IIRC, Dijkstra's THE operating system for the Electrologica X8 was completed before the hardware was available. It was the first multitasking operating system (

IIRC, Dijkstra's THE operating system for the Electrologica X8 was completed before the hardware was available. It was the first multitasking operating system (albeit with a fixed set of tasks) and the first use of semaphores. Supposedly the initial version only had trivial coding errors.