ARX was an unreleasedMach-likeoperating system written inModula-2+^[3] developed byAcorn Computers Ltd in the Acorn Research Centre (ARC)United Kingdom (UK) and later byOlivetti—which purchased Acorn—for Acorn's newArchimedes personal computers based on theARM architecturereduced instruction set computer (RISC)central processing unit (CPUs).

According to the project Application Manager^[4] Richard Cownie, during the project, while Acorn was developing the kernel, it used the C and Acorn Modula Execution Library (CAMEL) in the Acorn Extended Modula-2 (AEM2) compiler (ported fromModula-2ETH Zurich (ETH) usingEconet hardware). Though never released externally, CAMEL was ported to use onSun Microsystems Unix computers.^[5] In an effort to port Sun'sworkstations SunNeWS to the Archimedes, David Chase developed a compiler based on AEM2 for the programming languageModula-3.^[6]

ARX was apreemptivemultitasking,multithreading, multi-useroperating system. Much of the OS ran inuser mode and as a result suffered performance problems due to switches intokernel mode to performmutexes, which led to the introduction of the SWP instruction to the instruction set of the ARMv2a version of the ARM processor. It had support of a file system for optical (write once read many (WORM)) disks^[4] and featured a window system, a window toolkit (and a direct manipulationuser interface (UI) editor^[7]) and anInterscript-based text editor, for enriched documents written inInterpress (aHTML precursor). The OS had to be fitted in a 512 KBread-only memory (ROM)ROM image.^[8] This suggests that ARX had amicrokernel-type design.^{[according to whom?]}

It was not finished in time to be used in the Acorn Archimedes range of computers, which shipped in 1987 with an operating system named Arthur, later renamedRISC OS, derived from the earlierMachine Operating System (MOS) from Acorn's earlier8-bitBBC Micro range.^[9] Confusion persisted about the nature of ARX amongst the wider public and press, with some believing that ARX was Acorn's own Unix variant,^[10] with this view being refined in time to accommodate ARX as Acorn's own attempt to deliver a "UNIX look-alike" whose development had been abandoned in favour of a traditional Unix version for the Archimedes, which ultimately emerged asRISC iX.^[11]

The Acorn Research Centre was acquired byOlivetti.

• RISC iX

• ARX featuresArchived 2010-02-09 at theWayback Machine
• History of the addition of the SWP instruction to the ARM3 instruction set

• Acorn Computers operating systems
• ARM operating systems
• Discontinued operating systems
• Microkernel-based operating systems
• Microkernels

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