THE multiprogramming system

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For other uses, see THE.

The THE multiprogramming system was a computer operating system designed by a team led by Edsger 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[1] 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."[1]

The THE system apparently introduced the first forms of software-based memory segmentation (the Electrologica X8 did not support hardware-based memory management)[1], freeing programmers from being forced to use actual physical locations on the drum memory. It did this by using a modified ALGOL compiler (the only 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."[1]

The design of the THE multiprogramming system is significant for its use of a layered structure, in which "higher" layers only depend on "lower' layers:

  • Layer 0 was responsible for the multiprogramming aspects of the operating system. It decided which process was allocated to the CPU. It dealt with interrupts and performed the context switches when a process change was required. This is the lowest level.
  • Layer 1 was concerned with allocating memory to processes.
  • Layer 2 dealt with inter-process communication and communication between the operating system and the console. The system used semaphores for synchronization, with Dijkstra's Banker's algorithm avoiding deadlocks.[1]
  • 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 compilation, execution, and printing of users' 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 overall control of the system (called the system operator).

The constraint that higher layers can only depend on lower layers was imposed by the designers in order to make reasoning about the system (using quasi-formal methods) more tractable, and also to facilitate building and testing the system incrementally. The layers were implemented in order, layer 0 first, with thorough testing of the abstractions provided by each layer in turn. This formal design process was highly successful, as reported by Dijkstra:

We have found it is possible to design a refined multiprogramming system in such a way that its logical soundness can be proved a priori and that its implementation admits exhaustive testing. The only errors that showed up during testing were trivial coding errors (occurring with a density of only one error per 500 instructions), each of them located with 10 minutes (classical) inspection at the machine and each of them correspondingly easy to remedy.[2]

This division of the kernel into layers was similar in some ways to Multics' later ring-segmentation model. Several subsequent operating systems have used layering to some extent, including Windows NT and Mac OS X, although usually with fewer layers.

The code of the system was written in assembly language for the Dutch Electrologica X8 computer. This computer had 32K of core memory using 27-bit words[1], a 512K drum providing backing store for the LRU algorithm, paper tape readers, paper tape punches, and printers.

[edit] See also

[edit] References

  1. ^ a b c d e f g pg 512, "Chapter 13: Historical Perspective" of Operating System Concepts by Abraham Silberschatz and James L. Peterson, June 1988; ISBN 0-201-18760-4. 573 pages.
  2. ^ E. W. Dijkstra . EWD 196: The structure of the 'THE'-multiprogramming system (manuscript). Text [1], PDF [2]
  • E. W. Dijkstra . EWD 126: The Multiprogramming System for the EL X8 THE (manuscript). 14 June 1965. Text [3], PDF [4]
  • E. W. Dijkstra, The structure of the 'THE'-multiprogramming system, Communications of the ACM 11(5):341 – 346, 1968. (subscription)