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Acorn Electron

The Acorn Electron is a budget version of the BBC Micro educational/home computer made by Acorn Computers Ltd. It had 32 kilobytes of RAM, and its ROM included BBC BASIC along with its operating system.

The Electron was able to save and load programs onto audio cassette via a supplied converter cable that connected it to any standard tape recorder that had the correct sockets. It was capable of basic graphics, and could display onto either a television set, a colour (RGB) monitor or a "green screen" monitor.

At its peak, the Electron was the third best selling micro in the United Kingdom, and total lifetime game sales for the Electron exceeded those of the BBC Micro. There are at least 500 known games for the Electron and the true total is probably in the thousands.

The hardware of the BBC Micro was emulated by a single customized ULA chip designed by Acorn. It had feature limitations such as being unable to output more than one channel of sound where the BBC was capable of three-way polyphony (plus one noise channel) and the inability to provide teletext mode. The machine architecture also imposed a substantial speed decrease on applications running from RAM, although ROM applications ran at the same speed

The ULA controlled memory access and was able to provide 32K × 8 bits of addressable RAM using 4 × 64K × 1-bit RAM chips (4164).

History

The Electron was developed during 1983 as a cheap sibling for the BBC Micro with the intention of capturing the low cost Christmas sales market for that year. Although Acorn were able to shrink substantially the same functionality as the BBC into just one chip, manufacturing problems meant that very few machines were available for the Christmas period - to the extent that some shops reported eight presales for every delivered machine.

This was a blow from which the machine never fully recovered, although games sales for it would ultimately outstrip those of the BBC Micro. Following Olivetti's 1985 cash injection into Acorn the machine was effectively sidelined.

With hindsight, the machine was too lacking in RAM (a typical program would need to fit in only around 20 kB once display memory is subtracted) and processing power to take on the prevailing ZX Spectrum and Commodore 64. Despite this, several features that would later be associated with BBC Master and Archimedes were first features of Electron expansion units, including ROM cartridge slots and the Advanced Disc Filing System — a hierarchical improvement to the BBC's original Disc Filing System.

The Electron is commonly thought of by many retro computer enthusiasts as a failure. However, whilst it may not have been as popular as the Spectrum, Commodore 64, Amstrad CPC or even its own BBC sibling it did sell in sufficient numbers to ensure that new software was being produced right up until the early 1990s. This meant the Electron had a lifespan not much shorter than those more popular micros and much longer than competitors such as the Oric-1 and Dragon 32.

Popular upgrades

Acorn Plus 1

The Acorn Plus 1 added two ROM slots, an analogue interface and a parallel port. The analogue interface was normally used for joysticks, the parallel for a printer.

Access to ROM memory occurred at 2 MHz regardless of graphics mode so theoretically programs released on ROM could run at least twice as fast as those released on tape or disc. Despite this all of the games released on ROM were packaged as 'serial ROMS', from which the micro would load programs into main memory in exactly the same way as if it were loading from tape. This meant that programs did not need to be modified for their new memory location but gave no execution speed benefits whatsoever.

Acorn Plus 3

The Acorn Plus 3 was a hardware module that connected independently of the Plus 1 and provided a double-density 3½” disc drive connected through a WD1770 drive controller and an ADFS ROM. Because the WD1770 is capable of single density mode and uses the same IBM360 derived floppy disc format as the Intel 8271 found in the BBC Micro, it was also possible to run a DFS filing system with an alternate ROM.

P.R.E.S. Advanced Plus 3

The Advanced Plus 3 was very similar to the Acorn Plus 3 but packaged as a ROM cartridge for the Plus 1 with a disc drive connector at the head. This made it possible to connect a 5¼” floppy disc drive as used by BBC Micro owners or a more common 3½” drive.

Slogger/Elektuur Turbo Board

The Slogger and Elektuur Turbo Boards were born out of a hack initially devised at Acorn. By moving the lowest 8 KB of RAM outside of reach of the ULA, the CPU could always access it at 2 MHz. The tradeoff was that the screen could not be located in that 8 KB. In practice the operating system ROMs always put the screen into the top 24 KB and as a result this probably only broke compatibility with around 2% of software.

The Slogger Turbo Board was a professionally fitted upgrade whereas the Elektuur modification was described in an article in Dutch Electronics magazine Elektuur and intended for users to perform at home.

Speeding up the low portion of memory is particarly useful on 6502 derived machines because that processor has a faster addressing mode for the first 256 bytes and so it is common for software to put any variables involved in time critical sections of program into that region.

If Acorn had thought to include this small modification in the original Electron design it is likely the machine would have had a much greater impact as it would have nearly doubled the amount of motion possible in games and saved modes 0–3 (including the only 16 colour mode) from being nearly useless due to contended memory timings.

Slogger Master RAM Board

A development of the Turbo Board, the Master RAM Board duplicated the Turbo Board functionality and added a further option of running the micro with 32 kB of shadow RAM in addition to the ordinary 32 kB — giving 64 kB total. Some clever program counter catches meant that the ordinary system ROMs and any software using the OS calls could function without significant modification, making substantially more memory available for BASIC, View, Viewsheet and almost every other business application. By providing extra storage this modification also allowed some games and applications intended for the BBC Micro to function on the Electron despite the lack of a native Mode 7.

Applications could not directly address video memory in this mode without modification, so it was incompatible with most games, although there is no inherent reason why a game could not be written to function in shadow mode.

During its decline, Master RAM Boards were added to every Electron in an attempt to increase sales.

Jafa Systems Mode 7 Display Unit

Of the capabilities present in the BBC Micro but absent from the Electron, the teletext style mode 7 was particularly conspicuous because of the very low memory usage in that mode (just less than 1 kB) and the high number of BBC programs that used it. Jafa Systems provided a number of solutions to redress this deficiency.

The most basic solution was a pure software system supplied on a ROM cartridge that drew a low resolution approximation of the mode 7 display in a graphics mode. Although cheap and effective in enabling use of some software that only used official ROM entry points for text output, this solution proved very slow because the Electron had to be placed into an 80 byte pitch display to be able to get anywhere near to reproducing mode 7 and the CPU spent a lot of time drawing approximations of mode 7 characters and graphics that in a hardware solution would be achieved without any CPU processing. It also used up 20 kB of RAM for the graphics display rather than the 1 kB of a hardware mode 7.

Two solutions with additional hardware were provided. The first used the same graphics processor as the BBC Micro in mode 7 — the SAA5050 — but used software to ensure that it was fed with the correct graphics data. A software ROM would put the machine into an ordinary 40 byte pitch display. While the ULA would read the display from memory in the usual fashion, the SAA5050 would listen to the data it was reading and produce a mode 7 interpretation of the same information. When necessary the hardware would switch between the graphics output being produced by the micro and that being produced by the add-on.

The disadvantage to this system is that while the SAA5050 would expect to be repeatedly fed the same 40 bytes of data for every display scanline of every character row, the ULA would read a different set of 40 bytes for every display scanline in order to produce a full graphics display. A software ROM worked around this by duplicating the data intended for a mode 7 display in memory. Although this produced a mode 7 that barely impacted upon CPU performance and gave the same visual quality as the BBC Micro, it remained compatible only with software that used the ROM routines for outputting text and graphics and still used 10 kB of memory for the display.

A second version of the hardware add-on corrected these problems. By adding a CRTC6845 to the package, a full hardware solution was created that did not reduce CPU performance and only used 1 kB of memory for the display. A software ROM was still supplied, but this did no more than expand the hardware ROM so that it knew mode 7 now existed and was able to switch into it.

Merlin M2105

An unusual variant of the Electron was sold by British Telecom Business Systems as the BT Merlin M2105 Communications Terminal. This consisted of a de-badged Electron plus a large expansion unit containing 32 KB of RAM, 48 KB of ROM, a Centronics printer port and a modem. The ROM firmware provided dial-up communications facilities. These were used by the Interflora florists network in the UK for over a decade.

Technical information

Hardware

CPU: MOS Technology 6502A
Clock rate: variable. CPU runs at 2 MHz when accessing ROM and 1 MHz or 0.5897 MHz (depending on graphics mode) when accessing RAM due to sharing memory access with the video display circuits. The Electron is widely misquoted as operating at 1.79 MHz after measurements derived from speed testing against the thoroughly 2 MHz BBC Micro for various pieces of 'common software'
Coprocessor: Ferranti Semiconductor Custom ULA
RAM: 32 kB
ROM: 32 kB
Text modes: 20×32, 40×25, 40×32, 80×25, 80×32 (all text output produced by software in graphics modes)
Graphics modes: 160×256 (4 or 16 colours), 320×256 (2 or 4 colours), 640×256 (2 colours), 320×200 (2 colours — spaced display with two blank horizontal lines following every 8 pixel lines), 640×200 (2 colours — spaced display)
Colours: 8 colours (TTL combinations of RGB primaries) + 8 flashing versions of the same colours
Sound: 1 channel of sound, 7 octaves; built-in speaker. Software emulation of noise channel supported
Dimensions: 16×34×6.5 cm
I/O ports: Expansion port, tape recorder connector (1200 baud variation on the Kansas City standard for data encoding), aerial TV connector (RF modulator), composite video and RGB monitor output
Power supply: External PSU, 18V AC

Quirks

Like the BBC Micro, the Electron was constrained by limited memory resources. Of the 32 KB RAM, 3½ KB was allocated to the OS at startup and at least 10 KB was taken up by the display buffer in contiguous display modes.

Due to the timing of interrupts it was possible to disable either the top 100 or bottom 156 lines of the display with palette changes. Many games took advantage of this, gaining storage by leaving non-graphical data in the disabled area.

Other games would load non-graphical data into the display, leaving it visible as regions of apparently randomly coloured pixels.

Although page flipping was a hardware possibility, the limited memory forced most applications to do all their drawing directly to the visible screen, often resulting in graphical flicker or visible redraw. A notable exception is Players' Joe Blade series.

Tricks

FireTrack: smooth vertical scrolling

Although programs can alter the position of the screen in memory, the non-linear format of the display means that vertical scrolling can only be done in blocks of 8 pixels without further work.

FireTrack exploits a division in the way the Electron handles its display — of the seven available graphics modes, two are configured so that the final two of every ten scanlines are blank and are not based on the contents of RAM. If 16 scanlines of continuous graphical data are written to a character block aligned portion of the screen then they will appear as a continuous block in most modes but in the two non-continuous modes they will be displayed as two blocks of 8 scanlines, separated in the middle by two blank scanlines.

In order to keep track of its position within the display, the Electron maintains an internal display address counter. The same counter is used in both the continuous and non-continuous graphics modes and switching modes mid-frame does not cause any adjustment to the counter.

FireTrack switches from a non-continuous to a continuous graphics mode part way down the display. By using the palette to mask the top area of the display and taking care about when it changes mode it can shift the continuous graphics at the bottom of the display down in two pixel increments because the internal display counter is not incremented on blank scanlines during non-continuous graphics modes.

Exile: sampled speech

Exile turns the Electron's one channel output into a digital speaker for PCM output.

The speaker can be programmatically switched on or off at any time but is permanently attached to a hardware counter so is normally only able to output a square wave. But if set to a frequency outside the human audible range then the ear can't perceive the square wave, only the difference between the speaker being switched on and off. This gives the effect of a simple toggle speaker similar to that seen in the 48 kB ZX Spectrum. Exile uses this to output 1-bit audio samples.

Popular games

Although not as well supported by the biggest software publishers as rivals like the Commodore 64 and ZX Spectrum, a good range of games were available for the Electron. The traditional BBC Micro publishers such as Acornsoft, Superior Software and Micro Power offered the widest support. Notable popular games particularly associated with the Electron include:

Starship Command (Acornsoft, 1983)
Chuckie Egg (A'n'F, 1984)
Elite (Acornsoft, 1984)
the Repton series (Superior Software, 1985-1989)
Thrust (Superior Software, 1986)
Exile (Superior Software, 1988)

There were also many popular games officially converted to the Electron from arcade machines (including Crystal Castles, Tempest, Commando, Paperboy and Yie Ar Kung-Fu) or other home computer systems (including Impossible Mission, Jet Set Willy, The Way of the Exploding Fist, Tetris, The Last Ninja, Barbarian and SimCity).

There were also many original titles on the Electron that received little mention at the time (e.g. Bun Fun and Spy Snatcher).

Despite Acorn themselves effectively shelving the Electron in 1985, games continued to be developed and released by professional software houses until 1991. In addition to the 1,400 games released for the Acorn Electron (99% of these on cassette), several thousand extra public domain titles were released on disc through Public Domain libraries. Notable enterprises which produced discs of such software are BBC PD, Electron User Group and HeadFirst PD.

See also the list of Acorn Electron games for a fairly comprehensive list of games published for the machine and BBC Micro and Acorn Electron games for a list of games with information on Wikipedia.