The THE multiprogramming system or THE OS was a computer operating system designed by a team led by Edsger W. Dijkstra, described in monographs in 1965-66 and published in 1968. Dijkstra never named the system; "THE" is simply the abbreviation of "Technische Hogeschool Eindhoven", then the name (in Dutch) of the Eindhoven University of Technology of the Netherlands. The THE system was primarily a batch system that supported multitasking; it was not designed as a multi-user operating system. It was much like the SDS 940, but "the set of processes in the THE system was static". The THE system apparently introduced the first forms of software-based paged virtual memory (the Electrologica X8 did not support hardware-based memory management), freeing programs from being forced to use physical locations on the drum memory. It did this by using a modified ALGOL compiler (the only programming language supported by Dijkstra's system) to "automatically generate calls to system routines, which made sure the requested information was in memory, swapping if necessary". Paged virtual memory was also used for buffering input/output (I/O) device data, and for a significant portion of the operating system code, and nearly all the ALGOL 60 compiler. In this system, semaphores were used as a programming construct for the first time. Design The design of the THE multiprogramming system is significant for its use of a layered structure, in which "higher" layers depend on "lower" layers only: Layer 0 was responsible for the multiprogramming aspects of the operating system. It decided which process was allocated to the central processing unit (CPU), and accounted for processes that were blocked on semaphores. It dealt with interrupts and performed the context switches when a process change was needed. This is the lowest level. In modern terms, this was the scheduler. Layer 1 was concerned with allocating memory to processes. In modern terms, this was the pager. Layer 2 dealt with communication between the operating system and the system console. Layer 3 managed all I/O between the devices attached to the computer. This included buffering information from the various devices. Layer 4 consisted of user programs. There were 5 processes: in total, they handled the compiling, executing, and printing of user programs. When finished, they passed control back to the schedule queue, which was priority-based, favoring recently started processes and ones that blocked because of I/O. Layer 5 was the user; as Dijkstra notes, "not