operating system basic

7/27/2019 Operating System Basic

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1.0 INTRODUCTION TO OPERATINGSYSTEM

An operating system (OS) is software, consisting of programs and data thatruns on computers and manages the computer hardware and provides

common services for efficient execution of various application software.

For large systems, the operating system has even greater responsibilities

and powers. It makes sure those different programs and users running at

the same time do not interfere with each other. The operating system is

also responsible for security, ensuring that unauthorized users do not

access the system. Operating systems provide a software platform on top

of which other programs, called application programs, can run. The

application programs must be written to run on top of a particular

operating system. Handheld computer operating system is designed to

provide an environment in which a user can easily interface with the


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computer to execute programs. Thus, some operating systems are

designed to be convenient, others to be efficient, and others combination

of the two. Modern general-purpose computers, including personal

computers and mainframes, have an operating system to run other

programs, such as application software. Examples of operating systems for

personal computers include Microsoft Windows, Mac OS, UNIX, and Linux.

1.1 What is an Operating System?

An operating system is an important part of almost every computer

system. An Operating system mainly performs three functions

Convenience: It makes computer more convenient to use Efficiency: It allows the computer system resources to be used in an

efficient manner

Ability to solve: It should be constructed in such a way as topermit the effective development, testing and introduction of

new system functions at the same time interfacing with services.

It also provides basics for application program and act as an intermediate

between a user of a computer and the computer hardware. Without

operating System, a computer is basically a useless lump of metal. A

computer system can be divided roughly into four components: the

hardware, the operating system, the application program, and the users.


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The hardware, central processing unit (CPU), memory, and the

input/output (I/O) devices provide the basic computing resources. The

application program such as word processors, spreadsheets, compilers,

and web browsers-defines the ways in which these resources are used to

solve the computing problems of the user. The operating system controls

and co-ordinates the use of the hardware among the various application

programs for the various users.

1.2 History of Operating Systems

An operating system (OS) is a software program that manages the

hardware and software resources of a computer. The OS performs basic

tasks, such as controlling and allocating memory, prioritizing the

processing of instructions, controlling input and output devices, facilitating

networking, and managing files. First three digital computers were

designed by English Mathematician Charles Babbage in 1792-1871. It was

purely mechanical design.


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1.2.1 First Generation (1945-1955):- In First Generation

Computers with Vacuum tubes and plug boards were evolved. In these

days a single group of people designed, built, programmed, operated

and maintained each machine. Programming was done in MachineLanguage. The first computers did not have operating systems. In the early

1950s, a computer could execute only one program at a time. Each user

had sole use of the computer and would arrive at a scheduled time with

program and data on punched paper cards and tape. The program would

be loaded into the machine, and the machine would be set to work until

the program completed or crashed.

1.2.2 Second Generation (1955-65):- Transistors and batch

System. Here for the first time between the programmers and

maintenance persons, Transistors Were introduced. To run a job, a

programmer would first write the program on paper, then punch it on

cards, bring the card deck down to Input / Output room batch system

was to collect a tray full of jobs in the Input / Output room and then read

them onto a magnetic tape just to reduce the wasted time.

1.2.3 Third Generation (1965-1980):- Integrated Chips

and Multiprogramming In this generation, Integrated circuits were

used, providing a major price/performance advantage over second

generation machines, which were built up from individual transistors.

Concept of Multiprogramming was introduced. When the current job was

to wait for a tape or other Input / Output operation to complete the

Central Processing Unit simply sat idle until the Input / Output

finished. As the Central Processing Unit time is very precious this

wasted time is not significant.


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1.2.4 Fourth Generation (1980-present):- Personal

Computers With the development of LSI (Large Scale Integration)

circuits, chips containing thousands of transistors in a square

centimeter of silicon, the age of personal computers dawned. Personal

Computers are not that different from mini computers. The most

powerful Personal Computers used by business, universities and

government installations are usually called Work Stations (Large

Personal Computers). Usually they are connected together by hardware.

1.3Basic Functions of an operating systemAt the simplest level, an operating system does two things. It manages the

hardware and software resources of the system. In a desktop computer,

these resources include such things as the processor, memory, disk space,

etc. (On a cell phone, they include the keypad, the screen, the address

book, the phone dialer, the battery and the network connection.)It

provides a stable, consistent way for applications to deal with the

hardware without having to know all the details of the hardware. The firsttask, managing the hardware and software resources, is very important, as

various programs and input Methods compete for the attention of the

central processing unit (CPU) and demand memory, storage and

input/output (I/O) bandwidth for their own purposes. In this capacity, the

operating system plays the role of the good parent, making sure that each

application gets the necessary resources while playing nicely with all the

other applications, as well as husbanding the limited capacity of the

system to the greatest good of all the users and applications. The second

task, providing a consistent application interface, is especially important if

there is to be more than one of a particular type of computer using the

operating system, or if the hardware making up the computer is ever open

to change. A computer, without any software, is essentially of no use. If it

is with its software that it can store, process and retrieve information. An


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operating system is the program that controls all the other parts of a

computer system both the hardware and the software. There are many

important reasons for studying operating systems. User interacts with the

computer through operating system in order to accomplish his task since it

is his primary interface with a computer. It helps user to understand theinner function of a computer very closely. Many concepts and techniques

found in operating system have general applicability in other applications.

1.4 Feature of Operating System

Multi-User: Allows two or more users to run programs at the sametime. Some operating systems permit hundreds or even thousands

of concurrent users.

Multi Processing: Supports running a program on more than oneCPU.

Multi Tasking: Allows more than one program to run concurrently. Multithreading: Allows different parts of a single program to run

concurrently.

Real time: Responds to input instantly. General-purpose operatingsystems, such as DOS and UNIX, are not real-time.

The operating system is also responsible for security, ensuring thatunauthorized users do not access the system.

Performs many low-level tasks on behalf of users and applicationprograms.


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Provides a way for users to interact with the computer.

1.5 Qualities of Operating System

Usability Robustness: -accept all valid input without error, and gracefully

handles all invalid inputs.

Consistency Proportionality: - Simple, cheap and frequent things are easy. Forgiving: -Errors can be recovered from. Reasonable error messages. Convenient: - Not necessary to repeat things, or do awkward

procedures to accomplish things.

Powerful: - Has high-level facilities.1.6 Utilities of operating system

1.6.1 Resource Management/Allocation

CPU - determine which process the CPU works on. Main memory - determine which processes can utilize which

memory locations.

1.6.2 Types of resource management allocation

Extended Machine


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Resource Manager1.6.2.1 Extended Machine: - The program that hides the truth

about the hardware from the programmer and presented a nice, simple

view of named files that can be read and written is, of course, theoperating system. The function of the operating system is to present the

user with the equivalent of an extended machine or virtual machine

that is easier to program than the underlying hardware.

1.6.2.2 Resource Manager: - Operating systems as primarily

providing its users with a convenient interface are a top-down view. In

short, this view of the operating system holds that its primary task is tokeep track of who is using which resource, to grant resource requests,

to record for usage, and to solve conflicting requests from different

programs and users.

1.7 Operating system services

Program Creation: Operating system services editors anddebuggers, to assist programmer in creating programs. These

services are in the form of utility programs that are accessible

through the Operating System.

Program Execution: Program execution needs instructions anddata to be loaded in to main menu, Input/output devices and files to

be initialized and other resources to be prepared. The operating

system handles all these Tasks for the user.

Access to Input/output devices: Each Input/output devicerequires its own peculiar set of instructions of control signals for


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operation. Operating system takes care of details so, the

programmers can think in terms of simple read and writes.

System Access: Operating System controls access to the system asa whole and to specific system resources.

Error detection and response: Variety of errors can occur whicha computer system is running, i.e., internal and external hardware

errors, such as memory error, device failure error, various software

errors, inability of Operating System to grant request of an

application errors and so on.

Accounting: A good Operating System collects usage statisticsfor various resources and monitors performance parameters such

as response time, which can be used for billing purposes on a multi-

user system.

1.8 Operating System Concepts:Operating Systems are among the most complex pieces of software. Five

major achievements in the development of Operating System are

Process Memory Management Information protection and security Scheduling and Resource Management

1.8.1 Process: It is somewhat more general term than job.Process is a program that is in execution. Three major lines of computer


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system development created problems in timing and synchronization

that contributed to the development of the concept of the process.

The design of system software to co-ordinate the various activities

turned out to be difficult. With many jobs in progress at any one time,

each of which involved numerous steps to be performed in sequence, itbecame impossible to analyze all the possible combination of sequences

of events. So many errors were detected which were difficult to

diagnose because they needed to be distinguished from application

software errors and hardware errors. To tackle these problems, it is

required to monitor and control the various programs executing on

the Processor in a systematic way.

The concept of process provides the foundation process consists ofthe following three components.

An executable program

The associated data needed by the program

Execution context of program

1.8.1.1 Execution : context includes the information that theOperating System needs to manage the process and that the processor

needs to properly execute the process. If two processes A and B exist

in a portion of the main memory, each process is Recorded in process

list, which is maintained by Operating System. Process index register

contain the index in to the process list of the process currently

controlling the processor, Program counter points to the next

instruction in that process to be executed. Base and limit register

defines the region in memory occupied by the process. Thus process is

realized as a data structure. A process can either be executing or Awaiting

execution. The entire state of process is contained in its context.

1.8.2 Memory Management: Users need a computingenvironment that supports the flexible use of data, efficient and orderly


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control of storage allocation. An Operating System, to satisfy these

Requirements has five principal storage management responsibilities as

follows:

Process Isolation: Operating System must prevent independentprocess from interfacing with data and memory of each other.

Automatic Allocation and Management : Programs should bedynamically allocated memory across the memory is required.

Operating System can achieve efficiency by assigning memory to jobs

only as needed.

Support of Modular Programming : Programmers should beable to define program modules and to create, destroy and alterthe size of modules dynamically.

Protection and Access Control: Sharing of memory at any level ofmemory Hierarchy. Operating System must allow portions of

memory to be accessible in various ways by various users.

Long term storage: Many users and application require means forstoring information for extended periods. Operating Systems meet

these requirements with the concept of Virtual Memory and filesystem facilities. Virtual Memory is a facility that allows program to

address memory from a logical point of view without regard to

the amount of main memory Physically available.

1.8.3 Information protection and security

Operating System must support a variety of protection and security

mechanism to computer system and the information stored in them.Some overall protection policies are:

No sharing : In this case, processes are completely isolatedfrom each other and each process has exclusive control over

resources statically or dynamically assigned to it.


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Controlled information dissemination : In some systems,security classes are defined to enforce a particular dissemination

policy. Users and applications are given security clearances of a

certain level, whereas data and other resources (e.g., I/O

devices) are given security classifications.

Access Control : Is concerned with regulating user access to thetotal system, sub systems, and data, and regulating process

access to various resources and objects within the system.

Information flow control: Regulates the flow of data within thesystem and its delivery to users.

1.8.4 Scheduling and Resource Management

A key task of the Operating system is manage the various resources

available to it (main memory space, I/O devices, processors) and to

schedule their use by the various active processes.

Any resource allocation and scheduling policy must consider the Followingthree factors:

Fairness : Typically, we would like all processes that arecompeting for the Use of a particular resource to be given

approximately equal and fair access to That resource. This is

especially so for jobs of the same class, that is, jobs of similar

demands, which are charged the same rate.

Differential responsiveness : On the other hand, the operatingsystem may need to discriminate between different classes of

jobs with different service requirements. The operating system

should attempt to make allocation and scheduling decisions to meet

the total set of requirements. The operating system should also view

these decisions dynamically. For example, if a process is waiting for


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the use of an I/O device, the operating system may wish to schedule

that process for execution as soon as possible to free up the device

for later demands from other processes.

Efficiency : Within the constraints of fairness and efficiency, theoperating system should attempt to maximize throughput,

minimize response time, and in the case of time sharing,

accommodate as many users as possible

1.9 System Structure

As more and more features have been added to operating systems

and as the underlying hardware has become more complex and versatile,

the size and complexity of operating systems has grown. The size of afull-featured operating system and the difficulty of the task it

addresses have led to three unfortunate but all too common

problems. First, operating systems are chronically late in being delivered.

This goes for new operating systems and for upgrades of older

systems. Second, the systems have latent bugs that show up in the

field and must be fixed and reworked. And finally, performance is often

not what was expected. To manage the complexity of operating Systems

and to overcome these problems, much attention has been given over the

years to the software structure of the operating system. Certain points

seem obvious. The software must be modular. This helps to organize the

software development process and limits the task of diagnosing and

fixing errors. The modules must have well-defined interfaces to each

other, and the interfaces must be as simple as possible. Again, this

eases the Programming task. It also makes the task of system evolution

easier. With clean, minimal interfaces between modules, one module

can be changed with minimal impact on other modules.

2.0 Classification of Operating Systems


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2.1 Evolution of Operating Systems

An operating system may processes its task serially (sequentially) or

concurrently (several task simultaneously). It means that the resources of

computer system may be dedicated to a single program until its

completion or they may be allocated among several programs in different

stages of execution. The feature of operating system to execute multiple

programs in interleaved fashion or different time cycles is called as

multiprogramming system.

2.1.1 Serial processing:-

The process of development and preparation of a program in such

environment is slow and cumbersome due to serial processing and

numerous manual processing. In a typical sequence first the editor is calledto create a source code of user program written in programming language,

translator is called to covert a source code into binary code and then finally

loader is called to load executable program into main memory for

execution. If syntax errors are detected, the whole process must be

restarted from the beginning.

2.1.2 Batch Processing

The next logical step in the evolution of operating system was to

automate the sequencing of operations involved in program execution and

in the mechanical aspects of program development. Jobs with similar

requirement were batched together and run through the computer as a

group. But there was one problem in making batch processing efficient

that is during this transition from one job to the next, the CPU sat idle. To


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overcome this idle time, a small program called a resident monitor was

created which is always resident in the memory. It automatically

sequenced one job to another job. Resident monitor acts according to the

directives given by a programmer through control cards which contain

information like marking of jobs beginning and endings, commands forloading and executing programs etc. These commands belong to job

control language.

2.1.3 Multiprogramming

A single user cannot always keep CPU or I/O device busy at all.

Multiprogramming offers a more efficient approach to increase systemperformance. System supporting multiprogramming approach allows more

than utilization. More number of programs competing for system

resources better will be resource utilization. Multiprogramming has been

employed to increase the resource utilization of a computer system and to

support multiple simultaneous interactive users (terminals).

2.2 Types of operating system: -

Operating system can be classified into following types based on number

of users using the same system simultaneously.

2.2.1 Single-user, single tasking

As the name implies, this operating system is designed to manage the

computer so that one user can effectively do one thing at a time. The PalmO.S. for Palm handheld computers is a good example of a modern Single-

user, single-task operating system.

2.2.2 Single-user, multi-tasking


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This is the type of operating system most people use on their desktop

and laptop computers today. Windows 98 and the Mac O.S. are both

examples of an operating system that will let a single user have several

programs in operation at the same time. For example, it's entirely possible

for a Windows user to be writing a note in a word processor whiledownloading a file from the Internet while printing the text of an e-mail

message.

2.2.3 Multi-user, multi-tasking

A multi-user operating system allows many different users to take

advantage of the computer's resources simultaneously. The operatingsystem must make sure that the requirements of the various users are

balanced, and that each of the programs they are using has sufficient and

separate resources so that a problem with one user doesn't affect the

entire community of users. It's important to differentiate here between

multi-user operating systems and single-user operating systems that

support networking. Windows 2000 and Novell Netware can each support

hundreds or thousands of networked users, `but the operating systems

themselves aren't true multi-user operating systems. The systemadministrator is the only user for Windows 2000 or Netware. The network

support and the entire remote user logins the network enables are, in the

overall plan of the operating system.

2.2.4 Real-Time operating System

Instruments and industrial systems need Real Time OperatingSystems (RTOS). An RTOS typically has very little user-interface capability,

and no end-user utilities, since the system will be a sealed box when

delivered for use. A very important part of an RTOS is managing the

resources of the computer so that a particular operation executes in

precisely the same amount of time every time it occurs. In a complex

machine, having a part move more quickly just because system resources


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are available may be just as catastrophic as having it not move at all

because the system is busy.

2.2.5 Batch

This strategy involves reading a series of jobs (called a batch) into the

machine and then executing the programs for each job in the batch. This

approach does not allow users to interact with programs while they

operate.

2.2.6 Timesharing

This strategy supports multiple interactive users. Rather than

preparing a job for execution ahead of time, users establish an interactive

session with the computer and then provide commands, programs and

data, as they are needed during the session.

2.2.7 Personal computing

This strategy supports a single user running multiple programs on a

dedicated machine. Since only one person is using the machine, more

attention is given to establishing predictable response times from the

system. This strategy is quite common today because of the popularity of

personal computers.

Dedicated:- This strategy supports real-time and process control systems.

These are the types of systems, which control satellites, robots, and air

traffic control. The dedicated strategy must guarantee certain response

times for particular computing tasks or the application is useless.


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3.0 Process Management

3.1 Introduction

An operating system executes a variety of programs, Batch system

jobs, Time-shared systems, user programs or tasks. Textbook uses the

terms job and process almost interchangeably. Process a program in

execution; process execution must progress in sequential fashion. A

process includes: program counter, stack, and data section etc.

3.2 Definition and concept of Process

Process Management concerns the control of programs within the

system. We use the term process to refer to a program that is loaded into

computer memory and is being executed i.e. is utilizing CPU time

resources. Recall that only the operating system can allocate system

resources, so the process will execute in either user mode or system mode

(system mode has direct access to resources). In order for different user

processes to exist, the operating system must be able to create and delete

both user and system processes.

3.3 Process StateAlthough each process is an independent entity, with its own

program counter and internal state, process often need to interact with

other process. One process may generate some output that another

process uses as an input. A process is more than the program code. It also

includes the current activity, as represent by the value of the program


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counter and the processors registers. Two processes may be associated

with same program. The operating system is responsible for managing all

the processes that are running on a computer and allocated each process a

certain amount of time to use the processor. To keep track of the state of

all the processes, the operating system maintains a table known as theprocess table. Inside this table, every process is listed along with the

resources the processes are using and the current state of the process.

Processes can be in one of three states: running, ready, or waiting. The

running state means that the process has all the resources it need for

execution and it has been given permission by the operating system to use

the processor. Only one process can be in the running state at any given

time. The remaining processes are either in a waiting state (i.e., waiting for

some external event to occur such as user input or a disk access) or a readystate (i.e., waiting for permission to use the processor). In a real operating

system, the waiting and ready states are implemented as queues, which

hold the processes in these states.

Different states of a process

3.4 Process Scheduling


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CPU scheduling is the basis of multi-programmed operating systems. By

switching the CPU among processes, the operating system can make the

computer more productive. The main function of process scheduling is to

schedule the process in such a way that none of the resource is wasted.

3.5 Real-Time Scheduling

3.5.1 Hardware real-time systems required completing a criticaltask within a guaranteed amount of time. Systems are required to

complete a critical task within a guaranteed amount of time. A process is

submitted along with a statement of the amount of time in which it needs

to complete or perform I/O. the scheduler then either admits the process,guaranteeing that the process will complete on time. Hard real time

systems are composed of special-purpose software running on hardware

dedicated to their critical process, and lack the full functionality of modern

computer and operating system.

3.5.2 Software real-time computing requires that criticalprocesses receive Priority over less fortunate ones. It is less restrictive. It

requires that critical processes receive priority over less fortunate ones.

Although adding software real-time functionality to a time sharing system

may cause an unfair allocation of resources and may result in longer

delays. Soft real time systems acceptably in an environment require careful

design of the scheduler and related aspect of the operating system. The

system must have priority scheduling, and real time process must have the

highest priority.

4.0 VIRTUAL MEMORY


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Memory management strategies like paging and segmentation help

implement the concept of multiprogramming. But they have a few

disadvantages. One problem with the above strategies is that they require

the entire process to be in main memory before execution can begin.

Another disadvantage is the limitation on the size of the process. Processes

whose memory requirement is larger than the maximum size of the

memory available will never be execute. Virtual memory is a technique

that allows execution of processes that may not be entirely in memory.

Also virtual memory allows mapping of a large virtual address space onto a

smaller physical memory. It also raises the degree of multiprogramming

and increases CPU utilization. Because of the above features, users arefreed from worrying about memory requirements and availability.

4.1 Need for Virtual Memory Technique

Every process needs to be loaded into physical memory for execution. One

brute force approach to this is to map the entire logical space of the

process to physical memory as in the case of paging and segmentation.Many a time the entire process need not be in memory during execution.

Code used to handle error and exceptional cases is executed only in case

errors and exceptional conditions occur which is usually a rare occurrence,

may be one or no occurrence in an execution.

Static declarations of arrays lists and tables declared with a largeupper bound but used with no greater than 10% of the limit.

Certain features and options provided in the program as a futureenhancement, never used, as enhancements are never

implemented.

Even though entire program is needed, all its parts may not beneeded at the same time because of overlays.


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All the examples show that a program can be executed even though it is

partially in memory. This scheme also has the following benefits:

Physical memory is no longer a constraint for programs andtherefore users can write large programs and execute them

Physical memory required for a program is less. Hence degree ofmultiprogramming can be increased because of which utilization

and throughput increase.

I/O time needed for load / swap is less.

5.0 CONCLUSION

An operating system (OS) is software that runs on computers and manages

the computer hardware and provides common services for efficient

execution. It acts as an interface between hardware and software

applications. Operating system is designed to provide an environment in

which a user can easily interface with computer to execute programs.

Operating system consists of graphical user interface (GUI). The operating

system is also responsible for security, ensuring that unauthorized users do

not access the system. Thus, some operating systems are designed to beconvenient, others to be efficient, and others combination of the two.


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