ARX was a Mach-like operating system written in Modula-2+ developed by Acorn Computers Ltd in the Acorn Research Centre (ARC) and UK later Olivetti Research Center and later on Software Technology Laboratory (when Olivetti bought out Acorn) at Palo Alto for their new ARM RISC processors based Archimedes computers range. According to the project Application Manager during the project, Acorn was developing for it, Compiler for Acorn Modula-2 Extended Language (CAMEL in Modula-2+) bootstrapping from an Acorn Extended Modula-2 (AEM2) compiler (ported from Modula-2 ETH using Econet hardware), though never released externally, it was ported to use it in SUN Unix machines, in an effort to port Sun's workstations Sun NeWS to the Archimedes, and after Olivetti bought out Acorn, developed a compiler based on CAMEL for the Modula-3 programming Language.ARX was a pre-emptive multitasking, multithreading, multi-user operating system. Much of the OS ran in user mode and as a result suffered performance problems due to switches into kernel mode to perform mutexes, which led to the introduction of the SWP instruction to the instruction set of the ARM3 version of the ARM processor. It had support of optical (WORM) disks file system and featured a window system, a window toolkit (and a direct manipulation User Interface editor) and an InterScript-based text editor, for enriched documents written in InterPress (a HTML precursor). The OS had to be fitted in a 512KB ROM image. This suggests that ARX had a microkernel-type design.It was not finished in time to be fitted to the Acorn Archimedes range of computers, which shipped in 1987 with the Arthur operating system, derived from the earlier MOS operating system from Acorn's earlier 8-bit BBC Micro range. Arthur was later superseded by RISC OS.The Acorn Research Centre was bought out by Olivetti.