RISC OS

RISC OS is a windowed operating system release first in 1987 by ARM. It uses the reduced instruction set computing architecture.

Risc OS is a graphical desktop environment, GUI, that runs programs in windows. Version 5, 32-bit, is open source. Version 6 is proprietary. RISC OS runs on ARM-32 and ARM-64 machines and most ARM compatible derivatives.

Links

 * Wikipedia article