Acorn Archimedes

The Acorn Archimedes is, or rather was, a range of computers produced from the late 1980s to mid 1990s by Acorn Computers ltd. of Cambridge, England. The successor to the original 6502-based BBC Micro, the Archimedes machines were designed around the original production 32-bit ARM2 processor and its chipset.

Initially released in August of 1987, it was the world's first microcomputer with a RISC processor; and this was at a time when RISC was still considered a valid design philosophy rather than a worn-out buzzword.

Operating systems running on Archimedes hardware range from the original stopgap Arthur operating system, through RISC OS (the definitive Archimedes OS) to ARM Linux, NetBSD, via such interesting waypoints as RISCiX and Nemesis.

Evolution of the Archimedes

The Archimedes specification and design evolved from various commercial failures, the Cambridge Workstation version of the unsuccessful Acorn Business Computer in particular. Staff at a short-lived Acorn research facility in Palo Alto were, inspired by work at Xerox, working on an operating system known as ARX, which was to be the operating system for the successor to the Acorn Business Computer. As can probably be guessed from the 'research' label, no working operating system ever emerged. Looking to cut their potential losses, Acorn approached the BBC with their designs, and a plan for a stopgap operating system (Arthur), offering the system as the true replacement for the BBC Micro.

"The BBC said it was interested, so we put some red function keys on it and changed the case colour. Our original research for the office automation machine had said it had to be grey."

-- Roger Wilson

Production Systems

The BBC liked the system, and so with a market assured, Acorn put the new machines into production. The initial product line consisted of the A305 and A310, essentially the same systems but with 512k and 1Mb of RAM respectively. The A410 and A440 were similar in design but included an on-board ST506 hard disc controller. The A410 had 1Mb of memory on board, the A440 had 4Mb, and a 20Mb hard disc drive. These were later revised, by the addition of larger hard discs and a faster version of the memory controller chip MEMC1a, to become A410/1 and A440/1. At the same time, the A305 and A310 were dropped, and the low-cost A3000 introduced (which, like the BBC Micro, and its contemporaries the Commodore Amiga and Atari ST, had the mainboard, disk drive and keyboard integrated into a single unit]).

The Archimedes Chipset

The Archimedes chipset used a unified 8MHz bus architecture, with a single main bank of RAM containing all program and data memory, as well as video and sound buffers. DMA to video and sound was handled by the VIDC (VIDeo Controller) chip, in conjuction with the MEMC (MEMory Controller, also known as 'Anna') chip. The MEMC provided basic DMA facilities, memory timing and refresh, controls, as well as an inverse page table memory mapping system, allowing sophisticated memory management of up to 4Mb of fast page mode DRAMs. This 4Mb limit quickly became painful to Archimedes owners. Many machines could handle more than 4Mb, but only with the addition of a separate MEMC chip for each additional 4Mb of RAM. The VIDC (also known, for reasons probably lost in the annals of time, as 'Arabella') could produce 12-bit colour outputs, and had a set of palette registers which provided 16 palette entries to 4-bit (16-colour) video modes, or provided part of the palette data for 8-bit/256-colour video modes. The fixed part of the palette data for 256-colour modes gave a very well-rounded display, and with a little colour dithering (built into RISC OS) was almost indistinguishable from most 16-bit displays.

The chipset was rounded out by the IOC ('Albion'), or Input/Output Controller, which provided miscellaneous input/output functions, timers and timer comparison interrupts.

The Last Archimedes

In 1990, a new flagship machine took over from the A440/1, namely the A540. The main new feature of this machine was the use of the new ARM3 processor instead of the ARM2 used in previous machines: this ran at 25 or 33MHz, and featured a unified 4Kb level 2 instruction/data cache; theoretically, it made the machine up to six times faster than the 8MHz ARM2s. This magnitude of speedup was frequently realised in practice, helped by the density of ARM instructions, and by reducing the CPU's dependency on main memory bandwidth also required for tasks such as video DMA. The A540 also featured (naturally) a larger hard disc, and an option to install an additional MEMC memory controller chip, giving access to up to 8Mb of RAM.

1991 saw the release of the A5000, which was the first machine to make use of a revised chipset, which replaced the MEMC and IOC chips with an IOMD (Input/Output and Memory Device) chip, and also came with a much revised version of RISC OS.

With the A5000, the 'Archimedes' moniker was officially dropped, making the A540 the last true 'Archimedes' machine; but since the product line continues, so shall I...

Hot on the heels of this machine was the A4, Acorn's only RISC OS laptop to see the light of day. Its design was based on an Olivetti laptop chassis with a 640x480 grayscale LCD monitor, and the same chipset as the A5000. A commercial failure due to its high price and lack of flexibility; although this could be said of almost any Acorn machine since the BBC Micro, it was particularly true of the conspicuously overpriced A4.

Mass Market Failures

The next machines, announced in late summer 1992, were aimed at reducing costs, both retail and manufacture. The basic Archimedes chipset had fallen behind the times. Silicon processes were shrinking while Acorn used essentially the same chips, without dramatically improving performance. The shrinking process sizes, however, gave an opportunity for a significant cost saving without extra work to further improve performance. The entire chipset, ARM2, VIDC, MEMC1a and IOC were integrated onto a single chip, vastly reducing the complexity of system boards and the chip count. Also, since memory technology had improved in the interim, the system bus and system chip run at 12MHz instead of the previous 8MHz). This all-in-one system chip, the ARM250, was used in the A3010, A3020 (both models billed as replacements for the A3000, differentiated by the expandability of RAM, inclusion of an on-board IDE controller only on the A3020, and the fact that the A3010 had green function keys rather than the traditional red.) and the A4000 which was in appearance much like a slim-line A5000.

Then things got quiet for a good long time, until the RiscPC was unveiled in 1994. The RiscPC was a breakthrough in its day, and brought workstation class features to the home computer market: massively expandable memory capabilities, a multi-processor bus capable of accepting both multiple ARM processors or an Intel 486 as a second processor to run Windows software. Processor cards included the new ARM6 and ARM7, and plans were in place for ARM8, although these were largely scuppered by the introduction of cards for Digital Semiconductors' (now Intel) StrongARM processor. The RiscPC featured a much improved video controller chip, VIDC20 which could support far higher resolution and colour depths than the VIDC10's 256 colour limitation. For the first time on an Acorn RISC OS machine, the video subsystem had its own private memory and data bus, which allowed higher bandwidth displays to be used without impacting the processor's performance unduly.

The weak point of the RiscPC's design, however, was its cripplingly slow CPU and system bus. The bus was still 32 bits wide, and ran at 16MHz. In comparison with the baseline Pentium system's bus of 64 bits and 33MHz, this just wasn't sufficient for a scalable system, and in particular was not sufficient to adequately support the 2-way multiprocessing which the bus architecture was capable of physically supporting.

The last machine to make it out of Acorn's workstation division before they closed completely was the A7000. This was similar in many ways to the A4000, being a small-footprint desktop case with an integrated system chip, the ARM7500 (or ARM7500FE with floating point accelerator). Several models were released, some intended as diskless network nodes, all models having ethernet interfaces built in.

Shortly before the closure of the workstation division, the development of the RiscPC's successor, named 'Phoebe' (Yes, after the character from Friends...) was almost completed. This machine featured standard PC components where custom ones had previously been required, and included such joys as PCI slots, proper 16-bit sound, and a yellow system case. None were ever sold.

And there ends the story of one of the greatest, most innovative lines of home computers ever built, and never sold.

Despite being relatively small compared to their industry peers, Acorn Computers were an innovative team. From their headquarters in Cambridge, they had already released the Atom, before following up in 1982 with their hugely successful Proton, otherwise known as the BBC Microcomputer. They had also branched out into new markets with the ABC, or Acorn Business Computer.

The ABC was not an astounding success, and certainly didn't meet the company's expectations. Nor did the Acorn Electron - a stripped-down BBC Micro intended for home use. While demand for the Electron was high, difficulties in producing the custom microchips used in its design meant few were sold - and, once stocks had risen to satisfactory levels, demand had long peaked.

One of the great strengths of the BBC Micro was the Tube, another Acorn development. The Tube was a fast bus that allowed for many different second processors to be installed, such as a Z80 or 80186. This allowed the BBC Micro to run different OSes, such as CP/M. Much of Acorn's development at this stage was focused on running more and better hardware off the Tube, but Acorn had seen the need for a powerful new solution if they were to succeed in the market from as early as the launch of the Atom in 1980.

The venerable 6502, upon which the Proton was based, was by 1983 quite long in the tooth. Acorn wanted a powerful, flexible new computer system, complete with a graphical user interface such as those Xerox had been experimenting with. Having asked Intel for 80186s to use, they had been turned away - and the 68000 was underwhelming.

The breakthrough came when Acorn engineers read the Berkeley RISC documents. The decision was swiftly made: if off-the-shelf processors don't make the grade, roll your own. Sophie Wilson, Acorn's virtuoso engineer, set to work, writing the processor's instruction set on a twin-6502 Beeb.

After the company was bought by Olivetti, full-scale efforts towards producing the new RISC processor - dubbed the Acorn RISC Machine, or ARM - were launched. VLSI Technology, a firm Acorn already contracted to produce various chips, such as ROMs, were asked to produce an ARM. On 26 April, 1985, the processor was delivered to Acorn - and, perhaps an omen of future success, it worked first time. The ARM1 was ready.

Using the ARM1 via the Tube as a co-processor in BBC Micros, the ARM2 was swiftly designed. The ARM2 was a 32-bit microprocessor, with 32-bit bus and 26-bit address space. The ARM2 was also Tube-compatible, and so it was decided that it should be implemented in development machines for the new Acorn computer: the Archimedes. The ARM2 was a top-secret project still at Acorn: even when courting Olivetti, they hadn't let on that it was under development.

The ARM Development System was released in 1986, at £4,000. A Tube-compatible co-processor for the BBC Master, it boasted an ARM2 which ran rings around the 286. It also came with three support chips, and 4MB of RAM. Acorn continued development of the Archimedes; now their wünderkind was ready, in the shape of the ARM, they needed the OS to run on it. This task was given to the Acorn Research Centre, or ARC, at Palo Alto, California.

Work on the OS, known as ARX, was slow. Acorn's goal of a UNIX-like OS would have had a truly modern platform, capable of pre-emptive multi-tasking, multi-threading, and multiple users. However, by 1987 it was clear that ARX would not be ready in time. A stop-gap was sought.

Hurridly, using the legendary BBC Basic which Sophie Wilson had ported to the ARM (which led to truly fast execution times), an OS was written, complete with GUI. With a gaudy, bright colour scheme reminiscent of early Commodore Amiga Workbench screens or Windows 2, it nonetheless got the job done. It featured an icon bar, displaying devices and applications, and windows for each currently-running application. Its name: ARTHUR, commonly believed to stand for "A RISC OS before THURsday".

Now Acorn had their hardware and software ready. In June, 1987, their ARM-based computer was launched: the Acorn Archimedes. The Archimedes, or Archie as they were affectionately known as (or, more often, simply Arc), came in four models to begin with, all equipped with an ARM2. The A305, with only 512k RAM and two expansion slots, was complimented by the A310 with a full megabyte. At the upper end, the A410 boasted 1MB RAM but four slots, whilst the top-of-the-line A420 boasted not only four slots and 2MB RAM, but a whopping 20MB hard disk. All four shipped with ARTHUR in ROM. Later, the A440 and A540 were released, with 4MB RAM and a 40MB hard drive, and 4MB and a 100MB hard drive, respectively.

The Archimedes was a desktop system, with the actual computer designed to sit on a desk, the monitor atop that. A keyboard plugged into this, and into that a three-buttoned mouse. The OS, to allow it to run on all systems (and not just those with hard drives), was burned onto ROMs. This allowed for swift boot times and operation, but was more difficult to upgrade. Nevertheless, Acorn began work on a proper, much less buggy OS for the Archimedes. The result of this was the RISC OS.

Originally to be named Arthur 2, this idea was scrapped when a film with a similar name was released. RISC OS 2 saw release in 1989, available on a set of ROMs to replace the original ARTHUR. RISC OS is a fast and easy-to-use OS, which featured many ideas ahead of their time. All applications are provided as folders, which carry all their resources. Double-clicking one in the Filer will run the app, whilst holding Shift when double-clicking will open the app's folder. All fonts are antialiased - a miracle in 1989. A command line could be accessed at any time just by tapping F12.

To go with the new OS, new hardware was designed, leading to the launch of the A3000. Still sporting the BBC branding, the Acorn Archimedes A3000 was an all-in-one unit like the Atari ST - floppy drive, computer and keyboard in one unit. The A3000 was an astounding success, with a powerful 8Mhz ARM2 and 1MB RAM, and sold well to both home and educational markets. The A3xx series was phased out.

ARM development hadn't stalled while Acorn worked on RISC OS - instead, they had produced a third iteration of the hardware. The ARM3 was a fast 25Mhz processor, and was used initially in the high-end A540. A new desktop machine, the A5000, soon saw launch, with an ARM3 to power it. This was again a traditional desktop case, and offered 4MB of RAM and a 40/80MB hard drive. Complimenting this was another version of RISC OS - RISC OS 3.0.

By now, Acorn had established dominance in the educational market. At home, they maintained a small, if reasonable marketshare - at least in the Commonwealth. Acorn weren't resting on their laurels, however, and decided in 1991 to continue to evolve their product lines. The A3000, with its ageing ARM2, was axed, replaced by two new derivitives: the A3010 (aimed towards home use, with a video port to connect to a TV set and a joystick port) and the A3020 (intended for educational use). The ARM2 was swapped for the ARM250 (literally ARM 2 and a half), which boasted higher clock speeds, and integrated some of the custom circuitry of the A3000.

The A5000 was joined by the A4000, a lower-profile machine. RISC OS 3.1 was now standard on all machines, the last major revision of the OS until the release of the Risc PC in 1994. Acorn's Archimedes line served them well for the late-80's and early 90's, powerful machines with a wonderful OS. However, Acorn's terrible efforts at marketing prevented them from gaining much acceptance outside of education, which would prove to be their downfall.