Acorn Electron

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The Acorn Electron

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Acorn Electron BASIC - the first thing displayed when an unexpanded Electron is switched on

The Acorn Electron was a budget version of the BBC Micro educational/home computer made by Acorn Computers Ltd. It had 32 kilobytes of RAM, and its ROM memory 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 to plug into the microphone socket of any tape recorder. It was capable of basic graphics, and could display onto either a television set 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.

The hardware used on the BBC Micro was emulated by a single chip (ULA – uncommitted logic array) designed by Acorn. It had feature limitations such as being unable to output more than one channel of sound instead of being three-way polyphonic, the inability to provide teletext mode and unable to run applications from RAM as fast as the 2MHz BBC Micro, 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 custom designed ULA, manufacturing problems meant that very few machines were available for the Christmas period with some shops reporting eight presales for every delivered machine.

This was a blow from which the machine never fully recovered, although across its lifespan more games were sold for the Electron than the BBC. With hindsight, the machine was too lacking in RAM (a typical program would need to fit in only around 20kb 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 machines were first features of Electron expansion units, including the ROM cartridge slots and the Advanced Disc Filing System - a hierarchical improvement to the DFS.

Popular upgrades

Acorn Plus 1

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

Access to ROM memory occurred at 2Mhz 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 was 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 that added a double density 3.5" 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 NEC8271 found in the BBC Micro it was also possible to run a DFS filing system with an alternate ROM.

Pres Advanced Plus 3

The Advanced Plus 3 was very similar to the Acorn Plus 3 but packaged as a ROM catridge for the Plus 1 with a disc drive connector at the head. This made it perfectly possible to connect the 5.25" floppy disc drives used by BBC Micro owners if the user wanted.

Slogger Turbo Board

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

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 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 32kb of shadow RAM in addition to the ordinary 32kb - giving 64kb 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.

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 couldn't be written to function in shadow mode.

During its decline, Master RAM Boards were added to every Electron to try and increase sales.

Technical information

• 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: 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), aerial TV connector (RF modulator), RGB video monitor output
• Power supply: External PSU, 18V

Popular games

A good range of games were available for the Electron from publishers such as Acornsoft and Superior Software, notably including:

• Elite
• Chuckie Egg
• The Repton series
• Exile
• Starship Command
• Thrust

See also the list of Acorn Electron games for a fairly comprehensive list of every game published for the machine and Category:Acorn Electron games for a list of games with information on Wikipedia.

Emulation

Two emulators of the machine exist, ElectrEm ([1]) for Windows/Linux/Mac OS X and Elkulator ([2]) for Windows/DOS.

External links

• Stairway To Hell
• The Acorn Preservation Project
• Acorn Electron World