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RISC OS (Reduced Instruction Set Computing Operating System) is a GUI operating system for ARM-processor based computers or similar devices. In certain countries it was an important operating-system before the dot-com computing era.

The OS was designed by Acorn for the 32-bit ARM based Acorn Archimedes, and released in its first version in 1987, as the Arthur operating system.

RISC OS was a rapid development of Arthur 1.2 after the failure of the ARX project. The first release was to be called Arthur 2, but was renamed to RISC OS 2, and was first sold in April 1989. It had co-operative multitasking with some limitations, but was not multithreading. It initially ran from a 512 KB ROM module. The WIMP interface offered all the standard features and fixed many of the bugs that had hindered Arthur. It lacked virtual memory and memory protection. The main advantage of the OS was its ROM; it booted very quickly and while it was easy to crash it was impossible to break. Its performance was further improved in that much of it was written in assembly language. The OS existed in modules and these could be updated by modules in RAM as improvements were made. Such is the modularity of RISC OS, that advanced users have been known to swap in their own replacement modules - one such module gives RISC OS the ability to do pre-emptive multitasking.

RISC OS 3 was released with the A5000 in 1991; it was almost four times the size of OS 2 and ran from a 2MB ROM. It improved multitasking and also placed some of the more popular base applications in the ROM. RISC OS 3.5 was sold from 1994 with the first RISC PC. When the RISC PC architecture was improved in 1995, the OS was updated to RISC OS 3.6 which featured much better hard-disk access and built in networking including TCP/IP, although the OS remained in ROM. In 1996 the RISC PC was upgraded to a StrongARM processor and the OS kernel was rewritten to take advantage, the new revision being dubbed RISC OS 3.7.

Acorn halted work in all areas except STBs in late 1998, and the company was renamed to Element 14. RISC OS development was halted with the 1996 OS 3.7. This led to a number of rescue efforts, including the creation of the ROX Desktop to provide a RISC OS-like interface on Unix and Linux systems. Two similar projects, Impulse and Eidos's Phoenix, both stalled.

In 1999 a new company called RISCOS Limited was founded. They licensed the rights to RISC OS from the new owner, Pace Micro, and continued the development of OS 3.8, releasing it as RISC OS 4 in July 1999. RISCOS Ltd's current version is RISC OS Select Edition 3 (RISC OS 4.37), several thousand copies were sold according to figures released by them. The less enhanced 4.03 version is still available at a lower cost.
RISC OS 5 is a separate evolution of RISC OS 3.x from Castle Technology Ltd for their Iyonix PC line of Acorn-clones, which run on the Intel XScale ARM processor. Although a wealth of software has now been updated, a few older applications can only be run on RISC OS 5 via an emulator, since a minor 26-bit ARM CPU function was removed by Intel from the XScale. Likewise, RISC OS 5 itself had to be ported to run properly on the new CPU.

In July 2003, Castle Technology Ltd bought RISC OS from Pace Micro [1]. RISCOS Ltd and Castle Technologies, at least for the time being, are maintaining separate development branches of the RISC OS operating system.

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