Chapter 1 - Introduction. In this chapter, the following content will be discussed: Overview of operating systems, OS and the computer system, memory utilization during operation of an OS, the program status word (PSW), privileged mode of CPU, state of the CPU,...
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Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 1 Copyright © 2008 What is an operating system ? • An operating system (OS) is a collection of programs that achieve effective utilization of a computer system by providing – Convenient methods of using a computer * Saves users’ time and boosts their productivity – Efficient use of the computer • An OS has several kinds of users – The OS meets diverse requirements of different kinds of users – Each user has a different view of what an OS is, and what it does Each of these views is called an abstract view Chapter 1: Introduction Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:2 Copyrightâ2008 Abstract views An abstract view focuses on the essential characteristics of a system – It contains some selected characteristics, and ignores others * It helps to control complexity in designing and understanding a system, by hiding unnecessary detail – For example, two abstract views of a car: * The driver’s view contains the rear-view mirror, steering, gear shift, and speedometer but ignores engine details * The mechanic’s view contains fluid levels, electrical system, … Q: Give examples of abstract views A: View of an abstract data type, object, Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 3 Copyright © 2008 Abstract views • Abstract views are used in – Software requirements specification * Specify what functions a system should perform The abstract view of a specific function helps in understanding the system data and fundamental operations on them – Software design and implementation * ‘Hiding’ the details of functions and subsystems while designing or implementing other functions and subsystems helps to control complexity Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 4 Copyright © 2008 A designer’s abstract view of an OS • User interface: Accepts commands from users: GUI, command line interface • Nonkernel programs: Implement user commands • Kernel: Core of the OS─controls the computer, provides functions & services Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 5 Copyright © 2008 A designer’s abstract view of an OS • The abstract view consists of three components – The kernel programs * interact with the computer’s hardware and implement the intended operation – The non-kernel programs * implement creation of programs and use of system resources by them These programs use kernel programs to control operation of the computer – The user interface * interprets the commands of a user and activates non-kernel programs to implement them Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 6 Copyright © 2008 Goals of an OS • Two primary goals of an OS are – Efficient use of the computer’s resources * To ensure cost-effectiveness of the computer – User convenience * A user should find it easy to use the computer • These two goals sometimes conflict – Prompt service can be provided through exclusive use of a computer; however, efficient use requires sharing of a computer’s resources among many users – An OS designer decides which of the two goals is more important under what conditions * That is why we have so many operating systems! Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 7 Copyright © 2008 User convenience • The computing environment influences the notion of user convenience – The computing environment is comprised of * The computer system * Its interfaces with other systems * Nature of computations performed by its users – Computing environments change with the architecture and use of a computer, e.g personal computing, online applications, embedded applications – Hence the notion of user convenience has several facets that depend on the computing environment Chapter1:Introduction Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:8 Copyrightâ2008 User convenience User convenience has several facets – Fulfillment of a necessity * Use of programs and files – Good service * Speedy response – Ease of Use * User friendliness – New programming model * e.g., Concurrent programming – Web-oriented features * e.g., Web-enabled servers – Evolution * Addition of new features, use of new computers Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 9 Copyright © 2008 Operation of an OS • During operation, an OS performs two primary functions – Program management * Perform program initiation/termination * Provide means through which many programs can work toward a common goal Helps in more convenient or faster fulfillment of user requirements – Resource management * Ensure efficient use of resources Use of CPU, I/O devices Chapter 1: Introduction Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:10 Copyrightâ2008 Managing memory The OS services a large number of processes simultaneously – It must make efficient use of memory – It must perform memory allocation and de-allocation speedily – These requirements may conflict, hence special techniques are used to implement effective memory management Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed SlideNo:25 Copyrightâ2008 Virtual Memory Users may wish to write very large programs Some of these programs may not fit in memory – The OS uses virtual memory to overcome this problem * Only some portion(s) of the code and data of a process are present in memory at any time * Other portions are loaded in memory when needed * Portions not presently needed may be removed from memory Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 26 Copyright © 2008 A schematic of virtual memory operation TheOSmaintainsinformationaboutportionsofaprocessthatareinmemory Amissingfrommemoryeventoccursifarequiredportionisnotinmemory TheOSnowloadsitinmemory Chapter1:Introduction Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:27 Copyrightâ2008 File processing • The file system ensures effectiveness of file processing – Provides convenient methods for a user to create, access and share files * Each file organization provides efficient access for a specific record access patterns – Provide assurance needed by users * Security and protection techniques to ensure that files are not accessed by unauthorized persons * Reliability techniques to ensure that files are not lost or damaged due to failures in the system Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 28 Copyright © 2008 Part II: Advanced topics • Process synchronization – Enables processes to work harmoniously towards common goals • Message passing – Facilitates communication among processes • Deadlock handling – Avoids indefinite waits • Implementation of file operations – Techniques to ensure efficiency and reliability • Multiprocessor operating systems – Ensure effective and efficient use of the multiple CPUs • Structure of operating systems – Provides portability and extensibility of operating systems Chapter1:Introduction Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:29 Copyrightâ2008 Process synchronization Process synchronization is employed for two purposes – Data sharing * Processes should share data or resources such that only one process uses a data item or a resource at any time It avoids data inconsistency – Coordination of activities * Processes must perform their operations in a desired order e.g., if a process wishes to use some data produced by another process, it must read the data only after the other process has assigned a value to it Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 30 Copyright © 2008 Two kinds of process synchronization (a) The Credit and Debit processes share the account balance, hence only oneofthemshouldaccessitatanytime (b)GenerateproducesasampleandAnalyzeanalyzesit.Analyzemust waitifGeneratehasnotyetproducedasample Chapter1:Introduction Dhamdhere:OperatingSystems AConceptưBasedApproach,2ed SlideNo:31 Copyrightâ2008 Deadlocks Some resources cannot be shared across processes; they must be used by only one process at a time – If a resource is in use by a process, another process wishing to use the resource must wait for the process to release it * Indefinite waits would arise if processes wait for each other This situation is called a deadlock It degrades response time and efficiency of use Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 32 Copyright © 2008 A deadlock involving three processes • Pk needs a resource that is currently allocated to Pi • Pi needs a resource that is currently allocated to Pj • Pj needs a resource that is currently allocated to Pk • Each of the processes waits indefinitely Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 33 Copyright © 2008 Part III: Distributed Operating Systems • A distributed system consists of several computer systems connected through a network – Terminology * A computer, which is called a host or node, exists in a site – A distributed OS manages computers in a distributed system in a well-integrated manner to provide two key benefits * high reliability Redundancy of resources in various computers is exploited to ensure that a process can complete despite failures * speed-up of a computation by performing some of its parts in different computer systems Chapter 1: Introduction Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:34 Copyrightâ2008 Issues in Distributed OS Individual computers in a distributed system have their own clocks and memories – Clocks in different computers are not synchronized, hence they may show different times – It is difficult to obtain a consistent view of data and activities * E.g., balances in two accounts, recorded while transferring funds between them, may not be mutually consistent (see next slide) • A distributed OS has to use special techniques to ensure consistency and reliability – Important during normal operation and recovery from a failure Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 35 Copyright © 2008 Consistency issues in a distributed OS • An observer in node Z records balances in accounts A and B while funds are being transferred • Z may record balance of X before $100 are transferred from it to Y, and balance of Y after the transfer—$100 are generated! • Z may record balance of X after $100 are transferred from it to Y, and balance of Y before the transfer—$100 are lost! Chapter 1: Introduction Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo:36 Copyrightâ2008 Issues in Distributed OS Theoretical issues – How to know the order in which events have occurred? * Important for FCFS allocation, etc – How to know the state of a system * Important for avoiding inconsistencies and performing ‘load balancing’ across nodes to obtain good performance • Distributed control algorithms – Parts of the algorithm run in different nodes * Obviates the need to collect ‘global’ state in one node * Special techniques are used to ensure consistency of actions – Used for resource allocation, scheduling, deadlock handling, etc Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 37 Copyright © 2008 Issues in Distributed OS • Recovery – User computations should not suffer due to node or link failures * OS restores computations in a failed node to a previous state * Special techniques are used to ensure consistency across nodes • Distributed file systems – Files can be accessed from any node * Special techniques are needed to ensure good performance when a non-local file is accessed * Special techniques are also needed to ensure reliability when nodes or network links fail Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 38 Copyright © 2008 Issues in Distributed OS • Distributed system security – Security threats arise from intruders, and viruses and worms – OS must provide measures to * Prevent intruders from corrupting or fabricating network messages * Enable processes to verify identities of other processes * Ensure data security Enable users to know who created a document or data Enable users to know whether documents or data have been tampered with Chapter 1: Introduction Dhamdhere: Operating Systems— A ConceptBased Approach, 2 ed Slide No: 39 Copyright © 2008 ... one of them should access it at any time (b) Generate produces a sample and Analyze analyzes it. Analyze must wait if Generate has not yet produced a sample Chapter 1: Introduction Dhamdhere: Operating Systems—... systems Chapter 1: Introduction Dhamdhere: Operating Systems— AConceptưBasedApproach,2ed SlideNo :13 Copyrightâ2008 Program management A computational structure is a configuration of several programs... programs so that their results are available to one another Chapter1 :Introduction Dhamdhere: OperatingSystems AConceptưBasedApproach,2ed SlideNo :14 Copyrightâ2008 A schematic of scheduling ProgramswaitingforCPUattentionareenteredinapool