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.