Source : Free On-Line Dictionary of Computing

RISC OS
     
        (Reduced Instruction Set Computer Operating System) {Acorn
        Computers}' {operating system} for their {Archimedes} family
        of {personal computer}s.  RISC OS replaced the {Arthur}
        operating system used on the first Archimedeses.  It is
        written in {ARM} {assembly code} and distributed on {ROM} so
        it takes up no disk space and takes no time to load.  It
        supports {cooperative multitasking} with memory management and
        includes a {graphical user interface} or "WIMP".  It is
        written in a highly modular style and makes extensive use of
        {vector}s so it is easy to modify and extend by loading new
        modules in {RAM}.  A large number of {system call}s (called
        "SWIs" - software interrupts) are available to the
        applications programmer and some of these are available as
        user comands via a built-in {command-line interpreter}.  RISC
        OS also supported {outline font}s when only {bitmap font}s
        were available on most other {platform}s.