(Reduced Instruction Set Computer Operating System) acorn computers' operating system 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. A large number of system calls (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 fonts when only bitmap fonts were available on most other platforms.