In the summer of 1987 Acorn User announced “Acorn has announced its long-awaited RISC technology micros, to be called the Archimedes series.” The makers of the BBC Micro had designed their own 32-bit processor, the Acorn RISC Machine, or ARM, and built a computer around it. It ran BBC BASIC programs “between 15 and 25 times faster than a model B”, showed up to 256 colours, and came with a mouse-driven desktop of windows and menus.
A year later ACE introduced it as “the world’s first 32-bit home micro” and worked out why so few people had bought one. At upwards of £800 it cost far more than an Atari ST or an Amiga, and in its first year it had only eight commercial games. Most Archimedes went to schools. The processor inside it did far better: Acorn spun off the ARM team in 1990, and ARM designs went on into Apple’s Newton and, later, into most of the world’s mobile phones.
Fast facts
- Manufacturer: Acorn Computers, Cambridge.
- Announced: 1987. The first 2,000 A305 and A310 machines were due between July and September, with volume deliveries from September.
- Launch prices: A305 (512K) £975 with a monochrome monitor; A310 (1MB) £1,235 with a colour monitor, according to Acorn User in August 1987.
- CPU: ARM2, 32-bit RISC, 8 MHz in the standard system (a 24 MHz master clock divided down).
- Memory: 512K to 4MB in the first range; 26-bit addressing, a 64MB address space.
- Display: up to 640×512; up to 256 colours on screen, with a palette drawn from 4,096.
- Sound: 8-bit samples fetched by DMA, with up to eight stereo positions.
- Operating system: Arthur, replaced by RISC OS in 1989.
Sophie Wilson designed ARM’s instruction set, while Steve Furber led the hardware design. Her biography follows the software models and wider team’s work that preceded a working chip.
Four chips
Acorn’s technical manual describes the machine as “the ARM chip set”: the ARM itself, a Memory Controller (MEMC), a Video Controller (VIDC) and an Input Output Controller (IOC). The processor does the computing, and the other three chips share the rest of the work between them.
| Chip | What it does |
|---|---|
| ARM | 32-bit processor with a 32-bit data bus and a “26-bit address bus, giving a 64 MB uniform address space” |
| MEMC | Drives the DRAM, generates “all the critical system timing signals, including processor clocks”, and feeds video and sound by DMA |
| VIDC | Turns memory into a picture through a colour palette, and plays sound |
| IOC | Keyboard, timers, interrupts and the expansion bus |
The memory controller sets the processor’s pace. MEMC takes a 24 MHz master clock and produces the clocks for everything else. VLSI Technology, which made the chips, describes a system in which a “24 MHz clock yields a basic processor cycle of 8 MHz”, built so that cheap 120 ns DRAM could keep up. MEMC also translates addresses: it maps pages of physical memory into a 32MB logical space “with three levels of protection”, so the operating system can move memory around and keep programs out of places they should not touch.
The video chip reads memory by itself. VIDC fetches the screen from main memory “under DMA control”, so there is no separate video RAM and the processor never copies pixels to a display chip. The screen is simply memory, 1, 2, 4 or 8 bits per pixel.
What “256 colours” meant
The palette explains a detail that surprised programmers used to other machines. VIDC’s palette has only 16 entries, each 13 bits wide, which is where the 4,096 colours come from (4 bits each of red, green and blue). In the 16-colour modes a programmer can pick any 16 of the 4,096. In the 256-colour modes, VIDC uses the palette only for the lowest bits of each colour; the top bits come straight from the pixel value. So the 256 colours on screen are a fixed pattern that the 16 palette entries can only nudge, not 256 free choices. David Braben’s Zarch used them to shade its landscape darker with distance, something the 16-colour Atari ST version had to drop.
Sound from the video chip
VIDC also makes the sound. The same chip that turns memory into pictures has an exponential digital-to-analogue converter and eight stereo image registers. MEMC fetches sample bytes from memory into VIDC by DMA, double-buffered so that software can refill one half while the other plays, and an interrupt tells the program when to refill.
The “eight channels” are positions in that single stream of bytes. Each successive sample byte takes the stereo position set for its slot, from one to eight in turn, and VLSI’s data manual explains how to program the registers when only four or two channels are used. Software combines the voices itself, so the ARM’s speed decided how many voices a program could afford. Acorn’s manual gives output filters “optimised for a 20 kHz sample rate”.
Arthur and RISC OS
The first Archimedes ran Arthur. Acorn’s first batch did not even have all its operating system in ROM: Acorn User reported that parts would come on disc until the full 512K ROM was ready. Arthur kept most of the BBC Micro’s star commands, included BBC BASIC V, and could run older BBC software through a 6502 emulator, but programs that broke the BBC’s rules, including most games, would not run.
RISC OS replaced Arthur in the spring of 1989, bringing a new desktop that ran several programs at once. Dealers in Acorn User’s April 1989 issue advertised it as “available April 89”, and software houses were already promising versions for “the new Desktop and multi-tasking”. RISC OS is still developed today, and runs on the Raspberry Pi.
Games and the market
The Archimedes launched with a game on its Welcome disk: a demo of David Braben’s landscape flying game, which became Zarch from Superior Software in 1987 and was converted for the Atari ST and Amiga as Virus. In May 1988 ACE called Zarch the highest-rated game it had reviewed. But the market stayed small. In October 1988 ACE counted just eight commercial Archimedes games in the machine’s first year, estimated there were fewer than one tenth as many Archimedes in Britain as STs, and noted that “a large proportion of these are in schools or colleges”. Superior Software’s Richard Hanson had complained of disappointing sales of Zarch and Conqueror.
The library grew in the early 1990s, largely through conversions. Krisalis brought Lemmings, James Pond, SWIV and Lotus Turbo Challenge II to the machine, and by 1996 Acorn software lists carried its version of Cannon Fodder, first released on the Amiga. Some games were written for the Archimedes first. Fourth Dimension’s Chocks Away put the player in a Tiger Moth biplane, and Star Fighter 3000, written by Acorn programmers and released on the Archimedes late in 1994, was converted to the 3DO for Krisalis in 1995.
Successors
Acorn kept the four-chip design for years. By 1990 the A500 series used the ARM3, which Acorn’s manual says has 4K of on-chip cache and is typically two to three times faster than the ARM2. In 1992 the A3010, A3020 and A4000 put the processor and its companion chips onto one integrated ARM chip. The Risc PC of 1994 replaced the design, and the last A3020 and A4000 machines were built in 1995; Acorn User reported that they did not meet new EC CE electrical regulations and would be sold only while stocks lasted.
ARM after Acorn
In November 1990 Acorn, Apple and VLSI Technology set up Advanced RISC Machines Ltd in Cambridge to carry the processor on without the computers. When Apple announced its Newton in 1992, it said the handheld would use the ARM 610 from Advanced RISC Machines, a processor that gave it “the equivalent power of leading desktop computers” while using less battery power than a small torch.
Acorn itself did not last. It closed its workstation division in 1998 and was broken up in 1999 and 2000. ARM became a public company in 1998, was bought by SoftBank in 2016, survived an attempted takeover by Nvidia that was abandoned in 2022, and returned to the stock market in 2023. The instruction set Acorn designed for the Archimedes is the ancestor of the processors in most of the world’s phones.
Sources
- Acorn User, issue 61, August 1987, p. 11: “RISC revealed”, the Archimedes announcement (models, prices, delivery dates, BASIC speed, 640×512 and 256 colours from 4,096, WIMP desktop, 6502 emulator, operating system partly on disc, Braben’s game on the Welcome disc).
- Acorn User, issue 81, April 1989: RISC OS “available April 89” and software for “the new Desktop and multi-tasking”.
- Acorn User, issue 164, January 1996: the end of the A3020 and A4000.
- Acorn User, issue 182, June 1997: the A3010, A3020 and A4000 and the Risc PC.
- Acorn User, issue 164, January 1996: dealer price list including Cannon Fodder (Krisalis).
- Acorn Computers, Acorn Archimedes 500 Series / Acorn R200 Series Technical Reference Manual (1990), pp. 5-6 and 13: the four-chip set, MEMC’s clocks and address translation, VIDC video and sound, the sound filters and interrupt, the ARM3.
- VLSI Technology, Acorn RISC Machine (ARM) Family Data Manual (1990): system overview (24 MHz and 8 MHz, 5-6 MIPS), VL86C310 VIDC palette, pixel formats, 8-bit colour encoding, stereo image registers and sound frequency register.
- ACE, issue 13, October 1988, pp. 116-117: “the world’s first 32-bit home micro”; price, games, sales and schools.
- ACE issue 1, October 1987, and issue 8, May 1988: Zarch and Virus (see David Braben).
- Acorn Action game guide, January 1994: Chocks Away (Fourth Dimension).
- Edge, issue 21, June 1995: Starfighter 3000 for the 3DO (Krisalis), from the Archimedes original.
- Apple 2000, July 1992: Apple’s Newton announcement and the ARM 610 from Advanced RISC Machines Ltd.
- Wikipedia, “Arm Holdings”, “Acorn Computers” and “RISC OS” (consulted September 2026; web, secondary): the founding of Advanced RISC Machines in November 1990, the 1998 listing, SoftBank (2016), the Nvidia bid (2020-2022) and the 2023 listing; the closure of Acorn’s workstation division in 1998 and the company’s break-up; RISC OS on the Raspberry Pi. https://en.wikipedia.org/wiki/Arm_Holdings, https://en.wikipedia.org/wiki/Acorn_Computers, https://en.wikipedia.org/wiki/RISC_OS