Operating Systems(KCS-401)

UNIT-1
Introduction
An Operating System (OS) is an interface between a computer user and
computer hardware.
An operating system is a software which performs all the basic tasks like file
management, memory management, process management, handling input
and output, and controlling peripheral devices such as disk drives and printers.
Examples of Operating System are –
• Windows (GUI based, PC)
• GNU/Linux (Personal, Workstations, ISP, File and print server, Three-tier client/Server)
• macOS (Macintosh), used for Apple’s personal computers and work stations (MacBook,
iMac).
• Android (Google’s Operating System for smartphones/tablets/smartwatches)
• iOS (Apple’s OS for iPhone, iPad and iPod Touch)

Definitions
An operating system is a program that acts as an interface between the user and the
computer hardware and controls the execution of all kinds of programs.
A more common definition is that the operating system is the one program running at all
times on the computer (usually called the kernel), with all else being application programs.

An operating system is concerned with the allocation of resources and services, such as
memory, processors, devices, and information. The operating system correspondingly
includes programs to manage these resources, such as a traffic controller, a scheduler,
memory management module, I/O programs, and a file system.

Functions of an operating System:

 • Memory Management
• Processor Management
• Device Management
• File Management
• Security
• Control over system performance
• Job accounting
• Error detecting aids
• Coordination between other software and users
Memory Management
Memory management refers to management of Primary Memory or Main
Memory. Main memory is a large array of words or bytes where each word or
byte has its own address.
Main memory provides a fast storage that can be accessed directly by the
CPU. For a program to be executed, it must in the main memory. An Operating
System does the following activities for memory management −
• Keeps tracks of primary memory, i.e., what part of it are in use by whom,
what part are not in use.
• In multiprogramming, the OS decides which process will get memory
when and how much.
• Allocates the memory when a process requests it to do so.
• De-allocates the memory when a process no longer needs it or has been
terminated.
Processor Management
In multiprogramming environment, the OS decides which process gets the
processor when and for how much time. This function is called process
scheduling. An Operating System does the following activities for processor
management −
• Keeps tracks of processor and status of process. The program
responsible for this task is known as traffic controller.
• Allocates the processor (CPU) to a process.
• De-allocates processor when a process is no longer required.
Device Management
An Operating System manages device communication via their respective
drivers. It does the following activities for device management −
• Keeps tracks of all devices. Program responsible for this task is known as
the I/O controller.
• Decides which process gets the device when and for how much time.
• Allocates the device in the efficient way.
• De-allocates devices.
File Management
A file system is normally organized into directories for easy navigation and
usage. These directories may contain files and other directions.
An Operating System does the following activities for file management −
• Keeps track of information, location, uses, status etc. The collective
facilities are often known as file system.
• Decides who gets the resources.
• Allocates the resources.
• De-allocates the resources.
Other Important Activities
Following are some of the important activities that an Operating System
performs −
• Security − By means of password and similar other techniques, it
prevents unauthorized access to programs and data.
• Control over system performance − Recording delays between request
for a service and response from the system.
• Job accounting − Keeping track of time and resources used by various jobs
and users.

• Error detecting aids − Production of dumps, traces, error messages, and

other debugging and error detecting aids.

• Coordination between other softwares and users − Coordination and

assignment of compilers, interpreters, assemblers and other software to the
various users of the computer systems.
Types of Operating System:

Operating systems are there from the very first computer generation and they
keep evolving with time. In this chapter, we will discuss some of the important
types of operating systems which are most commonly used.
Batch operating system
The users of a batch operating system do not interact with the computer
directly. Each user prepares his job on an off-line device like punch cards and
submits it to the computer operator. To speed up processing, jobs with similar
needs are batched together and run as a group. The programmers leave their
programs with the operator and the operator then sorts the programs with
similar requirements into batches.
The problems with Batch Systems are as follows −

• Lack of interaction between the user and the job.

• CPU is often idle, because the speed of the mechanical I/O devices is
slower than the CPU.
• Difficult to provide the desired priority.
Time-sharing operating systems
Time-sharing is a technique which enables many people, located at various
terminals, to use a particular computer system at the same time. Time-sharing
or multitasking is a logical extension of multiprogramming. Processor's time
which is shared among multiple users simultaneously is termed as
timesharing.
The main difference between Multiprogrammed Batch Systems and
TimeSharing Systems is that in case of Multiprogrammed batch systems, the
objective is to maximize processor use, whereas in Time-Sharing Systems, the
objective is to minimize response time.
Multiple jobs are executed by the CPU by switching between them, but the
switches occur so frequently. Thus, the user can receive an immediate
response. For example, in a transaction processing, the processor executes
each user program in a short burst or quantum of computation. That is, if
nusers are present, then each user can get a time quantum. When the user
submits the command, the response time is in few seconds at most.
The operating system uses CPU scheduling and multiprogramming to provide
each user with a small portion of a time. Computer systems that were
designed primarily as batch systems have been modified to time-sharing
systems.
Advantages of Timesharing operating systems are as follows −

• Provides the advantage of quick response.

• Avoids duplication of software.
• Reduces CPU idle time.
Disadvantages of Time-sharing operating systems are as follows −

• Problem of reliability.
• Question of security and integrity of user programs and data.
• Problem of data communication.
Distributed operating System
Distributed systems use multiple central processors to serve multiple realtime
applications and multiple users. Data processing jobs are distributed among
the processors accordingly.
The processors communicate with one another through various
communication lines (such as high-speed buses or telephone lines). These are
referred as loosely coupled systems or distributed systems. Processors in a
distributed system may vary in size and function. These processors are
referred as sites, nodes, computers, and so on.
The advantages of distributed systems are as follows −

• With resource sharing facility, a user at one site may be able to use the
resources available at another.
• Speedup the exchange of data with one another via electronic mail.
• If one site fails in a distributed system, the remaining sites can
potentially continue operating.
• Better service to the customers.
• Reduction of the load on the host computer.
• Reduction of delays in data processing.
Network operating System
A Network Operating System runs on a server and provides the server the
capability to manage data, users, groups, security, applications, and other
networking functions. The primary purpose of the network operating system
is to allow shared file and printer access among multiple computers in a
network, typically a local area network (LAN), a private network or to other
networks.
Examples of network operating systems include Microsoft Windows Server
2003, Microsoft Windows Server 2008, UNIX, Linux, Mac OS X, Novell
NetWare, and BSD.
The advantages of network operating systems are as follows −

• Centralized servers are highly stable.

• Security is server managed.
• Upgrades to new technologies and hardware can be easily integrated
into the system.
• Remote access to servers is possible from different locations and types
of systems.
The disadvantages of network operating systems are as follows −

• High cost of buying and running a server.

• Dependency on a central location for most operations.
• Regular maintenance and updates are required.
Real Time operating System
A real-time system is defined as a data processing system in which the time
interval required to process and respond to inputs is so small that it controls
the environment. The time taken by the system to respond to an input and
display of required updated information is termed as the response time. So in
this method, the response time is very less as compared to online processing.

Dual Mode operations in OS

An error in one program can adversely effect many processes, it might modify
data of another program, or also can effect the operating system. For
example, if a process stuck in infinite loop then this infinite loop could effect
correct operation of other processes. So to ensure the proper execution of the
operating system there are two modes of operation:
User Mode –
When the computer system run user application like creating a text document
or using any application program, then the system is in user mode. When the
user application requests for a service from the operating system or an
interrupt occurs or system call, then there there will be a transition from user
to kernel mode to fulfill the requests.
Note: To switch from kernel mode to user mode, mode bit should be 1.

Given below image describes what happen interrupt occurs:

Kernel Mode –
When system boots then hardware starts in kernel mode and when operating
system is loaded then it start user application in user mode. To provide
protection to the hardware, we have privileged instructions which execute only
in kernel mode. If user attempt to run privileged instruction in user mode then
it will treat instruction as illegal and traps to OS. Some of the privileged
instructions are:
1. Handling Interrupts
2. To switch from user mode to kernel mode.
3. Input Output management.

Operating System Properties

Batch processing
Batch processing is a technique in which an Operating System collects the
programs and data together in a batch before processing starts. An operating
system does the following activities related to batch processing −
• The OS defines a job which has predefined sequence of commands,
programs and data as a single unit.
• The OS keeps a number a jobs in memory and executes them without
any manual information.
• Jobs are processed in the order of submission, i.e., first come first served
fashion.
• When a job completes its execution, its memory is released and the
output for the job gets copied into an output spool for later printing or
processing.

Advantages
• Batch processing takes much of the work of the operator to the
computer.
• Increased performance as a new job get started as soon as the previous
job is finished, without any manual intervention.
Disadvantages

• Difficult to debug program.

• A job could enter an infinite loop.
• Due to lack of protection scheme, one batch job can affect pending jobs.
Multitasking
Multitasking is when multiple jobs are executed by the CPU simultaneously by
switching between them. Switches occur so frequently that the users may
interact with each program while it is running. An OS does the following
activities related to multitasking −
• The user gives instructions to the operating system or to a program
directly, and receives an immediate response.
• The OS handles multitasking in the way that it can handle multiple
operations/executes multiple programs at a time.
• Multitasking Operating Systems are also known as Time-sharing systems.
• These Operating Systems were developed to provide interactive use of a
computer system at a reasonable cost.
• A time-shared operating system uses the concept of CPU scheduling and
multiprogramming to provide each user with a small portion of a
timeshared CPU.
• Each user has at least one separate program in memory.

• A program that is loaded into memory and is executing is commonly

referred to as a process.
• When a process executes, it typically executes for only a very short time
before it either finishes or needs to perform I/O.
• Since interactive I/O typically runs at slower speeds, it may take a long
time to complete. During this time, a CPU can be utilized by another
process.
• The operating system allows the users to share the computer
simultaneously. Since each action or command in a time-shared system
tends to be short, only a little CPU time is needed for each user.
 • As the system switches CPU rapidly from one user/program to the next,
each user is given the impression that he/she has his/her own CPU,
whereas actually one CPU is being shared among many users.
Multiprogramming
Sharing the processor, when two or more programs reside in memory at the
same time, is referred as multiprogramming. Multiprogramming assumes a
single shared processor. Multiprogramming increases CPU utilization by
organizing jobs so that the CPU always has one to execute.
The following figure shows the memory layout for a multiprogramming
system.

An OS does the following activities related to multiprogramming.

• The operating system keeps several jobs in memory at a time.
• This set of jobs is a subset of the jobs kept in the job pool.
• The operating system picks and begins to execute one of the jobs in the
memory.
• Multiprogramming operating systems monitor the state of all active
programs and system resources using memory management programs
to ensures that the CPU is never idle, unless there are no jobs to
process.
Advantages

• High and efficient CPU utilization.

• User feels that many programs are allotted CPU almost simultaneously.
Disadvantages

• CPU scheduling is required.

 • To accommodate many jobs in memory, memory management is
required.
Interactivity
Interactivity refers to the ability of users to interact with a computer system.
An Operating system does the following activities related to interactivity −

• Provides the user an interface to interact with the system.

• Manages input devices to take inputs from the user. For example,
keyboard.
• Manages output devices to show outputs to the user. For example,
Monitor.
The response time of the OS needs to be short, since the user submits and
waits for the result.
Real Time System
Real-time systems are usually dedicated, embedded systems. An operating
system does the following activities related to real-time system activity.

• In such systems, Operating Systems typically read from and react to

sensor data.
• The Operating system must guarantee response to events within fixed
periods of time to ensure correct performance.
Distributed Environment
A distributed environment refers to multiple independent CPUs or processors
in a computer system. An operating system does the following activities
related to distributed environment −
• The OS distributes computation logics among several physical
processors.
• The processors do not share memory or a clock. Instead, each processor
has its own local memory.
• The OS manages the communications between the processors. They
communicate with each other through various communication lines.
Spooling
Spooling is an acronym for simultaneous peripheral operations on line.
Spooling refers to putting data of various I/O jobs in a buffer. This buffer is a
special area in memory or hard disk which is accessible to I/O devices.
An operating system does the following activities related to distributed
environment −
• Handles I/O device data spooling as devices have different data access
rates.
• Maintains the spooling buffer which provides a waiting station where
data can rest while the slower device catches up.
• Maintains parallel computation because of spooling process as a
computer can perform I/O in parallel fashion. It becomes possible to
have the computer read data from a tape, write data to disk and to
write out to a tape printer while it is doing its computing task.

Advantages

• The spooling operation uses a disk as a very large buffer.

• Spooling is capable of overlapping I/O operation for one job with
processor operations for another job.

Operating System Services

An Operating System provides services to both the users and to the programs.

• It provides programs an environment to execute.

• It provides users the services to execute the programs in a convenient
manner.
Following are a few common services provided by an operating system −

• Program execution
• I/O operations
• File System manipulation
• Communication
• Error Detection
• Resource Allocation
• Protection
Program execution
Operating systems handle many kinds of activities from user programs to
system programs like printer spooler, name servers, file server, etc. Each of
these activities is encapsulated as a process.
A process includes the complete execution context (code to execute, data to
manipulate, registers, OS resources in use). Following are the major activities
of an operating system with respect to program management −

• Loads a program into memory.

• Executes the program.
• Handles program's execution.
• Provides a mechanism for process synchronization.
• Provides a mechanism for process communication.
• Provides a mechanism for deadlock handling.
I/O Operation
An I/O subsystem comprises of I/O devices and their corresponding driver
software. Drivers hide the peculiarities of specific hardware devices from the
users.
An Operating System manages the communication between user and device
drivers.

• I/O operation means read or write operation with any file or any specific
I/O device.
• Operating system provides the access to the required I/O device when
required.
File system manipulation
A file represents a collection of related information. Computers can store files
on the disk (secondary storage), for long-term storage purpose. Examples of
storage media include magnetic tape, magnetic disk and optical disk drives
like CD, DVD. Each of these media has its own properties like speed, capacity,
data transfer rate and data access methods.
A file system is normally organized into directories for easy navigation and
usage. These directories may contain files and other directions. Following are
the major activities of an operating system with respect to file management −

• Program needs to read a file or write a file.

• The operating system gives the permission to the program for operation
on file.
• Permission varies from read-only, read-write, denied and so on.
• Operating System provides an interface to the user to create/delete
files.
• Operating System provides an interface to the user to create/delete
directories.
• Operating System provides an interface to create the backup of file
system.
Communication
In case of distributed systems which are a collection of processors that do not
share memory, peripheral devices, or a clock, the operating system manages
communications between all the processes. Multiple processes communicate
with one another through communication lines in the network.
The OS handles routing and connection strategies, and the problems of
contention and security. Following are the major activities of an operating
system with respect to communication −

• Two processes often require data to be transferred between them

• Both the processes can be on one computer or on different computers,
but are connected through a computer network.
• Communication may be implemented by two methods, either by Shared
Memory or by Message Passing.
Error handling
Errors can occur anytime and anywhere. An error may occur in CPU, in I/O
devices or in the memory hardware. Following are the major activities of an
operating system with respect to error handling −

• The OS constantly checks for possible errors.

• The OS takes an appropriate action to ensure correct and consistent
computing.
Resource Management
In case of multi-user or multi-tasking environment, resources such as main
memory, CPU cycles and files storage are to be allocated to each user or job.
Following are the major activities of an operating system with respect to
resource management −

• The OS manages all kinds of resources using schedulers.

• CPU scheduling algorithms are used for better utilization of CPU.
Protection
Considering a computer system having multiple users and concurrent
execution of multiple processes, the various processes must be protected
from each other's activities.
Protection refers to a mechanism or a way to control the access of programs,
processes, or users to the resources defined by a computer system. Following
are the major activities of an operating system with respect to protection −

• The OS ensures that all access to system resources is controlled.

• The OS ensures that external I/O devices are protected from invalid
access attempts.
• The OS provides authentication features for each user by means of
passwords.

System Call

In computing, a system call is the programmatic way in which a computer

program requests a service from the kernel of the operating system it is
executed on. A system call is a way for programs to interact with the
operating system. A computer program makes a system call when it makes a
request to the operating system’s kernel. System call provides the services of
the operating system to the user programs via Application Program
Interface(API). It provides an interface between a process and operating
system to allow userlevel processes to request services of the operating
system. System calls are the only entry points into the kernel system. All
programs needing resources must use system calls.

Services Provided by System Calls :

1. Process creation and management
2. Main memory management
3. File Access, Directory and File system management
4. Device handling(I/O)
5. Protection 6. Networking, etc.
Types of System Calls : There are 5 different categories of system calls – 1.
Process control: end, abort, create, terminate, allocate and free memory.
2. File management: create, open, close, delete, read file etc.
3. Device management
4. Information maintenance
5. Communication

1) Process Control:
• A running program needs to be able to stop execution either normally or
abnormally.
• When execution is stopped abnormally, often a dump of memory is
taken and can be examined with a debugger.
• Following are functions of process control:
i. end, abort ii.
load, execute
iii. create process, terminate process iv. get
process attributes, set process attributes v. wait for
time vi. wait event, signal event vii. allocate and free
memory 2) File management :
• We first need to be able to create and delete files. Either system call
requires the name of the file and perhaps some of the file's attributes.
 • Once the file is created, we need to open it and to use it. We may also
read, write, or reposition. Finally, we need to close the file, indicating
that we are no longer using it.
• We may need these same sets of operations for directories if we have a
directory structure for organizing files in the file system.
• In addition, for either files or directories, we need to be able to
determine the values of various attributes and perhaps to reset them if
necessary. File attributes include the file name, a file type, protection
codes, accounting information, and so on Functions:
o create file, delete file o

open, close file o read, write,

reposition o get and set file attributes
3) Device Management:
• A process may need several resources to execute - main memory, disk
drives, access to files, and so on. If the resources are available, they can
be granted, and control can be returned to the user process. Otherwise,
the process will have to wait until sufficient resources are available.
• The various resources controlled by the OS can be thought of as devices.
Some of these devices are physical devices (for example, tapes), while
others can be thought of as abstract or virtual devices (for example,
files).
• Once the device has been requested (and allocated to us), we can read,
write, and (possibly) reposition the device, just as we can with files.
• In fact, the similarity between I/O devices and files is so great that many
OSs, including UNIX, merge the two into a combined file-device
structure.
• A set of system calls is used on files and devices. Sometimes, 1/0 devices
are identified by special file names, directory placement, or file
attributes.
Functions: o request device, release
device o read, write, reposition
o get device attributes, set device attributes o

logically attach or detach devices

 4).Information Maintenance
• Many system calls exist simply for the purpose of transferring
information between the user program and the OS. For example, most
systems have a system call to return the current time and date.
• Other system calls may return information about the system, such as the
number of current users, the version number of the OS, the amount of
free memory or disk space, and so on.
• In addition, the OS keeps information about all its processes, and system
calls are used to access this information. Generally, calls are also used to
reset the process information.
Functions:
• get time or date, set time or date
• get system data, set system data
• get and set process, file, or device attributes 5).Communication
• There are two common models of interprocess communication: the
message-passing model and the shared-memory model. In the
messagepassing model, the communicating processes exchange
messages with one another to transfer information.
• In the shared-memory model, processes use shared memory creates and
shared memory attaches system calls to create and gain access to
regions of memory owned by other processes.
• Recall that, normally, the OS tries to prevent one process from accessing
another process's memory. Shared memory requires that two or more
processes agree to remove this restriction. They can then exchange
information by reading and writing data in the shared areas.
Functions: o create, delete communication
connection o send, receive messages o
transfer status information o Attach and
Detach remote device

Examples of Windows and Unix System Calls –

 WINDOWS UNIX

CreateProcess() fork()
exit()
ExitProcess() wait()

WaitForSingleObject()
Process Control

CreateFile() open()
read()
ReadFile() write()
close()
WriteFile()

CloseHandle()
File Manipulation

SetConsoleMode() ioctl()
read()
ReadConsole() write()

WriteConsole()
Device Manipulation

GetCurrentProcessID() getpid()
alarm()
SetTimer() sleep()

Sleep()
Information Maintenance

CreatePipe() pipe()
shmget()
CreateFileMapping() mmap()

MapViewOfFile()
Communication
Protection

SetFileSecurity() chmod()

InitlializeSecurityDescriptor() umask()

SetSecurityDescriptorGroup() chown()
Structure
Simple Structure

In MS-DOS, applications may bypass the operating system.

• Operating systems such as MS-DOS and the original UNIX did not have well-defined
structures.
• There was no CPU Execution Mode (user and kernel), and so errors in applications
could cause the whole system to crash.

Monolithic Approach

• Functionality of the OS is invoked with simple function calls within the kernel, which is
one large program.
• Device drivers are loaded into the running kernel and become part of the kernel.
A monolithic kernel, such as Linux and other Unix systems.

Layer Approach:

This approach breaks up the operating system into different layers.

• This allows implementers to change the inner workings, and increases
modularity.
• As long as the external interface of the routines don’t change, developers
have more freedom to change the inner workings of the routines.
• With the layered approach, the bottom layer is the hardware, while the highest
layer is the user interface.
o The main advantage is simplicity of construction and debugging. o
The main difficulty is defining the various layers.
o The main disadvantage is that the OS tends to be less efficient
than other implementations.
Microkernel

This structures the operating system by removing all nonessential portions of the
kernel and implementing them as system and user level programs.

• Generally they provide minimal process and memory management, and a

communications facility.
• Communication between components of the OS is provided by message passing.

The benefits of the microkernel are as follows:

• Extending the operating system becomes much easier.

• Any changes to the kernel tend to be fewer, since the kernel is smaller.
• The microkernel also provides more security and reliability.

Main disadvantage is poor performance due to increased system overhead from

message passing.
A Microkernel architecture.