RISC OS
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RISC OS

RISC OS (/rɪsk.ˈɛs/) is an operating system designed to run on ARM computers. Originally designed in 1987 by Acorn Computers of England, it was made for use in its new line of ARM-based Archimedes personal computers and was then shipped with other computers produced by the company. Despite the demise of Acorn, RISC OS continues to be developed today by the RISC OS Open community on version 5.0 of the system that was open sourced in 2018.

RISC OS is a modular operating system and takes its name from the reduced instruction set computer (RISC) architecture it supports. It incorporates a graphical user interface and a windowing system. Between 1987 and 1998, RISC OS shipped with every ARM-based Acorn computer including the Archimedes line, Acorn's R line (with RISC iX as a dual-boot option), RiscPC, A7000, and prototype models such as the Acorn NewsPad and Phoebe computer. A version of the OS, named NCOS, was used in Oracle's Network Computer and compatible systems.

After the break-up of Acorn, development of the OS was forked and continued separately by several companies, including RISCOS Ltd, Pace Micro Technology, Castle Technology, and RISC OS Developments. Since then, it has been bundled with several ARM-based desktop computers such as the Iyonix PC and A9home. Most recent stable versions run on the ARMv3/ARMv4 RiscPC, the ARMv5 Iyonix, ARMv7 Cortex-A8 processors and Cortex-A9 processors and the low-cost educational Raspberry Pi series of computers, with the exception of the Raspberry Pi 5.

The first version of RISC OS was originally released in 1987 as Arthur 1.20. The next version, Arthur 2, became RISC OS 2 and was released in April 1989. RISC OS 3.00 was released with the A5000 in 1991, and contained many new features. By 1996, RISC OS had been shipped on over 500,000 systems.

Acorn officially halted work on the OS in January 1999, renaming themselves Element 14. In March 1999 a new company, RISCOS Ltd, licensed the rights to develop a desktop version of RISC OS from Element 14, and continued the development of RISC OS 3.8, releasing it as RISC OS 4 in July 1999. Meanwhile, Element 14 had also kept a copy of RISC OS 3.8 in house, which they developed into NCOS for use in set-top boxes. In 2000, as part of the acquisition of Acorn Group plc by MSDW Investment, RISC OS was sold to Pace Micro Technology, who later sold it to Castle Technology Ltd.

In May 2001, RISCOS Ltd launched RISC OS Select, a subscription scheme allowing users access to the latest RISC OS 4 updates. These upgrades are released as soft-loadable ROM images, separate to the ROM where the boot OS is stored, and are loaded at boot time. Select 1 was shipped in May 2002, with Select 2 following in November 2002 and the final release of Select 3 in June 2004. In the same month, RISC OS 4.39, dubbed RISC OS Adjust, was released. RISC OS Adjust was a culmination of all the Select Scheme updates to date, released as a physical set of replaceable ROMs for the RiscPC and A7000 series of machines.

Meanwhile, in October 2002, Castle Technology released the Acorn clone Iyonix PC. This ran a 32-bit (in contrast to 26-bit) variant of RISC OS, named RISC OS 5. RISC OS 5 is a separate evolution of RISC OS based upon the NCOS work done by Pace. The following year, Castle Technology bought RISC OS from Pace for an undisclosed sum. In October 2006, Castle announced a shared source license plan, managed by RISC OS Open Limited, for elements of RISC OS 5.

In October 2018, RISC OS 5 was re-licensed under the Apache 2.0 license.

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