BBC Micro Totally Explained

Everything about The Bbc Microcomputer totally explained

The BBC Microcomputer System, or BBC Micro, was a series of microcomputers and associated peripherals designed and built by Acorn Computers Ltd 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.
   Having won the Literacy Project tender as the Acorn Proton, the BBC Micro became a cornerstone of computing in British education in the 1980s, being chosen by most schools and changing Acorn's fortunes to an extremely influential ITV documentary series The Mighty Micro, in which Dr Christopher Evans from the National Physical Laboratory predicted the coming (micro) computer revolution and its impact 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 their 1981 TV series The Computer Programme. The list of topics included programming, graphics, sound and music, Teletext, controlling external hardware, artificial intelligence, etc. It decided to badge a micro, then drew up a fairly ambitious (for its time) specification and asked for takers.
   The BBC discussed the issue with Sir Clive Sinclair, who tried to offer the unsuccessful Grundy NewBrain micro to them, but it was rejected. The BBC made appointments to see several other British computer manufacturers, including Dragon. The Acorn Proton not only was the only machine to come up to the BBC's specification, but also exceeded it in nearly every parameter. Research Machines had, until this time, been one of the leaders in UK educational computer market. One of the main advantages which helped the BBC Micro in the educational market was its durable construction. The machine's casing and keyboard was solidly built compared to that of the ZX Spectrum, being able to cope with all the abuse that schoolchildren could throw at it.
   The Model A and the Model B were initially priced at £235 and £335 respectively, but rising almost immediately to £299 and £399 due to increased costs. 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 Acorn attempted to counter this by producing a cut down version (although it did have the 32 kB RAM of the Model B rather than the 16 kB of the Model A), intended more for game playing, the Acorn Electron in 1983; games written specially for the Electron's more limited hardware could usually also be run on the Model B.

Description

Hardware features, Models A and B

The Model A had 16 KB (16 KiB) of user RAM; the Model B had 32 KB of user RAM. 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 (4 MHz), with alternating access given to the CPU and the video display circuits. This gave the BBC Micro a fully unified memory address structure with no speed penalties. Most competing micros with memory mapped display incurred CPU speed penalties depending on the actions of the video circuits (for example the Amstrad CPC and to a lesser extent the ZX Spectrum) or kept video memory completely separate from the CPU address pool (for example the MSX).
   The machine included a number of extra I/O 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 called the "Tube" allowed a second processor to be added; several types of processor were offered by Acorn including 68000 versions. It was later used in third-party add-ons, including a Zilog Z80 board and disk drive that allowed the BBC machine to run CP/M programs.
   The Tube interface allowed Acorn to use ARM CPU equipped BBC Micros as software development tools when creating the Acorn Archimedes. This resulted in the ARM development kit for the BBC Micro in 1986, priced at around £4000. In 2006 a kit with an ARM7TDMI CPU running at 64 MHz, with 16MB of RAM was released for the BBC Micro and Master, using the Tube interface to turn the old 8 bit micros into 32 bit RISC machines just as Acorn had done two decades previously. Among the software titles to run on the Tube were an enhanced version of Elite (see below) and a CAD package which required a second 6502 CPU and a 5 dimensional joystick called a "Bitstick".
   The Model A and the Model B were built on the same PCB and a Model A could be upgraded to a Model B without too much difficulty. Users wishing to run 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 which could be achieved without soldering. To do a full upgrade with all the external ports did however require soldering the connectors to the motherboard.
   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 partly rectified by a soldering-capable person, by soldering a resistor across two pads.

Export models

Two export models were developed; one for the US, with Econet and speech hardware as standard; the other for the Federal Republic of Germany. Both were fitted with RF shielding as required by the respective countries, and they were still based on the Intel 8271 floppy drive controller. The ROM sockets were moved from under the keyboard to the middle of the right-hand side, where a panel could be unscrewed from the shielding to reach them.
The version of BASIC included in the US model, BASIC III, was modified to accept both COLOUR and COLOR keywords in typed-in programs, but always print the American spelling in program listings. However the height of the graphics display was reduced from 256 scan lines per field to 200 to fit the NTSC raster, seriously affecting applications written for British computers. After the failed US marketing campaign the frustrated machines were remanufactured for the British market and sold off, resulting in a third 'export' variant.

Hardware features: B+64 and B+128

Acorn introduced the Model B+ in mid 1985, increasing the total RAM to 64 KB and including floppy disk support as standard, but this had modest market impact. 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 much-needed memory increase provided by this new 1985 'Beeb' was a welcome development, but was seen as an eighteen months or so too late to challenge the increased specifications of new rival microcomputer systems. 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 running 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. A piece of software that used copy protection techniques which involved direct access to the controller, simply wouldn't run on the new system.
There was also a long-running problem late on in the B/B+'s life infamous amongst B+ owners, when Superior Software released Repton Infinity, which refused to run on the B+. A string of unsuccessful replacements were issued before one compatible with both was finally released.

Software and expandability

   Large numbers of games were written for the Beeb, including the original version of the classic Elite. A range of hardware add-ons and expansions was available, and the machine had provisions for floppy disk drives and Econet networking hardware. There were also sockets for the addition of extra ROM chips.
   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 like vector graphics, keyboard macros, cursor-based editing, sound queues and envelopes, normally private to BASIC, were made available to any application. BASIC itself, being in a separate ROM, could be replaced with any equivalent language.
   Acorn strongly discouraged programmers from directly accessing the hardware and system variables, favouring official API calls. This was ostensibly to make sure programs kept working when moved to the Tube coprocessor, but it also made BBC Micro software more portable across the Acorn range. Whereas untrappable PEEKs and POKEs were commonly used on other computers to reach the system elements, both BBC BASIC and assembly language programs could set up the CPU registers or a parameter block, and call an operating system routine. In this way the MOS could translate the request for the devices and memory layout of the local machine (especially the Electron and Archimedes) or send it across the Tube interface, as direct access was impossible from the coprocessor.
   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 rose to popularity; 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.

BBC BASIC

The built-in ROM-resident BBC BASIC programming language interpreter was by far the most sophisticated of its time, and wholly supported the machine's educational focus. Advanced programs could be written without resorting to unstructured programming or assembly language (necessary with many competing computers). Should one want or need to do some assembly programming, BBC BASIC featured a built-in assembler.
When the BBC Micro was released competing PCs used Microsoft BASIC, or variants typically designed to resemble it. BBC BASIC had the following advantages:

Support for named procedures, rather than relying upon GOTO/GOSUB;
Support for IF .. THEN .. ELSE
Support for high-resolution graphics (albeit with a clumsy syntax);
Support for four-channel sound (again, with a clumsy syntax);
In-line support for assembly language, rather than having to POKE machine code stored in DATA lines;
Long variable names (Microsoft BASIC at that stage supported long names, but with only the first two characters recognised, so that effectively it was limited to two-character names);
Support for pointer-based programming, like C, although using a syntax borrowed from BCPL.

Successor machines and the retro scene

In 1986, Acorn followed up with the BBC Master series, which offered memory sizes from 128 KB and many other refinements which improved on the 1981 original. It attracted more interest than the B+ upgrades, although at heart it was 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.
   However, Acorn had produced their own 32-bit RISC CPU in 1985, the ARM2 and were working on building a personal computer around it. This was released in 1987 as four models in the Archimedes series, with the lower-specified two models (with 512 KB and 1 MiB respectively) released as BBC Microcomputers. Although the Archimedes ultimately wasn't a major success, the ARM family of processors has gone on to become the dominant processor architecture in mobile embedded consumer devices, particularly mobile phones
   The last model, the BBC A3000, was released in 1989 as essentially a 1 MiB Archimedes back in a single case form factor. The BBC closed the Computer Literacy Project two years later. As of 2005, thanks to its ready expandability and I/O functions, there are still numbers of BBCs in use, and a retrocomputing community of dedicated users finding new things to do with the old hardware. A BBC B+ was observed running the communications link in an unattended water pumping station in Oxhey in 1995. They still survive in a few interactive displays in museums across the country, and Jodrell Bank was reported to have used a BBC Micro to steer one of its satellite dishes in 2004. There are also a number of BBC Micro emulators for many OSes, so that even the original hardware is no longer necessary.
   In March 2008, the creators of the BBC Micro met at the Science Museum in London. The museum plans to hold an exhibition about the computer and its legacy in 2009.

Specifications (Model A to Model B+128)

	Model A	Model B	Model B+64	Model B+128
Processor	MOS Technology 6502A at 2 MHz		MOS Technology 6512A at 2 MHz
RAM	16 KB	32 KB	64 KB composed of 32 KB standard memory, 20 KB video (Shadow) memory and 12 KB extended (special Sideways) memory.	128 KB composed of 32 KB standard memory, 20 KB video (Shadow) memory and 76 KB extended (Sideways) memory.
ROM	32 KB of ROM composed of a 16 KB MOS (Machine Operating System) chip, and 16 KB read-only paged space defaulting to the BBC BASIC chip. Four paged 16KB ROM sockets standard, expandable to 16.		48 KB of ROM composed of 16 KB MOS, 16 KB DFS, and 16 KB read-only paged space defaulting to the BBC BASIC.
Keyboard	Full-travel keyboard with a top row of ten red-orange function keys $f_0-f_9$ . These generated Teletext control characters when pressed with CTRL or SHIFT, and could be programmed with keyboard macros. The arrow keys and BREAK could also serve as function keys.
Display	As Model B except RGB (Optional upgrade, soldering required).	6-pin DIN digital RGB connector +5V/0V, 1v p-p composite colour or monochrome video (link S39) and built-in UHF (PAL) RF modulator.
Graphics	As Model B, but Modes 0, 1, 2, and 3 not available due to lack of memory.	Configurable graphics in Modes 0-6 (see table below) based on the Motorola 6845 CRT controller or Mode 7, a special Teletext mode, based a Mullard SAA5050 Teletext chip and only taking 1 KB of RAM.
Sound	Four independent sound channels (one noise and three melodic) using the Texas Instruments SN76489 sound chip. Phoneme-based speech synthesis using the Texas Instruments TMS5220 with a custom Acorn ROM (the "PHROM", a TMS6100) of Kenneth Kendall's voice (optional).
Tape storage	Tape interface (with a relay operated motor control), using a variation of the Kansas City standard data encoding scheme running at 1200 or 300 baud.
Disk storage	Optional floppy disk interface based on the Intel 8271 chip, also requiring the installation of the DFS (disk filing system) ROM (and of soldered connector on Model A). (5.25" floppy drive usually used).		floppy disk controller based on the Western Digital WD1770 controller and DFS ROM as standard.
Hard disc storage	None (lack of memory).	Additional ADFS ROM required, external drive unit connected to the 1 MHz Bus interface. (Winchester Hard disc drives in 5 MB, 10 MB or 20 MB sizes. Maximum of 512MB per drive, up to four drives).
Serial Interface	Optional upgrade, soldering required.	5-pin 'domino'-DIN RS-423 serial port.
Parallel interface	Optional upgrade, soldering required.	26-pin IDC Centronics-compatible parallel port.
User port	Optional upgrade, soldering required.	20-pin IDC "user port" with 8 general purpose digital I/O pins and two special/trigger sensitive digital pins used for control purposes (for eg a turtle when using the Logo programming language).
Analogue interface	Optional upgrade, soldering required.	DB15 pin with four 8/12 bit analogue inputs based on uPD7002 IC (suitable for two joysticks), two inputs suitable for pushbuttons and an input for a light pen.
1 MHz Bus	Optional upgrade, soldering required.	34-pin IDC connector for generic expansion on a "daisy-chain" (used for connecting hard disks, sound synthesisers etc).
The Tube®	Optional upgrade, soldering required.	40-pin IDC connector for external second CPU. Options included a second 6502, a Zilog Z80, the ARM Evaluation System, or a National Semiconductor 32016 (the latter was either branded "BBC Microcomputer System - 32016 Second Processor" or "Acorn Computer - Cambridge Co-Processor"), other vendors added 6809, 6800, 68000 and 68008 with the addition of a co-processor adapter, it's possible to connect a 10MHz 80186 co-processor, that would normally reside inside a BBC Master, to a BBC Micro, thus enjoying a limited degree of PC compatibility.
Network (Optional extra)	Econet large-scale low-cost networking system - around 100 kbit/s using the Motorola 6854 (standard on US model).

The case was designed by industrial designer Allen Boothroyd of Cambridge Product Design Ltd.

Display modes

One of the features of the BBC Micro was that its video output could be switched (either by command or by software) between a number of different modes. These modes varied from 20 column text up to 80 column text, which allowed a range of different video displays to be usable - a domestic black and white TV for a large text mode or a high-resolution RGB display for 80 column text or detailed colour graphics. A Teletext mode was also included as an original requirement, due to the BBC use of broadcast teletext (Ceefax).

Mode 7 was a Teletext mode and extremely economical on memory: it also made the computer useful as a Prestel terminal, which made it popular with travel agents, banks and stock traders for a while. Train time displays at UK stations were driven by BBC Master computers until around the late 1990s when they were gradually phased out.

Modes 0 to 6 could display a choice of colours from a logical palette of sixteen, though only eight colours were available; the eight basic RGB colours (0-black, 1-red, 2-green, 3-yellow, 4-blue, 5-magenta, 6-cyan, 7-white) and eight colours in a flashing state, (8-black/white, 9-red/cyan, 10-green/magenta, 11-yellow/blue, 12-blue/yellow, 13-magenta/green, 14-cyan/red, 15-white/black)

Modes 3 and 6 were special software (framebuffer) text modes. To save RAM, the count of lines was reduced from 32 to 25. As this would reduce the height of the frame, filler rows were created between each line of text when the frame was output, where no pixels were read from the framebuffer. This creates characteristic black lines between the rows of text when a different background colour is set, and a blank gap at the bottom of the display with the left-over pixels. The screen mode is otherwise held in memory as a regular graphics mode.

In addition, the BBC B+ and the later Master allowed 'shadow modes', where the framebuffer was stored in 20 KB of an alternative RAM bank ('shadowing' the main memory, hence shadow RAM), leaving the main memory up to 0x8000 free for user programs. This feature was enabled by setting bit 7 of the mode variable, for example by requesting modes 128–135.

Optional extras

Serial ROM cartridge filing system via a slot to the left of the keyboard - usually fitted as part of the Speech Upgrade

Speech synthesis hardware based around the Texas Instruments TMS5220 - standard on US model. The speech chips featured sampled phonemes spoken by BBC newscaster Kenneth Kendall. (Very few people bothered with this upgrade - the synthesiser's abilities were rather limited, and some games programmers succeeded in producing versatile software speech synthesis using only the standard sound hardware)

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 the clip for Erasure's Oh L'Amour.

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.

The BBC Micro provided in-game graphics for the BBC TV show "The Adventure Game", where the BREAK key on the keyboard was covered by a plastic box to prevent accidental pressing.

Numerous 80s episodes of Doctor Who feature text, graphics, and sound effects generated by a BBC Micro computer. Such episodes include The Five Doctors (first broadcast in 1983) and The Twin Dilemma (first broadcast in 1984).

During the 80s and 90s a BBC Micro was used on the television programme Mastermind to display the contestants' scores.

Further Information

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