The BBC Micro

The BBC Micro, named for the UK's public service broadcaster, was an influential and feature rich 8-bit microcomputer produced by Acorn. Released in late 1981 it was commissioned by the BBC to accompany a series of television programmes intended to introduce the public to computing. The machine saw widespread adoption in education with a presence in almost every UK school at some stage throughout the 1980s. At least a million were sold and possibly up to 1.5 million.

History

Amongst others, the BBC had become concerned that the UK was falling behind with the rapid rise of microcomputer technology through the 1970s. This was highlighted in a BBC Horizon documentary called Now the Chips are Down, first broadcast in March 1978. The film had some influence on the then James Callaghan led Labour government to enquire with the BBC if something could be done to better inform the public about the emergence of computer technology. By late 1979 it had been agreed that the BBC would produce a series of programmes about computer technology. BBC Producer David Allen created a general computer introductory series called The Silicon Factor, which were shown in early 1980. This would be followed up with a more practical series of programmes including demonstrations of real computers to begin broadcasting from 1981. The endeavour would become known as the BBC's Computer Literacy project.

While it has now declined in use, the BASIC programming language was at the time the logical choice for education, serving a similar market niche as Python does today as an accessible way to program a computer. However, a multitude of slightly incompatible versions existed with varying features. Advisor to the BBC, John Coll, highlighted that a standardised dialect would be essential such that inexperienced viewers could type in programs from the TV show and expect them to work precisely as shown. This led the BBC to look for a specific computer to accompany the project that would be recommended.

The UK government actually partially owned an electronics company called Sinclair Radionics. This was not however the same Sinclair company that produced the ZX81 and Spectrum but an entirely separate concern that had been nationalised when Clive Sinclair entered financial difficulties. By 1979 Sinclair Radionics had been working on a machine called the NewBrain and this seemed like the most appropriate candidate for the public sector BBC to choose as the machine to base the TV series around. In the midst of a changing political climate, the computer was subsequently transferred to Newbury Laboratories, but unfortunately it became obvious that it was never going to be ready in time.

By December 1980, with the TV production due to commence the following year, John Coll and David Allen gave up on the NewBrain and scrambled to draw up a specification for a suitable replacement, approaching a number of alternative UK computer companies to enhance the possibility of something being available in time. In addition to Sinclair, Acorn, Nascom, Research Machines, Tangerine and Transam were all approached.

When the BBC called on Acorn in January 1981 to enquire if they had an appropriate machine, they did not. However, employee Steve Furber had a design in his desk drawer that was coincidentally close to the BBC's requirements. Recollections differ as to how complete the design was, but agree on the fact that it hadn't got much beyond the paper stage and there was certainly no actual hardware that could be shown. The BBC were on a very tight schedule and desperate for concrete progress. Acorn had a week to get something working or be out of the running. Astonishingly, they managed to achieve the near impossible deadline and successfully demonstrated a working computer. Legend has it that the timing was so tight the Acorn team were still building the prototype as the BBC delegation arrived at their offices. Of all they had seen, the BBC were most impressed with the Acorn machine and they got the deal.

The BBC Micro would make its first television appearance on 11th January 1982 in The Computer Programme. In the first of the series presenter Chris Serle begins by demonstrating it running a breakout clone and in a somewhat advanced application for 1982 purchases an airline ticket online via a modem. Subsequently, BASIC programming is introduced and Chris is taught to write small programs. As the series goes on progressively more complex programming concepts are introduced with Chris shown how to write the relevant code.

A number of applications for computers are discussed with examples running on the BBC Micro. In one episode Chris uses his skills to control a robot arm via BASIC. The benefits and drawbacks of different storage mediums are described and the BBC Micro is shown controlling a laser disc player that it is suggested could contain an encyclopedia (similar to CD-ROM based systems that would appear in later years); from there moving on to the concept of information retrieval. Electronic messaging is demonstrated (similar to email) and the BBC Micro is shown performing speech recognition with the presenters presciently speculating about home automation uses.

Specification

The BBC Micro was originally sold in two flavours, with more models to follow in later years. There was a cost-cut Model A with 16Kb of RAM, that also lacked a number of expansion ports, and the Model B with 32KB. The Model A was originally priced starting at at £225 with the Model B at £335, but this rapidly increased to £299 and £399 respectively by only February 1982 (equivalent to £1050 - £1400 in 2025), with Acorn citing the original pricing as being too optimistic. These were also base prices with a number of interface chips not populated (see below). Acorn somewhat naturally expected the cheaper Model A to be most popular however they were wildly wrong. In practice the Model B massively outsold the Model A by a ratio of 20:1 or more. Both machines also used an identical PCB and the Model A could be easily upgraded to the B. Anecdotally, this was so frequently done it is difficult for the retro computing enthusiast community to find an original unmodified Model A in the current era.

The computer was based around the popular MOS 6502A CPU running at 2Mhz. This was a widely used processor in many competing home micros but the BBC Micro had a trick that made it faster than virtually all the contemporary competition using the same CPU. To begin with, many 6502 based home computers, such as the Commodore 64, used a slower 1Mhz version. Of those that did run at 2Mhz, most would use 2Mhz memory and would compromise the CPU performance by yielding some proportion of the bus time to the video display to allow it to access the RAM. The BBC though used newly available faster 4816 memory chips from Hitachi that could operate at 4Mhz. This allowed plenty of time to serve both the CPU and the video display simultaneously allowing the CPU to run continuously at the maximum speed, delivering one of the fastest 8-bit machines of the day.

Video display was offered in several software switchable modes, each of which applied a different compromise on resolution and available colours in order to maximise the availability of the limited RAM, depending on what the running program required. The highest resolution video mode was 640x256 pixels in monochrome (mode 0) which used a sizeable 20KB of the 32KB of memory. This was a particularly high resolution for the time. Mode 0 was unavailable on the cheaper Model A as it didn't have enough RAM. The bitmap graphics mode with the most colours was mode 2, which offered 8 colours at 160x256 pixels and again used 20KB of RAM. The machine did however also offer text only modes which used less RAM. There was mode 7 which required only 1KB and offered 40x25 characters in 8 colours. The machine also had an 80 column mode (80 characters wide) which was at the time perceived as important for running business software.

Audio was delivered via a Texas Instruments SN76489, which was actually the same sound chip that would be used in the Sega Master System games console some years later together with a number of other home computers and a wide variety of arcade machines. This was a programmable four channel sound generator that delivered relatively advanced audio for 1981. It allowed for three simultaneous square wave tones at 16 independently controllable volume levels and one noise channel. The latter could be used for percussion sound effects and game explosion sounds and such.

The BBC Micro was not designed with any method of playing sampled audio, but as with many other machines of the era, clever programmers found a way. On the BBC this was achieved by playing the highest pitch tone the SN76489 could produce and then rapidly altering the volume in software to produce the desired waveform, which could be done at about 10Khz.

Expansion

The Model B was particularly noted for the generous provision of expansion interfaces and options. Competing consumer machines of the era might often have had a general purpose edge connector offering access to the system buses, a television output, cassette and perhaps a joystick input but not much else. The BBC Micro went far beyond this.

At this time home computers would routinely offer UHF or/and composite video output for connection to an analogue domestic television but the BBC Micro additionally offered an RGB monitor output as standard. This was a side-effect of a BBC requirement to be able to output a broadcast quality video signal for use in television production when the machine was being demonstrated to the public. However, the fact that the interface was standard meant that the machine was commonly seen paired with a colour monitor, which produced a sharp high resolution image that could not be achieved on a television.

A parallel printer port was available on the underside, compatible with the then popular "centronics" standard that enabled the majority of dot-matrix printers to be directly attached. Competing home computers would often require purchase of an additional interface. The parallel port was similar to that found on earlier PCs of the 80s and 90s.

An RS423 serial interface was provided as standard which enabled connection to a modem and some printers would also be attached via serial. The port could usually be made to be compatible with the more commonplace RS232 serial devices using an appropriate cable. An advantage of RS423 was that it could be driven over much longer distances than RS232 with better noise immunity. This enabled a peripheral to be potentially located some distance from the machine. It was also cheaper to implement than RS232 within the design of the BBC Micro.

A floppy disk drive interface together with a power supply for the drive was a standard feature on the main board, which many competing computers did not consider at all. However, the machine was widely sold without the relevant chips installed at a reduced price. Home users may have elected to take this option as a disk drive was an expensive device at the time, typically costing over £200 in 1981 (about £760 in 2025), and the associated controller chips were also not cheap. Even by 1983, the typical retail price for the disk controller chips alone was about £95 (about £318 in 2025). Some entire home computers could be found for less than even the cost of the disk chips. With a disk drive being outside the budget of many, the corresponding expensive interface may have gone completely unused. The chips could be added back in at a later stage though if subsequently required and kits were available from dealers.

For those unable to afford a disk system, a cassette interface was provided, which was then commonly used as a low-cost storage alternative. This worked at 1200 baud which was at the faster end of tape speeds at the time. Unusually, the BBC Micro had a cassette motor controller and could remotely control the tape player automatically. On most home machines the user had to start and stop the tape by pressing mechanical buttons under the instruction of the computer.

There was an analogue to digital input that could measure four analogue voltages over a range of 0 - 1.8v with 12-bit resolution, yielding 4096 discrete values per channel, and could sample at a rate of about 100Hz. This was was not a widely seen feature on other machines. The original intent behind this port was educational. A student could attach potentiometers, temperature sensors, light sensors and other electronic measurement devices and experiment with the effect, however it was often used for analogue joysticks. The port was easily accessible from BASIC with a specific keyword provided - ADVAL which was used for reading the current value from any of the four ADC channels.

A light-pen input was available on the analogue connector. The BBC micro was designed prior to the widespread adoption of the mouse as the de facto graphical pointing device and light pens were a common accessory at the time. The user would touch a wired pen-shaped device to the monitor wherever they wanted to click. This would look for a flash from the CRT electron beam passing by and software would calculate the position of the pen on the screen based on where the beam was when it saw the flash. Mice subsequently appeared when this form of input was popularised.

The BBC micro had a connector called the "1Mhz extension bus", allowing wide (although not complete) access to the internal system buses. The concept of this connector is somewhat like that of a PCIe expansion slot on a modern PC, allowing relatively low level access to the inner workings of the machine and was particular used for devices that required high speed data transfer. This operated at 1Mhz rather than the 2Mhz frequency of the CPU because many devices of the time (including some of the chips within the BBC micro itself) were unable to operate at 2Mhz. Additionally the higher frequency may not have been stable on an inexpensive unshielded ribbon cable that external devices would often use. There was also an analogue audio input on the same connector which could be mixed with the internally produced sound.

The User Port

The "user port" was a particularly thoughful and useful addition for an educational machine as it enabled hobbyist electronics to be easily connected. Connecting anything to a computer system bus, such as to the expansion edge connector of a typical home computer, required a high degree of electronics knowledge and computer skill with understanding of how to implement address decoding, latches and knowledge of system timings. Many components would also be required that a typical school probably wouldn't have.

The user port significantly reduced the barrier to entry for the education and home user to experiment with computer controlled electronics projects, requiring minimal knowledge and few parts. The consistency of having a standardised port always accessed in the same way on every machine also facilitated knowledge sharing in magazines. This placed the possibility of building computer controlled devices within the capability of most teachers and students. For example, the student could simply connect up some LEDs directly to the user port and make them flash with very little electronics skill needed and this could be achieved within the scope of one lesson without having to teach a whole array of complex computer concepts. A control program could be written in BASIC and sending a value to the user port was just one line of code.

The user port provided 8 data lines, each of which could be invididually configured to work bi-directionally as either an input or an output. It was implemented using a MOS 6522 chip which also provisioned a number of other more sophisticated facilities (such as timers) and these were made available on the port.

The simplicity of the user port was widely exploited by commercial electronics with a large array of add-on devices being produced for it as it. Lego produced a controller for the port, known as "Interface A", that was compatible with Lego Technic motors. Students could build robots out of Lego and control them via the provided software (called Lego Lines) or else write their own code in BASIC. As well as motors there were also optical sensors for the toy that could be used to measure how far robots had travelled, bump sensors and lamps. Other computers also received the same Lego controller but often an additional add-on card was needed to use it. On the BBC the required electronics was already built into the user port.

The concept of the user port as an accessible connector for hobbyist electronics was taken forward into the modern era by the popular Raspberry Pi computer, with the idea for the widely used GPIO header being inspired by the BBC Micro. The GPIO layout has now become somewhat of a quasi standard for small single board computers as a great deal of electronics has been built that is compatible with it.

The Tube

An advanced feature of the BBC Micro was that a variety of additional CPUs could be connected and the intended purpose of "The Tube" interface was to facilitate communication with those processors. Additional CPUs could have a completely dissimilar architecture to the 6502 and run entirely their own operating system independently of the BBC Micro. A Zilog Z80, an Intel 80186 and a National Semiconductor 32016 add-on were released by Acorn themselves in addition to an upgraded 3Mhz 6502 that was faster than the BBCs internal processor and had 64K of RAM. Several third party manufacturers also released products for the Tube, particularly lower cost competing Z80s, but also including a Motorola 68000.

A common use for a Z80 was running the CP/M operating system from Digital Research, which in 1981 was a near necessity for business with a huge catalogue of software available. Many competing computers of the time made CP/M compatibility a priority. The computer literacy project had envisioned the BBC Micro to be a CP/M machine in their original draft specification and this add-on was to some extent delivered to satisfy that requirement.

CP/M usage saw a decline through the early 1980s and the Intel 80186 second processor would emerge part-way through the BBC Micro's lifespan in around 1985, mainly for running Microsoft DOS, which was displacing it as the most common business operating system. The processor was more commonly seen paired with the subsequent BBC Master than the Model B, which was at this point an outdated model. The 80186 was rarely used in IBM compatibles as IBM themselves did not select it for their range, although a few PC clones were released that used it.

The National Semiconductor 32016 was a lesser known 32-bit CISC CPU with an external 16-bit data bus. It was somewhat overshadowed by the similar and more famous Motorola 68000, although offered some benefits. It had a fully 32-bit ALU whereas the 68000 did not and could out-perform it in some calculations. It had unified data and address registers and an orthogonal instruction set making it easier to code. The add-on ran at 6Mhz and included an FPU, which while now a standard CPU feature, was then much less common and inclusion of this gave a hint as to the market to which this device was intended. The adapter was aimed at scientific computing and used an obscure Acorn developed operating system called Panos bundled with programming languages such as LISP and FORTRAN that would appeal to research users.

When a second processor was active, the BBC would usually be used somewhat like a display terminal handling video, keyboard and disk I/O. It was not typically possible to run a program on the internal 6502 concurrently with the second processor and the user would use one or the other but not both simultaneously. BBC Micro programmers were encouraged to use O/S APIs in their code rather than directly accessing the memory because it ensured compatibility with the 6502 second processor.

The Tube itself is a 2Mhz asynchronous parallel interface onto the 6502 system buses. A custom Tube chip in the second processor units provides an abstraction layer isolating the BBC Micro from the inner workings of the alternative processors and mediating access to it. The Tube chip is placed at some convenient location in the 2nd processor memory map and delivers access to operating system calls on the BBC Micro. The 2nd processor would boot from ROM and often a monitor was available which would allow BBC Micro operating system commands to be typed at the command line. From there another operating system (e.g. CP/M) could be started. The 2nd processor operating system could call the BBC Micro O/S via routines in the ROM that would handle the Tube protocol.

The Tube was used by Acorn to develop the ARM processor, which then stood for Acorn RISC Machine and is now one of the most successful and widely used CPUs. There are usually several ARMs in most smartphones and a wide variety of other devices. Many servers are also now using ARM CPUs in preference to Intel x86. The ARM was originally developed for use in the Acorn Archimedes range that succeeded the BBC Micro.

Econet Networking

The BBC Micro had designed-in provision on the main board for Acorn's "Econet" networking standard with a dedicated position on the rear of the case for the connector. As with the disk controller, the network chip set was initially relatively expensive and so the machine was also widely sold without the network chips fitted and they could be added in later if required. Home users, with typically only one machine, would not have have found a network adapter tremendously useful in 1981. In education though it was an entirely different story and Econet networks were very common, possibly even ubqititous, in UK schools with Econet ports a feature of many classrooms walls.

As well as the educational benefit, Econet made good economic sense for a school due to the high cost of disk drives. Using the network, a single BBC Micro could share a drive between several students and this was an initial major use case. An Econet adapter, whilst not cheap, cost a lot less than a disk drive with the associated interface chips and a substantial cost saving could be made by using the network in this way. Printer sharing was also available, which were similarly expensive devices at the time, and hard disk based network attached storage follow.

Econet is a bus network with all computers connected along a single cable. A cable could be particularly long, up to about half a kilometer, and handle 253 BBC Micros. This was more than sufficient for the typical school that might have owned a few dozen machines at best but even so Econet did support considerably larger networks. Each cable was considered a network "segment" and up to 127 segments could be connected together through a network bridge box, allowing over 32,000 machines on a single LAN if required. The bridge boxes were embedded computers containing a 6502 and managed transmission of data between two network segments. In the parlance of modern IP networks, a segment behaves like a subnet and a bridge box is similar to a router. Multiple bridge boxes could themselves be connected together in a network on one of the two Econet interfaces, with the network segment being served on the other.

Teletext and Telesoftware

Teletext was a popular digital data service transmitted over television broadcast signals in the UK and many other countries. The idea was originally developed by Philips and first broadcast on an experimental basis from 1973 by the BBC, offering primarily text based information but also some low resolution character based graphics. ITV, the other major UK broadcaster at the time, joined in with an incompatible system but a unified standard was agreed between the two broadcasters by 1974 and the BBC started offering a public service that year. Teletext would gradually become a standard built-in feature of UK televisions as decoder prices dropped, providing access to news, sports results, TV listings, financial data and magazine articles on-demand.

Information was offered as a series of numbered pages linked together, somewhat like the modern web. The user would start at a home page (often page 100) which would act as an index linking through to other pages. The user would type in the number of the page they required on a television remote control and it would be displayed on the screen. That page may then contain the numbers of other linked pages. The series of pages displayed was dependent on the TV channel being viewed at the time. Teletext would either replace the television picture or be video mixed over the top with the television channel still viewable underneath.

The BBC Micro had a dedicated chip for rendering teletext, the Mullard SAA5050. It may seem unlikely now, since they are now almost entirely made in the far East, but amazingly this was actually a UK manufactured chip produced in Southampton. The BBC Micro video mode 7 was used to display teletext using this device.

An adapter was available for the BBC Micro that could receive teletext transmissions and features were available on the machine to store the pages to disk. However, it could also be used to download software over the air; this was known as "Telesoftware". Software for the BBC Micro was broadcast over television via the teletext system from 1983 through to 1989. Telesoftware could be seen on a standard television although it usually displayed as a jumble of characters that made little sense without the relevant decoder.

The BBC Micro presented Telesoftware as a filing system, the Acorn TFS, and logically behaved something like a continuous loop tape. The user would discover the page number on which a program of interest resided using the teletext index and could execute it straight from the air.

Speech Synthesiser

There was a mysterious perforated region to the left of the BBC Micro Model B keyboard. This was a somewhat ill conceived design choice for schools as it proved near irresistibly tempting for bored students to attempt to push it in. The purpose of this perforation was not exactly obvious and nor was it labelled, but it was actually intended to be part of the Acorn speech synthesis system.

The BBC Micro did not have a built-in speech synthesiser as standard but did have designed-in provision for an official one on the motherboard and an upgrade could be purchased which consisted of two chips. The first chip was a Texas Instruments TMS5220 that implemented the speech synthesiser and is a later descendent of the chip used in the then popular Speak & Spell toy but with a number of improvements. The second chip was a "phrase ROM" that contained digitised speech data. The perforated cut-out was intended to allow easy installation of additional phrase ROMs (such as different voices) via cartridges without needing to remove the cover of the computer. However, it's not obvious that any such cartridges were ever actually produced.

It's somewhat surprising the BBC Micro did not have a dedicated cartridge port as it is one of the most inexpensive interfaces to supply, often requiring little more than a connector, and many competing machines did have one, as well as of course games consoles. This was however provided on the subsequent BBC Master. Occasionally, the perforated cut-out was re-purposed by 3rd party companies to allow easy installation of general ROM chips or catridges unrelated to speech, with Viglen producing one such adapter.

The phrase ROM installed within the machine featured the voice of BBC news reader Kenneth Kendall who also had an appearance in the 1968 movie, 2001: A Space Odyssey as a news reader of the future.

Several competing, and easier to install, speech synthesisers would be released for the BBC. Ultimately, a technically very impressive and easy to use synthesiser published by Superior Software (called Speech!, written by David Hoskins) would render the Acorn upgrade moot as it was implemented entirely in software and required no additional hardware at all.

Hard Disks

If floppy drives were expensive in 1981 then hard drives were an outlandish extravagence. While it was quite possible to buy one for a home computer, hard disks were more usually seen paired with high-end business computers. Within only the next few years though they dropped in price to levels more achievable for a school.

Acorn officially released the Winchester Disk System in 1984 although other manufacturers had released BBC Micro compatible hard disks since at least 1983. The Acorn models originally came with a choice of 10Mb or 30MB capacity. "Winchester" was then a commonly used industry term referring to the most popular design of hard disk mechanism but the word has since fallen out of use. The Acorn device was a relatively bulky external unit containing a 5.25" hard disk (often a Seagate ST-412) and a controller with SCSI interface on two separate boards, together with a power supply. The unit interfaced via the BBC Micro 1Mhz bus port.

The Acorn drive was rarely marketed to the home user and a release price is difficult to find, but with reference to contemporary drive costs in 1984 for other systems the 10MB model might be estimated to have been priced perhaps somewhere in the region of £1500 - £2000 (about £6400 in 2025). It was hence far too expensive to provision one drive per computer. With a school lucky to afford a single unit, a far more common arrangement was sharing access over Econet and one BBC Micro would act as a file server to the network. Subsequently, Acorn produced "FileStore" network attached storage devices that would plug directly into an Econet network without requiring a separate computer.

BBC BASIC

It was then common practice to supply a computer with a programming language and several were available for the BBC Micro. Multiple langauges could be installed simultaneusly on "langauge ROMs". The default language that shipped with the machine was BBC BASIC. Developed by Sophie Wilson, it was recongised for both good performance and being one of the more sophisticated versions of the language with a more expansive feature set than was usually found on contemporary machines.

BBC BASIC offered structured programming with procedures and functions, many competing BASICs of the time only offered program flow control by reference to line numbers with the GOTO statement. A variety of bitmap graphics commands were available such as line drawing and filled polygons as well as sound production commands, features notably absent on the Commodore 64 BASIC. It supported high precision floating point arithmetic, which could not be guaranteed at the time, such as the original Apple II BASIC was missing floating point and later shipped with Microsoft BASIC to correct this. A variety of string manipulation features were available, error handling capabilities and disk file commands, which were often absent on competing implementations.

A novel feature of BBC BASIC was it allowed 6502 assembly language to be freely intermingled with BASIC. This enabled critical loops in a program to be accelerated by writing appropriate assembly whilst allowing the majority of the program to be coded in the easier to write BASIC. Assembly was inserted into a program by placing it between a pair of square brackets. On other machines, programs usually had to be either BASIC or assembly but could not be both.

Operating System

When first switched on, as was the common convention of other home computers of the era, the BBC would boot to a prompt where BASIC commands could be entered. This somewhat obfuscated the fact that the machine did have an underlying operating system: the Acorn MOS or Machine Operating System. Command line tools could be run from BASIC by prefixing the relevant command with an asterisk. Adding extra ROM chips would usually result in extra operating system commands appearing. For example, if the floppy disk interface was installed, there would be relevant commands available to control the device, much as would be expected in DOS. Some examples of the O/S commands are as follows:

• *CAT - lists the contents of a floppy disk (like ls in Linux)
• *COMPACT - defragments the disk
• *DELETE - removes a file
• *DRIVE - switch to a different floppy drive
• *EXEC - execute a batch file of commands
• *INFO - display file info (somewhat like ls -l in Linux)
• *LIST - displays a text file (like cat in Linux)
• *SPOOL - re-direct terminal display to a file (like sending stdout to a file in Linux)

A significant and well documented library of helpful machine code routines were also included and available to the assembly programmer. Many functions of BBC BASIC could also be called from assembly, making the BASIC routines an additional operating system library.

Later Models

The Model A and B were followed up with a series of variants. The Acorn Electron was a cost-cut BBC Micro launched in 1983 that was squarely intended to compete with the considerably cheaper Sinclair products and open up the home market. It lacked a number of interfaces found on the Model B, but several upgrades were available that could add the features back in. It sold modestly well, in the region of 200,000 machines, but had to be heavily discounted and did not succeed in usurping the more popular Spectrum.

The Model B+ 64K was released in 1985 and as the name implies increased the RAM to 64K in an attempt to keep the machine current with respect the competition. It used a slightly higher performing 65C12 processor that was fully compatible with the original 6502A but added a few extra instructions. It also included a more advanced disk controller and an extra two ROM sockets. A Model B+ with 128K was also released shortly afterwards. These versions share the same case as the Model B but with stickers to highlight the upgraded specification. Internally they used a different PCB design though, one benefit of which was the ROM slots were easier to access.

The BBC Master 128 followed in 1986 and had a relatively long lifespan, remaining in production until 1993 when 8-bit technology was well past its peak. This version was routinely seen in education together with the Model B. It used a 65C12 CPU with 128K of RAM and a significantly expanded 128K ROM in addition to two cartridge ports. The machine was no longer sold without a disk interface and all versions included one. There was an internal Tube interface that could accept a second processor within the case and also an internal serial interface intended for a modem.

The expanded ROMs in the BBC Master included an integrated wordprocessor called View accessed by typing *WORD and a spreadsheet called ViewSheet accessed by typing *SHEET. There was also an extended BBC BASIC with new features and the latest version of the MOS operating system.

Two variations of the Master were available that made use of the internal second processor slot. The Master 512 included a 10Mhz 80186 second processor card and 512K of RAM that enabled MS-DOS compatability and also use of GEM graphical operating system from Digital Research. GEM was bundled with the machine and also "DOS Plus" which was a Digital Research variation of DOS that was compatible with MS-DOS 2.11 and could also run CP/M-86 software. A mouse was included with the package for use with GEM. There was a Master Turbo which included a faster 6502 and was around twice as fast as a stock BBC Master. Both of the systems were really bundle deals as opposed to distinct machines and a stock Master 128 could be upgraded to either.