1. The Structure of “THE” – Multiprogramming System by Edsger W. Dijksta Technological University, Eindhoven, Netherlands. Presented by Navya Jammula

2. What is the paper about? The Conception, Construction and Verification of a multiprogramming system – “THE” (Technische Hogeschool Eindhoven)

3. Outline  Goal  Hardware configuration used  System Components  Synchronization Primitives  Experience  Mistakes  Concerns

4. Goal  Process smoothly a continuous flow of user programs.

5. Objectives  Reduction of turn-around time for programs of short duration  Economic use of peripheral devices  Efficient use of Memory and Processor.  Support programs that need the flexibility of general purpose computer without the capacity and the processing power

6. Tool  Dutch Electrologica EL X8 computer 32K core memory ( cycle time 2.5usec) 512K words drum (1024 words per track, 40msec rev ) An indirect addressing (suited for stack implementation) Low capacity channels supporting peripherals  (3 paper tape readers and punches, printer, plotter, and 2 teleprinter) A sound control of interrupts and peripherals

7. System Components  Storage Allocation  Processor Allocation  System Hierarchy

8. Storage Allocation  strict distinction between memory units and corresponding Information Units  Pages Core Pages Drum Pages  Segments A segment fits in a page.

9. Storage Allocation Contd.  Number of possible segment identifiers is much larger than the total number of pages in primary and secondary store.  The segment identifier gives fast access to a so-called “segment variable” in core whose value denotes Whether the segment is still empty or not, If not empty, in which page (or pages) it can be found.

10. Consequences  No need to return the segment to the same drum page from which it originally came Can select one from among the free drum pages with minimum latency time.  Total absence of a drum allocation problem A program need not occupy consecutive drum pages

11. Processor Allocation  In a single sequential process only the time succession of various states has a logical meaning, but not the actual speed with which the sequential process is performed.  Hence the whole system can be arranged as a society of sequential processes, progressing with undefined speed ratios

12. Society of Sequential Processes  Each user program accepted by the system  Each input peripheral (buffering input streams in synchronism with the execution of the input commands),  Each output peripheral (unbuffering output streams in synchronism with the execution of the output commands);  Segment Controller associated with the drum  Message Interpreter associated with the console keyboard.

13.  How do these processes co-operate with each other Using Explicit Mutual Synchronization statements. Semaphores Private Semaphores

14. System Hierarchy  “THE” system admits a strict hierarchical structure.  THE has six Layers, levels 0 – 5  Each layer builds an abstraction hiding the details below it  Each layer uses only the functionality of the lower layers

15. Level 0  Responsible for processor allocation to one of the processes whose dynamic progress is logically permissible  It deals with interrupts and performs the context switches when a process change is required.  A priority rule is incorporated to achieve quick response of the system  Above level 0 the number of processors actually shared is no longer relevant  This is the first level of abstraction

16. Level 1  The segment controller is present at this level.  Responsible to cater to the bookkeeping resulting from the automatic backing store  At all higher levels identification of information takes place in terms of segments, the actual storage pages had lost their identity  This is the next level of abstraction.

17. Level 2  At level 2 we find Message Interpreter taking care of the allocation of the console keyboard.  Above level 2 it is as if each process had its private conversational console  In fact they all share one physical resource and the sharing is provided via mutual synchronization.  At higher levels the actual console teleprinter loses its identity  This is the next level of abstraction

18. Levels 3 – 5  Level 3  Contains the Sequential Processes associated with buffering of input streams and unbuffering of output streams.  Manages all I/O between the devices attached to the computer  Level 4  Consists of the independent – user programs.  Level 5  Consists of the System Operator.

19. Synchronizing Primitives  Mutual synchronization of parallel sequential processes is implemented via "semaphores.“  Semaphores are special purpose integer variables initialized with the value 0 or 1  Parallel processes will access the semaphores via two specific operations P-operation "P(sem)" decreases the value of the semaphore called "sem“ by 1. V-operation. "V(sem)" increases the value of the semaphore called "sem“ by 1

20. Semaphores Contd.  If a semaphore value is nonpositive its absolute value equals the number of processes booked on its waiting list.  The P-operation represents the potential delay, the complementary V-operation represents the removal of a barrier.  P- and V-operations are "indivisible actions";  If the semaphore value resulting from a V – operation is negative, its waiting list originally contained more than one process.  It is logically immaterial-which of the waiting processes is then removed from the waiting list.

21. Semaphores in action begin semaphore mutex; mutex := 1; parbegin begin L1 : P (mutex ) ; critical section 1; remainder of cycle 1; go to L1 end; begin L2: P (mutex); critical section 2; V (mutex); remainder of cycle 2; go to L2 end parend end

22. Private Semaphores  Each sequential process has associated with it a number of private semaphores, initialized to 0. Max value equals 1, Min value equals -1.  No other process can ever perform a P-operation on them.

23. A process reaches a stage where the permission for dynamic progress depends on current values of state variables P(mutex) ; "inspection and modification of state variables including a conditional V(private semaphore)"; V (mutex) ; P(private semaphore).  If the resource is available, It performs the operation "V (private semaphore) “ The semaphore value then changes from 0 to 1  Else this V-operation is skipped, leaving to other processes, to perform this V – operation at a suitable moment

24. A process reaches a stage where as a result of its progress possibly one (or more) blocked processes should now get permission to continue P (mutex) ; "modification and inspection of state variables including zero or more V-operations on private semaphores of other processes"; V(mutex).

25. Proving Harmonious Co-operation Homing Position: The neutral point in which all cyclic processes are when the system is at rest.  A single task cannot give rise to an infinite number of task generations processes can only generate tasks for processes at lower levels of the hierarchy so that circularity is excluded  System cannot be at rest if there are still pending tasks  All processes will eventually be in homing position (no deadlock)

26. Experience  Conception  Construction  Verification

27. Conception, Construction  Conception All the concepts are born It took long time. Learnt that the society of mutually synchronized processes in time behavior can satisfy all the requirements.  Construction Done in rather traditional manner. Change of specifications has been rare.

28. Verification  Starting at level 0 the system was tested, each time adding (a portion of) the next level only after the previous level had been thoroughly tested. Force the system into all different relevant states Verify that the system continued to react according to specification  Hierarchical structure made testing easy Else the number of "relevant states" would have exploded to such a height that exhaustive testing would have been an illusion  The hardware failures couldn’t effect testing due to the hierarchical structure

29. Not Intended  Supports multi tasking but not multi access system  No common data base via which independent users can communicate  Does not cater for user programs written in machine language

30. Mistakes Made  Paying too much attention to eliminating what was not the real bottle neck.  Trying for “Perfect Installation”  Late Debugging.

31. Concerns  Production speed severely slows down working with half – time people.  People lose time and energy in switching over and the group loses decision speed. Similar to context switching in OS  This type of work is very difficult, and that every effort to do it with other than the best people is doomed to either failure or moderate success at enormous expense. This strongly supports the argument that we need wizards for multi programming

32. Conclusion  The “THE” system apparently introduced the first forms of software-based memory segmentation freeing programmers from being forced to use actual physical locations on the drum memory  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  Hierarchical structure of the system in which layers of abstraction are used to virtualize resources.  A refined multiprogramming system whose logical soundness can be proved a priori and that its implementation admits exhaustive testing.

33. Why read this paper?  No abstraction available, writing machine code, by hand, in order to deal with all the possible combinations of CPU/Peripheral device states  Low-level, interrupt driven, concurrent OS code which is hard to write even with tool and language support and decades of experience.  Building and testing abstractions and then using them as building blocks to construct an entire OS was critical,  Virtualizing the CPU and memory and I/O devices etc, and developing a blocking synchronization construct (semaphores) was a brilliant way of gaining control over this enormous level of complexity.  Concept of Combining the layered hierarchical approach and the use of processes with homing states based on the semaphores, had a major impact on testing and verification  Also, Many of the concepts presented here we take for granted today.

34. Questions?