notes on operating system2013 formatted

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INTRODUCTION TO MODERN OPERATING SYSTEM- CHAPTER 1

WHAT IS AN OPERATING SYSTEM?

AN OPERATING SYSTEM ACTS AS AN INTERFACE BETWEEN THE USER AND THE COMPUTER SYSTEM.

The 1960s definition of an operating system is the software that controls the hardware. However, today, due to microcode we need a better definition. We see an operating system asthe programs that make the hardware useable. In brief, an operating system is the set ofprograms that controls a computer. Some examples of operating systems are UNIX, Mach, MS-DOS, MS-Windows, Windows/NT, Chicago, OS/2, MacOS, VMS, MVS, and VM.

Controlling the computer involves software at several levels. We will differentiate kernelservices, library services, and application-level services, all of which are part of the operatingsystem. Processes run Applications, which are linked together with libraries that performstandard services. The kernel supports the processes by providing a path to the peripheraldevices. The kernel responds to service calls from the processes and interrupts from thedevices. The core of the operating system is the kernel, a control program that functions inprivileged state (an execution context that allows all hardware instructions to be executed),reacting to interrupts from external devices and to service requests and traps from processes.Generally, the kernel is a permanent resident of the computer. It creates and terminatesprocesses and responds to their request for service.

Operating Systems are resource managers. The main resource is computer hardware in theform of processors, storage, input/output devices, communication devices, and data. Some ofthe operating system functions are: implementing the user interface, sharing hardware among

users, allowing users to share data among themselves, preventing users from interfering withone another, scheduling resources among users, facilitating input/output, recovering fromerrors, accounting for resource usage, facilitating parallel operations, organizing data for secureand rapid access, and handling network communications.

The operating system (OS) is the first thing loaded onto the computer -- without theoperating system, a computer is useless.

Operating systems can now be found on many of the devices we use every day, fromcell phones to wireless access points.

The purpose of an operating system is to organize and control hardware and softwareso that the device it lives in behaves in a flexible but predictable way.

Not all computers have operating systems. The computer that controls the microwaveoven in your kitchen, for example, doesn't need an operating system. It has one set oftasks to perform, very straightforward input to expect (a numbered keypad and a fewpre-set buttons) and simple, never-changing hardware to control.

For a computer like this, an operating system would be unnecessary baggage, driving upthe development and manufacturing costs significantly and adding complexity where
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none is required. Instead, the computer in a microwave oven simply runs a single hard-wired program all the time (EMBEDDED SYSTEMS).

All desktop computers have operating systems. The most common are the Windowsfamily of operating systems developed by Microsoft, the Macintosh operating systemsdeveloped by Apple and the UNIX family of operating systems (which have beendeveloped by a whole history of individuals, corporations and collaborators). There arehundreds of other operating systems available for special-purpose applications,including specializations for mainframes, robotics, manufacturing, real-time controlsystems and so on.

Operating system are made of portable code instead of permanent physical circuits sothat changes can be made regularly as an when required without scrapping the wholedevice

For a desktop computer user, this means you can add a new security update, systempatch, new application or even an entirely new operating system rather than junk yourcomputer and start again with a new one when you need to make a change.

OBJECTIVES OF OPERATING SYSTEMS

Modern Operating systems generally have following three major goals. Operating systemsgenerally accomplish these goals by running processes in low privilege and providing servicecalls that invoke the operating system kernel in high-privilege state.

TO HIDE DETAILS OF HARDWARE BY CREATING ABSTRACTION:-

An abstraction is software that hides lower level details and provides a set ofhigher-level functions. An operating system transforms the physical world of devices,instructions, memory, and time into virtual world that is the result of abstractions builtby the operating system. There are several reasons for abstraction.First , the code needed to control peripheral devices is not standardized. Operatingsystems provide subroutines called device drivers that perform operations on behalf ofprograms for example, input/output operations.

Second , the operating system introduces new functions as it abstracts thehardware. For instance, operating system introduces the file abstraction so thatprograms do not have to deal with disks.

Third , the operating system transforms the computer hardware into multiplevirtual computers, each belonging to a different program. Each program that is runningis called a process. Each process views the hardware through the lens of abstraction.Fourth , the operating system can enforce security through abstraction.
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TO ALLOCATE RESOURCES TO PROCESSES (MANAGE RESOURCES):-An operating system controls how processes (the active agents) may access resources(passive entities).

PROVIDE A PLEASANT AND EFFECTIVE USER INTERFACE :-

The user interacts with the operating systems through the user interface and usuallyinterested in the look and feel of the operating system. The most importantcomponents of the user interface are the command interpreter, the file system, on-linehelp, and application integration. The recent trend has been toward increasinglyintegrated graphical user interfaces that encompass the activities of multiple processeson networks of computers.

One can view Operating Systems from two points of views: Resource manager andExtended machines . Form Resource manager point of view Operating Systems manage thedifferent parts of the system efficiently and from extended machines point of view OperatingSystems provide a virtual machine to users that is more convenient to use. The structurallyOperating Systems can be design as a monolithic system, a hierarchy of layers, a virtualmachine system, an exokernel, or using the client-server model.

Operating System Also Known as the Resource Manager Means Operating System will

Manages all the Resources those are Attached to the System means all the Resource like

Memory and Processor and all the Input output Devices those are Attached to the System are

Known as the Resources of the Computer System and the Operating system will Manage all the

Resources of the System. The Operating System will identify at which Time the CPU will perform

which Operation and in which Time the Memory is used by which Programs. And which Input

Device will respond to which Request of the user means When the Input and Output Devices

are used by the which Programs. So this will manage all the Resources those are attached to the

Computer System.

The basic concepts of Operating Systems are processes, memory management, I/Omanagement, the file systems, and security.
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OPERATING SYSTEM FUNCTIONS

PROCESS MANAGEMENT

A process is a program in execution: (A program is passive, a process active.)

A process has resources (CPU time, files) and attributes that must be managed.

One (or more) threads are the schedulable entities within a process.

Management of processes includes:

Thread Scheduling (priority, time management, . . . )

Creation/termination

Block/Unblock (suspension/resumption )

Synchronization

Communication

Deadlock handling

Debugging

The Operating System also Treats the Process Management means all the Processesthose are given by the user or the Process those are System s own Process are

Handled by the Operating System . The Operating System will Create the Priorities foe

the user and also Start or Stops the Execution of the Process and Also Makes the Child

Process after dividing the Large Processes into the Small Processes.


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MAIN MEMORY MANAGEMENT

Operating System also Manages the Memory of the Computer System means Providethe Memory to the Process and Also Deallocate the Memory from the Process. And also

defines that if a Process gets completed then this will deallocate the Memory from theProcesses.

Allocation/de-allocation for processes, files, I/O.

Maintenance of several processes at a time

Keep track of who's using what memory

Movement of process memory to/from secondary storage.

FILE MANAGEMENT

A file is a collection of related information defined by its creator. Commonly, files representprograms (both source and object forms) and data.

The operating system is responsible for the following activities in connections with filemanagement:

File creation and deletion. Directory creation and deletion.

Support of primitives for manipulating files and directories.

Mapping files onto secondary storage.

File backup on stable (nonvolatile) storage media.


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I/O MANAGEMENT

Buffer caching system

Generic device driver code

Drivers for each device - translate read/write requests into disk positioncommands.

SECONDARY STORAGE MANAGEMENT

Operating System also Controls the all the Storage Operations means how the

data or files will be Stored into the computers and how the Files will be

Accessed by the users etc. All the Operations those are Responsible for Storing

and Accessing the Files is determined by the Operating System Operating System

also Allows us Creation of Files, Creation of Directories and Reading and Writing

the data of Files and Directories and also Copy the contents of the Files and the

Directories from One Place to Another Place.

Disks, tapes, optical, ...

Free space management ( paging/swapping )

Storage allocation ( what data goes where on disk )

Disk scheduling

NETWORKING

Communication system between distributed processors.

Getting information about files/processes/etc. on a remote machine.

Can use either a message passing or a shared memory model.

PROTECTION

Of files, memory, CPU, etc.

Means controlling of access

Depends on the attributes of the file and user


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SYSTEM PROGRAMS(User Interface)

Command Interpreters -- Program that accepts control statements (shell, GUIinterface, etc.)

Compilers/linkers

Communications (ftp, telnet, etc.)

Extended Machine : Operating System also behaves like an Extended Machine meansOperating system also Provides us Sharing of Files between Multiple Users, also

Provides Some Graphical Environments and also Provides Various Languages for

Communications and also Provides Many Complex Operations like using Many

Hardwares and Softwares.

Mastermind: Operating System also performs Many Functions and for those Reasonswe can say that Operating System is a Mastermind. It provides Booting without an

Operating System and Provides Facility to increase the Logical Memory of the Computer

System by using the Physical Memory of the Computer System and also provides various

Types of Formats Like NTFS and FAT File Systems.

Operating System also controls the Errors those have been Occurred into the Programand Also Provides Recovery of the System when the System gets Damaged Means When

due to Some Hardware Failure , if System Doesnt Works properly then this Recover the

System and also Correct the System and also Provides us the Backup Facility. And

Operating System also breaks the large program into the Smaller Programs those arealso called as the threads. And execute those threads one by one

TYPES OF OPERATING SYSTEM

BASED ON ENVIRONMENT

CUI

GUI

BASED ON FUNCTIONS

REAL-TIME OPERATING SYSTEM

SINGLE-USER, SINGLE TASK

SINGLE-USER, MULTI-TASKING


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MULTI-USER , MULTI-TASKING

MULTIPROCESSING O/S

MULTITHREADING O/S

NETWORKING O/S

BATCH PROCESSING O/S

DISTRIBUTED O/S

Command user Interface :-This operating system is totally command dependent. Onecan work on this environment only if they are familiar with the respective O/S commandEg. MS-DOS

GUI: - Short for Graphical User Interface , a GUI Operating System contains graphics andicons and is commonly navigated by using a computer mouse. See the GUI definition fora complete definition. Below are some examples of GUI Operating Systems.

System 7.xWindows 98Windows CE

A real-time operating system (RTOS):- is an operating system (OS) intended to servereal-time application requests. It must be able to process data as it comes in, typically

without buffering delays. Processing time requirements (including any OS delay) aremeasured in tenths of seconds or shorter. RTOS is specially designed for embeddedenvironments such as consumer devices, automobiles and robotics

RTOS) - Real-time operating systems are used to control machinery, scientificinstruments and industrial systems.

An RTOS typically has very little user-interface capability, and no end-user utilities, sincethe 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 aparticular operation executes in precisely the same amount of time, every time itoccurs.

A common example of an RTOS is an HDTV receiver and display. It needs to read adigital signal, decode it and display it as the data comes in. Any delay would benoticeable as jerky or pixelated video and/or garbled audio.
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Some of the best known, most widely deployed, real-time operating systems

LynxOS

OSE

QNX

RTLinux

VxWorks

Windows CE

Single user single tasking : -As the name implies, this operating system is designed tomanage the computer so that one user can effectively do one thing at a time. The Palm

OS for Palm handheld computers is a good example of a modern single-user, single-taskoperating system. E.g. MS-DOS

An example of a this kind of O/S would be the O/S of a basic mobile phone or an mp3player.

There can only be one user using the device and that person is only using one of itsapplications at a time

An MP3 player contains a computer to handle all of its functions. It has a small amountof memory and a number of specialist silicon chips . The operating system is installed inmemory and runs as soon as you switch on the device.

Single user Multi-Tasking :-This is the type of operating system most people use ontheir desktop and laptop computers today. Microsoft's Windows and Apple's MacOSplatforms are both examples of operating systems that will let a single user have severalprograms in operation at the same time. For example, it's entirely possible for aWindows user to be writing a note in a word processor while downloading a file fromthe Internet while printing the text of an e-mail message.

Another word for multi-tasking is multiprogramming E.g. Windows 95.

Multi-user Multi-tasking :- A multi-user operating system allows many different users totake advantage of the computer's resources simultaneously. The operating system mustmake sure that the requirements of the various users are balanced, and that each of theprograms they are using has sufficient and separate resources so that a problem with
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one user doesn't affect the entire community of users. Unix, VMS and mainframeoperating systems, such as MVS, are examples of multi-user operating systems.

Examples of this kind of O/S include various versions of UNIX, LINUX, IBMs z/OS, OS390,MVS and VM

Network Operating System:- Operating system that supports networking

examples of a network O/S include Windows Vista, Windows 8 and Mac O/S X , Unix,Linux and all other mainframes O/S

A network O/S system should have the following features :-

Deal with users logging on

Maintain the network connection to the server

Expand the file system to view folders on other computers

Provide security to separate user accounts from each other

Batch Operating System :-


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Batch processing is a technique in which Operating System collects one programs and datatogether in a batch before processing starts. Operating system does the following activitiesrelated to batch processing.

OS defines a job which has predefined sequence of commands, programs and data as a

single unit.

OS keeps a number a jobs in memory and executes them without any manualinformation.

Jobs are processed in the order of submission i.e first come first served fashion.

When job completes its execution, its memory is released and the output for the jobgets copied into an output spool for later printing or processing.

Advantages

Batch processing takes much of the work of the operator to the computer.

Increased performance as a new job gets started as soon as the previous job finishedwithout any manual intervention.

Disadvantages

Difficult to debug program.

A job could enter an infinite loop.

Due to lack of protection scheme, one batch job can affect pending jobs.


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Distributed operating system:-

This is one of the modern operating system

It is a O/S that supports distributed computing

Distributed systems allow users to share resources on geographically dispersed hostsconnected via a computer network. Services could be provided either through a Client-Server Model or a Peer-to-Peer model

A distributed system is a collection of loosely coupled processors interconnected by acommunication network.

The processors in a distributed system may vary in size and function. They may includemicroprocessors, workstations, minicomputers and large general purpose computersystems

They are referred as sites, nodes, computers, machines and hosts

There are two types distributed System

o Parallel Computingo Distributed Computing

In the most simple form Parallel Computing is a method where several individual(autonomous) systems (CPU's) work in tandem to resolve a common computing

workload.Distributed Computing is where several dis-associated systems are working separatelyto resolve a multi-faceted computing workload.

An example of parallel computing would be two servers that share the workload ofrouting mail, managing connections to an accounting system or database, solving amathematical problem, etc....

Distributed Computing would be more like the SETI (Search for extra-terresterialintelligence). Program, where each client works a separate "chunk" of information, and

returns the completed package to a centralized resource that's responsible formanaging the overall workload.

If you think of ten men pulling on a rope to lift a load, that is parallel computing. If tenmen have ten ropes and are lifting ten different loads from one place to consolidate atanother place, that would be distributed computing.


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In Parallel Computing all processors have access to a shared memory. In distributedcomputing, each processor has its own private memory

Distributed Operating System is a model where distributed applications are running onmultiple computers linked by communications. A distributed operating system is an

extension of the network operating system that supports higher levels of

communication and integration of the machines on the network.

This system looks to its users like an ordinary centralized operating system but runs onmultiple, independent central processing units (CPUs).

These systems are referred as loosely coupled systems where each processor has its own

local memory and processors communicate with one another through various

communication lines, such as high speed buses or telephone lines. By loosely coupled

systems, we mean that such computers possess no hardware connections at the CPU -

memory bus level, but are connected by external interfaces that run under the control

of software. The Distributed Os involves a collection of autonomous computer systems, capable of

communicating and cooperating with each other through a LAN / WAN. A Distributed Os

provides a virtual machine abstraction to its users and wide sharing of resources like as

computational capacity, I/O and files etc.
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The structure shown in fig contains a set of individual computer systems andworkstations connected via communication systems, but by this structure we can not

say it is a distributed system because it is the software, not the hardware, that

determines whether a system is distributed or not.

The users of a true distributed system should not know, on which machine theirprograms are running and where their files are stored. LOCUS and MICROS are the best

examples of distributed operating systems.

Using LOCUS operating system it was possible to access local and distant files in uniformmanner. This feature enabled a user to log on any node of the network and to utilize the

resources in a network without the reference of his/her location. MICROS provided

sharing of resources in an automatic manner. The jobs were assigned to different nodes

of the whole system to balance the load on different nodes.

Below given are some of the examples of distributed operating systems:

l. IRIX operating system; is the implementation of UNIX System V, Release 3 for Silicon

Graphics multiprocessor workstations.

2. DYNIX operating system running on Sequent Symmetry multiprocessor computers. 3. AIX operating system for IBM RS/6000 computers.

4. Solaris operating system for SUN multiprocessor workstations.

5. Mach/OS is a multithreading and multitasking UNIX compatible operating system;

6. OSF/1 operating system developed by Open Foundation Software: UNIX compatible. Distributed systems provide the following advantages: 1 Sharing of resources.

2 Reliability. 3 Communication.

4 Computation speedup.

Distributed systems are potentially more reliable than a central system because if asystem has only one instance of some critical component, such as a CPU, disk, or

network interface, and that component fails, the system will go down. When there are

multiple instances, the system may be able to continue in spite of occasional failures. In

addition to hardware failures, one can also consider software failures. Distributed

systems allow both hardware and software errors to be dealt with.


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A distributed system is a set of computers that communicate and collaborate each otherusing software and hardware interconnecting components. Multiprocessors (MIMD

computers using shared memory architecture), multicomputers connected through

static or dynamic interconnection networks (MIMD computers using message passing

architecture) and workstations connected through local area network are examples ofsuch distributed systems.

A distributed system is managed by a distributed operating system. A distributedoperating system manages the system shared resources used by multiple processes, the

process scheduling activity (how processes are allocating on available processors), the

communication and synchronization between running processes and so on. The

software for parallel computers could be also tightly coupled or loosely coupled. The

loosely coupled software allows computers and users of a distributed system to be

independent each other but having a limited possibility to cooperate. An example of

such a system is a group of computers connected through a local network. Every

computer has its own memory, hard disk. There are some shared resources such files

and printers. If the interconnection network broke down, individual computers could be

used but without some features like printing to a non-local printer.

ADVANTAGES OF DISTRIBUTED SYSTEM

Resource Sharing

Computation Speedup Reliability

Communication

Resource Sharing :- If number of different sites are connected to one another, then auser at one site may be able to use the resources available at another. For. E.g. a user atsite A may be using a laser printer located at site B

Computation Sped : - If a particular computation can be partitioned into

subcomputations that can run concurrently then a distributed system allows us todistribute the subcomputations among various sites.

There also exist automated load sharing in which the distributed operating systemautomatically moves jobs among several sites that are lightly loaded.
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Reliability :- If one site fails in a distributed system the remaining sites can continueoperating giving the system better reliability.

Data is replicated among several sites in a distributed system so that even if one sitefails data still can be access from other sites.

Communication : -When several sites are connected to one another by a communicationnetwork, the users at different sites have the opportunity to exchange information likepass message among themselves, collaborate on project by transferring the files of theproject, logging in to each others remote systems to run programs and exchanging mailto coordinate the work.

DISTRIBUTED OPERATING SYSTEM

With the advent of computer networks, in which many computers are linked together

and are able to communicate with one another, distributed computing became feasible.A distributed computation is one that is carried out on more than one machine in acooperative manner. A group of linked computers working cooperatively on tasks,referred to as a distributed system, often requires a distributed operating system tomanage the distributed resources

The operating systems commonly used for distributed computing systems can bebroadly classified into two types

Network operating systems

-Distributed operating systems.

The three most important features commonly used to differentiate between these twotypes of operating systems are

o System image,

o Autonomy, and

o Fault tolerance capability.

o System image : Under network OS, the user views the distributed system as a collectionof machines connected by a communication subsystem. i.e the user is aware of the factthat multiple computers are used. A distributed OS hides the existence of multiplecomputers and provides a single system image to the users.
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o Autonomy: A network OS is built on a set of existing centralized OSs and handles theinterfacing and coordination of remote operations and communications between theseOSs. So, in this case, each machine has its own OS. With a distributed OS, there is asingle system-wide OS and each computer runs part of this global OS.

o Fault Tolerance capability: -Fault tolerance capability: A network operating systemprovides little or no fault tolerance capability in the sense that if 10% of the machines ofthe entire distributed computing system are down at any moment, at least 10% of theusers are unable to continue with their work. On the other hand, with a distributedoperating system, most of the users are normally unaffected by the failed machines andcan continue to perform their work normally, with only a 10% loss in performance of theentire distributed computing system.

o Therefore, the fault tolerance capability of a distributed operating system is usually very

high as compared to that of a network operating system

Multimedia System :- Supports multiple kinds of applications

Multimedia applications : Streaming audio, video games, etc.

Traditional applications : Editors, compilers, web servers, etc.

Different applications have different requirements For e.g. interactiveapplications like Editors, compilers require low or average response time

While soft real time applications like streaming media , virtual games requireshigh response time

Multimedia O/S divides resources according to application requirements. E.g. 30% of CPU to streaming and 20% to http server, etc.

Multimedia data consist of audio and video files as well as conventional files.

These data differ from conventional data in that multimedia data such as frame of videomust be delivered according to certain time restrictions (for e.g., 30 frames per second)

Multimedia applications may also include live webcasts(broadcast over the world wideweb) of speeches or sporting events and even live webcams that allow a viewer nManhattan to observe customers at a caf in Paris.

Multimedia applications also run on smaller devices. Eg. Stock trader may have stockquotes delivered wirelessly and in real time to his PDA.


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A mobile operating system:- , also referred to as mobile OS , is the operating system thatoperates a smartphone, tablet, PDA, or other digital mobile devices. Modern mobileoperating systems combine the features of a personal computer operating system witha touchscreen, cellular, Bluetooth, WiFi, GPS mobile navigation, camera, video camera,

speech recognition, voice recorder, music player, Near field communication, InfraredBlaster, and other features.

E.g. Android from googleinc., Blackberry10 from Blackberry, iOS from Apple Inc. NokiaAsha Platform from Nokia, Symbian OS from Nokia etc.

Clustered Systems : Cluster computing is a type of distributed computing. Distributedcomputing just means coordinating a number of computers to accomplish a single task.Cluster computing means the computers are specifically organized just to work togetherto accomplish a single task.

For example, massively parallel "grid computing" projects like seti@home andfolding@home are examples of distributed computing but they are not clustercomputing. Here, the computers all work together to accomplish a task, so this isdistributed computing. But they are not specifically arranged for this purpose (thearrangement is haphazard and uncoordinated with computers all over the place beingrandomly added and removed from the set working on the problem), so they are not acluster and this is not cluster computing.

What is SETI@home? SETI@home is a scientific experiment that uses Internet-connected computers in theSearch for Extraterrestrial Intelligence (SETI). You can participate by running a freeprogram that downloads and analyzes radio telescope data.E.g. http://setiathome.berkeley.edu/sah_about.php

Folding@home (FAH or F@h) is a distributed computing project for disease researchthat simulates protein folding, computational drug design, and other types of moleculardynamics. The project uses the idle processing resources of thousands of personalcomputers owned by volunteers who have installed the software on their systems. Itsprimary purpose is to determine the mechanisms of protein folding, which is the

process by which proteins reach their final three-dimensional structure, and to examinethe causes of protein misfolding.
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Web Based Computing

That answer is just as easy - because they are cheaper. Computers cost less if you don'thave to have to include a hard drive. They also cost less if you don't have to buy andinstall software on them, and that is the real driving force behind the switch. Less

obvious is that users can use more than one of them rather seamlessly. Consider thenew Surface Tablet from Microsoft. It runs a web-computer based version of Windowsthat looks virtually identical to Windows 8 which runs on real computers. This meansusers don't have to learn two systems. But it means more than that, it means that userscan use web, or cloud based apps instead of those they install locally, e.g. Google Docs,Facebook, Flickr, etc. to create, manipulate and store their work, which they can thenaccess using their phone, Chromebook, or any other web based device. computers withweaker processors and no hard drives that run applications off the "cloud instead oflocally and generally have touch screens to make using them easier. They're also

generally lighter and smaller than so-called "real" computers.

Effect of web based computing :All of this is evident as reports of a slowdown in sales ofreal computers contrasts with those noting bigger numbers for iOS, Android and nowWindows 8/RT based machines, which are essentially all the same breed: computersthat do little except allow users to browse the web, access social media and play musicor video files.

It's the newest trend in personal computing, and it's been going on for long enough now,that most have heard the rumblings

Cloud Computing : In science, cloud computing is a synonym for distributed computingover a network and means the ability to run a program on many connected computersat the same time.

Cloud computing is a type of computing that relies on sharing computing resources rather than having local servers or personal devices to handle applications.

It is called cloud computing because the data and applications exist on a cloud of WEBservers.
http://www.zdnet.com/news/moving-apps-to-the-cloud-why-when-and-how/6344653http://voices.yahoo.com/topic/1269/google.htmlhttp://www.nytimes.com/2012/09/08/technology/intel-downgrades-sales-expectations.html?_r=0http://www.ip-192.com/2012/11/05/tablet-samsung-apple/http://voices.yahoo.com/topic/67863/social_media.htmlhttps://en.wikipedia.org/wiki/Distributed_computinghttp://www.webopedia.com/TERM/D/device.htmlhttp://www.webopedia.com/TERM/A/application.htmlhttp://www.webopedia.com/TERM/A/application.htmlhttp://www.webopedia.com/TERM/D/device.htmlhttps://en.wikipedia.org/wiki/Distributed_computinghttp://voices.yahoo.com/topic/67863/social_media.htmlhttp://www.ip-192.com/2012/11/05/tablet-samsung-apple/http://www.nytimes.com/2012/09/08/technology/intel-downgrades-sales-expectations.html?_r=0http://voices.yahoo.com/topic/1269/google.htmlhttp://www.zdnet.com/news/moving-apps-to-the-cloud-why-when-and-how/6344653


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TRANSLATION PROGRAMS

o ASSEMBLERS

o COMPILERS

o INTERPRETERS

o LINKERS

Assembler :- A program that translates programs from assembly language to machinelanguage.

An assembler is a program that takes basic computer instructions and converts theminto a pattern of bits that the computer's processor can use to perform its basicoperations. Some people call these instructions assembler language and others use the

term assembly language .

Compilers ad Interpreters : -Historically, most programs have been written in "higher-level" languages such as COBOL, FORTRAN, PL/I, and C. These languages are easier tolearn and faster to write programs with than assembler language. The program thatprocesses the source code written in these languages is called a compiler.

A compiler is a program that translates a source program written in some high-levelprogramming language (such as Java) into machine code for some computerarchitecture (such as the Intel Pentium architecture). The generated machine code canbe later executed many times against different data each time.

Linker :- Also called link editor and binder, a linker is a program that combines objectmodules to form an executable program. Many programming languages allow you towrite different pieces of code, called modules , separately. This simplifies theprogramming task because you can break a large program into small, more manageablepieces. Eventually, though, you need to put all the modules together. This is the job ofthe linker.

In addition to combining modules, a linker also replaces symbolic addresses with realaddresses. Therefore, you may need to link a program even if it contains only onemodule.
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COMPONENT OF COMPUTER SYSTEM

o Hardwareo Provides basic computing resources (CPU, memory, I/O devices).o Operating Systemo Controls and coordinates the use of hardware among application programs.o Application Programso Solve computing problems of users (compilers, database systems, video games,

business programs such as banking software).o Userso People, machines, other computers

OPERATING SYSTEM VIEW

o Resource allocatoro to allocate resources (software and hardware) of the computer system and manage

them efficiently.o Control programo Controls execution of user programs and operation of I/O devices.o Kernelo The program that executes forever (everything else is an application with respect to the

kernel).o Monitors and Small Kernelso special purpose and embedded systems, real-time systemso Batch and multiprogramming

o Timesharing


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o workstations, servers, minicomputers, timeframeso Transaction systems

o Personal Computing Systems

o Mobile Platforms, devices (of all sizes)

COMPONENT OF O/S


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DEVICE DRIVERS

USER INTERFACE

SYSTEM UTILITIES :


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OPERATING SYSTEM STRUCTURES-CHAPTER 2

SYSTEM COMPONENTS

Even though, not all systems have the same structure many modern operating systems share

the same goal of supporting the following types of system components.

PROCESS MANAGEMENT

The operating system manages many kinds of activities ranging from user programs to systemprograms like printer spooler, name servers, file server etc. Each of these activities isencapsulated in a process. A process includes the complete execution context (code, data, PC,registers, OS resources in use etc.).

It is important to note that a process is not a program. A process is only ONE instant of aprogram in execution. There are many processes can be running the same program. The fivemajor activities of an operating system in regard to process management are

Creation and deletion of user and system processes. Suspension and resumption of processes. A mechanism for process synchronization. A mechanism for process communication. A mechanism for deadlock handling. Process - fundamental concept in OS Process is a program in execution. Process needs resources - CPU time, memory, files/data and I/O devices.

OS is responsible for the following process management activities. Process creation and deletion Process suspension and resumption Process synchronization and interprocess communication Process interactions - deadlock detection, avoidance and correction

MAIN-MEMORY MANAGEMENT

Primary-Memory or Main-Memory is a large array of words or bytes. Each word or byte has itsown address. Main-memory provides storage that can be access directly by the CPU. That is tosay for a program to be executed, it must in the main memory.

The major activities of an operating in regard to memory-management are:

Keep track of which part of memory are currently being used and by whom. Decide which process are loaded into memory when memory space becomes available. Allocate and deallocate memory space as needed. Main Memory is an array of addressable words or bytes that is quickly accessible.


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Main Memory is volatile. OS is responsible for: Allocate and deallocate memory to processes. Managing multiple processes within memory - keep track of which parts of memory are

used by which processes. Manage the sharing of memory between processes.

Determining which processes to load when memory becomes OS Task: Secondary Storage and I/O Management Since primary storage is expensive and volatile, secondary storage is required for

backup. Disk is the primary form of secondary storage. OS performs storage allocation, free-space management and disk scheduling. I/O system in the OS consists of Buffer caching and management Device driver interface that abstracts device details Drivers for specific hardware devices

FILE MANAGEMENT

A file is a collected of related information defined by its creator. Computer can store files on thedisk (secondary storage), which provide long term storage. Some examples of storage mediaare magnetic tape, magnetic disk and optical disk. Each of these media has its own propertieslike speed, capacity, data transfer rate and access methods.

A file systems normally organized into directories to ease their use. These directories maycontain files and other directions.

The five main major activities of an operating system in regard to file management are:

1. The creation and deletion of files.2. The creation and deletion of directions.3. The support of primitives for manipulating files and directions.4. The mapping of files onto secondary storage.5. The back up of files on stable storage media.6. File is a collection of related information defined by creator - represents programs and

7. File creation and deletion8. Directory creation and deletion9. Supporting primitives for file/directory manipulation.10. Mapping files to disks (secondary storage).11. Backup files on archival media (tapes).


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I/O SYSTEM MANAGEMENT

I/O subsystem hides the peculiarities of specific hardware devices from the user. Only thedevice driver knows the peculiarities of the specific device to whom it is assigned.

SECONDARY-STORAGE MANAGEMENT

Generally speaking, systems have several levels of storage, including primary storage,secondary storage and cache storage. Instructions and data must be placed in primary storageor cache to be referenced by a running program. Because main memory is too small toaccommodate all data and programs, and its data are lost when power is lost, the computersystem must provide secondary storage to back up main memory. Secondary storage consistsof tapes, disks, and other media designed to hold information that will eventually be accessedin primary storage (primary, secondary, cache) is ordinarily divided into bytes or wordsconsisting of a fixed number of bytes. Each location in storage has an address; the set of alladdresses available to a program is called an address space.

The three major activities of an operating system in regard to secondary storage managementare:

1. Managing the free space available on the secondary-storage device.2. Allocation of storage space when new files have to be written.3. Scheduling the requests for memory access.

NETWORKING

A distributed system is a collection of processors that do not share memory, peripheral devices,or a clock. The processors communicate with one another through communication lines callednetwork. The communication-network design must consider routing and connection strategies,and the problems of contention and security.

Connecting processors in a distributed system

Distributed System is a collection of processors that do not share memory or a clock.

Processors are connected via a communication network.

Advantages:Allows users and system to exchange information

provide computational speedup

increased reliability and availability of information


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PROTECTION SYSTEM

If a computer systems has multiple users and allows the concurrent execution of multipleprocesses, then the various processes must be protected from one another's activities.Protection refers to mechanism for controlling the access of programs, processes, or users to

the resources defined by a computer systems.

Protection mechanisms control access of programs and processes to user and systemresources.Protect user from himself, user from other users, system from users.Protection mechanisms must:Distinguish between authorized and unauthorized use.

Specify access controls to be imposed on use.

Provide mechanisms for enforcement of access control.

Security mechanisms provide trust in system and privacy

authentication, certification, encryption etc.

COMMAND INTERPRETER SYSTEM

A command interpreter is an interface of the operating system with the user. The user givescommands with are executed by operating system (usually by turning them into system calls).The main function of a command interpreter is to get and execute the next user specifiedcommand. Command-Interpreter is usually not part of the kernel, since multiple command

interpreters (shell, in UNIX terminology) may be support by an operating system, and they donot really need to run in kernel mode. There are two main advantages to separating thecommand interpreter from the kernel.

1. If we want to change the way the command interpreter looks, i.e., I want to change theinterface of command interpreter, I am able to do that if the command interpreter isseparate from the kernel. I cannot change the code of the kernel so I cannot modify theinterface.

2. If the command interpreter is a part of the kernel it is possible for a malicious process togain access to certain part of the kernel that it showed not have to avoid this uglyscenario it is advantageous to have the command interpreter separate from kernel.


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An O/S provides an environment within which programs are executed

O/S vary greatly in their makeup since they are organized along many different lines

The design of a O/S is a major task

It is important that goals of the system be well defined before the design begins

Operating system can be viewed from various vantage points

Services

Interface to users and programmers

Components

OPERATING SYSTEM SERVICES

Following are the five services provided by operating systems to the convenience of the users.

Program Execution

The purpose of a computer systems is to allow the user to execute programs. So the operatingsystems provides an environment where the user can conveniently run programs. The userdoes not have to worry about the memory allocation or multitasking or anything. These things

are taken care of by the operating systems.

Running a program involves the allocating and deallocating memory, CPU scheduling in case ofmultiprocess. These functions cannot be given to the user-level programs. So user-levelprograms cannot help the user to run programs independently without the help from operatingsystems.

I/O Operations

Each program requires an input and produces output. This involves the use of I/O. Theoperating systems hides the user the details of underlying hardware for the I/O. All the user

sees is that the I/O has been performed without any details. So the operating systems byproviding I/O makes it convenient for the users to run programs.

For efficiently and protection users cannot control I/O so this service cannot be provided byuser-level programs.


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File System Manipulation

The output of a program may need to be written into new files or input taken from some files.

The operating systems provides this service. The user does not have to worry about secondarystorage management. User gives a command for reading or writing to a file and sees his hertask accomplished. Thus operating systems makes it easier for user programs to accomplishedtheir task.

This service involves secondary storage management. The speed of I/O that depends onsecondary storage management is critical to the speed of many programs and hence I think it isbest relegated to the operating systems to manage it than giving individual users the control ofit. It is not difficult for the user-level programs to provide these services but for abovementioned reasons it is best if this service s left with operating system.

Communications

There are instances where processes need to communicate with each other to exchangeinformation. It may be between processes running on the same computer or running on thedifferent computers. By providing this service the operating system relieves the user of theworry of passing messages between processes. In case where the messages need to be passedto processes on the other computers through a network it can be done by the user programs.The user program may be customized to the specifics of the hardware through which themessage transits and provides the service interface to the operating system.

Error Detection

An error is one part of the system may cause malfunctioning of the complete system. To avoidsuch a situation the operating system constantly monitors the system for detecting the errors.This relieves the user of the worry of errors propagating to various part of the system andcausing malfunctioning.

This service cannot allowed to be handled by user programs because it involves monitoring andin cases altering area of memory or deallocation of memory for a faulty process. Or may berelinquishing the CPU of a process that goes into an infinite loop. These tasks are too critical tobe handed over to the user programs. A user program if given these privileges can interferewith the correct (normal) operation of the operating systems.

Services for ensuring the efficient operation of the system itself

Resource Allocation (With Multiple Users)

Accounting


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Protection and Security (user area and o/s area)

OPERATING SYSTEM INTERFACES

Command interpreter

Graphical User Interface

System Calls

A System Call is the main way a user program interacts with the OperatingSystem.System calls provide an interface to the services made available by an O/S.Thesecalls are generally available as routines written in C and C++ System calls provide aninterface between the process an the operating system. System calls allow user-level

processes to request some services from the operating system which process itself is notallowed to do. In handling the trap, the operating system will enter in the kernel mode,where it has access to privileged instructions, and can perform the desired service on thebehalf of user-level process. It is because of the critical nature of operations that theoperating system itself does them every time they are needed. For example, for I/O aprocess involves a system call telling the operating system to read or write particular areaand this request is satisfied by the operating system.

System programs provide basic functioning to users so that they do not need to write their own

environment for program development (editors, compilers) and program execution (shells). Insome sense, they are bundles of useful system calls.

TYPES OF SYSTEM CALLS

Process Control

File Management

Device Management

Information maintenance Communication

PROCESS CONTROL SYSTEM CALL

End, abort

Load, execute


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Create process, terminate process

Get process attribute, set process attributes

Wait

Allocate and free memory

FILE MANAGEMENT CALLS

Create file

Delete file

Read, write operation

Get file attributes, set file attributes

DEVICE MANAGEMENT CALLS

Request device, release device

Read, write

Get device attributes and set devices attributes

Logically attach or detach devices

INFORMATION MAINTAINENCE CALLS

Get time or date, set time or date

Get system data, set system data

Get process

Set process

COMMUNICATION CALLS

Create, delete communication connection

Send, receive messages

Transfer of status information

Attach or detach remote devices


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HOW A SYSTEM CALL WORKS

Obtain access to system space

Do parameter validation

System resource collection ( locks on structures )

Ask device/system for requested item

Suspend waiting for device

Interrupt makes thread ready to run

Wrap-up

Return to user

TYPES OF OPERATING SYSTEM STRUCTURE

An operating system is usually large and complex. Therefore, it should be engineeredcarefully. Possible ways to structure an operating system:

o Simple, single-user, MS-DOS,MacOS, Windows (Only one or two levels of code)o Monolithic, multi-user, UNIX,Multics, OS/360o Layered,T.H.E. operating system (Lower levels independent of upper levels)o Client/Server (microkernel), Chorus/MiX(OS built from many user-level

processes)o Modules (Solaris)(Core Kernel with dynamically loadable modules)


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Simple structure :

Simple structure has interfaces and functionality not separated For eg. Application program hasthe liberty of accessing basic I/O routines to write data directly or to display data to disk leavingit vulnerable for the O/S to get corrupted and crash through some malicious program.

MS-DoS is an example here it does not provide any hardware protection and MS-DOS,applications may bypass the operating system.

Operating systems such as MS-DOS and the original UNIX did not have well-definedstructures.

There is no CPU Execution Mode (user and kernel), and so errors in applications cancause the whole system to crash.
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Monolithic structure :-

Functionality of the OS is invoked with simple function calls within the kernel, which isone large program.

Device drivers are loaded into the running kernel and become part of the kernel.

Layered Approach:-

This approach breaks up the operating system into different layers.

This allows implementers to change the inner workings, and increases modularity. As long as the external interface of the routines dont change, developers have more

freedom to change the inner workings of the routines. With the layered approach, the bottom layer is the hardware, while the highest layer is

the user interface.o The main advantage is simplicity of construction and debugging.o The main difficulty is defining the various layers.o The main disadvantage is that the OS tends to be less efficient than other

implementations.


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In layered system program of O/S is defined as hierarchy of layers

Layer function

5 The operator/user

4 user program

3 process manage

2 memory

1 device

0 Hardware

The major difficulty with layered approach is to appropriately define the layers. Becausea layer can use the services only of the lower level layers.

A final problem with layered implementation is that it tends to be less efficient than theother types


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Microkernel Structure:-

This structures the operating system by removing all nonessential portions of the kernel and

implementing them as system and user level programs.

Generally they provide minimal process and memory management, and acommunications facility.

Communication between components of the OS is provided by message passing.

The benefits of the microkernel are as follows:

Extending the operating system becomes much easier. Any changes to the kernel tend to be fewer, since the kernel is smaller.

The microkernel also provides more security and reliability.

Main disadvantage is poor performance due to increased system overhead from messagepassing.


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The organizing structure currently in vogue is the

microkernel OS.

Goal is minimize what goes in the kernel, and implement much of the OS as user-levelprocesses. This results in:

better reliability

ease of extension and customization

mediocre performance (unfortunately)

First microkernel system was Hydra (CMU, 1970)

Examples of microkernel systems are the CMU Mach system, Chorus (French Unix-likesystem), and in some ways Microsoft NT/Windows.

Move as much functionality as possible from the kernel into user space.

Only a few essential functions in the kernel:

primitive memory management (address space)

I/O and interrupt management

Inter-Process Communication (IPC)

basic scheduling

Other OS services are provided by processes running in user mode (vertical servers):


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device drivers, file system, virtual me mory

Benefits of Microkernel

Extensibility/Reliability

o easier to extend a microkernelo easier to port the operating system to new architectureso more reliable (less code is running in kernel mode)o more secureo small microkernel can be rigorously tested.

Portability

o changes needed to port the system to a new processor is done in themicrokernel, not in the other services.

Modular Struture:-

Most modern operating systems implement kernel modules:

Uses object-oriented approach. Each core component is separate.

Each talks to the others over known interfaces.

Each is loadable as needed within the kernel.

Overall, similar to layers but with more flexibility.


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VIRTUAL MACHINE:-

A Virtual Machine (VM) takes the layered and microkernel approach to its logicalconclusion.

It treats hardware and the operating system kernel as though they were all hardware.

A virtual machine provides an interfaceidentical to the underlying bare hardware.

The operating system host creates the illusion that a process has its own processor and(virtual memory).

Each guest provided with a (virtual) copy of underlying computer.

The resources of the physical computer are shared to create the virtual machines:

CPU scheduling can create the appearance that users have their own processor.

Spooling and a file system can provide virtual card readers and virtual lineprinters.

A normal user timesharing terminal serves as the virtual machine operatorsconsole.


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Advantages/Disadvantages

The VM concept provides complete protection of system resources since each virtualmachine is isolated from all other virtual machines. This isolation permits no directsharing of resources.


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A VM system is a perfect vehicle for OS researchand development. System development is done on the virtual machine, instead of on aphysical machine and so does not disrupt normal system operation.

The VM concept is difficult to implement due to the effort required to provide an exactduplicate to the underlying machine.

A virtual machine takes the layered approach to its logical conclusion. It treatshardware and the operating system kernel as though they were all hardware. Avirtual machine provides an interface identical to theunderlying bare hardware. Theoperating system creates the illusion

of multiple processes, each executing on its own processor with its own (virtual)memory.

The resources of the physical computer are shared to create the virtual machines.

1. CPU scheduling can create the appearance that users have their own processor. 2. Spooling and a file system can provide virtual card readers and virtual lineprinters.

3. A normal user time-sharing terminal serves as the virtual machine operatorsconsole.

System Models

Non-virtual Machine Virtual Machine Advantages/Disadvantages of Virtual Machines

The virtual-machine concept provides complete protection of system resources sinceeach virtual machine is isolated from all other virtual machines. This isolation,however, permits no direct sharing of resources. A virtual-machine system is a


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perfect vehicle for operating-systems research and development. Systemdevelopment is done on the virtual machine, instead of on a physical machine and sodoes not disrupt normal system operation. The virtual machine concept is difficult toimplement due to the effort required to provide an exact duplicate to theunderlyingmachine.

Java Virtual Machine Compiled Java programs are platform-neutral byte codes executed by a Java Virtual

Machine (JVM). JVM consists of 1. class loader 2. class verifier 3. runtime interpreter Just-In-Time (JIT) compilers increase performance

Java Virtual Machine

Virtual Machines The concept of virtual machines is closely related to layering.In a typical multi-user system, users are expected to know that the machine is shared by otherusers, and that resources such as devices are shared between all the users.In virtual machine operating systems an addition layer of abstraction is placed between the

users and the system so that it appears to the user that they are using a machine dedicated tothem.Usually it is the case that a more powerful machine is used to host several virtual machines. Forexample, the 80386 and later Intel CPUssupported virtual 8086 machines. Thus, an operatingsystem designed for the 80386 could actually run several copies of MS-DOS and appear to theuser to be several different PCs at the same time.Another example of a virtual machine system is the IBM 370 running the VM operating system.


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This allowed users to work as if they had a dedicated (but smaller, less powerful) 370completely at their disposal.

A virtual machine provides an interface identical to the underlying bare hardware.

The operating system creates the illusion of multiple processes, each executing on itsown processor with its own (virtual) memory.

A Virtual Machine is a software that creates a virtualized environment between thecomputer platform and the end user in which the end user can operate software.

A virtual machine provides an interface identical to the underlying bare hardware.

The operating system creates the illusion of multiple processes, each executing on itsown processor with its own (virtual) memory.

Virtualization is an abstraction layer that decouples the physical hardware from theoperating system to deliver greater IT resource utilization and flexibility.

It allows multiple virtual machines, with heterogeneous operating systems to run inisolation, side-by-side on the same physical machine.

The host software that provides virtualization is often referred to as a virtual machinemonitor (VMM) or hypervisor.

The VMM gives each virtual machine an illusion of a complete computer to itself.


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Common Virtualisation used today

Run legacy software on non-legacy hardware

Run multiple operating systems on the same hardware

Create a manageable upgrade path

Manage outages (expected and unexpected) dynamically


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Chapter 3- Process Management

DEFINITION

The term "process" was first used by the designers of the MULTICS in 1960's. Since then, theterm process, used somewhat interchangeably with 'task' or 'job'. The process has been givenmany definitions for instance

A program in Execution.

An asynchronous activity. The 'animated sprit' of a procedure in execution. The entity to which processors are assigned. The 'dispatchable' unit.

and many more definitions have given. As we can see from above that there is no universallyagreed upon definition, but the definition " Program in Execution " seem to be most frequentlyused. And this is a concept are will use in the present study of operating systems.

Now that we agreed upon the definition of process, the question is what the relation betweenprocess and program is. It is same but with different name or when this beast is sleeping (not

executing) it is called program and when it is executing becomes process. Well, to be veryprecise. Process is not the same as program. In the following discussion we point out some ofthe difference between process and program. As we have mentioned earlier.

Process is not the same as program. A process is more than a program code. A process is an'active' entity as oppose to program which consider to be a 'passive' entity. As we all know that


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a program is an algorithm expressed in some suitable notation, (e.g., programming language).Being a passive, a program is only a part of process. Process, on the other hand, includes:

Current value of Program Counter (PC) Contents of the processors registers

Value of the variables The process stack (SP) which typically contains temporary data such as subroutine

parameter, return address, and temporary variables. A data section that contains global variables.

A process is the unit of work in a system.

In Process model, all software on the computer is organized into a number of sequentialprocesses. A process includes PC, registers, and variables. Conceptually, each process has itsown virtual CPU. In reality, the CPU switches back and forth among processes. (The rapidswitching back and forth is called multiprogramming).

The process state consist of everything necessary to resume the process execution if it issomehow put aside temporarily. The process state consists of at least following:

Code for the program. Program's static data. Program's dynamic data. Program's procedure call stack.

Contents of general purpose registers. Contents of program counter (PC) Contents of program status word (PSW). Operating Systems resource in use.

PROCESS OPERATIONS

PROCESS CREATION

In general-purpose systems, some way is needed to create processes as needed duringoperation. There are four principal events led to processes creation.

System initialization. Execution of a process Creation System calls by a running process. A user request to create a new process.


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Initialization of a batch job.

Foreground processes interact with users. Background processes that stay inbackground sleeping but suddenly springing to life to handle activity such as

email, webpage, printing, and so on. Background processes are called daemons.This call creates an exact clone of the calling process.

A process may create a new process by some create process such as'fork'. It choose to does so, creating process is called parent process and thecreated one is called the child processes. Only one parent is needed to create achild process. This creation of process (processes) yields a hierarchical structureof processes like one in the figure. Notice that each child has only one parent buteach parent may have many children. After the fork, the two processes, theparent and the child, have the same memory image, the same environmentstrings and the same open files. After a process is created, both the parent andchild have their own distinct address space. If either process changes a word inits address space, the change is not visible to the other process.


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PROCESS STATES

A process goes through a series of discrete process states.

New State The process being created. Terminated State The process has finished execution. Blocked (waiting) State When a process blocks, it does so because logically it cannot

continue, typically because it is waiting for input that is not yet available. Formally, aprocess is said to be blocked if it is waiting for some event to happen (such as an I/Ocompletion) before it can proceed. In this state a process is unable to run until someexternal event happens.

Running State A process is said t be running if it currently has the CPU, that is, actuallyusing the CPU at that particular instant.

Ready State A process is said to be ready if it use a CPU if one were available. It isrunable but temporarily stopped to let another process run.

Logically, the 'Running' and 'Ready' states are similar. In both cases the process is willing to run,only in the case of 'Ready' state, there is temporarily no CPU available for it. The 'Blocked' stateis different from the 'Running' and 'Ready' states in that the process cannot run, even if the CPUis available.

PROCESS STATE TRANSITIONS

Following are six(6) possible transitions among above mentioned five (5) states

Transition 1 occurs when process discovers that it cannot continue. If running process initiates an I/Ooperation before its allotted time expires, the running process voluntarily relinquishes the CPU.

This state transition is:

Block (process- name): Running Block.

Transition 2 occurs when the scheduler decides that the running process has run long

enough and it is time to let another process have CPU time.


Time-Run-Out (process- name): Running Ready.

Transition 3 occurs when all other processes have had their share and it is time for thefirst process to run again


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Dispatch (process- name): Ready Running.

Transition 4 occurs when the external event for which a process was waiting (such asarrival of input) happens.


Wakeup (process- name): Blocked Ready.

Transition 5 occurs when the process is created.


Admitted (process- name): New Ready.

Transition 6 occurs when the process has finished execution.


Exit (process- name): Running Terminated.

PROCESS CONTROL BLOCK

A process in an operating system is represented by a data structure known as a process controlblock (PCB) or process descriptor. The PCB contains important information about the specificprocess including

The current state of the process i.e., whether it is ready, running, waiting, or whatever. Unique identification of the process in order to track "which is which" information. A pointer to parent process. Similarly, a pointer to child process (if it exists). The priority of process (a part of CPU scheduling information). Pointers to locate memory of processes.

A register save area. The processor it is running on.

The PCB is a certain store that allows the operating systems to locate key information about aprocess. Thus, the PCB is the data structure that defines a process to the operating systems.


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Process Control block is used for storing the collection of information about the Processes and

this is also called as the Data Structure which Stores the information about the process. The

information of the Process is used by the CPU at the Run time. The various information which is

Stored into the PCB as followings:

1) Name of the Process.

2) State of the Process. Means Ready, Active, Wait.

3) Resources allocated to the Process

4) Memory which is provided to the Process.

5) Scheduling information.

6) Input and Output Devices used by the Process.

7) Process ID or a Identification Number which is given by the CPU when a Process Request

for a Service.
http://ecomputernotes.com/fundamental/introduction-to-computer/what-is-cpuhttp://ecomputernotes.com/fundamental/introduction-to-computer/what-is-cpu


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THREADS

Despite of the fact that a thread must execute in process, the process and its associated

threads are different concept. Processes are used to group resources together and threads arethe entities scheduled for execution on the CPU.

A thread is a single sequence stream within in a process . Because threads have some of theproperties of processes, they are sometimes called lightweight processes . In a process, threadsallow multiple executions of streams. In many respect, threads are popular way to improveapplication through parallelism. The CPU switches rapidly back and forth among the threadsgiving illusion that the threads are running in parallel. Like a traditional process i.e., processwith one thread, a thread can be in any of several states (Running, Blocked, Ready orTerminated). Each thread has its own stack. Since thread will generally call different proceduresand thus a different execution history. This is why thread needs its own stack. An operatingsystem that has thread facility, the basic unit of CPU utilization is a thread. A thread has orconsists of a program counter (PC), a register set, and a stack space. Threads are notindependent of one other like processes as a result threads shares with other threads theircode section, data section, OS resources also known as task, such as open files and signals.

PROCESSES VS THREADS

As we mentioned earlier that in many respect threads operate in the same way as that ofprocesses. Some of the similarities and differences are:

Similarities

Like processes threads share CPU and only one thread active (running) at a time. Like processes, threads within processes, threads within a processes execute

sequentially. Like processes, thread can create children. And like process, if one thread is blocked, another thread can run.

Differences

Unlike processes, threads are not independent of one another. Unlike processes, all threads can access every address in the task . Unlike processes, thread are design to assist one other. Note that processes might or

might not assist one another because processes may originate from different users.

WHY THREADS?


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Following are some reasons why we use threads in designing operating systems.

1. A process with multiple threads make a great server for example printer server.2. Because threads can share common data, they do not need to use interprocess

communication.

3. Because of the very nature, threads can take advantage of multiprocessors.

Threads are cheap in the sense that

1. They only need a stack and storage for registers therefore, threads are cheap to create.2. Threads use very little resources of an operating system in which they are working. That

is, threads do not need new address space, global data, program code or operatingsystem resources.

3. Context switching are fast when working with threads. The reason is that we only haveto save and/or restore PC, SP and registers.

But this cheapness does not come free - the biggest drawback is that there is no protectionbetween threads.

USER-LEVEL THREADS

User-level threads implement in user-level libraries, rather than via systems calls, so threadswitching does not need to call operating system and to cause interrupt to the kernel. In fact,the kernel knows nothing about user-level threads and manages them as if they were single-

threaded processes.

ADVANTAGES:

The most obvious advantage of this technique is that a user-level threads package can beimplemented on an Operating System that does not support threads. Some other advantagesare

User-level threads does not require modification to operating systems. Simple Representation:

Each thread is represented simply by a PC, registers, stack and a small control block,all stored in the user process address space.

Simple Management:This simply means that creating a thread, switching between threads and

synchronization between threads can all be done without intervention of the kernel.Fast and Efficient:

Thread switching is not much more expensive than a procedure call.


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DISADVANTAGES:

There is a lack of coordination between threads and operating system kernel. Therefore,process as whole gets one time slice irrespect of whether process has one thread or1000 threads within. It is up to each thread to relinquish control to other threads.

User-level threads requires non-blocking systems call i.e., a multithreaded kernel.Otherwise, entire process will blocked in the kernel, even if there are runable threadsleft in the processes. For example, if one thread causes a page fault, the process blocks.

KERNEL-LEVEL THREADS

In this method, the kernel knows about and manages the threads. No runtime system is neededin this case. Instead of thread table in each process, the kernel has a thread table that keepstrack of all threads i

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