1. The Structure of the “The” –Multiprogramming System
Edsger W. Dijkstra
Presented By: Nick Tyrrell

2. Paper Introduction May, 1968
Response to a call for papers on timely research and development efforts
Six person team
3 Guiding Principles
Select an ambitious project
Select a machine with a good basic design
Experience != Wisdom

3. EL X8 32K core memory 512K words drum size
Good peripheral and interrupt control
Several low capacity channels
Tape readers/writers, printers, etc
Lack of other awkward features

4. The System Goals Quick turn-around for short term programs
Economic peripheral use
Economic backing store and processor use
Flexibility as a general purpose computer

5. System Survey Storage Allocation Processor Allocation System Hierarchy
Semaphores
System Hierarchy

6. Storage Allocation
Prior system implementations used core memory for page-wise access to information
Segments
Hold the information
Can be on any page in memory
Location discovered via segment variables in core memory

7. Storage Allocation cont…
This approach
reduces drum allocation time
Allows for flexible program storage

8. Processor Allocation Society of sequential processes
User programs
Input peripherals
Output peripherals
Segment controller
Message Interpreter
This society requires synchronization

9. Semaphores Integer variable initialized to 1 or 0 Atomic Methods
P() decrements variable and places process on a waiting list if new value is negative
V() increments variable and removes a process from waiting list if new value is non-positive

10. Semaphores cont… Example mutex = 1; P(mutex); critical section
V(mutex);

11. Private Semaphores
Same as semaphores except only one process will ever perform a P() operation (block) on it while other process can perform a V() (signal) operation on it
Allowed values are -1,0,1

12. Private Semaphores Used to block process while configuring environment
There is a free reader, and a process needing a reader.
Block while the connection is made
Resume after connection

13. System Hierarchy Levels of abstraction Processor Allocation
Segment Control
Message Interpreter
Input and Output Buffering
Independent-user Programs
The Operator

14. Processor Allocation Level
Process Scheduling
Real-Time clock interrupt handler
Prioritization
Above this level, individual processor identity is lost

15. Segment Controller Separate process Drum interrupt handler
Provides bookkeeping for backing store
Above this level, pages lose their identity and all information is accessed via segment addressing

16. Message Interpreter Keyboard interrupt handler
Outputs a request to print the character to the printer
Provides a communication mechanism between running processes and the user

17. Message Interpreter cont…
Above this level, each process appears to have its own conversation consol
One conversation at a time
Multiple conversations are synchronized
Also above this level, the teleprinter looses its identity

18. Input and Output Buffering
Buffering of input streams
Un-buffering of output streams
Notifies the consol when a malfunction is detected
Above this level, the I/O devices loose their individual identity

19. Other Levels The next level is represented by user programs
User programs run on top of the prior Levels
The final level is the Operator

20. Design Experience Two main mistakes
Assuming a “perfect installation”
No thought to debugging during design
The system was proven logically sound prior to construction

21. Design Experience cont…
Conception
Took a long time
Construction
Plain machine code
Verification
Testing was done from the bottom level up
Each level was exhaustively tested prior to the next

22. Dijkstra’s Conclusion
Nothing new in program verification
Effective structuring minimizes testing
He hopes industry will follow this example