Operating systems (For CBSE School Students)

Operating
Systems
By: Gaurav Aggarwal

Operating System
• An interface between Applications and hardware
• It manages computer hardware resources and
provides services for computer programs
• Application programs require an operating system
to function.

Operating System
• A program or a software that governs the
functioning of other programs
• It keeps track of files and folder on the disk
• Manages CPU Time
• Control devices
• Allows convenient usage; hides the tedious stuff

Types of Operating
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System
• Tasks
o Uni tasking (DOS)
o Multi tasking : All modern OS
• Users
o Single User: Mac, Windows
o Multi User: Linux

Types of Operating
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System
• Single User, Single Task
o A user can do only one thing at a time
o MS DOS
• Single User, Multi Task
o Most commonly Used
o A user can do multiple tasks
o Windows, MAC
• Multi User, Multi Task
o Multiple users can do multiple tasks
o Linux, Unix

Types of Operating
System – Real Time OS
• Aims at executing real-time applications
• Objective: quick and predictable response to
events
• Event-driven or Time sharing design
• Event-driven system switches between tasks based
on their priorities or external events
• Time-sharing operating systems switch tasks based
on clock interrupts
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Types of Operating
System – Distributed
• A distributed operating system manages a group of
independent computers
• Makes them appear to be a single computer
• Emerged because of development of networked
computers
• Example: Processor on one system and hard disk on
another
• Architecture can be implemented on Unix and
Linux
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Types of Operating
System – Mobile
• Mobile OS operates Smartphone, Tablet and other
mobile devices
• They have features of Personal Computer
Operating System along with features to manage
other hardware
o Cellular, camera, Near field communication, infrared, etc.
• IOS, Android, Windows Phone, Firefox OS, etc
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FOSS
• Free Open Source Software
• Free – Means Liberty and not related to Price or cost
• Open – Source code is available and any body can
contribute to the development. Organization
independent
• Example: Linux

OS - Wake up call
• When you turn on the power to a PC, the first
program that runs is a set of instructions kept in the
computer's read-only memory (ROM).

OS - Wake up call
• These instructions are
called Power on Self Test
(POST)
• It checks to make sure
everything is functioning
properly.
• It checks the CPU,
memory, and basic input-output
systems (BIOS) for
errors.

OS - Wake up call
• Once successful, the software will begin to
activate the computer's disk drives.
• The BIOS attempts to access the first sector of the
drive designated as the boot disk.
• The BIOS confirms there's a bootstrap loader, or
boot loader, in that first sector of the boot disk,
and it loads that boot loader into memory (RAM)

OS - Wake up call
• The bootstrap loader or boot loader is a small
program that has a single function: It loads the
operating system into memory and allows it to
begin operation.
• Once the boot loader is in memory, the BIOS hands
over its work to the boot loader, which in turn
begins loading the operating system into memory.

OS - Wake up call
• When the boot loader finishes its task, it turns control
of the PC over to the operating system.
• Then, the OS is ready for user interaction.

OS - Memory Storage and
Management

Management
• Besides managing
hardware and software
resources on the system,
the OS must manage
resources and memory.
• Each process must have
enough memory in which
to execute, and It can
neither run into the memory
space of another process,
Nor be run into by another
process.

Management
• The different types of memory in the system must be
used properly so that each process can run most
effectively.
o Cache
o Ram
o Disk

Cache Memory
• A section of a computer's memory which
temporarily retains recently accessed data in order
to speed up repeated access to the same data.
• It provides rapid access without having to wait for
systems to load.
• Very Costly

RAM
• Random access memory
(RAM) is the best known
form of computer
memory.
• RAM is considered
"random access" because
you can access any
memory cell directly if you
know the row and column
that intersect at that cell.

RAM
• The more RAM your computer has, the faster
programs can function.
• Remember, that if the power is turned off, then
all data left in RAM, that has not been saved to
the hard drive, is lost.

Virtual Memory
• Virtual Memory – a method
of using hard disk space to
provide extra memory. It
simulates additional RAM.
• In Windows, the amount of
virtual memory available,
equals the amount of free
RAM plus the amount of
disk space allocated to the
swap file.

Virtual Memory – Swap
File
• A swap file is an area of your hard disk that is set aside for
virtual memory. Swap files can be either temporary or
permanent.

Kernel
• Core or nucleus of an operating system
• Interacts with the hardware
• First program to get loaded when the system starts
and runs till the session gets terminated
• Different from BIOS which is hardware dependent.
• Kernel is software dependent

Kernel types
• Monolithic
o All OS related code are stuffed in a single
module
o Available as a single file
o Advantage : Faster functioning
• Micro
o OS components are isolated and run in their own
address space
o Device drivers, programs and system services run
outside kernel memory space
o Supports modularity
o Lesser in size

Shell
• Program that interacts with kernel
• Bridge between kernel and the user
• Command interpreter
• User can type command and the command is
conveyed to the kernel and it will be executed

Types of Shell
• Sh – simple shell
• BASH – Bourne Again Shell
• KSH – Korne Shell
• CSH – C Shell
• SSH – Secure Shell
• To use a particular shell type the shell name at the command
prompt.
o Eg $csh – will switch the current shell to c shell
• To view the available shells in the system, type cat /etc/shells
at the command prompt
• To view the current shell that is being used, type echo $SHELL
at the command prompt

Operating System
User 1
User 2

Shell Scripting
• Shell scripting is the most useful and powerful
feature in Linux
o Minimizes typing of repetitive command
o Can schedule jobs to run in the system
o Can initiate back up activities for system administration
o Similar to batch files in DOS, but more powerful than Batch files

Working with shell script
• Open a file with extension .sh using vi editor
• We can type any number of commands that we
use to type at command prompt
• Save the file
• Execute the file
o sh file.sh
o ./file.sh (if the file has execution permission)

4 Freedoms with FOSS
• Freedom to run the software anywhere
• Freedom to study how the programs work. i.e
source code will be accessible
• Freedom to redistribute copies
• Freedom to improve the software
• If a software has all these 4 freedoms, then it is a
FOSS

Copyleft
• Termed by Richard Mathew Stallman
• Copyleft statement:
o “Copying and redistribution are permitted in any medium provided this
notice is preserved.”

Linux
• Developed in 1991 by Linus Torvalds
• Used in most of the computers, ranging from
super computers to embedded system
• Multi user
• Multi tasking
• Time sharing
• Monolithic kernel

Free Software Foundation
• Founded by Richard Stallman in 1983
• Organisation that started developing copylefted
programs
• Project – GNU Project
o GNU Not Unix
o Recursive expansion

Interrupts
• Interrupt transfers control to the interrupt service routine
generally, through the interrupt vector, which contains
the addresses of all the service routines.
• Interrupt architecture must save the address of the
interrupted instruction.
• Incoming interrupts are disabled while another interrupt
is being processed to prevent a lost interrupt.
• A trap is a software-generated interrupt caused either by
an error or a user request.
• An operating system is interrupt driven.

Modes
• Superviser Mode
o Unrestricted access to hardware
• Protected Mode
o Everything Else

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PROCESSES
PROCESS CONCEPT:
A program is passive; a process active.
Attributes held by a process include
•hardware state,
•memory,
•CPU,
•progress (executing)
WHY HAVE PROCESSES?
Resource sharing ( logical (files) and physical(hardware) ).
Computation speedup - taking advantage of multiprogramming – i.e. example
of a customer/server database system.

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PROCESSES PROCESS STATE
 New The process is just being put together.
 Running Instructions being executed. This running process holds the CPU.
 Waiting For an event (hardware, human, or another process.)
 Ready The process has all needed resources - waiting for CPU only.
 Suspended Another process has explicitly told this process to sleep. It will be
awakened when a process explicitly awakens it.
 Terminated The process is being torn apart.

PROCESS CONTROL BLOCK:
CONTAINS INFORMATION ASSOCIATED WITH EACH
PROCESS:
It's a data structure holding:
 PC, CPU registers,
 memory management information,
 accounting ( time used, ID, ... )
 I/O status ( such as file resources ),
 scheduling data ( relative priority, etc. )
 Process State (so running, suspended, etc. is simply
a field in the PCB ).
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PROCESSES Process State

PROCESSES
Scheduling
Components
The act of Scheduling a process means changing the active PCB pointed to by the CPU.
Also called a context switch.
A context switch is essentially the same as a process switch - it means that the memory,
as seen by one process is changed to the memory seen by another process.
See Figure on Next Page
SCHEDULING QUEUES:
(Process is driven by events that are triggered by needs and availability )
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Ready queue = contains those processes that are ready to run.
I/O queue (waiting state ) = holds those processes waiting for I/O service.

3:
Proce
sses
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PROCESSES
Scheduling
Components
The CPU switching
from one process to
another.

Process Scheduling
• First Come – First Serve
o Long process may make others starve
• Shortest Job First
o Long process may starve
• Round Robin
o Lots of time used in switching
• Priority Scheduling
o Low priority Job Starve

Thread
• A thread of execution is the smallest sequence of
programmed instructions that can be managed
independently by an operating system schedule
• Processes are typically independent, while threads
exist as subsets of a process

Multitasking and
Multithreading
• Multitasking refers to a computer's ability to
perform multiple jobs concurrently
o more than one program are running concurrently,
e.g., UNIX
• A thread is a single sequence of execution
within a program
• Multithreading refers to multiple threads of
control within a single program
o each program can run multiple threads of control
within it, e.g., Web Browser

Threads and Processes
CPU
main
run
Process 1 Process 2 Process 3 Process 4
GC

What are Threads Good
For?
• To maintain responsiveness of an application during
a long running task.
• To separate independent tasks and make the
execution faster.
o If a thread waits for I/O, other thread can execute in meanwhile

Cloud Computing
• A type of outsourcing of computer services
• Similar to the way in which the supply of electricity is
outsourced. Users can simply use it.
• They do not need to worry where the electricity is
from, how it is made, or transported. Every month,
they pay for what they consumed.
• Idea behind cloud is similar

Cloud Computing
• The user can simply use storage, computing power,
or specially crafted development environments,
without having to worry how these work internally.
• Cloud computing is a systems architecture model
for Internet-based computing.
• It is the development and use of computer
technology on the Internet

The Average Cloud user
• Connects to the cloud via the Internet
o Does computing tasks, or
o Runs applications, or
o Stores Data

The Cloud Requires
• An Internet connection
• An account - Created with a user name and a
password
• Agree to Terms

So what is the cloud?
Computing and software resources that are delivered on demand, as service.
(2013, January) A Walk in the Clouds. Cloud Computing, CDW-G Reference Guide., 3-5.

Cloud Computing – Simple Definition
Cloud Computing = Software as a Service
+ Platform as a Service
+ Infrastructure as a Service
+ Data as a Service

+ Data as a Service
• Software as a Service (SaaS)
̵ From end user’s point of view
̵ Apps are located in the cloud
̵ Software experiences are delivered through the Internet
̵ Example: Google Docs

+ Data as a Service
• Platform as a Service (PaaS)
̵ From developer’s point of view (i.e. cloud users)
̵ Cloud providers offer an Internet-based platform to
developers who want to create services but don't want to
build their own cloud
̵ Example: Web Hosting

+ Data as a Service
• Infrastructure as a Service (IaaS)
̵ Cloud providers build datacenters
• Power, scale, hardware, networking, storage, distributed systems, etc
̵ Datacenter as a service
̵ Cloud users rent storage, computation, and maintenance from
cloud providers (pay-as-you-go; like utility)

Computer Storage
• Computers have internal or hard drive storage(C:
Drive)
• CPU has a drive for storing programs, documents,
pictures, videos, presentations, etc…
Standard Computer Tower
or Central Processing Unit
(CPU)
Inside the Computer

External Storage
 Allows your content to become mobile
 Save to the storage device
 Take device to any compatible computer
 Open and use content
CD/DVD Thumb Drive External Hard Drive
SD Card Micro SD Card

Networked Storage
• Multiple work stations talk to one unit that stores
information and data.
• Data is not saved to the C: drive, but to a network
drive
• Can retrieve the data stored to the network from
any of the connected workstations.

Cloud Storage
• Create an Account –
User name and
password
• Content lives with the
account in the cloud
• Log onto any
computer with Wi-Fi to
find your content

Software
and
application
s
More than
Storage

Document Creation
• Google Docs
• SkyDrive
• Box

Advantages of Cloud Computing
• Lower computer costs:
o You do not need a high-powered and high-priced
computer to run cloud computing's web-based
applications.
o Since applications run in the cloud, not on the desktop
PC, your desktop PC does not need the processing
power or hard disk space demanded by traditional
desktop software.
o When you are using web-based applications, your PC
can be less expensive, with a smaller hard disk, less
memory, more efficient processor...
o In fact, your PC in this scenario does not even need a
CD or DVD drive, as no software programs have to be
loaded and no document files need to be saved.
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• Improved performance:
o With few large programs hogging your computer's
memory, you will see better performance from your
PC.
o Computers in a cloud computing system boot and
run faster because they have fewer programs and
processes loaded into memory…
• Reduced software costs:
o Instead of purchasing expensive software
applications, you can get most of what you need for
free-ish!
• most cloud computing applications today, such as the Google
Docs suite.
o better than paying for similar commercial software
• which alone may be justification for switching to cloud
applications.
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• Instant software updates:
o Another advantage to cloud computing is that you are no
longer faced with choosing between obsolete software and
high upgrade costs.
o When the application is web-based, updates happen
automatically
• available the next time you log into the cloud.
o When you access a web-based application, you get the
latest version
• without needing to pay for or download an upgrade.
• Improved document format compatibility.
o You do not have to worry about the documents you create
on your machine being compatible with other users'
applications or OSes
o There are potentially no format incompatibilities when
everyone is sharing documents and applications in the cloud.
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• Unlimited storage capacity:
o Cloud computing offers virtually limitless storage.
o Your computer's current 1 Tbyte hard drive is small
compared to the hundreds of Pbytes available in the
cloud.
• Increased data reliability:
o Unlike desktop computing, in which if a hard disk
crashes and destroy all your valuable data, a
computer crashing in the cloud should not affect the
storage of your data.
• if your personal computer crashes, all your data is still out
there in the cloud, still accessible
o In a world where few individual desktop PC users
back up their data on a regular basis, cloud
computing is a data-safe computing platform!
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• Universal document access:
o That is not a problem with cloud computing, because
you do not take your documents with you.
o Instead, they stay in the cloud, and you can access
them whenever you have a computer and an Internet
connection
o Documents are instantly available from wherever you
are
• Latest version availability:
o When you edit a document at home, that edited
version is what you see when you access the
document at work.
o The cloud always hosts the latest version of your
documents
• as long as you are connected, you are not in danger of having an
outdated version
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• Easier group collaboration:
o Sharing documents leads directly to better
collaboration.
o Many users do this as it is an important advantages of
cloud computing
• multiple users can collaborate easily on documents and
projects
• Device independence.
o You are no longer tethered to a single computer or
network.
o Changes to computers, applications and documents
follow you through the cloud.
o Move to a portable device, and your applications and
documents are still available.
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Disadvantages of Cloud Computing
• Requires a constant Internet connection:
o Cloud computing is impossible if you cannot
connect to the Internet.
o Since you use the Internet to connect to both
your applications and documents, if you do not
have an Internet connection you cannot access
anything, even your own documents.
o A dead Internet connection means no work and
in areas where Internet connections are few or
inherently unreliable, this could be a deal-breaker.
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• Does not work well with low-speed connections:
o Similarly, a low-speed Internet connection, such as
that found with dial-up services, makes cloud
computing painful at best and often impossible.
o Web-based applications require a lot of bandwidth
to download, as do large documents.
• Features might be limited:
o This situation is bound to change, but today many
web-based applications simply are not as full-featured
as their desktop-based applications.
• For example, you can do a lot more with Microsoft
PowerPoint than with Google Presentation's web-based
offering
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• Can be slow:
o Even with a fast connection, web-based
applications can sometimes be slower than
accessing a similar software program on your
desktop PC.
o Everything about the program, from the interface
to the current document, has to be sent back
and forth from your computer to the computers
in the cloud.
o If the cloud servers happen to be backed up at
that moment, or if the Internet is having a slow
day, you would not get the instantaneous access
you might expect from desktop applications.
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• Stored data might not be secure:
o With cloud computing, all your data is stored on the
cloud.
• The questions is How secure is the cloud?
o Can unauthorised users gain access to your
confidential data?
• Stored data can be lost:
o Theoretically, data stored in the cloud is safe,
replicated across multiple machines.
o But on the off chance that your data goes missing,
you have no physical or local backup.
• Put simply, relying on the cloud puts you at risk if the cloud
lets you down.
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Operating systems (For CBSE School Students)

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