1. The Structure of the “THE”-Multiprogramming System
Edsger W. Dijkstra
Technological University, Eindhoven, The Netherlands
Communications of the ACM, 11(5): , 1968
Presented by: Amin Almassian
CS510 - Concepts of Operating Systems, Fall 2013
Portland State University

2. About the author Edsger Wybe Dijkstra (1930-2002)
A pioneer in the area of distributed computing.
His foundational work:
Concurrency primitives (such as the semaphore),
Concurrency problems (such as mutual exclusion and deadlock),
Reasoning about concurrent systems, and self-stabilization
Winner of ACM's A.M. Turing Award in 1972
The Edsger W. Dijkstra Prize in Distributed Computing is named for him
Graphs “shortest path” algorithm: shortest route between two cities in the Netherlands. “Without pencil and paper you are almost forced to avoid all avoidable complexities.”
“In 1955 when I decided not to become a physicist, to become a programmer instead. At the time programming didn't look like doing science, it was just a mixture of being ingenious and being accurate.” [1]

3. Multiprogramming System Objectives
Not intended as a multi-access system
To process smoothly a continuous flow of user programs
Making economic use of peripheral devices
Reduction of turn-around time for programs of short duration
Automatic control of backing store to be combined with economic use of the central processor
Feasibility to use the machine for multiple apps economically

4. Hardware Configuration EL X8
Core memory: 2.5µsec, 27 bits; 32K
Storage: drum of 512K words, 1024 words per track, 40 msec
An indirect addressing mechanism well suited for stack implementation
A sound system for commanding peripherals and controlling of interrupts
low capacity channels
3 paper tape readers at 1000char/see;
3 paper tape punches at 150char/sec;
2 teleprinters (a plotter, a line printer)

5. Storage Allocation Memory units (pages): core pages and drum pages
Information units: segments (a segment fits in a page)
Segment variable: Segment variable value tells if the segment is empty or not
Segment identifier gives fast access to a segment variable
If the segment is not empty, the value denotes which page(s) the segment can be found
Consequences:
A core page can be dumped onto a free drum page (the one with the minimum latency time) to free up the core page
Drum page occupation does not have to be consecutive.

6. Process Allocation
A society of sequential processes (the concept of process abstraction)
logical meaning for a process:
The time succession of various states,
Not the actual speed of execution
Mutual synchronization
Allows for cooperation between sequential processes
Processor switches from process to process,
Temporal delaying the progress of the processes (blocking)

7. L2: Message Interpreter
System Hierarchy
L5: Operator
L4: User Program
L3: Buffering I/O
L2: Message Interpreter
L1: Segment Controller
L0: Process Allocation

8. Level 0: Process Allocation
Is in charge of allocation of the processor for processes
Takes advantage of real-time clock interrupts to regain the control of the processor
Provides an abstraction:
The number of processors actually shared is no longer relevant.
The actual processor that had lost its identity having disappeared from the picture.
Priority rule included

9. Level 1: Segment Controller
Is in charge of memory storage and allocation
Consists of a sequential process synchronized with the drum interrupt and sequential processes of higher levels
Provides a level of abstraction:
Higher levels identify information in terms of segments
the actual storage pages that had lost their identity having disappeared from the picture.
At higher levels, the actual storage pages have lost their identity

10. Level 2: Message Interpreter
A mediator between the operator and any of the higher level processes
When a key is pressed, the character along with an interrupt is sent to the system
Sends output commands to printer
Provides an abstraction level:
Processes share the same physical console
Above this level, each process thinks it has its own private console (the advantage of using mutual synchronization)

11. Provides an Abstract level:
Level 3: Buffering I/O
Is in charge of buffering of input streams and Un-buffering of output streams by using sequential processes
The sequential processes associated with the peripherals are of a level above the message interpreter, because they must be able to converse with the operator (e.g. in the case of detected malfunctioning).
Provides an Abstract level:
Abstracts the actual peripherals as “logical communication units” to higher levels

12. Level 4 : The independent user programs
Level 5: Operator
Level 4: User Program
Level 4 : The independent user programs
Level 5: the operator (not implemented by the author’s team).

13. Synchronizing Primitives
Semaphores Initialized with the value of 0 or 1
P-operation decreases value of a semaphore by 1
If sem ≥ 0  process can continue
If sem< 0  process is stopped and is put on waiting list
V-operation increases value of a semaphore by 1
If sem >0  no effect
If sem ≤ 0  a process on the waiting list is removed and continues progressing once allocated to a processor
If there is more than one process on the waiting list it is undefined which process is removed
Semaphore [2]

14. Mutual Exclusion
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 parend
End
Mutex: {-(n-1), …,-1, 0, 1}
Allows for straightforward extension to more than two parallel processes

15. Private Semaphors Analogous to condition synchronization mechanisms
Each sequential process has associated with a number of private semaphores (range between -1 and 1)

16. Private Semaphors
Whenever a process reaches a stage where the permission for dynamic progress depends on current values of state variables, it follows the pattern:
P(mutex) ;
"inspection and modification of state variables including a conditional V(private semaphore)";
//if wants to continue // V(private-semaphore)
//If (private semaphore > 0) V (mutex);
P(private semaphore)

17. Private Semaphors
Whenever a process reaches a stage where as a result of its progress possibly one (or more) blocked processes should now get permission to continue, it follows the pattern:
P (mutex) ;
"modification and inspection of state variables including zero or more V- operations on private semaphores of other processes";
V(mutex).

18. Proving the Harmonious Cooperation
Homing position
Accepted a task
Perform the task
Blocked
A Free resource needed
Homing position
Accepted a task
Perform the task
Blocked
Unstable Situation

19. Proving the Harmonious Cooperation
1. a process generate a finite number of tasks for other processes.
processes can only generate tasks for processes at lower levels of the hierarchy so that circularity is excluded.
2. It is impossible that all processes have returned to their homing position while there is still pending tasks. (This is proved via instability of the situation)
3. After the acceptance of an initial task all processes eventually will be (again) in their homing position. (proof by induction on the level of hierarchy, starting at the lowest level)

20. Summary and Conclusion
Old concepts yet state-of-the-art ( )
Segments == Segments
Core page and drum pages == Paged virtual memory abstraction
Sequential Process abstraction == Process/Thread
Semaphores == Semaphores
hierarchical implementation == hierarchical implementation
Hierarchical structure has been a great advantage to verification and testing
Abstraction at each layer
Proof of logical soundness at each level of abstraction
Small components results in small test cases => Easier to test

21. Refrences
[1] Thomas J. Misa and Philip L. Frana An interview with Edsger W. Dijkstra. Commun. ACM 53, 8 (August 2010), DOI= /
[2] Animations for Operating Systems, Sixth Edition by William Stallings, available at