intro to OS

Operating Systems/Introduction to Operating System Lecture Notes
PCP Bhatt/IISc, Bangalore M1/V1/June 04/1
Module 1: Introduction to Operating System
Operating System (or shortly OS) primarily provides services for running applications on
a computer system.
Need for an OS:
The primary need for the OS arises from the fact that user needs to be provided with
services and OS ought to facilitate the provisioning of these services. The central part of a
computer system is a processing engine called CPU. A system should make it possible
for a user’s application to use the processing unit. A user application would need to store
information. The OS makes memory available to an application when required. Similarly,
user applications need use of input facility to communicate with the application. This is
often in the form of a key board, or a mouse or even a joy stick (if the application is a
game for instance).
The output usually provided by a video monitor or a printer as some times the user may
wish to generate an output in the form of a printed document. Output may be available
in some other forms. For example it may be a video or an audio file.
Let us consider few applications.
• Document Design
• Accounting
• E-mail
• Image processing

• Games
We notice that each of the above application requires resources for
• Processing information
• Storage of Information
• Mechanism to inputting information
• Provision for outputting information
• These service facilities are provided by an operating system regardless of the
nature of application.
The OS offers generic services to support all the above operations. These operations in
turn facilitate the applications mentioned earlier. To that extent an OS operation is
application neutral and service specific.
User and System View:
From the user point of view the primary consideration is always the convenience. It
should be easy to use an application. In launching an application, it helps to have an icon
which gives a clue which application it is. We have seen some helpful clues for launching
a browser, e-mail or even a document preparation application. In other words, the human
computer interface which helps to identify an application and its launch is very useful.
This hides a lot of details of the more elementary instructions that help in selecting the
application. Similarly, if we examine the programs that help us in using input devices like
a key board – all the complex details of character reading program are hidden from the
user. The same is true when we write a program. For instance, when we use a
programming language like C, a printf command helps to generate the desired form of
output. The following figure essentially depicts the basic schema of the use of OS from a
user stand point. However, when it comes to the view point of a system, the OS needs to
ensure that all the system users and applications get to use the facilities that they need.

Also, OS needs to ensure that system resources are utilized efficiently. For instance,
there may be many service requests on a Web server. Each user request need to be
serviced. Similarly, there may be many programs residing in the main memory. The
system need to determine which programs are active and which need to await some
form of input or output. Those that need to wait can be suspended temporarily from
engaging the processor. This strategy alone enhances the processor throughput. In
other words, it is important for an operating system to have a control policy and
algorithm to allocate the system resources.
The Evolution:
It would be worthwhile to trace some developments that have happened in the last
four to five decades. In the 1960s, the common form of computing facility was a
mainframe computer system. The mainframe computer system would be normally
housed in a computer center with a controlled environment which was usually an air
conditioned area with a clean room like facility. The users used to bring in a deck of
punched cards which encoded the list of program instructions.
The mode of operation was as follows:
User would prepare a program as a deck of punched cards.
The header cards in the deck were the “job control” cards which would indicate
which compiler was to be used (like Fortran / Cobol compilers).
The deck of cards would be handed in to an operator who would collect such jobs
from various users.
COMPUTER HARDWARE
OPERATING SYSTEM
SYSTEM AND APPLICATION PROGRAMS
USER 1 USER 2 USER n………

The operators would invariably group the submitted jobs as Fortran jobs, Cobol
jobs etc. In addition, these were classified as “long jobs” that required
considerable processing time or short jobs which required a short and limited
computational time.
Each set of jobs was considered as a batch and the processing would be done for a batch.
Like for instance there may be a batch of short Fortran jobs. The output for each job
would be separated and turned over to users in a collection area. This scenario clearly
shows that there was no interactivity. Users had no direct control. Also, at any one time
only one program would engage the processor. This meant that if there was any input or
output in between processing then the processor would wait idling till such time that the
I/O is completed. This meant that processor would idling most of the time as processor
speeds were orders of magnitude higher than the input or output or even memory units.
Clearly, this led to poor utilization of the processor. The systems that utilized the CPU
and memory better and with multiple users connected to the systems evolved over a
period of time as shown in Table1.1.
.
At this time we would like to invoke Von - Neumann principle of stored program
operation. For a program to be executed it ought to be stored in the memory. In the
scheme of things discussed in the previous paragraph, we notice that at any time only one
program was kept in the memory and executed. In the decade of 70s this basic mode of
operation was altered and system designers contemplated having more than one program
resident in the memory. This clearly meant that when one program is awaiting

completion of an input or output, another program could, in fact, engage the CPU..
Late 60’s and early 70’s
Storing multiple executables (at the same time) in the main memory is called
multiprogramming. With multiple excutables residing in the main memory, the
immediate consideration is: we now need a policy to allocate memory and processor time
to the resident programs. It is obvious that by utilizing the processor for another process
when a process is engaged in input or output the processor utilization and, therefore, its
output are higher. Overall, the multiprogramming leads to higher throughput for this
reason.
Multiprogramming
While multiprogramming did lead to enhanced throughput of a system, the systems still
essentially operated in batch processing mode.
1980’s
In late 70s and early part of the decade of 80s the system designers offered some
interactivity with each user having a capability to access system. This is the period when
the timeshared systems came on the scene.
Basically, the idea is to give every user an illusion that all the system resources were

available to him as his program executed. To strengthen this illusion a clever way was
devised by which each user was allocated a slice of time to engage the processor. During
the allocated time slice a users’ program would be executed. Now imagine if the next turn
for the same program comes quickly enough, the user would have an illusion that the
system was continuously available to his task. This is what precisely time sharing
systems attempted – giving each user a small time slice and returning back quickly
enough so that he never feels lack of continuity. In fact, he carries an impression that the
system is entirely available to him alone.
Timeshared systems clearly require several design considerations. These include the
following: How many programs may reside in the main memory to allow, and also
sustain timesharing? What should be the time slice allocated to process each program?
How would one protect a users’ program and data from being overwritten by another
users’ program? Basically, the design trends that were clearly evident during the decade
of 1970-80 were: Achieve as much overlapping as may be feasible between I/O and
processing. Bulk storage on disks clearly witnessed a phenomenal growth. This also
helped to implement the concept to offer an illusion of extended storage. The concept of
“virtual storage” came into the vogue. The virtual storage essentially utilizes these disks
to offer enhanced addressable space. The fact that only that part of a program that is
currently active need be in the main memory also meant that multi-programming could
support many more programs. In fact this could be further enhanced as follows:
1. Only required active parts of the programs could be swapped in from disks.
2. Suspended programs could be swapped out.
This means that a large number of users can access the system. This was to satisfy the
notion that “computing” facility be brought to a user as opposed to the notion that the
“user go to compute”. The fact that a facility is brought to a user gives the notion of a
utility or a service in its true sense. In fact, the PC truly reflects the notion of “computing
utility” - it is regarded now as a personal productivity tool.

Swapping of program parts main memory - disc, vice-versa
It was in early 1970s Bell Laboratory scientists came up with the now well known OS:
Unix. Also, as the microcomputers came on scene in 1980s a forerunner to current DOS
was a system called CP/M. The decade of 1980s saw many advances with the promise of
networked systems. One notable project amongst these was the project Athena at MIT in
USA. The project forms the basis to several modern developments. The client-server
paradigm was indeed a major fall out. The users could have a common server to the so
called X-terminals.
The X windows also provided many widgets to support convenient human computer
interfaces. Using X windows it is possible to create multiple windows. In fact each of the
windows offers a virtual terminal. In other words it is possible to think about each of
these windows as a front-end terminal connection. So it is possible to launch different
applications from each of the windows. This is what you experience on modern day PC
which also supports such an operating environment.
CP/M based computer
Networking topologies like star, ring and general graphs, as shown in the figure, were
being experimented with protocols for communication amongst computers evolved. In
In our discussions, we shall discuss many of the above
issues in greater detail as we move to later chapters. On
the micro-computer front the development was aimed at
relieving the processor from handling input output
responsibilities. The I/O processing was primarily handled
by two mechanisms: one was BIOS and the other was the
graphics cards to drive the display. The processor now
was relieved from regulating the I/O. This made it
possible to utilize the processor more effectively for other
processing tasks. With the advent of 1990s the computer
communication was pretty much the order of the day.

particular, the TCP/IP suite of network protocols were implemented. The growth in the
networking area also resulted in giving users a capability to establish communication
between computers. It was now possible to connect to a remote computer using a telnet
protocol. It was also possible to get a file stored in a remote location using a file transfer
(FTP) protocol. All such services are broadly called network services.
x x x
(a) Star (b) Ring (c) Tree
Let’s now briefly explore where the OS appears in the context of the software and
application.
Let’s consider a scenario where we need to embed the computer system in an industrial
application. This may be regulating the temperature of a vessel in a process control. In a
typical process control scenario
• Monitoring – initializes and activates the hardware.
• Input – Reads the values from sensors and stores it in register.
• Decision – checks whether the readings are within the range.
• Output – responds to the situation.
• Scenario: A temperature monitoring chemical process.

• What we need: A supervisory program to raise an alarm when temperature goes
beyond a certain band.
• The desired sequence of operational events: Measure input temperature,
process the most recent measurement, perform an output task.
The computer system may be employed in a variety of operational scenarios like a bank,
airlines reservation system, university admissions and several others. In each of these we
need to provide the resources for
• Processing
• User access to the system
• Storage and management of information
• Protection of information against accidental and intentional misuse
• Support for data processing activities
• Communication with I/O devices
• Management of all activities in a transparent manner.
Let’s now review What Does an OS Do?
• Power On Self Test (POST)
• Resource management

• Support for multi-user
• Error Handling
• Communication support over Network
• (Optional) Deadline support so that safety critical application run and fail
gracefully
Operational View:
Let’s briefly look at the underlying principle of operation of a computer system. Current
systems are based on The Von-Neumann principle. The principle states that a program is
initially stored in memory and executed by fetching an instruction at a time.
The basic cycle of operation is
Fetch an instruction (Fetch)
Interpret the instruction (Decode)
Execute the instruction (Execute)
Modern systems allow multiple users to use a computer system. Even on a stand alone
PC there may be multiple application which are running simultaneously. For instance, we
have a mail program receiving mails, a clock to display time while we may be engaged in
browsing a word process.
In other words OS need to schedule the processor amongst all the application
simultaneously without giving an impression that the processor time is being divided and
scheduled per an application.
An Operational Overview:
• Processor – schedule and allocate processor time.
Operating Cycle Memory Map

• Memory – executes program and access data
• Input output devices
• Communication with devices
• Mutual exclusion – schedule the usage of shared device and fair access
• Shell of an OS
• Human computer interface (HCI/CHI)
A Modern Computer System
The peripheral devices communicate in a mode known as interrupt mode .Typically
human input is considered important and often uses this mode. This is so because human
desire to guide the operation. For instance, we use a mouse click or a key board input.
These require immediate attention by needing an appropriate interruption service.

Processes and Tools:
Processes:
Most OSs support a notion that a program in execution may be regarded as a process.
Clearly, with multiple applications active at the same time there are many processes that
are active. The OS needs to manage all these processes. Often applications may spawn
processes that need to communicate with each other. Such inter process communication
forms the primary basis of distributed computing.
With the coming together of communications network and computers it is possible to
create processes on one machine and seek services from another machine. The machine
seeking the services is called client machine and the machine offering services is called
server machine. When you use a web browser, your computer is the client and the
machine which provides the content for browsing is the web server. All modern OSs
support client-server operations for service provisioning.
Tools:
One important issue is: how does one use the services efficiently? In addition to the
device handling utilities, most OSs provide many other general purpose utilities as a set
of packaged tools. The tools help us to think in terms of higher level of operations. In the
absence of tools and tool kits we would be required to write a fresh program each time.

As an example, consider a sort utility in UNIX. It supports identification of fields in a file
with multi field structured data. Sort allows us to sort on a chosen field on a file. Imagine
if we had to write a sort program every time we had a set of records making a file. Also
look at the support we derive from the compression software tools like compression of
files for long-term storage or transmission (Zip tools in windows environment). It would
be stupendous task to write a compression program for every file transfer.
Trends in Computing:
In this section we shall examine other trends in computing. The application scenario can
considerably influence the types of services that need to be managed by an operating
system. One such example is the emergence of parallel computing.
Parallel Computing:
There are many problem solving situations like weather forecasting, image processing,
statistical data analysis and simulation, pharmaceutical drug synthesis and others where
using a single processor becomes a limiting factor
Parallel Computing
File Server Recording PC
Switch
Processing Nodes
The emergence of micro-computer, in particular Apple
mackintosh, offered improvement in the manner in which
human computer interfaces could be utilized for I/O and
also for launching the applications. Apple was the first
computer system to use mouse clicks to launch application.
It also was the first environment to offer a “drag and drop”
facility. In fact this set trends to offer icon driven convenient
ways of launching applications.
Apple Mac
Computer
For this class of problems one has to resort to
parallel processing. In a parallel processing
environment we may have a problem split in such
a way that different parts of the same problem use
a different processor with minimal communication.
Parallel processing together with the network
based services supports a mode of computing
which is referred some times as distributed
computing. In distributed computing environment
it is also possible to support distribution of data,
files and execution of processes.

In some other situation a computer system may be a part of a control system. For
instance, the system may be utilized for regulating a utility – like electrical power supply.
There is usually an elaborate instrumentation to keep track of the health of plant or a
process. In these situations the instrumentation monitors physical quantities like pressure,
temperature or speed and the process needs to respond in a real-time mode to be able to
regulate the operation of the system.
Real - Time Systems:
Another category of real-time systems are those that are used to offer services like ATM
(Automatic teller machine) or airlines reservation systems. The systems of the latter kind
are often called transaction oriented systems. This is because the basic information
processing involves a transaction.
ATM
temperature in a process vessel exceeds a certain limit. The operating system used in such
real-time systems is often referred to as RTOS.
In addition, to the above kind of systems, we are now witnessing emergence of a class of
systems that are embedded within an application.
For instance, it is not unusual to find up to four microprocessors in a car. These may be
used for regulating fuel injection, cruise control or even operation of brakes. Similarly,
there are microcontrollers embedded in washing machines. The presence of these systems
As an example, consider processing a request like
withdrawing money. It is a financial transaction,
which requires updating accounts within an
acceptable response time. Both the process control
systems and transaction oriented systems are real
time systems in which the response time to a request
is critical. The main objective is to assure timeliness
in response. The transaction oriented systems may
even use the computer network extensively and
often. The process control systems may be bus
based or may have local area network as well. Both
these systems invariably recognize an event and
offer a response. In a transaction oriented system
like ATM, it may be a card input. In a process
control system an event may be detected when the

is totally transparent to the user. He does not experience the presence of the system while
using the gadgets. This is because the operating environment only requires minimal
control buttons and response panels. The embedded systems are designed completely
with a different philosophy. These are not general purpose computing environments.
Instead, these have dedicated mode of operation. In these cases the operating system is
not aimed at raising through put with general purpose utility back-ups. Instead these
systems are designed with minimal and dedicated services. The minimal service
provisioning is done using a minimal OS kernel often called micro-kernel. A well known
Simputer
Wireless Systems:
Wireless systems in general allow access from any where any time. These are also called
ubiquitous systems. The ubiquity comes from the fact that unlike a wired system, the
medium of communication is air which can be utilized from anywhere anytime.
Finally, the other major trend which we are witnessing now is driven by Web – the world
wide web. All modern systems are internet compliant and allow a user to connect to the
rest of the world. Web offers commercial organizations to offer their merchandise on the
web. Also, it gives the consumers an opportunity to seek services using the web. In our
country now Web is quite extensively utilized for railway reservation and also for the air
ticket booking. Web can also offer other services. For example, down loading music is
common. Web can, and in due course of time will, offer services which are currently
offered by operating systems. In fact, then we will have the paradigm where “network is
the computer” as was proclaimed by the SUN CEO, Scott McNealy a few years ago.
embedded system is a common mobile phone and a personal
digital assistant (PDA). One important Indian contribution in
this class of systems is a Simputer. A Simputer can be utilized
as a PDA. Simputer can also be used in a dedicated
application. Simputers have also been upgraded to support
wireless connectivity.

intro to OS

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intro to OS