Introduction to OS, types of operating systems

Operating System Definition and Function
In the Computer System (comprises of Hardware and software), Hardware can only
understand machine code (in the form of 0 and 1) which doesn't make any sense to a
naive user.
We need a system which can act as an intermediary and manage all the processes and
resources present in the system.
 An Operating System can be defined as an
interface between user and hardware. It is
responsible for the execution of all the
processes, Resource Allocation, CPU
management, File Management and many other
tasks.
 The purpose of an operating system is to
provide an environment in which a user can
execute programs in convenient and efficient
manner.

Structure of a Computer System
A Computer System consists of:
Users (people who are using
the computer)
Application Programs
(Compilers, Databases, Games,
Video player, Browsers, etc.)
System Programs (Shells,
Editors, Compilers, etc.)
Operating System ( A special
program which acts as an
interface between user and
hardware )
Hardware ( CPU, Disks,

What does an Operating system do?
 Process Management
 Process Synchronization
 Memory Management
 CPU Scheduling
 File Management
 Security

Functions of Operating System
 It helps with memory management. It keeps a track of the files being saved in the
Main memory and the primary memory of the computer device
 Whenever a computer is turned on, the Operating system automatically starts to
work. Thus, the booting and rebooting process of a computer device is also an
important function of the OS
 It provides a user interface
 Managing of basic peripheral devices is done by the operating system
 Using the password protection option of an operating system, the data in the device
can be kept secure
 It coordinates with the software and the user
 Easy navigation and organisation of files and programs are managed by the OS
 Any kind of program which needs to be run through the system is done by the
operating system
 If any kind of error or bug is found during the program is detected using the operating
system

Components of an Operating Systems
Components of an Operating Systems
There are two basic components of an Operating System.
 Shell
 Kernel
Shell
 Shell is the outermost layer of the Operating System and it handles the interaction
with the user.
 The main task of the Shell is the management of interaction between the User and
OS.
 Shell provides better communication with the user and the Operating System Shell
does it by giving proper input to the user it also interprets input for the OS and
handles the output from the OS.
 It works as a way of communication between the User and the OS.

Kernel
 The kernel is one of the components of the Operating System which
works as a core component.
 The rest of the components depends on Kernel for the supply of the
important services that are provided by the Operating System.
 The kernel is the primary interface between the Operating system and
Hardware.
Functions of Kernel
The following functions are to be performed by the Kernel.
 It helps in controlling the System Calls.
 It helps in I/O Management.
 It helps in the management of applications, memory, etc.

Types of Operating System
1. Batch Operating System
 There is no direct communication between the computer and the OS
 There is an intermediate, the Operator, which needs to distribute the work into
batches and sort similar jobs
 Multiple users can use it
 Can easily manager a large amount of work
2. Real-Time Operating System
 It has a data processing system
 The processing time is very small between the user’s command and the output
 Used in fields where the response needs to be quick and rapid
3. Time-Sharing Operating System
 Multiple people at various terminals can use a program at the same time
 The main motive is to minimize the response time

4. Distributed Operating System
 When two or more systems are connected to each other and one can open files which are not
present in their system but in other devices connected in the network
 Its usage has now increased over the years
 They use multiple central processors to serve real-time applications
 Failure of one system does not affect the other systems connected in the network
5. Embedded Operating System
 These special Operating systems are built into larger systems
 They generally are limited to single specific functions like an ATM
6. Network Operating System
 They have one main server which is connected to other client servers
 All the management of files, processing of data, access to sharing files, etc. are performed over this
small network
 It is also a secure operating system for working with multiple users
7. Mobile Operating System
 With the advancement in the field of technology, smartphones now are released with an Operating
system.
 They are designed in a manner that they can help a small device work efficiently

Batch Processing Operating System

System calls in Operating System
The interface between a process and an operating system is provided by
system calls. In general, system calls are available as assembly
language instructions.
They are also included in the manuals used by the assembly level
programmers.
System calls are usually made when a process in user mode requires
access to a resource.
Then it requests the kernel to provide the resource via a system call.

System Calls Are Required In The Following Situations
If a file system requires the creation or deletion of files. Reading and
writing from files also require a system call.
Creation and management of new processes.
Network connections also require system calls. This includes sending
and receiving packets.
Access to a hardware devices such as a printer, scanner etc. requires a
system call.

Types of System Calls
Process Control
These system calls deal with processes such as process creation, process
termination etc.
File Management
These system calls are responsible for file manipulation such as creating
a file, reading a file, writing into a file etc.
Device Management
These system calls are responsible for device manipulation such as
reading from device buffers, writing into device buffers etc.
Information Maintenance
These system calls handle information and its transfer between the
operating system and the user program.
Communication
These system calls are useful for interprocess communication. They also
deal with creating and deleting a communication connection.

Types of System Calls Windows Linux
Process Control
CreateProcess()
ExitProcess()
WaitForSingleObject()
fork()
exit()
wait()
File Management
CreateFile()
ReadFile()
WriteFile()
CloseHandle()
open()
read()
write()
close()
Device Management
SetConsoleMode()
ReadConsole()
WriteConsole()
ioctl()
read()
write()
Information
Maintenance
GetCurrentProcessID()
SetTimer()
Sleep()
getpid()
alarm()
sleep()
Communication
CreatePipe()
CreateFileMapping()
MapViewOfFile()
pipe()
shmget()
mmap()

Types of Operating Systems Structures
Simple/Monolithic Structure
Micro-Kernel Structure
Hybrid-Kernel Structure
Layered Structure
Modular Structure
Virtual Machine

Simple/Monolithic
structure
 Operating systems do not
have well-defined structures
and are small, simple, and
limited.
 The interfaces and levels of
functionality are not well
separated.
 MS-DOS is an example of
such an operating system.

Micro-Kernel Structure
 A microkernel is an operating system structure that aims to keep the
kernel as small and lightweight as possible.
 It provides only essential services, such as process scheduling and inter-
process communication, while moving most non-essential services, like
device drivers, into user space.
Hybrid-Kernel
Structure
Hybrid-kernel structure is nothing but just a combination of
both monolithic-kernel structure and micro-kernel structure.
Basically, it combines properties of both monolithic and
micro-kernel and make a more advance and helpful
approach.
It implement speed and design of monolithic and modularity

Layered Structure
 An OS can be broken into pieces and
retain much more control over the
system.
 In this structure, the OS is broken into a
number of layers (levels). The bottom
layer (layer 0) is the hardware, and the
topmost layer (layer N) is the user
interface.
 These layers are so designed that each
layer uses the functions of the lower-level
layers.
 This simplifies the debugging process, if
lower-level layers are debugged and an
error occurs during debugging, then the
error must be on that layer only, as the
lower-level layers have already been
debugged.

Modular Structure
It is considered as the best approach
for an OS. It involves designing of a
modular kernel.
The kernel has only a set of core
components and other services are
added as dynamically loadable
modules to the kernel either during
runtime or boot time.
It resembles layered structure due to
the fact that each kernel has defined
and protected interfaces, but it is more
flexible than a layered structure as a
module can call any other module.
For example Solaris OS

VMs (Virtual Machines)
 A virtualization-based kernel, also known as a hypervisor, is an operating system
structure that enables the execution of multiple operating systems concurrently on the
same hardware.
 A virtual machine (VM) is a virtual environment which functions as a virtual computer
system with its own CPU, memory, network interface, and storage, created on a
physical hardware system.
 Multiple OS systems use the same hardware and partition resources between virtual
computers.
 Separate Security and configuration identity.
 Ability to move the virtual computers between the physical host computers as
holistically integrated files.

Introduction to OS, types of operating systems

Inter Process Communication (IPC)
A process can be of two types:
 Independent process.
 Co-operating process.
 Inter-process communication (IPC) is a mechanism that allows processes
to communicate with each other and synchronize their actions.
 The communication between these processes can be seen as a method of
co-operation between them.
 Processes can communicate with each other through both:
1.Shared Memory
2.Message passing

Unit II - Process Synchronization
Critical Section Problem in OS
 Critical Section is the part of a program which
tries to access shared resources.
 That resource may be any resource in a
computer like a memory location, Data
structure, CPU or any IO device.
 The critical section cannot be executed by more
than one process at the same time; operating
system faces the difficulties in allowing and
disallowing the processes from entering the
critical section.
 The critical section problem is used to design a
set of protocols which can ensure that the Race
condition among the processes will never arise.

 Mutex is special a binary
semaphore that synchronizes the
access to shared resources like
memory or I/O. It is a locking
mechanism.
 In case multiple threads want to
access the critical section, mutex
allows only one thread at a time to
be in the critical section.
 Mutex ensures that the code in the
critical section (which has shared
resources) being controlled will only
be used by a single thread at a
time.
Mutex & Semaphores

wait (mutex); //locks
…..
Critical Section
…..
signal (mutex); //unlocks
 The producer-consumer problem is
an example of a multi-process
synchronization problem. The
problem describes two processes, the
producer and the consumer that
share a common fixed-size buffer and
use it as a queue.
 The producer’s job is to generate data,
put it into the buffer, and start again.
 At the same time, the consumer is
consuming the data (i.e., removing it
from the buffer), one piece at a time.
What is the Producer-Consumer Problem?

Semaphore
A semaphore is a non-negative integer variable that is shared between
various threads. Semaphore works upon signaling mechanism, in this a thread
can be signaled by another thread. Semaphore uses two atomic operations for
process synchronisation:
 Wait (P)
 Signal (V)

 Semaphores are just normal variables used to coordinate the
activities of multiple processes in a computer system.
 They are used to enforce mutual exclusion, avoid race
conditions, and implement synchronization between
processes.
 The process of using Semaphores provides two operations:
wait (P) and signal (V).
 The wait operation decrements the value of the semaphore,
and the signal operation increments the value of the
semaphore.
 When the value of the semaphore is zero, any process that
performs a wait operation will be blocked until another
process performs a signal operation.

 Race Conditions: It occurs when some common shared
resources are accessed by multiple processes or threads, leading
to unpredictable behavior.
 Critical Section: section of code where shared resources are
accessed. Only one process or thread should be allowed to
execute the critical section simultaneously.
 Deadlocks: a situation occurs when two or more processes are
holding the resources and do not release them causing a
deadlock to occur because another is waiting for that resources.
Deadlock prevention and detection mechanisms are critical in
process synchronization.
 Starvation: One process is being unable to access resources due
to other processes holding onto them for extended periods, often
due to priority scheduling or unfair resource allocation.

Types of Semaphores
Semaphores are of two Types
 Binary Semaphore: This is also known as a mutex lock.
It can have only two values – 0 and 1.
 Its value is initialized to 1.
 It is used to implement the solution of critical section
problems with multiple processes.
 Counting Semaphore: Its value can range over an
unrestricted domain.
 It is used to control access to a resource that has multiple
instances.

Classical Problems of Synchronization with
Semaphore Solution
These problems are used for testing nearly every newly
proposed synchronization scheme. The following problems of
synchronization are considered as classical problems:
1. Bounded-buffer (or Producer-Consumer) Problem,
2. Dining-Philosophers Problem,
3. Readers and Writers Problem,
4. Sleeping Barber Problem

Dining-Philosophers Problem
The Dining Philosopher Problem states
that K philosophers seated around a
circular table with one chopstick
between each pair of philosophers.
There is one chopstick between each
philosopher.
A philosopher may eat if he can pickup
the two chopsticks adjacent to him.
One chopstick may be picked up by
any one of its adjacent followers but
not both.
This problem involves the allocation of
limited resources to a group of
processes in a deadlock-free and
starvation-free manner.

Sleeping Barber Problem
Barber shop with one barber, one barber chair and N chairs to wait in.
When no customers the barber goes to sleep in barber chair and must
be woken when a customer comes in.
When barber is cutting hair new customers take empty seats to wait, or
leave if no vacancy.
This is basically the Sleeping Barber Problem.

 The problem is to write a program that coordinates the actions of the
customers and the barber in a way that avoids synchronization problems,
such as deadlock or starvation.
 One solution to the Sleeping Barber problem is to use semaphores to
coordinate access to the waiting chairs and the barber chair. The solution
involves the following steps:
 Initialize two semaphores: one for the number of waiting chairs and one for
the barber chair. The waiting chairs semaphore is initialized to the number of
chairs, and the barber chair semaphore is initialized to zero.
 Customers should acquire the waiting chairs semaphore before taking a seat
in the waiting room. If there are no available chairs, they should leave.
 When the barber finishes cutting a customer’s hair, he releases the barber
chair semaphore and checks if there are any waiting customers. If there are,
he acquires the barber chair semaphore and begins cutting the hair of the
next customer in the queue.

The Need For CPU Scheduling Algorithm
 CPU scheduling is the process of deciding which process will own the CPU to use while
another process is suspended.
 The main function of the CPU scheduling is to ensure that whenever the CPU remains idle,
the OS has at least selected one of the processes available in the ready-to-use line.
 In Multiprogramming, if the long term scheduler picks more I/O bound processes then most
of the time, the CPU remains idol.
 The task of Operating system is to optimize the utilization of resources.
 If most of the running processes change their state from running to waiting then there may
always be a possibility of deadlock in the system.
 Hence to reduce this overhead, the OS needs to schedule the jobs to get the optimal
utilization of CPU and to avoid the possibility to deadlock.

Objectives of Process Scheduling Algorithm
Utilization of CPU at maximum level. Keep CPU as busy as possible.
Allocation of CPU should be fair.
Throughput should be Maximum. i.e. Number of processes that complete their
execution per time unit should be maximized.
Minimum turnaround time, i.e. time taken by a process to finish execution should be
the least.
There should be a minimum waiting time and the process should not starve in the ready
queue.
Minimum response time. It means that the time when a process produces the first
response should be as less as possible.

Completion,
Turn Around,
Response and
Waiting
Times
Completion time is the time when a
process completes execution.
Turn Around time is the total time
between process arrival and completion.
Response time is the time between process
arrival and first response from the system.
Waiting time is the total time a process
spends waiting in the ready queue.

Terminologies Used in CPU Scheduling
 Arrival Time: Time at which the process arrives in the ready queue.
 Completion Time: Time at which process completes its execution.
 Burst Time: Time required by a process for CPU execution.
 Turn Around Time: Time Difference between completion time and
arrival time.
 Turn Around Time = Completion Time – Arrival Time
 Waiting Time(W.T): Time Difference between turn around time and
burst time.
 Waiting Time = Turn Around Time – Burst Time

There are mainly two types of scheduling methods:
Preemptive Scheduling: Preemptive scheduling is used when a process switches from
running state to ready state or from the waiting state to the ready state.
Non-Preemptive Scheduling: Non-Preemptive scheduling is used when a process
terminates , or when a process switches from running state to waiting state

 First Come First Serve
 Shortest Job First – Pre-emptive & Non-Pre-emptive
 Priority Scheduling
 Round Robin Scheduling
Types of CPU Scheduling Algorithms

 Jobs are executed on first come, first serve basis.
 It is a non-preemptive scheduling algorithm.
 Easy to understand and implement.
 Its implementation is based on FIFO queue.
 Poor in performance as average wait time is high.
First Come First Serve (FCFS)

Shortest Job Next (SJN)
This is also known as shortest job first, or SJF
This is a non-preemptive, pre-emptive scheduling algorithm.
Best approach to minimize waiting time.
Easy to implement in Batch systems where required CPU time is known in advance.
Impossible to implement in interactive systems where required CPU time is not
known.
The processer should know in advance how much time process will take.

Priority Based Scheduling
 Priority scheduling is a non-preemptive algorithm and one of the most
common scheduling algorithms in batch systems.
 Each process is assigned a priority. Process with highest priority is to be
executed first and so on.
 Processes with same priority are executed on first come first served
basis.
 Priority can be decided based on memory requirements, time
requirements or any other resource requirement.

Given: Table of processes, and their Arrival time, Execution time, and priority. Here we are
considering 1 is the lowest priority.

Round-Robin Scheduling
 It is a Preemptive type of CPU scheduling algorithm in OS.
 The round-robin algorithm generally focuses on the Time Sharing
technique.
 Round robin Scheduling is the simplest and one of the oldest algorithms.
 This algorithm is a real-time algorithm as it responds to an event within
a specific time limit.
 Round robin is a widely used algorithm in traditional OS.

Process No Arrival Time (AT) Burst Time (BT)
P1 0 5
P2 1 4
P3 2 2
P4 4 1

Process No Arrival Time
(AT)
Burst Time
(BT)
Completion
Time (CT)
Turn Around
Time (TAT)
Waiting Time
(WT)
Response
Time (RT)
P1 0 5 3 1
0
12 12 7 0
P2 1 4 2
0
11 10 6 1
P3 2 2
0
6 4 2 2
P4 4 1
0
9 5 4 4
Total Turn Around Time = 31 ms
Average Turn Around Time = 31/4 = 7.75 ms
Total Waiting Time = 19 ms
Average Waiting Time = 19/4 = 4.75 ms
Total Response Time = 7 ms
Average Response Time = 7/4 = 1.75 ms

Introduction to OS, types of operating systems

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Introduction to OS, types of operating systems