RISC OS is an operating system first developed by Acorn Computers Ltd to run on computers based around the ARM processor that was originally designed in-house by the company.

The operating system

Since it was initially released (with the name Arthur) in 1987 as the supplied operating system on Acorn's Archimedes computer range, there have been a number of versions - and today, despite the closure of Acorn in 1998, the OS - which is now open source - continues to be developed and is available for several ARM-based systems, including the very popular Raspberry Pi.

While it is not as fully featured as some other operating systems commonly in use today, such as Microsoft Windows, macOS, or various flavours of Linux, it does include a number of features that both differentiate it and make it a joy to use. These are just a couple of examples:

Context sensitive menus
There are no menu bars fixed to the top of windows (or the top of the screen) taking up space, with 'standard' entries, and separate pop up menus for context sensitive items. Instead all menus are pop-up, with a standardised format, and contextual entries as necessary - a standard feature of RISC OS since the 1980s.
Windows don't jump to the front
In most other operating systems, when you click on a window to make it the 'active' one, that window comes to the front, obscuring other windows. Sometimes this doesn't matter, but sometimes it can be a nuisance, such as when you need to work in one window, but see the contents of another. With RISC OS, when you click on a window, it remains where it is - so you can work in a window, with another one partially in front of it if you choose.

The ARM processor

The ARM processor was initially conceived at Acorn Computers in the early 1980s when they were looking to develop a new system based around something more powerful than the 6502 used in their 8-bit computers such as the BBC Micro. The first ARM processor was powered up in 1985, and a couple of years later the ARM-based Archimedes computer systems were launched.

ARM at that time stood for Acorn RISC Machine, and the processor was a Reduced Instruction Set Computer. This meant it had fewer instructions than a typical processor, all of which could be encoded in a single word of memory and operated in a single processor cycle - this meant that the instructions were able to be executed much faster.

With the processor eventually moving out of Acorn to its own company, it has evolved and grown, and due to its low power consumption and efficient design it has gone on to become ubiquitous; there are versions at the heart of most current phones and tablet computers, as well as in embedded computer systems in cars and general household appliances. Acorn Computers may be long gone, but their legacy lives on in many things around us.