Arthur was an operating system with graphic user surface of the company Acorn, who on their arm processor - based computers one used, approximately on the Acorn Archimedes.
The first version of Arthur appeared 1987.
It was not multitaskingable and the graphic user surface was completely in BBC BASIC written.
Was replaced Arthur of RISC OS 2, which was written contrary to Arthur in assembler. Most applications, which were written for Arthur, ran also under RISC OS.