1. The Structure of the "THE"- Multiprogramming Edsger W. Dijkstra Technological University, Eindhoven, The Netherlands System Andrew Edwards

2. Goal A reduction of turn-around time for programs of short duration Economic use of peripheral devices Automatic Control of backing store to be combined with economic use of the central processor The economic feasibility to use the machine for those applications for which only the flexibility of a general purpose computer is needed, but (as a rule) not the capacity nor the processing power.

3. Terminology “Pages”, “core pages”, and “drum pages” “Segments” Identification method which allows lookup of segments to be added to core “Segment Variable”

4. Processor Allocation Treat both input and output peripherals as though they had their own sequential streams Whole system is based on concept of “sequential processes”

5. LevelFunctionality 0Allocated a process to a processor Real time clock interrupts is processed 1Sequential process in terms of the drum 2Stream from Keyboard 3Sequential process of buffering input Sequential process of unbuffering output 4Independent User Programs 5Operators

6. Design Implemented bottom up Designed so that testing would require the fewest test cases in each transition

7. Semaphores Special purpose variables for keeping track of mutual exclusion After initialization can only be decremented (lock is now held) or incremented (lock released) Negative numbers cause processes to be put on a waiting list

8. Semaphores 2 COROLLARY 1. If a semaphore value is nonpositive its absolute value equals the number of processes booked on its waiting list. COROLLARY 2. The P-operation represents the potential delay, the complementary V- operation represents the removal of a barrier.

9. Semaphores 3 P and V operations are indivisible and are always done sequentially If a V operation is done it does not matter which process is removed from the waiting list Processes can only become blocked from trying to access a semaphore which is non positive

10. Mutual Exclusion Allows only one process to be in a critical section at a time. Critical sections are used always for the inspection and modification of state variables

11. Mutual Exclusion Example begin semaphore mutex; mutex := 1; parbegin begin L1 : P (mutex ) ; critical section 1; V(mutex); 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 Paraeud end

12. Private Semaphores Semaphore associated with an individual process Used as a state tracker to check if the process should be allowed to continue or be stopped temporarily

13. Private Semaphores 2 P(mutex) ; "inspection and modification of state variables including a conditional V(private semaphore)"; V (mutex) ; P(private semaphore).

14. Harmonious Cooperation “Homing position” when all processes are at rest Initial tasks can only spawn a finite amount of tasks – Due to processes only being able to call lower processes All processes cannot be in the homing position if there is an unaccepted task. After the acceptance of an initial task all processes will reach the state of the “homing position”

15. References "The Structure of the "THE" Multiprogramming System" by E. W. Dijkstra, Communications of the ACM, 11(5):341--346, 1968.