The Free On-line Dictionary of Computing (30 December 2018):

RISC OS

    (Reduced Instruction Set Computer Operating
   System) The operating system originally developed by Acorn
   Computers for their Archimedes family of personal
   computers.

   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
   vectors so it is easy to modify and extend by loading new
   modules in RAM.  Many system calls (called "SWIs" -
   software interrupts) are available to application programmers
   and some of these are available as user comands via a built-in
   command-line interpreter.  RISC OS also supported outline
   fonts when only bitmap fonts were available on most other
   platforms.

   Following the virtual demise of Acorn, development of RISC OS
   4 was taken over by RISCOS Ltd on 1999-03-05 and released on
   1999-07-01.

   (2004-09-21)