(Reduced Instruction Set Computer Operating System) AcornComputers' 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.