1. GANDHI INSTITUTE FOR EDUCATION & TECHNOLOGY
Baniatangi, Bhubaneswar
FOR 6TH SEM (CSE, ECE, EEE)
Prepared By:
Santosh Kumar Rath

2. Operating System
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Introduction to Operating System:
An operating system is a program that acts as an intermediary between a user of a computer
and the computer hardware. The purpose of an operating system is to provide an environment
in which a user can execute program.
(i) Hardware
System Components
An operating system is an important part of almost every computer system. A computer system
can be divided roughly into four components:
(ii) Operating system
(iii) Applications programs
(iv) The users
1. Hardware
 The hardware- the central processing unit (CPU), the memory, and the input /output
(I/O) devices - provides the basic computing resources for the system.
2. Operating System
 An operating system is a control program.
 A control program controls the execution of user programs to prevent errors and
improper use of the computer.
 It provides a basis for the application program and acts as an intermediary between
the user and computer.
3. Application Program
 It help the user to perform the singular or multiple related tasks.
 It help the program to solve in real work.
 The applications programs- such as compilers, database systems, games, and business
programs- define the ways in which these resources are used to solve the computing
problems of the users.
Simple Batch Systems

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To speed up processing, jobs with similar needs were batched together and were run through
the computer as a group. Thus, the programmers would leave their programs with the
operator. The operator would sort programs into batches with similar requirements and, as the
computer became available, would run each batch. The output from each job would be sent
back to the appropriate programmer.
Demerits
 The lack of interaction between the user and the job while that job is executing.
 In this execution environment, the CPU is often idle.
SPOOLING
The expansion of spooling is the simultaneous peripheral operation on-line .For
example if two or more users issue the print command, and the printer can accept the requests
.Even the printer printing some other jobs. The printer printing one job at the same time the
“spool disk” can load some other jobs.
‘Spool disk’ is a temporary buffer, it can read data from the secondary storage
devices directly. Simultaneously the CPU executing some other job in the spool disk , at the
same time the printer printing the third job. So three jobs are running simultaneously.
Multi-programming Systems:
It is a technique to execute the number of the program simultaneously by a single
processor. In this case the number of the processes are reside in main memory at a time. The
operating system picks and begins to execute one of the job in the main memory.
In non-multiprogramming system , the CPU can execute only one program at a time , if
the running program waiting for any I/O device , the CPU become idle, so it will effect on the
performance of the CPU. But in the multiprogramming the CPU switches from that job to
another job in the job pool. So the CPU is not idle at any time.
This idea is common in other life situations. A lawyer does not have only one client at a time.
Rather, several clients may be in the process of being served at the same time..)
Multiprogramming is the first instance where the operating system must make decisions for the
users.
 Multiprogramming increases CPU utilization .
Advantages:-
 CPU is never idle , so the performance of CPU will increase.
 Throughput of the CPU may also increase.

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Time-Sharing Systems
Time sharing, or multitasking, is a logical extension of multiprogramming. Multiple jobs are
executed by the CPU switching between them, but the switches occur so frequently that the
users may interact with each program while it is running.
Time-sharing systems were developed to provide interactive use of a computer system at a
reasonable cost. A time-shared operating system uses CPU scheduling and multiprogramming
to provide each user with a small portion of a time-shared computer.
Time-sharing operating systems are CTTS , Multics , Cal ,Unix .
Distributed Systems
 Increases computation speed
A distributed system is a collection of physically separate, possibly heterogeneous computer
systems that are networked to provide the users with access to the various resources that the
system maintains. The processor can’t share the memory or clock , each processor has its.
Advantages:
 Increases functionality
 Increases the data availability, and reliability.
Network Operating System :
A network operating system is an operating system that provides features such as file sharing
across the network and that includes a communication scheme that allows different processes
on different computers to exchange messages. A computer running a network operating
system acts autonomously from all other computers on the network, although it is aware of the
network and is able to communicate with other networked computers.
Parallel System:
A system consisting of more than one processor and it is tightly coupled then it is known as
parallel system. In parallel systems number of the processor executing their job parallel.
Tightly Coupled:-The system having more than one processor in close communication ,sharing
the computer bus , the clock , and some times memory and peripheral devices. These are
referred to as the “tightly coupled” system.

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Advantages:-
• Increases throughput.
• Increases reliability.
A real-time operating system (RTOS) is an
Special-Purpose Systems:
The discussion thus far has focused on general-purpose computer systems that we are all
familiar with. There are, however, different classes of computer systems whose functions are
more limited and whose objective is to deal with limited computation domains.
Real-Time Embedded Systems
operating system (OS) intended to serve real-
time application requests. It must be able to process data as it comes in, typically without buffering
delays. A real-time system is used when rigid time requirements have been placed on the
operation of a processor or the flow of data; thus, it is often used as a control device in a
dedicated application. Sensors bring data to the computer. The computer must analyze the
data and possibly adjust controls to modify the sensor inputs. Systems that control scientific
experiments, medical imaging systems, industrial control systems, and certain display systems
are realtime systems. Some automobile-engine fuel-injection systems, home-appliance
controllers, and weapon systems are also real-time systems.
A real-time system has well-defined, fixed time constraints. Processing must be done within the
defined constraints, or the system will fail. For instance, it would not do for a robot arm to be
instructed to halt after it had smashed into the car it was building. A real-time system functions
correctly only if it returns the correct result within its time constraints. Contrast this system with
a time-sharing system, where it is desirable (but not mandatory) to respond
quickly, or a batch system, which may have no time constraints at all.

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SYSTEM STRUCTURES
 User interface: All OS have a user interface(UI).Interfaces are of three types-
Command Line Interface: uses text commands and a method for entering them Batch
interface: commands and directives to control those commands are entered into
files and those files are executed. Graphical user interface: This is a window
system with a pointing device to direct I/O, choose from menus and make
selections and a keyboard to enter text.
Operating-System Services
OS provides an environment for execution of programs. It provides certain services to
programs and to the users of those programs. OS services are provided for the
convenience of the programmer, to make the programming task easier.
One set of SOS services provides functions that are helpful to the user –
 Program execution: System must be able to load a program into memory
and run that program. The program must be able to end its execution either
normally or abnormally.
 I/O operations: A running program may require I/O which may involve a file
or an I/O device. For efficiency and protection, users cannot control I/O devices
directly.
 File system manipulation: Programs need to read and write files and
directories. They also need to create and delete them by name, search for a
given file, and list file information.
 Communications: One process might need to exchange information with another
process.Such communication may occur between processes that are executing on
the same computer or between processes that are executing on different
computer systems tied together by a computer network. Communications may be
implemented via shared memory or through message passing.
 Error detection: OS needs to be constantly aware of possible errors. Errors may
occur in the CPU and memory hardware, in I/O devices and in the user program. For
each type of error, OS takes appropriate action to ensure correct and consistent
computing.

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Another set of OS functions exist for ensuring efficient operation of the system. They are-
a. Resource allocation: When there are multiple users or multiple jobs running at the same
time, resources must be allocated to each of them. Different types of resources such
as CPU cycles, main memory and file storage are managed by the operating system.
b. Accounting: Keeping track of which users use how much and what kinds of
computer resources.
c. Protection and security: Controlling the use of information stored in a multiuser or
networked computer system. Protection involves ensuring that all access to system
resources is controlled. Security starts with requiring each user to authenticate himself or
herself to the system by means of password and to gain access to system resources.
An example to illustrate how system calls are used:
System Calls
System calls provide an interface to the services made available by an operating system.
Writing a simple program to read data from one file and copy them to another file-

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a) First input required is names of two files – input file and output file. Names can
be specified in many ways- One approach is for the program to ask the user for the names of
two files. In an interactive system, this approach will require a sequence of system
calls, to write a prompting message on screen and then read from the keyboard the
characters that define the two files. On mouse based and icon based systems, a menu of
file names is displayed in a window where the user can use the mouse to select the
source names and a window can be opened for the destination name to be specified.
b) Once the two file names are obtained, program must open the input file and
create the output file. Each of these operations requires another system call. Possible error
conditions –When the program tries to open input file, no file of that name may exist
or file is protected against access. Program prints a message on console and terminates
abnormally. If input file exists, we must create a new output file. If the output file with the
same name exists, the situation caused the program to abort or delete the existing file and
create a new one. Another option is to ask the user(via a sequence of system calls) whether to
replace the existing file or to abort the program.
When both files are set up, a loop reads from the input file and writes to the output file
(system calls respectively). Each read and write must return status information regarding
various possible error conditions. After entire file is copied, program closes both files,
write a message to the console or window and finally terminate normally.
Application developers design programs according to application programming interface
(API). API specifies set of functions that are available to an application programmer.
The functions that make up the API typically invoke the actual system calls on behalf
of the application programmer. Benefits of programming rather than invoking actual system
calls:
 Program portability – An application programmer designing a program using an API
can expect program to compile and run on any system that supports the same API.

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 Actual system calls can be more detailed and difficult to work with than the
API available to an application programmer.
The run time support system ( a set of functions built into libraries included with a compiler) for
most programming languages provides a system call interface that serves as a link to
system calls made available by OS. The system call interface intercepts function calls
in the API and invokes the necessary system call within the operating system. A number is
associated with each system call and the system call interface maintains a table indexed
according to these numbers. System call interface then invokes the intended system call
in the OS kernel and returns the status of the system call and return any values.
System calls occur in different ways, depending on the computer in use – more
information is required than simply the identity of the desired system call. The exact
type and amount of information vary according to the particular OS and call.
Three general methods are used to pass parameters to OS-
I. Pass the parameters in registers
II. Storing parameters in blocks or tables in memory and the address of the block id passed as
a parameter in a register
III. Placing or pushing parameters onto the stack by the program and popping off the stack by
the OS.
• end, abort
Types of system calls
Five major categories:
1) Process control
• load, execute
• create process, terminate process
• get process attributes, set process attributes
• wait for time
• wait event, signal event
• allocate and free memory
2) File Management

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• create file, delete file
• open, close
• read, write, reposition
• get file attributes, set file attributes
3) Device management
• request device, release device
• read, write, reposition
• get device attributes, set device attributes
• logically attach or detach devices
4) Information maintenance
• get time or date, set time or date
• get system data, set system data
• get process, file or device attributes
• set process, file or device attributes
5) Communications
• create, delete communication connection
• send, receive messages
• transfer status information
• attach or detach remote devices

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System programs provide a convenient environment for program development and
execution. They can be divided into these categories-
File management: These programs create, delete, copy, rename, print, dump, list and
manipulate files and directories.
Status information: Some programs ask the system for the date, time, and amount
of available memory or disk space, number of users.
File modification: Text editors may be available to create and modify the content of files
stored on disk or other storage devices.
Programming language support: Compilers, assemblers, debuggers and interpreters for
common programming languages are often provided to the user with the OS.
Program loading and execution: Once a program is assembled or compiled, it must
be loaded into memory to be executed. System provides absolute loaders, relocatable
loaders, linkage editors and overlay loaders.
Communications: These programs provide the mechanism for creating virtual connections
among processes, users and computer systems.
System Programs

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In addition to systems programs, OS are supplied with programs that are useful in
solving common problems or performing operations. Such programs include web
browsers, word processors and text formatters, spread sheets, database systems etc.
These programs are known as system utilities or application programs.
PROCESS MANAGEMENT
Current day computer systems allow multiple programs to be loaded into memory and
executed concurrently. Process is nothing but a program in execution. A process is the unit
of work in a modern time sharing system. A system is a collection of processes:
operating system processes executing operating system code and user processes
executing user code. By switching CPU between processes, the operating system can make
the computer more productive.
Overview
A batch system executes jobs whereas a time shared system has user programs or
tasks. On a single user system, user may be able to run several programs at one
time: a word processor, a web browser and an e-mail package. All of these are called
processes.
The Process
A process is a program in execution. A process is more than a program code sometimes known
as text section. It also includes the current activity as represented by the value of the program
counter and the contents of the processor’s registers. A process generally also includes the
process stack which contains temporary data and a data section which contains global
variables.
A process may also include a heap which is memory that is dynamically allocated
during process run time. Program itself is not a process, a program is a passive entity
such as a file containing a list of instructions stored on disk (called an executable file)
whereas process is an active entity with a program counter specifying the next
instruction to execute and a set of associated resources. A program becomes a
process when an executable file is loaded into memory. Although two processes may be
associated with the same program, they are considered two separate execution sequences.
Process State

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As a process executes, it changes state. The state of a process is defined in part by the
current activity of that process. Each process may be in one of the following states-
New: The process is being created.
Running: Instructions are being executed.
Waiting: The process is waiting for some event to occur.
Ready: The process is waiting to be assigned to the processor.
Terminated: The process has finished execution.
Process Control Block
Each process is represented in the operating system by a process control block (PCB)
also called task control block. It contains many pieces of information associated with a
specific process including:

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Process State: The state may be new, ready, running, waiting, halted etc.
Process Number: Unique number which is assigned to a program at time of its execution.
Program Counter: The counter indicates the address of the next instruction to be executed for
this process.
CPU registers: The registers vary in number and type depending on the computer architecture.

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CPU scheduling information: This information includes a process priority, pointers to scheduling
queues, and other scheduling parameters.
Memory management information: This information may include such information as the value
of base and limit registers etc.
Accounting information: This information includes the amount of CPU and real time used, time
limits etc.
I/O status information: This information includes the list of I/O devices allocated to the process,
etc.
Threads
Process is a program that performs a single thread of execution. A single thread of
control allows the process to perform only one task at one time.
Process Scheduling
The objective of multi programming is to have some process running at all times to
maximize CPU utilization. The objective of time sharing is to switch the CPU among
processes so frequently that users can interact with each program while it is running.
To meet these objectives, the process scheduler selects an available process for
program execution on the CPU. For a single processer system, there will never be more than
one running process.

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Scheduling Queues
Job queue: As processes enter the system, they are put inside the job queue, which consists of
all processes in the system.
Ready queue: The processes that are residing in main memory and are ready and
waiting to execute are kept on a list called the ready queue. This queue is stored as a
linked list. A ready queue header contains pointers to the first and final PCB’s in the list. Each
PCB includes a pointer field that points to the next PCB in the ready queue.
Device queue: When a process is allocated the CPU, it executes for a while and eventually
quits, is interrupted or waits for the occurrence of a particular event such as the completion of
an I/O request. The list of processes waiting for a particular I/O device is called a device
queue. Each device has its own device queue.
Queuing diagram
A common representation for process scheduling is queuing diagram. Each rectangular box
represents a queue. Two types of queues are present: ready queue and a set of
device queues. Circles represent the resources that serve the queues and the arrows
indicate the flow of processes in the system.

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A new process is initially put in the ready queue. It waits there until it is selected for execution
or is dispatched. Once the process is allocated the CPU and is executing, one of the
following events might occur –
a) The process could issue and I/O request and then be placed in the I/O queue.
b) The process could create a new sub process and wait for the sub process’s termination.
c) The process could be removed forcibly from the CPU as a result of an interrupt
and be put back in the ready queue.
In a batch system, more processes are submitted than can be executed immediately.
These processes are spooled to a mass storage device (disk) where they are kept for later
execution. The long term scheduler or job scheduler selects processes from this pool
and loads them into memory for execution. The short term scheduler or CPU scheduler
selects from among the processes that are ready to execute and allocates the CPU to one of
them. The distinction between these two lies in the frequency of execution. The long
term scheduler controls the degree of multi programming (the number of processes in memory).
Most processes can be described as either I/O bound or CPU bound. An I/O bound process is
one that spends more of its time doing I/O than it spends doing computations. A CPU bound
process generates I/O requests infrequently using more of its time doing computations.
Schedulers
A process migrated among the various scheduling queues throughout its life time. The OS must
select for scheduling purposes, processes from these queues and this selection is
carried out by scheduler.

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Long term scheduler must select a good mix of I/O bound and CPU bound processes.
A system with best performance will have a combination of CPU bound and I/O bound
processes.
Some operating systems such as time sharing systems may introduce
additional/intermediate level of scheduling. Idea behind medium term scheduler is that
sometimes it can be advantageous to remove processes from memory and thus reduce
the degree of multiprogramming. Later the process can be reintroduced into memory
and its execution can be continued where it left off. This scheme is called swapping. The
process is swapped out and is later swapped in by the medium term scheduler.
Context switch
Interrupts cause the OS to change a CPU from its current task and to run a kernel routine.
When an interrupt occurs, the system needs to save the current context of the process currently
running on the CPU so that it can restore that context when its processing is done, essentially
suspending the process and then resuming it. The context is represented in the PCB of
the process, it includes the value of CPU registers, process state and memory
management information. We perform a state save of the current state of the CPU and then
state restore to resume operations.
Switching the CPU to another process requires performing a state save of the current process
and a state restore of a different process. This task is known as context switch. Kernel saves
the context of the old process in its PCB and loads the saved context of the new process
scheduled to run. Context switch time is pure overhead. Its speed varies from machine to
machine depending on the memory speed, the number of registers that must be copied
and the existence of special instructions. Context switch times are highly dependent on
hardware support.

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Multithreading
Thread is a basic unit of CPU utilization. Support for threads may be provided either at the
user level for user threads or by the kernel for kernel threads. User threads are
supported above the kernel and are managed without kernel support whereas kernel
threads are supported and managed directly by the operating system.
In a single processor system, only one process can run at a time; others must wait until CPU is
free and can be rescheduled. The objective of multi programming is to have some
process running at all times to maximize CPU utilization. Under multi programming,
several processes are kept in memory at one time. When one process has to wait, the OS
Benefits of multi threaded programming are:
Responsiveness: Multi threading an interactive application may allow a program to
continue running even if part of it is blocked or is performing a lengthy operation,
thereby increasing responsiveness to the user.
Resource interaction: Threads share the memory and the resources of the process to which they
belong. The benefit of sharing code and data is that it allows an application to
have several different threads of activity within the same address space.
Economy: Allocating memory and resources for process creation is costly. Because threads
share resources of the process to which they belong, it is more economical to create and
context switch threads.
Utilization of multi processor architecture: The benefits of multi threading can be
greatly increased in a multi processor architecture where threads may be running in parallel
on different processors. Multi threading on a multi CPU machine increases concurrency.
User threads are supported above the kernel and are managed without kernel
support whereas kernel threads are supported and managed directly by the operating
system.
CPU scheduling is the basis of multi programmed operating systems. By switching CPU among
processes, the OS can make the computer more productive. On operating systems that
support threads, it is kernel level threads and not processes that are scheduled by operating
system. But process scheduling and thread scheduling are often used interchangeably.

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takes the CPU away from that process and gives the CPU to another process. As CPU is one
of the primary computer resources, its scheduling is central to operating system design.
Three common ways of establishing a relationship between user level and kernel level
threads are:
Many to one model - Maps many user level threads to one kernel thread. Thread
management is done by the thread library in user space hence it is efficient but
entire process will block if a thread makes a blocking system call. As only one thread can
access the kernel at a time, multiple threads are unable to run in parallel on multi processors.
One to one model - Maps each user thread to kernel thread. It allows more
concurrency by allowing another thread to run when a thread makes a blocking
system call; it also allows multiple threads to run in parallel on multi processors.
Disadvantage is that creating a user thread requires creating the corresponding kernel thread.
Many to many model - Multiplexes many user level threads to a smaller or equal
number of kernel level threads. The number of kernel threads may be specific to
either a particular application or a particular machine. Developers can create as many user
threads as necessary and the corresponding kernel threads can run in parallel on a

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multi processor. Also, when a thread performs a blocking system call, the kernel can
schedule another thread for execution.
On operating systems that support threads, it is kernel level threads that are being scheduled
by operating system. User level threads are managed by thread library. To run on a CPU,
user level threads must be mapped to an associated kernel level thread although this
mapping may be indirect and may use a light weight process.
PROCESS SCHEDULING:
Process is an executing program with a single thread of control. Most operating systems
provide features enabling a process to contain multiple threads of control. A thread is a basic
unit of CPU utilization; it comprises of a thread ID, a program counter, a register set
and a stack. It shares with other threads belonging to the same process its code
section, data section and other operating system resources such as open files and signals. A
traditional or heavy weight process has a single thread of control.
CPU – I/O burst cycle:
Process execution consists of a cycle of CPU execution and I/O wait. Processes
alternate between these two states. Process execution begins with CPU burst followed by an
I/O burst and so on. The final CPU burst ends with a system request to terminate execution.

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An I/O bound program has many short CPU bursts. A CPU bound program might
have a few long CPU bursts.
CPU scheduler:
Whenever the CPU becomes idle, the operating system must select one of the
processes in the ready queue to be executed. The selection process is carried out by
the short term scheduler or CPU scheduler. The scheduler selects a process from the
processes in memory that are ready to execute and allocates the CPU to that process.
The ready queue is not necessarily a first in first out queue. A ready queue can be
implemented as a FIFO queue, a priority queue, a tree or an unordered linked list.
All the processes in the ready queue are lined up waiting for a chance to run on the
CPU. The records in the queue are process control blocks of the processes.
Pre-emptive scheduling:
CPU scheduling decisions may take place under the following four conditions-

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a) When a process switches from the running state to the waiting state
b) When a process switches from the running state to the ready state
c) When a process switches from the waiting state to the ready state
d) When a process terminates
When scheduling takes place under conditions 1 and 4, scheduling scheme is non - pre emptive
or co-operative. Else it is called pre emptive.
Under non pre emptive scheduling, once the CPU has been allocated to a process,
the process keeps the CPU until it releases the CPU either by terminating or by
switching to the waiting state. Pre emptive scheduling incurs cost associated with access to
shared data. Pre emption also affects the design of the operating system kernel.
Dispatcher:
The dispatcher is the module that gives control of the CPU to the process selected by
the short term scheduler. This function involves-
a) Switching context
b) Switching to user mode
c) Jumping to the proper location in the user program to restart that program
Dispatcher should be as fast as possible since it is invoked during every process switch. The
time it takes for the dispatcher to stop one process and start another running is
called dispatch latency.
Scheduling criteria:
Different CPU scheduling algorithms have different properties. Criteria for comparing CPU
scheduling algorithms-
a) CPU utilization: Keep the CPU as busy as possible
b) Through put: One measure of work of CPU is the number of processes that are completed
per time unit called through put.

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c) Turnaround time: The interval from the time of submission of a process to the time
of completion is the turnaround time. Turnaround time is the sum of the periods spent
waiting to get into memory, waiting in the ready queue, executing on the CPU and doing I/O.
d) Waiting time: It is the sum of the periods spent waiting in the ready queue.
e) Response time: The time it takes for the process to start responding but is not the time it
takes to output the response.
It is desirable to maximize CPU utilization and through put and to minimize
turnaround time, waiting time and response time.
Scheduling algorithms:
First Come First Serve:
This is the simplest CPU scheduling algorithm. The process that requests the CPU first
is allocated the CPU first. The implementation of FCFS is managed with a FIFO queue.
When a process enters the ready queue, its PCB is linked onto the tail of the queue.
When the CPU is free, it is allocated to the process at the head of the queue. The running
process is then removed from the queue.
The average waiting time under FCFS is often quite long. The FCFS scheduling algorithm is non
pre-emptive.
Once the CPU has been allocated to a process, that process keeps the CPU until it
releases the CPU either by terminating or by requesting I/O. It is simple, fair, but
poor performance. Average queuing time may be long.

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Shortest Job First:
This algorithm associates with each process the length of the process’s next CPU burst. When
the CPU is available, it is assigned to the process that has the smallest next CPU burst. If the
next CPU bursts of two processes are the same, FCFS scheduling is used to break the tie.
Priority
The SJF is a special case of the general priority scheduling algorithm. A priority is
associated with each process and CPU is allocated to the process with highest priority.
Equal priority processes are scheduled in FCFS order. An SJF algorithm is simply a
priority algorithm where the priority is the inverse of the predicted next CPU burst.
:

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The larger the CPU burst, the lower the priority. Priorities are generally indicated by some
fixed range of numbers usually 0 to 7.
Priorities can be defined either internally or externally. Internally defined priorities use
some measurable quantity to computer the priority of a process. External priorities are
set by criteria outside the operating system.
Priority scheduling can be either pre emptive or non pre emptive. When a process arrives at
the ready queue, its priority is compared with the priority of the currently running
process. A preemptive priority scheduling algorithm will preempt the CPU is the
priority of the newly arrived process is higher than the priority of the currently
running process. A non preemptive priority scheduling algorithm will put the new process at
the head of the ready queue.
The major problem with priority scheduling algorithm is indefinite blocking or
starvation. A process that is ready to run but waiting for the CPU can be considered
blocked. A priority scheduling algorithm can leave some low priority processes waiting
indefinitely. A solution to the problem of indefinite blockage of low priority processes
is aging. Aging is a technique of gradually increasing the priority of processes that wait in
the system for a long time.
Round Robin
The process may either have a CPU burst of less than 1 time quantum. Then, the
process itself will release the CPU voluntarily. The scheduler will then proceed to the next
process in the ready queue. Else if the CPU burst of the currently running process is longer than
1 time quantum, the timer will go off and will cause an interrupt to the operating
:
The round robin scheduling algorithm is designed for time sharing systems. It is similar to FCFS
scheduling but preemption is added to switch between processes. A small unit of time
called a time quantum or time slice is defined. A time quantum is generally from 10 to 100
milli seconds. The ready queue is treated as a circular queue. The CPU scheduler goes around
the ready queue allocating the CPU to each process for a time interval of up to 1
time quantum. To implement RR scheduling, ready queue is implemented as a FIFO
queue. New processes are added to the tail of ready queue. The CPU scheduler
picks the first process from the ready queue, sets the timer to interrupt after 1 time
quantum and dispatches the process.

27. Operating System
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system. A context switch will be executed and the process will be put at the tail of the
ready queue. The CPU scheduler will then select the next process in the ready queue.
In the RR scheduling algorithm, no process is allocated the CPU for more than 1 time quantum
in a row. If a process’s CPU burst exceeds 1 time quantum, that process is preempted and is
put back in the ready queue. Thus this algorithm is preemptive. The performance of the RR
algorithm depends heavily on the size of time quantum. If the time quantum is extremely
large, the RR policy is same as the FCFS policy. If the time quantum is extremely
small, the RR approach is called processor sharing.
Algorithm evaluation
Selection of an algorithm is difficult. The first problem is defining the criteria to be
used in selecting an algorithm. Criteria are often defined in terms of CPU utilization,
response time or through put. Criteria includes several measures such as-
a) Maximizing CPU utilization under the constraint that the maximum response time is
1 second.
b) Maximizing through put such that turnaround time is linearly proportional to total
execution time.

28. Operating System
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