Taking a RISC
In late 1983, Acorn started developing a new 32-bit processor called ARM.
The name stood for Acorn RISC Machine, with RISC standing for Reduced Instruction Set Computing. The idea of RISC had been around since the 1970s.
Acorn were inspired by the design of the 6502, which was a popular 8-bit microprocessor.
The 6502 was used in machines such as the Apple II, and Acorn’s own BBC Micro. It had a simple design, without the slow and complex instructions found in other chips, such as the Z80. That enabled it perform well.
Acorn began by designing an instruction set for the ARM. Rather than rushing to make a hardware prototype, they simulated it in software, and debugged it.
Some of the work was even done using BBC Micros (fitted with a second 6502 running at 3 MHz). After 18 months, the first ARM1 chip was built in 1985.
Fast and Efficient
The ARM used fewer transistors than other popular microprocessors, so it consumed far less power — although that would only become a selling point at a later date, for battery-powered devices.
Virtually all of the ARM’s instructions executed in just one clock cycle. That made it much faster than the 68000 used in the Apple Macintosh, and machines such as the Amiga and Atari ST, which I’d been reading about in the college library.
In contrast to the ARM, the 68000 had a CISC architecture (Complex Instruction Set Computing). It took a minimum of four clock cycles, even for the simplest instructions. Many instructions took eight or more cycles.
In 1987, Acorn released their first ARM-based computers. These featured the ARM2 chip. which had a few improvements over the ARM1.
Some of the machines were called BBC Archimedes, and others were called Acorn Archimedes. They were significantly more expensive than computers such as the Atari ST, but they were also more powerful.
The operating system was initially called Arthur. Later the name became RISC OS. It had cooperative multitasking and a graphical user interface.
There was an emulator for the BBC Micro, as well as the BBC BASIC programming language. That made it easier for people to upgrade from earlier Acorn machines.
Sound and Graphics
Sound was 8-bit digital audio, rather like the Macintosh and Amiga. Both the Archimedes and Amiga had a single stereo output. Internally though, the Archimedes supported up to 8 stereo channels, whereas the Amiga only had two channels for each of its two outputs.
A number of graphics modes were available. It could display up to 256 colours from a palette of 4096 at 640 x 512, or 16 colours at 800 x 600. That was more than the Amiga’s 32 colours (ignoring its 4096-colour HAM mode).
Computers like the Amiga and later versions of the Atari ST had blitters, which accelerated certain graphics operations. The Archimedes didn’t have a blitter — but its ARM processor coupled with fast memory meant that it still performed well.
The ARM CPU core evolved over the years, and can now be found inside most of the world’s mobile phones. As if that weren’t enough, an even greater number can be found within a wide range of other products, making ARM yet more popular still.
In my job in recent years, I’ve worked on embedded systems which use ARM CPU cores. But back in the 80s and 90s, I have to admit that I didn’t pay much attention to what Acorn did after the BBC Micro, with the ARM and Archimedes.
The Archimedes certainly looks like a very interesting machine. It’s also incredibly significant from a historical point of view, given the number of ARM chips that are in use these days. I would certainly like to try one, and take a closer look at RISC OS.
Do you have memories of using the Archimedes?
Share your thoughts in the comments section below.
(Article updated 30 March 2018 to embed the videos listed above.)