Acorn Archimedes Computers

Except for its earliest models (see Acorn 8-bit Computers), Acorn had built its computers around the 6502 microprocessor, which was also used by Apple, Atari, Commodore, and others. Seeing the end of the 8-bit era approaching, Acorn knew that it was time to move to a new architecture

Acorn computer line

Unable to find an existing solution that would match the speed of 6502-based machines, Acorn decided to adopt the Berkeley RISC concept in a processor of its own design. The RISC concept is a Reduced Instruction Set Computer, where the CPU would be optimized for a limited set of instructions that it could perform very efficiently.

The Acorn RISC Machine (ARM) project officially began in October 1983, although advance work had already been done on the RISC concept. The first ARM chip (a.k.a. ARM1 was produced on April 26, 1985, and it worked perfectly the first time it was powered up. The 32-bit ARM1 was used as a co-processor in the BBC Micro to run simulations and CAD software so the RISC team could design support chips for the new CPU as well as the ARM2 CPU.

It turns out that Acorn’s fledgling RISC processor was not only far more powerful than the Motorola 68000 used in Amiga and Atari ST computers, but it was also remarkably energy efficient, which is a big reason later versions of the ARM design became so popular in handheld devices – including most of today’s smartphones.

1983 and 1984

Especially in the US, 1983 had been a very trying year for the home computer industry. IBM had taken over the business world, and Commodore was dropping the price of the Commodore 64 from US$595 to an eventual US$99 in big steps to flood the market. Commodore’s goal was to drive Texas Instruments out of the home computer market as revenge for TI destroying Commodore’s calculator business.

Commodore dropped the price of its entry-level VIC-20 so low that TI could only respond by selling its TI-99/4A at a loss – or give up on selling its entire inventory of home computers. Texas Instruments was the first casualty of the 1983 Home Computer Price War, and Atari became the second. In the end, Commodore hurt itself as much as anyone, and despite selling 10 million Commodore 64s, the company soon went bankrupt.

In an interesting and unexpected twist, Commodore got rid of its founder, Jack Tramiel, who went on to buy Atari from Warner Brothers in 1984. That was also the year that Apple introduced the Macintosh and nearly went bankrupt due to poor sales. Atari would go on to become a player in the next phase of the home computer market, primarily marketing its Atari ST series against the Macintosh and Commodore’s Amiga.

Acorn was also going through dire financial difficulties and became a subsidiary of Olivetti in February 1985.

Acorn’s First RISC Computers

The first ARM product was the ARM Development System, which was a BBC Master with an ARM as its second CPU. As the name implied, this machine would be used to develop software for Acorn’s forthcoming ARM-only computer.

Acorn Archimedes A3000

Acorn Archimedes A3000

The first ARM-based system designed for general use was the Acorn Archimedes, which came to market in June/July 1987. The ARM2 32-bit CPU had 26-bit addressing and ran at 8 MHz, which is comparable to the 8 MHz 32-bit 68000 CPU with 24-bit addressing used in Apple Macintosh, Atari ST, and Commodore Amiga computers.

The Development System had been designed for computing professionals, especially programmers, but the Archimedes was aimed directly at the consumer market – the very place the BBC Micro and Master had done so well.

RISC proved itself superior, with Archimedes taking the crown as the fastest microcomputer in the world at the time, able to achieve 4.5 MIPS (million operations per second), although some claimed 18 MIPS. The ARM2 processed about 3x as many instructions per clock cycle as the Intel 80386 and 40% more per clock cycle than the Motorola 68030.

Acorn/BBC Archimedes

Acorn released two families of Archimedes computers: Both had a 2-part case, a 3.5″ 800K floppy drive, an external keyboard, and a 3-button mouse. A cutting-edge feature was 8 channels of stereo sound, and 256-color graphics was standard. They shipped with Arthur OS in ROM (later replaced by RISC OS, which required swapping ROMs), BBC BASIC V, and a BBC Micro emulator.

The new computers used the VIDC1a video chip and supported a wide range of resolutions and color options:

  • 160 x 256 with 4, 16, or 256 colors
  • 320 x 256 with 2, 4, 16, or 256 colors
  • 640 x 256 with the same color options
  • 640 x 512 with the same color options
  • 800 x 600 with 2, 4, or 16 colors
  • 1152 x 896 with 2 colors (not officially supported)

Colors were chosen from a palette of 4,096 colors (12-bit, like the Amiga), except that in 256-color mode, 240 colors were fixed and 16 could be assigned in software.

The 300 Series, marketed as BBC Archimedes, did not include expansion slots, although a 2-slot backplane from Acorn could be added, and third-party 4-slot backplanes were also available. The A305 had 512 MB of system memory, the A310 a full megabyte.

The 400 Series, sold as Acorn Archimedes models, had four expansion slots and an ST506 controller for an internal hard drive. The A410 had 1 MB of RAM, and the A440 an impressive (for 1997) 4 MB.

Acorn added the A540 in September 1987, which included SCSI and the ability to work with a Genlock device to deal with video input and output. A 100 MB SCSI hard drive was standard, and it could be upgraded from 4 MB to 16 MB.

However, it was an uphill battle for Acorn. The Macintosh had been on the market since January 1984, the Atari ST since June 1985, and the Amiga since July 1985, giving them a 2-3 year head start. Even though Archimedes was over twice as powerful, it never developed a strong following beyond the UK market.

Archimedes: The Next Generation

In May 1989, Acorn introduced a new series of Archimedes models, the A3000 and A5000. The new models ran RISC OS 2, and improved version of the RISC OS available for earlier Archimedes models. Acorn also made RISC OS 2 ROMs available for its previous models.

The BBC A3000 ran an ARM2 at 8 MHz and included 1 MB of system memory. Unlike the A300, the A3000 was an all-in-one home computer design with an integrated keyboard. The A3000 had only one expansion slot, and it was physically different from the expansion slots used in the first generation of Archimedes computers. It was the last computer to bear the BBC name.

Electronically the signals were the same, so it was possible to build an adapter and use existing expansion cards in an external chassis.

The Acorn Archimedes A5000 shipped in 1991 with a 25 MHz ARM3 processor, 2-4 MB of system memory, and a 40 or 80 MB IDE hard drive. Like the original Archimedes series, this was a 2-part design with a detached keyboard. It was the first Acorn model to use a high-density floppy drive, storing up to 1.6 MB on a disk – a big step forward from 800K. A later version of the A5000 bumped the processor speed to 33 MHz, offered a choice of 4 or 8 MB of RAM, and could be purchased with an 80 or 120 MB hard drive.

The A5000 introduced RISC OS 3.0, but several bugs were detected, and Acorn soon moved to RISC OS 3.10 and then 3.11.

The Third Generation

Introduced in 1992, the next generation Archimedes was built around the ARM250 microprocessor, a single-chip design that provided the power of ARM3 with a built-in input/output controller, video and sound controller, and memory controller. The built-in cache was sacrificed for an external cache, and clock speed of the new processor was 12 MHz.

The A3010 was intended for the home market was a one-piece design similar to the A3000 but quite a bit more compact. It included a TV modulator and a 9-pin joystick port for gaming.

The A3020 was aimed at the home office and education markets and included a built-in 2.5″ laptop hard drive and a network interface port.

The A4000 was nearly identical to the A3020 but used a 3.5″ hard drive, which allowed higher capacity and speed.

All of these models could be upgraded to 4 MB, although in the case of the A3010 it required a third-party upgrade to overcome Acorn’s 2 MB limit.

Archimedes on the Go

Acorn also shipped the A4 laptop in 1992, running a 24 MHz ARM3 processor, so just the tiniest bit slower than the A5000. The A4 had a 640 x 480-pixel grayscale display and a monitor port that allowed it to be connected to a color display. What it lacked was a built-in pointing device (such as a trackball), so the user had to take along a mouse.

Beyond Archimedes


In April 1994, Acorn introduced its RiscPC with the RiscPC 600 running a 30 MHz ARM610 processor. The new model had two 72-pin SIMM slots for up to 256 MB of system memory. The RiscPC had a unique new case that could be grown by adding slices on top of the main box; each of these slices had two “podules” bays in the rear along with a 3.5″ and a 5.25″ drive bay on the front.

In 1995, the RiscPC 700 was launched with a 40 MHz ARM710 CPU, which was also available as an upgrade for the 600.

There was a huge leap forward in processing power in 1996 with the 200 MHz StrongARM upgrade card for RiscPC 600 and 700. And in 1997, the Acorn J233 StrongARM RiscPC pushed processor speed a bit further, to 233 MHz.

By now the memory bus was becoming a real bottleneck because it ran at 16 MHz and was originally designed for a 30 MHz CPU. After acquiring Acorn’s PC business, Castle launched the Kinetic RiscPC line in May 2000. With the new model was a new processor card that included memory, so memory access was no longer limited by a slow bus.


Acorn moved beyond the Archimedes name with the introduction of the A7000 in 1995, which was powered by a 32 MHz ARM7500 processor. Standard memory was 4 MB on the system board plus a single SIMM slot supporting up to a 128 MB SIMM for 132 MB total.

The A7000 series accepted one expansion card using the same bus as the Archimedes line. It shipped with RISC OS 3.60.

In 1987, the A7000+ shipped with a 48 MHz ARM7500FE processor. The A7000+ shipped with 8 MB on the logic board. After Castle acquired Acorn’s computer business, the Castle A7000+ was released, identical except for having a 56 MHz processor. The A7000+ shipped with RISC OS 3.71.

RISC OS 4 was available as an upgrade, and NetBSD and ARM Linux were available as alternative operating systems.

The End of RiscPC

In 2003, Castle Technology ended production of RiscPCs, nine years after Acorn had introduced the first Archimedes models.

However, RISC OS lives on, continuing to be developed by RISCOS Ltd. and the RISC OS Open community. Today you can run RISC OS 5 on a Raspberry Pi.

If you have used an Archimedes computer or RiscPC, consider joining the Acorn Archimedes group on Facebook.

Photos by Simon Inns, Creative Commons license, used with his kind permission.

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