About Acorn Computers and ARM Processors

There are more ARM processors on this world than with any other type of processor (including Intel compatible ones). Your cell phone most probably has an ARM CPU. Did you know that “ARM” originally stood for “Acorn RISC machine” and that the processors were designed for Acorn's desktop operating system?

  • For latest RISC OS news, try the Icon Bar, one of the Acorn-related newsgroups (for example comp.sys.acorn.advocacy), or the websites of RISC OS Ltd and Advanced RISC Machines (ARM).
  • Many of the JPEGs are only thumbnails - click on a picture to view it full size.
  • Disclaimer: All of this must be regarded as my personal view. Other people may have different thoughts about Acorn and their products! If you think that some of the information on this page is wrong, I'd be glad if you told me.
  • This information is not complete - the newer history of RISC OS is missing, as I no longer use RISC OS today.

[Acorn logo]


Acorn? Never heard about them... and why should I bother?

I am perfectly aware that your computer and your OS are far better - but if you learn a little about Acorn, you might find that even though the system really can be considered exotic, it is well-planned with consistent design and some clever details which were not present in any other OS at the time it was released.

The Acorn platform is probably one of the smallest computer platforms, consisting of some estimated 500,000 machines (excluding older 8-bit computers). A majority of them were sold in Great Britain, as Acorn Group were situated at Cambridge, and many were bought by British schools. However, there are also a lot of private users of Acorn computers, mostly - in order of importance - in Great Britain & (Northern) Ireland, Germany, France, Australia & New Zealand, the Netherlands and Italy. There are practically no Acorns to be found in the USA.


Being a small company in a market of industry giants who have much greater resources (for development, cheap production and for marketing), Acorn were always exposed to strong competition. The company was founded in 1978 by Hermann Hauser and Chris Curry. In the 1980s, practically all British schools were equipped with their "BBC" computers (and Acorn machines were also quite popular as home computers), but when Wintel PCs began to gain importance, more and more schools switched over to that platform. Still, the educational sector remained an important market for a long time.

See the articles on stairwaytohell.com and Robert McMordie's page for more information on the early days of Acorn.

[ART logo]

The company went through many a restructuring. Most importantly, seperate companies were founded for supporting the UK education market (Xemplar was owned in part by Acorn, in part by Apple), developing RISC OS (Acorn RISC Technologies; ART, although this didn't exist as a separate company for very long), working on the 32-bit processor architecture (Advanced RISC Machines; ARM) and on Acorn's NetComputer models (Acorn Network Computing; ANC, again, not for a long time).

As time went by, Acorn were able to sign contracts with various major companies, with a positive overall effect on their share price, e.g. Apple (UK education market), Digital Semiconductors (StrongARM processor) and Oracle (NetComputer).

Since about 1997, the company's focus slowly changed. The desktop market of RISC OS machines was large enough to sustain it, and a new desktop computer was being designed, but other markets looked more promising in the long run. Building on the experiences made when designing the Acorn NetComputer, Acorn concentrated on making their technology available for licensing to third parties, for things like interactive/digital TV and Multimedia Point of Sale Terminals.

Finally, on "Black Thursday" 17th September 1998, in a completely unexpected move Acorn announced that all work on desktop computers had ceased, which included the Phoebe workstation which had been scheduled for November, and that development would focus completely on the digital TV market from now on. In an attempt to get rid of the "educational" image, even the company name was changed to Element 14 early in 1999. (Element 14 is silicon.)

Later, the company was bought by Pace Computers Ltd. Pace were only interested in Acorn's digital TV expertise - no further development for desktop systems was expected from their side. Later, development of RISC OS was taken over by RISCOS Ltd.

For a very long time, Acorn had remained the only European company designing and manufacturing complete desktop computer systems, which at the time were considered a true alternative to the more popular systems by many (e.g. the Times and me - I think a few others too).


Hmmm... I bet because the computers were produced in such low numbers, they were quite expensive!

That depends on your point of view. Looking only at the numbers you are right: Compared to Intel-based computers, you payed more for the same amount of processing power. Additionally, you did not get the same support because of a much looser net of dealers. However, all this is outweighed by the good architecture design and the great OS - you don't save money, but you save yourself a lot of hassle and annoyance.

Acorn produced computers for a long time. Their first "BBC" models were based on the 8-bit MOS 6502 processor (very similar to the MOS 6510 found in the Commodore 64), but later models use 32 bit ARM processors.

Archimedes series

Acorn A3010

Acorn Archimedes computers were the first of Acorn's computers to use a 32-bit architecture. The later models featured graphics resolutions of up to 800 × 600 with up to 256 colours, 8-bit logarithmic stereo sound with eight channels, ARM3 processors (up to 25 MHz, I think) and up to 4 MB of RAM (16 MB with one model). Many of them could be found for a long time in British schools. The last OS version supporting them was RISC OS 3.11.

The A3010 was the first Acorn computer I owned. It ran at 12 MHz using an ARM250 and, unlike its brother A3020, didn't come with a built-in harddisc. Among others, there were also the A4000 and A5000, which looked more like "ordinary" PCs, i.e. with a separate keyboard, and the A4 (a laptop).

PocketBook series

[PocketBook II]
PocketBook II

The PocketBook notebooks, of which the PocketBook II is an example, are special in that they do not use ARM processors. Instead, they are 100% compatible with Psion notebooks, e.g. the Psion 3c. However, note that the later Psion 5 series of palmtops uses an ARM7100 processor.

RiscPC series (and A7000)

The first generation of RiscPCs was launched back in 1994. Afterwards, there were numerous, but not really significant improvements to the design.

The RiscPC has

  • two standard SIMM slots and one non-standard slot for 1 or 2 MB of VRAM (can also run without VRAM),
  • an IDE bus,
  • a VIDC20 chip combining video and sound output (up to 135 MHz pixel rate, 24bpp colours and CD quality sound) and
  • an IOMD chip that provides high-speed buffered serial/parallel input and output as well as memory-mapping.

The machine can be expanded almost infinitely: If you need to fit more than the two expansion cards the base model can hold, you can add a second, third and fourth slice, each of which can contain two more expansion cards and provides another 5¼" and another 3½" bay. (See the pictures below.) In practice, however, few people used more than two slices, which means the backplanes with more than 4 slots are extremely hard to come by - if at all. For the early RiscPC models, you usually also had to upgrade to a more powerful PSU when adding the second slice.

[RiscPC] [4-slice RiscPC] [2-slice RiscPC] [486 card for the RiscPC]

Probably the most remarkable feature of the RiscPC are its two processor slots: When upgrading your processor, you just need to replace a small processor card instead of the whole motherboard as with previous Acorn machines. This way, processor upgrades were quite cheap. (StrongARM upgrade: fivefold performance for £99!) The second slot may be used for another processor. Theoretically, things like DSPs and MPEG decoders could be connected, but the only available cards for the second slot are x86 Intel processors. Running Windows (but not OS/2) and Acorn's own RISC OS on the same machine at the same time, the processors share all the computer's resources, like memory, discs and I/O ports.
(By the way: It was really nice to see Windows run inside one of the windows of the RISC OS desktop, at a time when virtual machines on personal computers were unheard of.)

Acorn also introduced the A7000 (followed later by the A7000+), a cut-down version of the RiscPC with only one processor card slot, only one SIMM slot, VRAM soldered to the motherboard and a lower price. There are some improvements over the RiscPCs, most notably the integrated Floating Point Accelerator and support for EDO RAM. The downside is, in my opinion, the dull design of the case.

The first RiscPCs were sold with RISC OS 3.5, the last Acorn-supported versions are 4.0x.


The concept of the NetComputer was to offer low-cost PCs to people who had not previously owned a computer. NCs were envisioned as a kind of thin client which relied on Internet connectivity for much of its functionality. Some models were expected to connect to a TV to avoid the cost for a monitor.

Acorn built the reference NC model for Oracle, one of the companies driving the NC initiative. After Acorn introduced their NC, numerous other companies also designed their own NCs, but, remarkably, a fair number of these used ARM instead of Intel processors.

The Acorn NC models, just like the NCs of other companies, were not particularly successful. In the mid-1990s, the concept may just have been ahead of its time. These days, netbooks (small, low-cost notebooks) have filled the niche that NCs were targeting.

The CoNCord shown on this picture is the fastest NC and also the one with the most unusual design. (Rumour has it that this CoNCord was only a mock-up, but other models were real.)

Prototype machines which were never produced

Acorn developed machines that would only have been produced if someone had ordered large quantities of a model - this did not happen. They also designed a high-end desktop machine, then cancelled the whole project...

The Stork sub-notebook comes either with a monochrome LCD screen (like on this picture) or with a TFT screen, but can also connect to standard monitors. A docking station allows you to use it conveniently on your desktop. The Stork contains a harddisc, but floppy and CD ROM drives must be connected externally. Some nice details are its built-in trackball, the Freeze Mode which preserves memory contents for as long as five days with full batteries (this was before APM or APCI...) and the support for PCMCIA expansion cards. The computer weighs only 1.8 kg.

This notebook almost made it to production. Allegedly, an American company had already ordered a large number of machines, but withdrew later on.

The NewsPad is the result of Acorn's taking part in the European Union OMI-NewsPAD project (OMI = Open Microprocessor Initiative). Basically, the NewsPad machines are designed to replace ordinary newspapers, but of course they can do a lot more than that. The specification is quite similar to that of the Stork (harddisc, docking station, Freeze Mode etc.), except the NewsPad has a touch-sensitive screen, no keyboard (you can connect one to the docking station) and support for a bi-directional infrared link and for video/sound digitizing. The NewsPad weighs 2 kg.

One cannot help comparing this to much later devices like the Apple iPad. Again, it seems that the idea was ahead of its time.

The Phoebe RiscPC 2 workstation

The RiscPC design has a few shortcomings: It doesn't support newer technologies like EDO RAM and E-IDE, and the internal IDE bus only allows you to connect two devices. Additionally, the whole architecture was designed for the ARM610 processor running at 40 MHz, so when 202 MHz StrongARMs became available, the low bus speed suddenly represented a bottleneck which reduced the speed of the processor significantly.

These problems were addressed by Acorn in the design of a new desktop computer, Phoebe, which had the following features:

  • One processor soldered to the motherboard, with the option to add another one on a daughter board.
  • Higher bus speed of 66 MHz (128 MHz in PC terminology). The RiscPC bus is so slow at 16 MHz (32 MHz in PC terminology) that a 202 MHz StrongARM using the new bus will be nearly twice as fast straight away.
  • Support for SDRAM, E-IDE, 230kbps serial connections, about 200 MHz video bandwidth, MIDI in/out and sound sampling, PCI, IRDA, and Wintel compatibility through a special PC card
  • Very nice case (in my opinion), designed by the same company that also designed the Zip drive for Iomega. The colour of the case caused a quite heated debate on the Acorn newsgroups...

[link to large Phoebe picture] [Phoebe alone in all her yellow glory] [with monitor and Logitech mouse/keyboard] [with monitor and Logitech mouse/keyboard]

As mentioned before, Acorn decided to abandon the whole project only two months before its completion, at a time when prototypes were already up and running, although not at full speed. Even worse, they decided to discontinue all support for the RISC OS desktop market. Subsequently, several companies producing software and hardware for Acorns set up RISC OS Ltd, a company whose goal it was to license RISC OS from Acorn/Element 14 and to continue with its development.


With only so few machines made, surely there is very little software around!

Not really! Because RISC OS had been around for quite a few years (since 1988), there were many programs for it. It is true that on the PC you could usually choose between 20 programs doing the same thing whereas there were only two or three for RISC OS, but the quality of the programs was generally higher.

Looking at the commercial market, there were numerous companies (mostly in the UK) that developed for RISC OS. The software prices were about the same as on the PC market, and due to the much lower numbers of copies sold, the support was often excellent, with programmers available for contact over the Internet. Practically all major software (e.g. painting packages, word processors, spreadsheet) contained import/export filters to allow data exchange with PC programs.

After Acorn abandoned the desktop market in late 1998, development of new commercial software mostly stopped.

The games scene was not particularly lively, though some of the most popular PC games tended to find their way onto Acorn screens two to three years after their inital release on the Windows/Intel platform (See Acorn Arcade for much more information on games!). The demo scene was also rather small, but there really were a few good coders out there!

An argument in favour of RISC OS is the large amount of Freeware and Shareware that is available for it. Apart from ports of Freeware programs written in C (like PGP, PovRay, TeX, GNU C compiler, InfoZip and RasMol) there are excellent free text editors, Internet applications (browser, newsreader etc.) and painting and drawing programs, to mention only a few. There are also good programs by Acorn: An image conversion program, a drawing program, a complete Internet stack, a video player and more.

All in all, I was content with the available software. There was one caveat though: Unless you lived in the UK, you really needed Internet access if you always wanted to be up to date. (There were magazines and PD libraries, but most of them were in the UK.)

Operating system

Oh, such a small company will never have the resources to develop a decent OS for their machines!

You just might be surprised if you gave it a try. At the time it was introduced, RISC OS was very competitive compared to its rival operating systems on other platforms. If you happen to stumble across an Acorn, just try it out!

Actually, there are several operating systems for Acorn computers: RISC OS is the one designed by Acorn for their own computers. Additionally, OSs have been ported to the Acorn platform: ARMLinux, RiscBSD, and RISC iX by Acorn themselves.


Here is a quick overview of Acorn's own operating system. Many "features" may hardly seem worth mentioning from today's point of view, but remember that RISC OS 3 was released in 1991, one year before Windows 3.11, and even its predecessor RISC OS 2 from 1989 had many of these features!

  • Single-user, co-operatively multitasking, but not multithreading (you can multithread within one task with the help of an extension module).
  • Provides a desktop with window environment. An icon bar shows icons for filing systems and programs. The Task Manager module lists all tasks together with the memory they take up and allows you to alter the amount of memory for applications that let you.
  • The OS is not loaded from disc, but comes in 4 MB of ROM. This saves you a lot of RAM, makes the machine more invulnerable against viruses, allows for machines without harddiscs (predestined for networks) and makes booting very fast - the minimum is about 3 seconds with a StrongARM processor! Replacing old parts of the OS without copying all of it to RAM is also possible: The ROM is subdivided into 4k pages, each of which can be replaced by a page in RAM. Alternatively, you can also replace one of the over 100 modules making up the OS.
  • RISC OS is only available on ROMs containing the British version. However, you can download the German version from Acorn's web site. This German RISC OS replaces all text inside RISC OS, but not the code - it only needs 350 kBytes of RAM to 'patch' 4 MBytes of ROM. As far as I know, RISC OS has only been translated into one other language apart from German, namely Welsh.
  • Consistent look and feel across all applications. In part, this is due to the OS providing many routines to easily implement it this way, and in part to Acorn's efforts at setting up very useful rules about how a program should behave. (E.g. what names the mouse buttons have and what effect they should usually have - the whole Style Guide is 130 pages long.) The result is that for any new programs you get, you hardly need to peek inside the manual - it's all self-explanatory.
  • Modules extending the OS (e.g. internet stack) can be loaded or removed any time, not just during booting.
  • RISC OS has always supported what Bill Gates had the nerve to call "Plug & Play" - unlike with PCs of that era, it is never necessary to configure interrupts etc. before an extension card works.
  • All filing systems (CD, HD, Floppy, ROM, RAM and soft-loaded ones) also install icons on the icon bar, which allow you to access them quickly. The RISC OS equivalent to Microsoft's Explorer is simply called the Filer. It opens a new window for each directory. Copying/moving files is achieved by dragging them from one directory to another. Similarly, to save a file from an application, you just have to drag the file icon to a directory window. Drag & Drop will also work between applications, e.g. you can write some text in a text editor and then directly 'save' it to a word processor window without saving to file or to some clipboard.
  • Nice window design. The excellent Outline font manager anti-aliases fonts in real-time as it draws them to the screen. You can choose any font for your system font, and in contrast to older Windows versions, those fonts are very readable even on a low-resolution screen... A special mode even allows anti-aliasing to work with multicoloured backgrounds:

    [Sample of font manager output]

  • Printing has been implemented in such a way that you do not need a new printer driver for each program you buy. Instead, programs print by making calls to the OS which will turn the graphics primitives either into PostScript or into bitmaps and send them to the parallel port, to a file or over a network. You only need one printer driver to allow all programs to print.
  • You can change the screen resolution and number of colours at any time.
  • A very fast BASIC interpreter (BBC BASIC) is supplied as part of RISC OS. Using it, you can create programs running in the desktop - you need not buy any expensive development software. The interpreter also contains an excellent ARM assembler. ("Yuk, BASIC?!" - Well, this flavour is fun to program!)

There is another remarkable detail that I would like to mention: As of version 3.5, RISC OS has been supplied with an anti-virus program (also in ROM) which prevents viruses from spreading - thus, older viruses do not spread at all any longer on new computers! (By the way: This anti-virus program is possible because there exist relatively few viruses for Acorn computers - about 150.)

On the other hand, there are also some flaws in RISC OS, most notably:

  • No support for multithreading, although additional, free software allows this.
  • Virtual memory is supported, but only with a commercial product from a third party, not as part of the OS.
  • Memory protection is almost non-existent, programs can (and sometimes do) take down the whole machine. This is especially true for "modules", which are considered OS extensions and run in a privileged processor mode.
  • In general, no major development of the OS has taken place for years. (Acorn did invest a lot of resources to make it use the capabilities of the RiscPC machines and then again to make it work with StrongARMs, but not much has changed for users.)

It is sad that this mature operating system (Acorn claim that they have over 500 man years experience in developing for the ARM processor) is only known to relatively few people...


Here are some screenshots of my StrongARM RiscPC running the RISC OS Wimp at 800 × 600 with 15bpp colour. By the way: The window border design can also be changed to whatever you like, except the defaults are nicer than most of the replacements I have seen.

[Screenshot] The RISC OS desktop, with a Filer window in the top left and the TaskManager window in the top right corner of the screen, and the free newsreader Messenger as well as a browser running. ARMLinux is also installed on the machine; the icons of its boot loader and the harddisc partition are on the pinboard. The DJ400 icon on the icon bar is the printer manager.
[Screenshot] The text editor Zap in HTML and C++ mode. The Internet stack is running and Acornet, a collection of free Internet programs, is loaded. The icon bar at the bottom is just filled with program symbols - if there are too many to fit, it starts to scroll sideways. As you can see from the Filer windows, under RISC OS dots are used to separate directory names, and if there are extensions, they are introduced with a slash - this can be a bit confusing if you are used to DOS/Unix filename conventions.
[Screenshot] The desktop with the commercial DTP program Impression Style in memory. Note that the FontManager enables all programs to use the high quality anti-aliased fonts - once you are used to this, working under Windows or X will inevitably make you think that something is missing... Additionally, a desktop solitaire game is loaded (ummmm). The little window titled ChangeFSI is a pop-up menu; different menus pop up depending on where the mouse pointer is when you press the middle mouse button.