The ACORN ELECTRON is a budget version of the BBC Micro
educational/home computer made by
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
For a short period, the Electron was reportedly the best selling
micro in the
* 1 History
* 2 Popular upgrades
* 2.1 Acorn Plus 1
* 2.1.1 Compatibility
* 2.2 Acorn Plus 2
* 2.3 Acorn Plus 3
* 2.3.1 ADFS quirks
* 2.4 First Byte Joystick Interface * 2.5 P.R.E.S. Advanced Plus 3 * 2.6 P.R.E.S. Advanced Plus 5 * 2.7 Slogger/Elektuur Turbo Board * 2.8 Slogger Master RAM Board * 2.9 Jafa Systems Mode 7 Display Unit * 2.10 Electron Second Processor
* 3 Merlin M2105
* 4 Technical information
* 4.1 Hardware * 4.2 Quirks
* 4.3 Tricks
* 4.3.1 FireTrack: smooth vertical scrolling * 4.3.2 Exile: sampled speech * 4.3.3 Frak! and Zalaga: Polyphonic music
* 5 Popular games * 6 Emulation * 7 Design team * 8 See also * 9 References * 10 External links
BASIC prompt after switch-on or hard reset (soft reset was similar but did not contain the Acorn trademark).
Acorn Computer released the
Acorn formally announced the Electron at the
Acorn User Exhibition in
August 1983, one year after Hauser mentioned the computer, by which
time its price had risen to £175 while the Spectrum cost £130. The
company expected to ship the Electron before Christmas, and sell
100,000 by February 1984. Reviews were generally favorable, noting its
excellent graphics compared to the
The reduced processor speed was attributable to the use of a 4 bit
wide memory system instead of the 8 bit wide memory system of the BBC
Micro to reduce cost. The Electron used just 4 64kbit RAM devices
instead of the 16 16kbit RAM devices deployed on the BBC Model B. In
The ULA continued to cause problems, delaying large-scale
manufacturing. By October 1983 Acorn had received orders for more than
150,000 units, and expected to produce 25,000 a month before
Christmas; the existing backlog would take more than six months to
fulfill. Demand for the Electron was high but only two of
WH Smith 's
London branches had inventory. As the company increased production
during 1984, however, the British home-computer market greatly
weakened; one Acorn executive later said, "after Christmas they were
just delivering and the company ended up with £43 million of
unsaleable stock". Acorn's Christmas 1984 sales were greatly below
expectations and in January 1985 the company reduced the Electron's
price to £129. In conjunction with an unsuccessful expansion into the
United States, by February Acorn's market capitalization declined 85%
from the previous year.
With hindsight, the machine lacked the 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 Spectrum and Commodore
64. Despite this, several features that would later be associated with
BBC Master and Archimedes were first features of Electron expansion
While it may not have been as popular as the Spectrum, Commodore 64
ACORN PLUS 1
The Acorn Plus 1 added two
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.
The Cartridge-port ROM-slots provided additional control lines, (compared to the lines available via the Edge-connector on the rear of the Electron), such as ROMSTBY, SNDIN, ROMQA, and some additional Voltage sources (+16V) etc. The total number of lines exposed via the Cartridge port almost matched those from the 1 MHz bus of the BBC.
Additional peripheral cartridge-holders by companies such as P.R.E.S.
(via their ARA/ARA2/ASR products) allowed \'Sideways ROM\' capability,
that allowed the standard Acorn ROM space to be programmatically
mapped out for alternative EPROMs, either physically via ZIF Sockets,
or 'virtually' via ROM-images loaded into (battery-backed) RAM in the
same ROM memory space. This enabled the Electron to achieve the same
functionality as that provided by the Expansion-ROM slots under the
keyboard and on the bottom-left of the
The addition of the Plus 1 added a number of new *FX and OSBYTE calls that allowed the OS to read the values from the analogue and parallel interfaces.
The Plus 1 needed memory page &D for its workspace, and some games used this space. To disable the Plus 1, after pressing BREAK, the following commands could be issued:
*FX163,128,1 ?&212=&D6 ?&213=&F1 ?"> Acorn Plus 3, showing the connector and disc drive
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. There were two versions of the Plus 3 produced: A Single-sided and a Double-sided drive version. 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, such as the P.R.E.S AP4 interface.
The Plus 3 reset PAGE to &1D00, reducing the amount of free RAM available to user. The ADFS system could be temporarily disabled (and PAGE reset to &E00) via the *NOADFS command. Alternative WD1770-based DFS and ADFS interfaces such as the P.R.E.S AP4 and 'ADFS E00' products left PAGE at Part no: unknown; input 220/240V AC/50 Hz; output 19V AC/0.737A/14W: USAGE: Electron, Electron+Plus1 * UPRATED Part no: 865-010; input 240V AC/50 Hz 50W; output 21V AC/1.75A/36.75W: USAGE: Electron+Plus3, Electron+Plus3+Plus1
The original Electron Edge-connector was repeated on the back of the Plus3, in addition to a secondary smaller edge-connector, that enabled additional drives to be connected (Shugart -compatible connection). These required their own power-supply. The secondary edge-connector could not power external drives.
Repair note: If the internal power-supply connector, used to power the existing internal 3.5" drive is damaged, and requires replacement, then the original AMP 800-930 4-pin connector, which was already in short supply during the original production run, may be replaced with a Molex 5264 50-37-5043 "Mini-SPOX" connector as an alternative.
If using the Plus3 in screen modes 0-3, the pseudo-variable TIME would be thrown off, as the interrupts were disabled during disk access in these modes.
Per a News article on page 9 of the October 1984 issue of Acorn User, the Plus 3 was originally designed to have used the Intel 8272 disk controller, (and not 8271, which were in short supply at the time).
The ADFS file format used the bytes Hugo to delimit the directory names on the disc, named after ADFS author Hugo Tyson. Another quirk was the presence of the file ZYSYSHELP which was required by the system, and created during formatting. This was a kludge. Acorn's v1.0 ADFS implementation on the Electron was unreliable when writing to the first few tracks of a floppy disc, so this was a "fix" and simply involved writing a file full of garbage to the suspect part. The ADFS would then skip it. Disc corruption could also occur if attempting to use the *COMPACT command without disabling the blinking cursor with the following command:
This was due to the fact that the *COMPACT command used screen memory (by default) as working space during the operation, and the software-implemented blinking cursor corrupted that memory space. An alternative would be to give arguments to make it use non-screen memory for workspace, for example *COMPACT 40 20 in MODE 6. Tyson recalls that this bug was found during late testing but not fixed in the initial ROM release in order to avoid late changes, as workarounds exist.
Disc formatting was done via the *EFORM command, vs the more familiar *FORM40/*FORM80 DFS commands. Note additionally that the *EFORM command differs from the equivalent *AFORM command for the 1770 ADFS on the BBC Microcomputer. This is possibly as a result of needing to create the ZYSYSHELP file on the Electron. The *EFORM command was only supplied on the Welcome disc that was shipped with the Plus3, and was not included in the ROM.
FIRST BYTE JOYSTICK INTERFACE
As a games machine the Electron shared the same failing as the Sinclair Spectrum in not having a joystick port. This was quickly remedied by First Byte Computers who developed an interface and software which allowed a "switched" joystick to be used with the majority of software titles. This interface became very popular and was sold by W.H. Smiths, Boots, Comet and hundreds of independent computer dealers.
P.R.E.S. ADVANCED PLUS 3
The P.R.E.S. Advanced Plus 3 with a 3½" drive
The Advanced Plus 3 was very similar to the Acorn Plus 3 but packaged
as an ADFS
P.R.E.S. ADVANCED PLUS 5
AP5 Provided User Port (all lines), 1 MHz bus and 'Tube' bus capability, enabling second processor usage. For the PRES5, and other similar interfaces, the User Port is usually implemented via a 6522 VIA chip.
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 particularly 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
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
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
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 each 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.
ELECTRON SECOND PROCESSOR
During the latter years, PMS produced a 2nd Processor specifically for the Electron. This provided an alternative to buying the combination of the P.R.E.S. Advanced Plus 5 and Acorn 6502 2nd Processor.
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.
The hardware of the
The edge-connector on the rear of the Electron exposed almost all the Bus lines, but not all. (The BBC Micro, courtesy of all its ports, exposed all lines.)
For Issue 1-4 motherboards, the ULA had an issue similar to those experienced by other socketed CPU's. Over time, the thermal heating and cooling could cause the ULA to rise slightly out of its socket just enough to cause the machine to start exhibiting 'hanging' or other startup-failure issues, such as a continuous 'startup beep'. This was despite a metal cover, and locking-bar mechanism designed to prevent this from occurring. Pushing down on the metal cover to reseat the ULA was normally sufficient to rectify these issues. Issue 5 and 6 boards utilized a different epoxy resin covering directly over the ULA, which resolved this issue.
The keyboard included a form of single-key keyword input, similar to that used on the Sinclair Spectrum , via the 'func' key. However, unlike the Spectrum, the single-keypress keyword-entry was optional, and keywords could be entered manually if preferred.
The ULA controlled memory access and was able to provide 32K × 8 bits of addressable RAM using 4 × 64K × 1-bit RAM chips (4164). Due to needing two accesses to each chip instead of one, and the complications of the video hardware also needing access, reading or writing RAM was much slower than on the BBC Micro. This meant that although ROM applications ran at the same speed, there was a substantial speed decrease on applications running from RAM.
Acorn ALF03 Data Recorder
* 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
Exile is an example of a game where the developers left non-graphical data visible in the display buffer to gain additional memory space.
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.
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 eight 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 Sinclair ZX Spectrum . Exile uses this to output 1-bit audio samples.
Frak! And Zalaga: Polyphonic Music
Aardvark Software's " Frak! " and "Zalaga" As part of the copy protection, illegal copies of the games would cause a pseudo-polyphonic rendition of Trumpet Hornpipe, the Captain Pugwash theme tune, to play endlessly rather than loading the game properly (Pugwash being a pirate). On the Electron version of Frak!, the tune was the main theme from "Benny Hill" ( Boots Randolph 's "Yakety Sax "). The polyphony was achieved via fast note-switching to achieve the necessary chords.
See also: List of Acorn Electron games
Although not as well supported by the biggest software publishers as
rivals like the
Starship Command (
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
Despite Acorn themselves effectively shelving the Electron in 1985, games continued to be developed and released by professional software houses until 1991. There were around 1,400 games released for the Acorn Electron, 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 .
Three emulators of the machine exist, ElectrEm for Windows /Linux
/macOS , Elkulator for Windows /
The operating system ROM locations 0xFC00-0xFDFF contain the
following text, which is different from the 'thanks' list in the
original BBC Model B : (C) 1983
Additionally, the last bytes of both the BASIC ROM and 'Plus 3 Interface' ADFS v1.0 ROM include the word "Roger", thought to be a reference to Roger Wilson .
The case was designed by industrial designer Allen Boothroyd of Cambridge Product Design Ltd.
* ^ "Spectrum Toppled". Electron User. Database Publications. 3
(5): 7. February 1986. Retrieved 2015-11-01.
* ^ A B C D Smith, Tony. "Acorn’s would-be ZX Spectrum killer,
the Electron, is 30". 2013-08-23. The Register. Retrieved 12 January
* ^ Electron Advanced User Guide - Appendix E
* ^ "