1. Sujata Ray Dey Maheshtala College Computer Science Department
Processes
Sujata Ray Dey
Maheshtala College
Computer Science Department

2. Processes Process Concept Process State Process Control Block
Process Scheduling
Schedulers
Context Switch
Idea on Co-operating processes
Processes Sujata Ray Dey (Comp. Sc.)

3. Process Concept Process – Program at the time of execution
Process Vs Programs –
Program consists of Instructions in any Programming Language
Process consists of instructions in machine code
Program reside in Secondary storage devices
Process reside in Main memory
Processes Sujata Ray Dey (Comp. Sc.)

4. Process State
State of Process is determined by the current activity of the process.
New : Process is being created
Running : Process is been executing
Waiting : Process is waiting for some event to occur
Ready : Process is waiting for processor
Terminate : Process has finished execution
Processes Sujata Ray Dey (Comp. Sc.)

5. Diagram of Process State
Processes Sujata Ray Dey (Comp. Sc.)

6. Process Control Block (PCB)
Each process is represented by its own process control block.
A PCB contains –
Process state – running, waiting etc.
Program Counter – location of next executable instruction.
CPU registers
CPU scheduling information
Memory management information
Accounting information
I/O status information
Processes Sujata Ray Dey (Comp. Sc.)

7. Diagram of a PCB
Processes Sujata Ray Dey (Comp. Sc.)

8. CPU switch from Process to Process
Processes Sujata Ray Dey (Comp. Sc.)

9. Process Scheduling Maximize CPU use
Process schedulers select the process to assign CPU
Scheduling queues –
Job queue : set of all processes in the system
Ready queue : set of all processes residing in main memory and waiting for execution
Device queue : set of processes waiting for an I/O device
Processes migrate among various queues
Processes Sujata Ray Dey (Comp. Sc.)

10. Queuing Diagram
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11. Schedulers
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU
It is invoked frequently  must be fast
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue
It is invoked infrequently  may be slow
The long-term scheduler controls the degree of multiprogramming
Processes can be described as either:
I/O-bound process – spends more time doing I/O than computations, many short CPU bursts
CPU-bound process – spends more time doing computations; few very long CPU bursts
Processes Sujata Ray Dey (Comp. Sc.)

12. Addition of Mid-Term Scheduler
The process is swapped out and swapped in by the medium term scheduler. Used to reduce the degree of multiprogramming. Process can be removed from memory and can be reintroduced into memory and execution can be continued where it left off.
Processes Sujata Ray Dey (Comp. Sc.)

13. Context Switch
When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process via a context switch
Context of a process represented in the PCB
Context-switch time is overhead; the system does no useful work while switching
The more complex the OS and the PCB  the longer the context switch
Time dependent on hardware support
Some hardware provides multiple sets of registers per CPU  multiple contexts loaded at once
Processes Sujata Ray Dey (Comp. Sc.)

14. Co-operating Processes
Independent – A process is independent if it cannot affect or affected by the other processes executing in the system.
Co-operating - Any process shares data with other processes is a co-operating process.
Need for process co-operation :
Information sharing
Computation speedup
Modularity
Convenience
Processes Sujata Ray Dey (Comp. Sc.)

15. Co-operating Processes contd..
Information sharing – An environment is provided to allow concurrent access to the shared resources.
Computation Speed up – Parallel execution is done by breaking a task into several subtasks.
Modularity – Construct a system in modular fashion.
Convenience – Even an individual user may have many tasks to work on at one time. Eg, user may be editing, printing, compiling in parallel
Processes Sujata Ray Dey (Comp. Sc.)

16. Thank You
Processes Sujata Ray Dey (Comp. Sc.)