depending on the actions of the video circuits (e.g. the Amstrad CPC and to a lesser extent the ZX Spectrum) or kept video memory completely separate from the CPU address pool (e.g. the MSX).

The machine included a number of extra input/output interfaces: serial and parallel printer ports; an 8-bit general purpose digital I/O port; a port offering four analogue inputs, a light pen input, and switch inputs; and an expansion connector (the "1 MHz bus") that enabled other hardware to be connected. Extra ROMs could be fitted (four on the PCB or sixteen with expansion hardware) and accessed via paged memory. An Econet network interface and a disk drive interface were available as options. All motherboards had space for the electronic components, but Econet was rarely fitted. Additionally, an Acorn proprietary interface named the "Tube" allowed a second processor to be added. Three models of second processor were offered by Acorn, based on the 6502, Z80 and 32016 CPUs. The Tube was later used in third-party add-ons, including a Zilog Z80 board and hard disk drive from Torch that allowed the BBC machine to run CP/M programs.

Separate pages, each with a codename, were used to control the access to the I/O:

The Tube interface allowed Acorn to use BBC Micros with ARM CPUs as software development machines when creating the Acorn Archimedes. This resulted in the ARM development kit for the BBC Micro in 1986, priced at around £4000. From 2006 a kit with an ARM7TDMI CPU running at 64 MHz, with as much as 64 MB of RAM, was released for the BBC Micro and Master, using the Tube interface to upgrade the old 8-bit micros into 32-bit RISC machines. Among the software that operated on the Tube were an enhanced version of the Elite video game (see below) and a computer-aided design system that required a second 6502 CPU and a 5-dimensional joystick named a "Bitstik"[1].

The Model A and the Model B were built on the same printed circuit board (PCB) and a Model A could be upgraded to a Model B without too much difficulty. Users wishing to operate Model B software needed only to add the extra RAM and the user/printer 6522 VIA (which many games used for timers) and snip a link, a task that could be achieved without soldering. To do a full upgrade with all the external ports did, however, require soldering the connectors to the motherboard. The original machines shipped with "OS 0.1", with later updates advertised in magazines, supplied as a clip-in integrated circuit, with the last official version being "OS 1.2". Variations in the Acorn OS exist as a result of home-made projects and modified machines can still be bought on internet auction sites such as eBay, as of 2011.

The BBC Model A was phased out of production with the introduction of the Electron, with chairman Chris Curry stating at the time that Acorn "would no longer promote it" (the Model A).

Early BBC Micros used linear power supplies at the insistence of the BBC's engineering specification, but these very hot-running PSUs were soon replaced in production by switched mode units.

An apparent oversight in the manufacturing process resulted in a significant number of Model Bs producing a constant buzzing noise from the built-in speaker. This fault could be rectified partly by soldering a resistor across two pads.

There were five developments of the main BBC micro circuit board that addressed various issues through the models production, from 'Issue 1' through to 'Issue 7' with variants 5 and 6 not being released. The 1985 'BBC Microcomputer Service Manual' from Acorn documented the details of the technical changes.

Per Watford Electronics comments in their '32K Ram Board Manual':

Early issue BBCs (Issue 3 circuit boards and before) are notorious for out of specification timings. If problems occur with this sort of machine, the problem can generally be cured by the use of either a Rockwell 6502A CPU chip, or by replacing IC14 (a 74LS245) with either another 74LS245 or the faster 74ALS245.

Export models

Two export models were developed: one for the US, with Econet and speech hardware as standard; the other for West Germany. Both were fitted with radio frequency shielding as required by the respective countries, and they were still based on the Intel 8271 floppy drive controller. From June 1983 the name was always spelled out completely – "British Broadcasting Corporation Microcomputer System" – to avoid confusion with Brown, Boveri & Cie in international markets.

US models included the BASIC III ROM chip, modified to accept the American spelling of COLOR, but the height of the graphics display was reduced to 200 scan lines to suit NTSC TVs, severely affecting applications written for British computers. After the failed US marketing campaign the unwanted machines were remanufactured for the British market and sold, resulting in a third 'UK export' variant.

Side product

In October 1984, the Acorn Business Computer (ABC)/Acorn Cambridge Workstation range of machines was announced, based primarily on BBC hardware.

B+64 and B+128

Acorn introduced the Model B+ in mid-1985, increasing the total RAM to 64 KB but this had modest market effect. The extra RAM in the Model B+ BBC Micro was assigned as two blocks, a block of 20 KB dedicated solely for screen display (so-called "Shadow" RAM) and a block of 12 KB of 'special' Sideways RAM. The B+128 came with an additional 64 KB (4 × 16 KB "Sideways" RAM banks) to give a total RAM of 128 KB.

The new B+ was incapable of operating some original BBC B programs and games, such as, for example, the very popular Castle Quest. A particular problem was the replacement of the Intel 8271 floppy disk controller with the Western Digital 1770 – not only was the new controller mapped to different addresses, it was fundamentally incompatible and the 8271 emulators that existed were necessarily imperfect for all but basic operation. A piece of software that used copy protection techniques involving direct access to the controller, simply would not operate on the new system. Acorn attempted to alleviate this, starting with version 2.20 of the 1770 DFS, via an 8271-backward- compatible Ctrl+Z+Break option.

There was also a long-running problem late in the B/B+'s commercial life infamous amongst B+ owners, when Superior Software released Repton Infinity, which refused to operate on the B+. A series of unsuccessful replacements were issued before one compatible with both was finally released.

BBC Master

During 1986, Acorn followed up with the BBC Master, which offered memory sizes from 128 KB and many other refinements which improved on the 1981 original. It had essentially the same 6502-based BBC architecture, with many of the upgrades that the original design had intentionally made possible (extra ROM software, extra paged RAM, second processors) now included on the circuit board as internal plug-in modules.

Software and expandability

The BBC Micro platform amassed a large software base of both games and educational programs for its two main uses as a home and educational computer. Notable examples of each include the original release of Elite and Granny's Garden. Programming languages and some applications were supplied on ROM chips to be installed on the motherboard. These loaded instantly and left the RAM free for programs or documents.

Although appropriate content was little-supported by television broadcasters, telesoftware could be downloaded via the optional Teletext Adapter and the third-party teletext adaptors that emerged.

The built-in operating system, Acorn MOS, provided an extensive API to interface with all standard peripherals, ROM-based software and the screen. Features private to some versions of BASIC, like vector graphics, keyboard macros, cursor-based editing, sound queues and envelopes, were placed in the MOS ROM and made available to any application. BBC BASIC itself, being in a separate ROM, could be replaced with any equivalent language.

BASIC, other languages and utility ROM chips resided in any of four 16 KB paged ROM sockets, with OS support for sixteen sockets via expansion hardware. The five (total) sockets were located partially obscured under the keyboard, with the leftmost socket hard-wired for the OS. While the original usage for the perforated panel on the left of the keyboard was for a Serial ROM or Speech ROM, a ZIF socket or edgecard connector could be installed in that location instead. The socket could be connected to one of the empty Sideways/PagedROM sockets via a header cable. The paged ROM system was essentially modular. A language-independent system of star commands, prefixed with an asterisk, provided the ability to select a language (for example *BASIC, *PASCAL), a filing system (*TAPE, *DISC), change settings (*FX, *OPT) or carry out ROM-supplied tasks (*COPY, *BACKUP) from the command line. The MOS recognised a handful of built-in commands, and polled the paged ROMs in descending order for service otherwise; if none of them claimed the command then the OS returned a Bad command error. Connecting an external EPROM programmer, one could write extensive programs, copy to programmable ROM (PROM) or EPROM, then invoke them without taxing user memory.

Not all ROMs offered star commands (ROMs containing data files, for instance), but any ROM could "hook" into certain vectors to enhance the system's functionality. Often the ROM was a device driver for mass storage combined with a filing system, starting with Acorn's 1982 Disc Filing System whose API became the de facto standard for floppy disc access. The Acorn Graphics Extension ROM (GXR) expanded the VDU routines to draw geometric shapes, flood fills and sprites. During 1985 Micro Power designed and marketed a Basic Extension ROM, introducing statements such as WHILE, ENDWHILE, CASE, WHEN, OTHERWISE, and ENDCASE, as well as direct mode commands including VERIFY.

Acorn strongly discouraged programmers from directly accessing the system variables and hardware, favouring official system calls. This was ostensibly to make sure programs kept working when migrated to coprocessors that utilised the Tube interface, but it also made BBC Micro software more portable across the Acorn range. Whereas untrappable PEEKs and POKEs were commonly used by other computers to reach the system elements, programs in either machine code or BBC BASIC would instead pass parameters to an operating system routine. In this way the MOS could translate the request for the local machine or send it across the Tube interface, as direct access was impossible from the coprocessor. Published programs largely conformed to the API except for games, which routinely engaged with the hardware for greater speed, and thus required a particular Acorn model.

As the early BBC Micros had ample I/O allowing machines to be interconnected, and as many schools and universities employed the machines in Econet networks, numerous networked multiplayer games were created. With the exception of a tank game, Bolo, few became popular, in no small measure due to the limited number of machines aggregated in one place. A relatively late but well documented example can be found in a dissertation based on a ringed RS-423 interconnect.


In line with its ethos of expandability Acorn produced its own range of peripherals for the BBC Micro, including:

  • Joysticks
  • Tape recorder
  • Floppy drive interface upgrade
  • Floppy drives (single and double)
  • Econet networking upgrade
  • Econet Bridge
  • Winchester disk system
  • 6502 Second Processor
  • Z80 Second processor (with CP/M and business software suite)
  • 32016 Second processor
  • ARM Evaluation System
  • Teletext adapter
  • Prestel adapter
  • Speech synthesiser
  • Music 500 synthesiser
  • BBC Turtle (robot)
  • BBC Buggy
  • IEEE 488 Interface
  • Other manufactures also produced an abundance of add-on hardware, some the most common being:

  • RGB monitors
  • Printers, plotters
  • Modems
  • BBC BASIC built-in programming language

    The built-in ROM-resident BBC BASIC programming language interpreter realised the system's educational emphasis and was key to its success; not only was it the most comprehensive BASIC compared to other contemporary implementations but it ran very efficiently and was therefore fast. Advanced programs could be written without resorting to non-structured programming or machine code (necessary with many competing computers). Should one want or need to do some assembly programming, BBC BASIC featured a built-in assembler that allowed a very easy mixture of BBC BASIC and assembler for whatever processor BBC BASIC was operating on.

    When the BBC Micro was released, many competing home computers used Microsoft BASIC, or variants typically designed to resemble it. Compared to Microsoft BASIC, BBC BASIC featured IF…THEN…ELSE, REPEAT…UNTIL, named procedures and functions, but retained Goto and GOSUB for compatibility. It also supported high-resolution graphics, four-channel sound, pointer-based memory access (borrowed from BCPL) and rudimentary macro assembly. Long variable names were accepted and distinguished completely, not just by the first two characters.

    Other languages

    Acorn had made a point of not just supporting BBC Basic but a number of contemporary languages, some of which were supplied as ROM chips to fit the spare 'Sideways-ROM' sockets on the motherboard. Other languages were supplied on tape or disk based.

    Programming Languages from Acorn:

  • ISO Pascal (2× 16 KB ROM + floppy disk)
  • S-Pascal (disk or tape)
  • BCPL (ROM plus further optional disk based modules)
  • Forth (16 KB ROM)
  • LISP (disk,tape or ROM)
  • Logo (2× 16 KB ROM)
  • Turtle Graphics (disk or tape)
  • Micro-PROLOG (16 KB ROM)
  • COMAL (16 KB ROM)
  • Microfocus CIS COBOL (running under CP/M on floppy disks via the Z80 second processor)
  • Successor machines

    Acorn produced their own 32-bit Reduced Instruction Set (RISC) CPU during 1985, the ARM1. Furber composed a reference model of the processor on the BBC Micro with 808 lines of BASIC, and ARM Holdings retains copies of the code for intellectual property purposes. The first prototype ARM platforms, the ARM Evaluation System and the A500 workstation, functioned as second processors attached to the BBC Micro's Tube interface. Acorn staff developed the A500's operating system in situ through the Tube until, one by one, the on-board I/O ports were enabled and the A500 ran as a stand-alone computer. With an upgraded processor this was eventually released during 1987 as four models in the Archimedes series, the lower-specified two models (512 KB and 1 MB) continuing the BBC Microcomputer brand with the distinctive red function keys. Although the Archimedes ultimately was not a major success, the ARM family of processors has become the dominant processor architecture in mobile embedded consumer devices, particularly mobile telephones.

    Acorn's last BBC-related model, the BBC A3000, was released in 1989. It was essentially a 1 MB Archimedes back in a single case form factor.

    Retro computing scene

    As of 2005, thanks to its ready expandability and I/O functions, there are still numbers of BBC Micros in use, and a retrocomputing community of dedicated users finding new tasks for the old hardware. They still survive in a few interactive displays in museums across the United Kingdom, and the Jodrell Bank observatory was reported to be still using a BBC Micro to steer its 42 ft radio telescope during 2004. The Archimedes came with 65Arthur, an emulator which BYTE stated "lets many programs for the BBC Micro run"; other emulators exist for many operating systems.

    In March 2008, the creators of the BBC Micro met at the Science Museum in London. There was to be an exhibition about the computer and its legacy during 2009.

    The UK National Museum of Computing at Bletchley Park uses BBC Micros as part of a scheme to educate school children about computer programming.

    In March 2012, the BBC and Acorn teams responsible for the BBC Micro and Computer Literacy Project met for a 30th anniversary party, entitled "Beeb@30". This was held at ARM's offices in Cambridge and was co-hosted by the Centre for Computing History.

    Continued development and support

    Long after the "venerable old Beeb" was superseded, additional hardware and software has been developed. Such developments have included Sprow's 1999 zip compression utility and a ROM Y2K bugfix for the BBC Master.

    There are also a number of websites still supporting both hardware and software development for the BBC micros and Acorn in general.

    Display modes

    Like the IBM PC with the contemporary Color Graphics Adapter, the video output of the BBC Micro could be switched by software between a number of display modes. These varied between 20 and 40-column text suitable for a domestic TV, to 80-column text best viewed with a high-quality RGB-connected monitor (The 80-column mode was often too blurred to view when using a domestic TV via the UHF output). The variety of modes offered applications a flexible compromise between colour depth, resolution and memory economy. In the first models, the OS and applications were left with the RAM left over from the display mode.

    Mode 7 was a Teletext mode, extremely economical on memory and an original requirement due to the BBC's own use of broadcast teletext (Ceefax). It also made the computer useful as a Prestel terminal. The teletext characters were generated on board, for use with monitors and TV sets without a Teletext receiver. Train time displays at UK stations were driven by BBC Master computers in this mode until around the late 1990s.

    Modes 0 to 6 (the 'ASCII' modes) could display colours from a logical palette of sixteen: the eight basic colours at the vertices of the RGB colour cube and eight flashing colours made by alternating the basic colour with its inverse. The palette could be freely reprogrammed without touching display memory. Modes 3 and 6 were special text-only modes that used less RAM by reducing the number of text rows and inserting blank scan lines below each row. Mode 6 was approximately the same size as Teletext. Modes 0 to 6 could show diacritics and other user defined characters. All modes besides the two text modes supported bitmapped graphics.

    The BBC B+ and the later Master provided 'shadow modes', where the 1–20 KB frame buffer was stored in an alternative RAM bank, freeing the main memory for user programs. This feature was requested by setting bit 7 of the mode variable, i.e. by requesting modes 128–135.

    Optional extras

    A speech synthesis upgrade based on the Texas Instruments TMS5220 featured sampled phonemes spoken by BBC newscaster Kenneth Kendall. The speech system was standard on the US model where it had an American vocabulary. Elsewhere it sold poorly and was eventually largely replaced by Superior Software's software-based synthesiser using the standard sound hardware.

    The speech upgrade also added two empty sockets next to the keyboard intended to take 16 KB serial ROM cartridges containing either extra speech phoneme data (in addition to the default speech ROM fitted to the motherboard), or general software accessed through the ROM Filing System. The original plan was that some games would be released on cartridges, but due to the limited sales of the speech upgrade, little or no software was ever produced for these sockets. The cut-out space next to the keyboard (nicknamed the "ashtray") was more commonly used to install other upgrades, such as a ZIF socket for conventional paged ROMs.

    Use in the entertainment industry

    The BBC Domesday Project, a pioneering multimedia experiment, was based on a modified version of the BBC Micro's successor, the BBC Master.

    Musician Vince Clarke of the British synth pop bands Depeche Mode, Yazoo, and Erasure used a BBC Micro (and later a BBC Master) with the UMI music sequencer to compose many hits. In music videos from the 1980s featuring Vince Clarke, a BBC Micro is often present or provides text and graphics such as a clip for Erasure's "Oh L'Amour". The musical group Queen used the UMI Music Sequencer on their record A Kind of Magic. The UMI is also mentioned in the CD booklet. Other bands who have used the Beeb for making music are A-ha and the reggae band Steel Pulse. Paul Ridout is credited as "UMI programmer" on Cars' bassist/vocalist Benjamin Orr's 1986 solo album, The Lace. Black Uhuru used the Envelope Generator from SYSTEM software (Sheffield) running on a BBC Micro, to create some of the electro-dub sounds on Try It (Anthem album 1983).

    The BBC Micro was used extensively to provide graphics and sound effects for many early 1980s BBC TV shows. These included, notably, The Adventure Game (the first two series, produced before the BBC Micro was released, used either an HP 9845 or an Apple ][ with a plastic box covering its RESET key to prevent accidental pressing by contestants); the children's quiz game "First Class" (where the onscreen scoreboard was provided by a BBC Micro nicknamed "Eugene"); and numerous 1980s episodes of Doctor Who including "Castrovalva", "The Five Doctors", and "The Twin Dilemma".


    BBC Micro Wikipedia

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