Original URL: http://www.theregister.co.uk/2012/06/01/acorn_archimedes_is_25_years_old/

Strong ARM: The Acorn Archimedes is 25

The first ARM computer

By Tony Smith

Posted in Vintage, 1st June 2012 06:00 GMT

Archaeologic The Acorn Archimedes is 25 years old this month. The first machines based on the company's ARM (Acorn Risc Machine) processor were announced in June 1987, the year after the 32-bit chip itself was launched.

Four versions of the Archimedes were released in 1987: the A305, A310, A410 and A440. The first two had 512KB and 1MB of memory, respectively. You could upgrade an A305 to an A310 simply by adding in the extra Ram.

Acorn promotes Archimedes 300 series

Acorn's Archimedes 310

The A410 had 1MB of memory too, but the A440 had a (then) whopping 4MB and came with a 20MB hard drive as well as the 800KB 3.5in floppy drive – which also supported 640KB discs for BBC Master compatibility – found on the other three models.

Upgrading the A305 or A310 to A410 level was a matter of adding in a "Podule" backplane circuit board, which contained the hard drive controller. You also had to add, of course, the hard drive. There was room for two Podules on the A300 series.

The new BBC Micro

The A300 series was pitched at home users – essentially as direct replacements for, respectively, the BBC Micro and 1986's BBC Master; the two A300 models had "British Broadcasting Corporation Microcomputer System" stamped on the keyboard, along with the familiar owl logo. The pricier A400 series were pushed at businesses and technical organisations. Their Archimedes of choice was quickly revealed to be the A440, a machine that would have set them back a colossal – now, let alone then – £2299. The own-brand colour monitor was £200 on top of that, though the monochrome screen was just 50 quid.

Despite the price, and that fact that it didn't become available until the Autumn of 1987, the A440 proved to be the most popular of the the two, and by early 1988 the A410 began to disappear from Acorn's marketing literature.

Acorn promotes Archimedes

Home computing in the late 1980s, Archimedes style
Source: Acorn Archimedes 300 series brochure

By the time of the A440's arrival, the A305 and A310 – priced at £799 and £875, respectively – were winning positive reviews after going on sale in late July/early August 1987.

The microcomputer bible of the time, Personal Computer World, reviewed the A500 – the prototype on which the shipping Archimedes were based, and writer Dick Pountain said it "felt like the fastest computer I have ever used, by a considerable margin".

Strong ARM

Powering the A305 and the other Archimedes was the 32-bit ARM Risc (Reduced Instruction Set Computing) chip, developed by Acorn as the replacement for the 6502 processor it had implemented in its Atom, BBC and Electron micros during the early 1980s.

Such was the power of the ARM, wrote Pountain, that the Archimedes' software sprites could bounce around the screen as smoothly as the hardware-managed sprites offered by the likes of the Amiga and Commodore 64.

David Braben's Lander from the Archimedes demo disk

From the Archimedes demo disk: David Braben's Lander... later known as Zarch

The Acorns shipped with a demo game written by David Elite Braben simply called Game 1 – it was also labelled Lander and was the foundation for Braben's Zarch game. You flew polygon-built ship over a 3D landscape, complete with real physics and particle effects. "Astonishing," Pountain called it.

"Nothing," he wrote, "could better illustrate the way a quantitative jump in processing power can lead to a quantitative step in applications. You just can't do this stuff on an [Intel] 8088, and it's doubtful on a [Motorola] 68000: the hefty calculations of landscape perspective and particle motion for each scene need to be performed in less than the screen refresh period or the illusion collapses."


Here, though, the illusion was maintained, all thanks to the power of the ARM. It contained 27 32-bit registers and could handle 44 instructions, the majority of which excuted in a single clock cycle. The CPU ran at 8MHz. It was attached to separate memory, video and I/O controllers, all designed by Acorn. The I/O chip was codenamed 'Albion'.

The VIDC – codenamed 'Arabella' and designed, like the other ARM auxiliary chips, by Tudor Brown, Mike Muller and others – supported colour resolutions of up to 640 x 512 – 16 colours; if you wanted 256, you had to drop to 640 x 256 – but could reach 1024 x 1024 if you were happy to adopt a single colour. It also handled sound output.

Inside the Archimedes A440. Source: Chris' Acorns

Inside the Archimedes A440
Source: Chris' Acorns

Novelly, the MEMC memory controller – 'Anna' – was able to support 32MB of memory – far more than it was economically feasible to add, so it was able to page the 4MB of physical Ram on and off a hard drive to provide rudimentary virtual memory, something we take for granted today. Ditto the application memory protection, a feature modern multitasking operating systems manage for their users.

Back then, the Archimedes' OS was called Arthur, only renamed RiscOS the following year, allegedly to avoid confusion with the movie Arthur 2: On the Rocks, also released in 1988, though this seems very unlikely.

Arthur is said to have stood for "A Risc operating system before THURsday".


The first machines shipped with Arthur 0.2, rapidly followed by 0.3 and soon updated to 1.2. Arthur was developed by a team headed by Paul Fellows – their work would be continued by William Stoye's group after Arthur became Risc OS. This being the era of Rom storage – of which the Archimedes has 512KB, on four chips – updating the OS required opening the machine's casing, removing the current Rom chips and replacing them with new ones.

Curiously, Acorn wanted its Arthur 0.2 Roms back, but users were free to bin chips holding version 0.3. Does anyone know why?

Arthur UI. Source: Matthew Pye

Arthur's Archimedes Desktop Manager
Source: Matthew Pye

Arthur comprised two parts: one for the machine itself, the other, separate, for the storage. That allowed Acorn to offer multiple filing systems. Two were supplied with the original Archimedes: ADFS (Advanced Disk Filing System), a revised version of the ADFS that shipped with the BBC Master, and ANFS (Advanced Network Filing System), to be used to access server-hosted files over Acorn's Econet network tech. Users could switch from one to the other and back again at the command line.

Eschewing filename extensions, Arthur was coded to map files to applications, allowing you to key in a filename and have the correct software load it up for viewing, editing or running.

Windows, Icons, Mouse

Unlike others operating systems, most notably MS-DOS – by now six years old – and CPM (even older), Arthur could load multiple files to different memory addresses.

Not that most users delved into this command-line jiggery pokery – the Archimedes hid it all behind its own colour graphical user interface, called Archimedes Desktop Manager, of the kind already seen on the Mac and in the early forms of Windows and Gem, but here written in… BBC Basic.

Advertising Risc OS. Source: Wikimedia

Advertising Archimedes' GUI
Source: Wikimedia

The Archimedes had a mouse too, and one that followed the WIMP UI pioneer Xerox's three-button layout: Select, Menu and Adjust.

To maintain compatibility with the BBC micros, Acorn equipped the Archimedes with 65EMU, a 6502 emulator. To offer Beeb buffs some extra familiarity, the A300 series' keyboard function keys were coloured red, as per the BBC Micro.

Archimedes experiments

The A300 and A400 machines would be Acorn's mainstay for the next two years. May 1989 saw the debut of the last BBC-branded machine, the BBC A3000 – in a new, wedge-shaped casing with an integrated keyboard – and, a month later, Acorn revamped the A400 series with an updated memory controller, upping the A440's hard drive to 40MB and introducing the 20MB A420 in the process.

The Archimedes A3000. Source: Chris' Acorns

All around the Archimedes A3000
Source: Chris' Acorns

Earlier that year, Acorn rolled out the R140, its first machine with the new Risc iX operating system – Unix on ARM, essentially. This would be followed by The R225 and R260 in June 1990, but they failed to win the support that Arthur – by now called Risc OS – had won, not least because of the price differential: at launch the R260 cost £5000, though that bought you 8MB of Ram and a 100MB SCSI hard drive. The A540 had one of those too, and while it only had 4MB of Ram, it nonetheless cost half as much as the R260.

Acorn killed off Risc iX in 1992. By then it had released the A5000, in September 1991. It had a 25MHz ARM 3 chip, later upgraded to 33MHz. Like the original Archimedes, it had separarate keyboard and system units, though the latter now looked more like a bog-standard PC. It was the professional-oriented successor to the A400 series

The A3000's wedge casing was retained for home and school users, and formed the basis for September 1992's A3010 and A3020. The A5000 was revised with an ARM 250 chip to become the A4000 at that time. The A3000 series had ARM 250s too, though the first A3010s went out with ARM 2s.

Acorn in decline

But by now the Archimedes family was more than five years old, and while it had built up a huge following in schools, even they, like many of its early users, were turning to other platforms, most notably the now ubiquitous Windows PC.

Archimedes morphed into the Risc PC line, a series of ARM-based boxes designed to run Microsoft Windows, or any other OS, on a x86 co-processor, and present it in a Risc OS desktop window.

Acorn Risc PC

Windows play: Acorn's Risc PC

Acorn spun ARM off as a separate business in November 1990 and then opened up shares in the subsidiary in 1998. The IPO brought Acorn £18m but it lost £9m during the first nine months of 1998, and the strategic review this prompted centred on killing off the Workstation division.

Acorn, one of the two key pioneers of UK home computing, would no longer make desktop computers. It would focus instead on Digital Signal Processor (DSP) chips and set-top boxes. By 1999, it wasn't even called Acorn any longer. It became Element 14, though the name lasted only a year, dropped when the company was acquired by Broadcom to become the comms chip giant's DSL division. ®

The author would like to thank the many retro-tech fans for archiving home computing adverts, photos and documentation from the 1980s and 1990s, without whom this feature would not be possible. Special thanks go to Chris Whytehead for his Chris' Acorns site, which provided invaluable source material. ®

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