2a Process Concepts

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Operating Systems

Operating Systems-

Process Concept in Operating Systems

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Prepared by:Dr. Subariah Ibrahim, Maznah Kamat, Noh Abd Samad, Muhalim Mohamed Amin

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Operating SystemsOperating Systems

Process Concepts

Thread Conce t

Process Scheduling

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Operating Systems

Chapter 2

PROCESS CONCEPTS

Definition

Process States

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Process Creation & Termination

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Definition A Process is an instance of program execution.

Two processes may come from the samero ram, however their executions are different.

Therefore each one is a different process. Example: If two users are running a word

processor, two processes will be created.

A program is a passive entity whereby it

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compr ses o program co ng, w c s a s a centity. Therefore, a program itself is not a

.

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Components of a Process

Context of a program execution:

-

registers, program counter Other information needed by the operating system

eg. Process state, scheduling information.

Program Data

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parameters, local variables, return address)

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Process States

Process states depict the current activity of a.

When a process is executed, the process will.

changes states after an event occurs.

causes a process to change states

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Two-State Model

ve- a e o e

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Process with Two-State Model

A process can be in one of the following states:

-

- Not-running

Enter ExitRunninNot

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unn ng

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Two-State Model : Queue Diagram

EnterQueue

Dispatch Exit

CPU

Pause

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Five-State Model

New The process is being created

Running Instructions are being executed

Read The rocess is waitin to be

assigned to a CPUWaiting / Blocked The process is waiting for some

event to occur, eg: I/O completion

Exit The process has finished execution

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Note: different operating system assign different names for the states.9


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Five-State Model:

AdmitDispatch

ew ea y unn ngInterru t terminate

event wait

event

occurs

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WaitingExit

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Operating Systems

Five-State Model:

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Process Description

OS controls all the events that occur in a computer

system. ere ore some ata structure s nee e to eep

track of this information

P1 P3P2Virtual

emory

Computer

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CPU I/O I/O I/O Memory

Resources

AllocatedRequest

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Information Table (cont)

OS needs to keep track on the status of each

computer system.

Done by keeping the information in tables asfollows:

Memory Table

I/O Table

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ProcessTable

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Information Table (cont)

Memory Tables Process

Memory

Devices I/O Tables

Image

for

Process 1

FilesProcesses File Tables

Process Tables

Process 1

Process

Image

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Process n

.

.or

Process n

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Process Control Block (PCB) (cont)

Each process has a PCB

Contents of PCB:

a rocess en ca on

Numeric identifiers that may be stored with the processcontrol block include:

Identifier of this process

Identifier of the process that created this process (parent

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User identifier

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(b) Processor State Information

General purpose registers

Control and status registers (Program counter; PSW-,

Process state, scheduling information such as priority,amount of time executed the last time

Memory management information eg. Pointers to page table

Resource ownership and utilization eg. opened files

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processes in a queue

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Operations on Process

OS needs to provide a mechanism for:(a) Process Creation

A process can create another process

parent process The process being created is known as a child

process

Every new process can create another process

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(b) Process Termination

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(a) Process Creation

Operations involved:

Keep the process information in a process table

Allocate space for the process Reasons for Creation:

Submission of a new batch job

A user logs on to a computer system

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printing

Spawned by existing process

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Operating Systems

Example: UNIX Process Hierarchy

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Example: UNIX Process Creation

the processes memory space with a new

program.

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(b) Process Termination

Reasons for Termination :

Program has completed User logs off

Quit an application

Error and fault conditions: Memory unavailable

ro ec on error

Arithmetic error

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Operations in Process Termination:

De-allocate resources21


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Context Switchin 1/2

Changes in information when CPU is switchedfrom one process to another

Store the current process state for the processthat leaves CPU in its PCB.

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allocated CPU from its PCB.

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Context Switchin 2/2

Time taken for context switching is considered asystem overhead.

The speed varies from machine to machine,de endin on the memor s eed, the number ofregisters that must be copied and the existence

of special instructions.

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T ical s eeds are a few milliseconds.

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Operating Systems

Context Switching between 2 processes

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When to Switch a Process

Clock interrupt

Means process has executed for the maximum allowable timeslice

The value of timer is zero

Memory fault memory address is in virtual memory so it must be brought into

main memory

Trap

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may cause process to be moved to Exit state

Su ervisor call/s stem call

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Cooperating Processes (1/2)

Concurrent Processes can be:

Independent processes

canno a ec or e a ec e y e execu on oanother process.

Cooperating processes

can affect or be affected by the execution of

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Cooperating Processes (2/2)

Advantages of process cooperation:

Computation speedup

Convenience (e.g. editing, printing, compiling)

Concurrent execution requires

process communication and process

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Operating Systems

Example:oncurrent

Execution ofrocesses(1/3)

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Operating Systems

Example:

Execution of

(2/3)

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Operating Systems

Example:

Execution of

(3/3)

100 = Starting address ofdispatch program.execution of dispatcherprocess;

1st & 3rd columns court

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2nd & 4th columns showsaddress of instruction being

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Operating Systems

Example: Concurrent Execution of Processes(Process States)

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2a Process Concepts

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