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OPERATING SYSTEM
What
is
an
Operating
System?
 A program that acts as an intermediary
between a user of a computer and the
computer hardware.
 Operating system goals:
 Execute user programs and make solving user
problems easier.
 Make the computer system convenient to use.
 Use the computer hardware in an efficient
manner.
What is an Operating System?
Not easy to define precisely…
Users
Applications
compilers
databases
word processors
Operating System
Hardware
CPU
memory
I/O devices
 OS:
Everything in system that isn’t an application or hardware
 OS:
Software that converts hardware into a useful form for applications
History of Operating Systems (1)
Early batch system
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bring cards to 1401
read cards to tape
put tape on 7094 which does computing
put tape on 1401 which prints output
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History of Operating Systems (2)
• First generation 1945 - 1955
– vacuum tubes, plug boards
• Second generation 1955 - 1965
– transistors, batch systems
• Third generation 1965 – 1980
– ICs and multiprogramming
• Fourth generation 1980 – present
– personal computers
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History of Operating Systems (3)
 Structure of a typical FMS job – 2nd generation
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Computer System Structure
 Computer system can be divided into four
components
 Hardware – provides basic computing resources
 CPU, memory, I/O devices
 Operating system
 Controls and coordinates use of hardware among various
applications and users
 Application programs – define the ways in which the
system resources are used to solve the computing
problems of the users

Word processors, compilers, web browsers, database
systems, video games
 Users
 People, machines, other computers
Four Components of a Computer System
Computer Startup
 bootstrap program is loaded at power-up or reboot
 Typically stored in ROM or EPROM, generally known as
firmware
 Initializates all aspects of system
 Loads operating system kernel and starts execution
Computer System Organization
 Computer-system operation
 One or more CPUs, device controllers connect through
common bus providing access to shared memory
 Concurrent execution of CPUs and devices competing
for memory cycles
Computer Hardware Review (7)
Structure of a large Pentium system
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Operating System Concepts (1)
 A process tree
 A created two child processes, B and C
 B created three child processes, D, E, and F
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Operating System Concepts (2)
Operating System Concepts (3)
Operating System Concepts (4)
Operating System Concepts (5)
Two processes connected by a pipe
Operating System Structure
 Multiprogramming needed for efficiency
 Single user cannot keep CPU and I/O devices busy at all times
 Multiprogramming organizes jobs (code and data) so CPU always has one
to execute
 A subset of total jobs in system is kept in memory
 One job selected and run via job scheduling
 When it has to wait (for I/O for example), OS switches to another job
 Timesharing (multitasking) is logical extension in which CPU switches jobs
so frequently that users can interact with each job while it is running, creating
interactive computing
 Response time should be < 1 second
 Each user has at least one program executing in memory process
 If several jobs ready to run at the same time  CPU scheduling
 If processes don’t fit in memory, swapping moves them in and out to run
 Virtual memory allows execution of processes not completely in memory
Operating-System Operations
 Interrupt driven by hardware
 Software error or request creates exception or trap
 Division by zero, request for operating system service
 Other process problems include infinite loop, processes modifying
each other or the operating system
 Dual-mode operation allows OS to protect itself and other system
components
 User mode and kernel mode
 Mode bit provided by hardware
 Provides ability to distinguish when system is running user code
or kernel code
 Some instructions designated as privileged, only executable in
kernel mode
 System call changes mode to kernel, return from call resets it to
user
VARIOUS PARTS OF AN O.S.
 Kernel (also known as the executive)
 Process manager
 Scheduler
 File manager
OS Components
 Kernel: Core components of the OS
 Process scheduler
 Determines when and for long each process executes
 Memory manager
 Determines when and how memory is allocated to processes
 Decides what to do when main memory is full
 File system
 Organizes named collections of data in persistent storage
 Networking
 Enables processes to communicate with one another

Protection and Security
.Protection – any mechanism for controlling access
of processes or users to resources defined by the
OS
 Security – defense of the system against internal and external attacks
 Huge range, including denial-of-service, worms, viruses, identity
theft, theft of service
 Systems generally first distinguish among users, to determine who can
do what
 User identities (user IDs, security IDs) include name and
associated number, one per user
 User ID then associated with all files, processes of that user to
determine access control
 Group identifier (group ID) allows set of users to be defined and
controls managed, then also associated with each process, file
 Privilege escalation allows user to change to effective ID with
more rights
WHAT DOES AN O.S. DO?
 initialize the hardware of the computer system
 provide basic routines for device control
 provide for the management, scheduling and
interaction of tasks
 maintain system integrity and handle errors
Where are o.s.found?
 There are many types of operating systems, the
complexity of which varies depending upon what type
of functions are provided, and what the system is
being used for. Some systems are responsible for
managing many users on a network. Other operating
systems do not manage user programs at all. These are
typically found in hardware devices like petrol pumps,
airplanes, video recorders, washing machines and car
engines.
SINGLE USER O.S.
 We are all familiar with the concept of sitting down at a
computer system and writing documents or performing
some task such as writing a letter. In this instance there is
one keyboard and one monitor that you interact with.
 Operating systems such as Windows 95, Windows NT
Workstation and Windows 2000 professional are essentially
single user operating systems. They provide you the
capability to perform tasks on the computer system such as
writing programs and documents, printing and accessing
files.
The advantage of having a multi-user operating system is that
normally the hardware is very expensive, and it lets a number
of users share this expensive resource. This means the cost is
divided amongst the users. It also makes better use of the
resources. Since the resources are shared, they are more likely
to be in use than sitting idle being unproductive.
DISADVANTAGE OF M.O.S.
 One problem with multi-user computer
systems is that as more users access it, the
performance becomes slower and slower.
Another disadvantage is the cost of
hardware, as a multi-user operating system
requires a lot of disk space and memory. In
addition, the actual software for multi-user
operating systems tend to cost more than
single-user operating systems.
MULTITASKING O.S.
 A multi-tasking operating system provides the ability
to run more than one program at once. For example, a
user could be running a word processing package,
printing a document, copying files to the floppy disk
and backing up selected files to a tape unit. Each of
these tasks the user is doing appears to be running at
the same time.
DISADVANTAGE OF MULTITASKING O.S.
 A multi-tasking operating system has the
advantage of letting the user run more than
one task at once, so this leads to increased
productivity. The disadvantage is that more
programs that are run by the user, the more
memory that is required.
What are input devices?
Input devices provide input signals such as commands to the
operating system. These commands received from input
devices instruct the operating system to perform some task or
control its behavior. Typical input devices are a keyboard,
mouse, temperature sensor, air-flow valve or door switch. In
the example of our simple security control system, the input
devices could be door switches, alarm keypad panel and
smoke detector units.
Output devices are instruments that receive commands
or information from the operating system. Typical
output devices are monitor screens, printers, speakers,
alarm bells, fans, pumps, control valves, light bulbs etc.
OPERATING SYSTEM UTILITIES
 Managing Files and Documents
 Development of Programs and Software
 Communicating between people and with other
computer systems
 Managing user requirements for programs, storage
space and priority
OPERATING SYSTEM
OVERVIEW
Characteristics
Other Characteristics include:
 Time Sharing - multiprogramming environment that's also interactive.
 Multiprocessing - Tightly coupled systems that communicate via shared memory. Used for
scientific applications. Used for speed improvement by putting together a number of off-the-shelf
processors.
 Distributed Systems - Loosely coupled systems that communicate via message passing. Advantages
include resource sharing, speed up, reliability, communication.
 Real Time Systems - Rapid response time is main characteristic. Used in control of applications
where rapid response to a stimulus is essential.
1: Operating Systems Overview
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Modern Operating systems generally have following
three major goals. Operating systems generally
accomplish these goals by running processes in low
privilege and providing service calls that invoke the
operating system kernel in high-privilege state.
The user interacts with the operating systems
through the user interface and usually interested
in the “look and feel” of the operating system.
The most important components of the user
interface are the command interpreter, the file
system, on-line help, and application
integration. The recent trend has been toward
increasingly integrated graphical user interfaces
that encompass the activities of multiple
processes on networks of computers.
An abstraction is software that hides lower level details
and provides a set of higher-level functions. An operating
system transforms the physical world of devices,
instructions, memory, and time into virtual world that is
the result of abstractions built by the operating system.
There are several reasons for abstraction.
First, the code needed to control peripheral devices is not
standardized. Operating systems provide subroutines called device
drivers that perform operations on behalf of programs for
example, input/output operations.
Second, the operating system introduces new functions as it
abstracts the hardware. For instance, operating system introduces
the file abstraction so that programs do not have to deal with disks.
Third, the operating system transforms the computer hardware
into multiple virtual computers, each belonging to a different
program. Each program that is running is called a process. Each
process views the hardware through the lens of abstraction.
An operating system controls how processes (the active
agents) may access resources (passive entities).
THANKS.