The Acorn Archimedes was the first general-purpose home computer based on the 32-bit ARM RISC CPU from Acorn Computers Ltd. The term 'Archimedes' is also commonly used to describe Acorn computers that were not officially called Archimedes, but were based on the same architecture. It was first released in the UK in 1987, and used an operating system called ARTHUR, allegedly standing for "A Risc-based operating system by THURsday" (later versions were named RISC OS). The current version of RISC OS is version 4, and it runs on the RISC PC.