Operating System - Processes
Process
A process is basically a program in execution. The execution of
a process must progress in a sequential fashion.
A process is defined as an entity which represents the basic
unit of work to be implemented in the system.
To put it in simple terms, we write our computer programs in a
text file and when we execute this program, it becomes a process which performs
all the tasks mentioned in the program.
When a program is loaded into the memory and it becomes a
process, it can be divided into four sections ─ stack, heap, text and data. The
following image shows a simplified layout of a process inside main memory −
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S.N.
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Component & Description
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1
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Stack
The process Stack contains
the temporary data such as method/function parameters, return address and
local variables.
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2
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Heap
This is dynamically
allocated memory to a process during its run time.
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3
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Text
This includes the current
activity represented by the value of Program Counter and the contents of the
processor's registers.
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4
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Data
This section contains the
global and static variables.
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Program
A program is a piece of code which may be a single line or
millions of lines. A computer program is usually written by a computer
programmer in a programming language. For example, here is a simple program
written in C programming language −
#include <stdio.h>
int main() {
printf("Hello, World! \n");
return 0;
}
A computer program is a collection of instructions that performs
a specific task when executed by a computer. When we compare a program with a
process, we can conclude that a process is a dynamic instance of a computer
program.
A part of a computer program that performs a well-defined task
is known as an algorithm. A collection of computer programs, libraries and
related data are referred to as a software.
Process Life Cycle
When a process executes, it passes through different states.
These stages may differ in different operating systems, and the names of these
states are also not standardized.
In general, a process can have one of the following five states
at a time.
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S.N.
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State & Description
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1
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Start
This is the initial state
when a process is first started/created.
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2
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Ready
The process is waiting to
be assigned to a processor. Ready processes are waiting to have the processor
allocated to them by the operating system so that they can run. Process may
come into this state after Start state or while running it by but
interrupted by the scheduler to assign CPU to some other process.
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3
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Running
Once the process has been
assigned to a processor by the OS scheduler, the process state is set to
running and the processor executes its instructions.
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4
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Waiting
Process moves into the
waiting state if it needs to wait for a resource, such as waiting for user
input, or waiting for a file to become available.
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5
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Terminated or Exit
Once the process finishes
its execution, or it is terminated by the operating system, it is moved to
the terminated state where it waits to be removed from main memory.
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Process Control Block
(PCB)
A Process Control Block is a data structure maintained by the
Operating System for every process. The PCB is identified by an integer process
ID (PID). A PCB keeps all the information needed to keep track of a process as
listed below in the table −
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S.N.
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Information & Description
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1
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Process State
The current state of the
process i.e., whether it is ready, running, waiting, or whatever.
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2
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Process privileges
This is required to
allow/disallow access to system resources.
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3
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Process ID
Unique identification for
each of the process in the operating system.
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4
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Pointer
A pointer to parent
process.
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5
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Program Counter
Program Counter is a
pointer to the address of the next instruction to be executed for this
process.
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6
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CPU registers
Various CPU registers where
process need to be stored for execution for running state.
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7
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CPU Scheduling Information
Process priority and other
scheduling information which is required to schedule the process.
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8
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Memory management
information
This includes the
information of page table, memory limits, Segment table depending on memory
used by the operating system.
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9
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Accounting information
This includes the amount of
CPU used for process execution, time limits, execution ID etc.
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10
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IO status information
This includes a list of I/O
devices allocated to the process.
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The architecture of a PCB is completely dependent on Operating
System and may contain different information in different operating systems.
Here is a simplified diagram of a PCB −
The PCB is maintained for a process throughout its lifetime, and
is deleted once the process terminates.
