Introduction to Operating Systems, Lecture notes of Operating Systems

Download Introduction to Operating Systems and more Lecture notes Operating Systems in PDF only on Docsity! 5 Operating System : An Overview UNIT 1 OPERATING SYSTEM : AN OVERVIEW Structure Page Nos. 1.0 Introduction 5 1.1 Objectives 6 1.2 What is an Operating System? 6 1.3 Goals of an Operating System 8 1.4 Generations of Operating Systems 8 1.4.1 0th Generation 1.4.2 First Generation (1951-1956) 1.4.3 Second Generation (1956-1964) 1.4.4 Third Generation (1964-1979) 1.4.5 Fourth Generation (1979-Present) 1.5 Types of Operating Systems 13 1.5.1 Batch Processing Operating System 1.5.2 Time Sharing 1.5.3 Real Time Operating System (RTOS) 1.5.4 Multiprogramming Operating System 1.5.5 Multiprocessing System 1.5.6 Networking Operating System 1.5.7 Distributed Operating System 1.5.8 Operating Systems for Embedded Devices 1.6 Desirable Qualities of OS 16 1.7 Operating Systems : Some Examples 16 1.7.1 DOS 1.7.2 UNIX 1.7.3 Windows 1.7.4 Macintosh 1.8 Functions of OS 18 1.8.1 Process Management 1.8.2 Memory Management 1.8.3 Secondary Storage Management 1.8.4 I/O Management 1.8.5 File Management 1.8.6 Protection 1.8.7 Networking 1.8.8 Command Interpretation 1.9 Summary 22 1.10 Solutions/Answers 22 1.11 Further Readings 24 1.0 INTRODUCTION Computer software can be divided into two main categories: application software and system software. Application software consists of the programs for performing tasks particular to the machine’s utilisation. This software is designed to solve a particular problem for users. Examples of application software include spreadsheets, database systems, desktop publishing systems, program development software, and games. On the other hand, system software is more transparent and less noticed by the typical computer user. This software provides a general programming environment in which programmers can create specific applications to suit their needs. This environment provides new functions that are not available at the hardware level and performs tasks 6 Introduction to Operating Systems and Process Management related to executing the application program. System software acts as an interface between the hardware of the computer and the application software that users need to run on the computer. The most important type of system software is the operating system. An Operating System (OS) is a collection of programs that acts as an interface between a user of a computer and the computer hardware. The purpose of an operating system is to provide an environment in which a user may execute the programs. Operating Systems are viewed as resource managers. The main resource is the computer hardware in the form of processors, storage, input/output devices, communication devices, and data. Some of the operating system functions are: implementing the user interface, sharing hardware among users, allowing users to share data among themselves, preventing users from interfering with one another, scheduling resources among users, facilitating input/output, recovering from errors, accounting for resource usage, facilitating parallel operations, organising data for secure and rapid access, and handling network communications. This unit presents the definition of the operating system, goals of the operating system, generations of OS, different types of OS and functions of OS. 1.1 OBJECTIVES After going through this unit, you should be able to: • understand the purpose of an operating system; • describe the general goals of an operating system; • discuss the evolution of operating systems; • describe various functions performed by the OS; • list, discuss and compare various types of OS, and • describe various structures of operating system. 1.2 WHAT IS AN OPERATING SYSTEM? In a computer system, we find four main components: the hardware, the operating system, the application software and the users. In a computer system the hardware provides the basic computing resources. The applications programs define the way in which these resources are used to solve the computing problems of the users. The operating system controls and coordinates the use of the hardware among the various systems programs and application programs for the various users. We can view an operating system as a resource allocator. A computer system has many resources (hardware and software) that may be required to solve a problem: CPU time, memory space, files storage space, input/output devices etc. The operating system acts as the manager of these resources and allocates them to specific programs and users as necessary for their tasks. Since there may be many, possibly conflicting, requests for resources, the operating system must decide which requests are allocated resources to operate the computer system fairly and efficiently. An operating system is a control program. This program controls the execution of user programs to prevent errors and improper use of the computer. Operating systems exist because: they are a reasonable way to solve the problem of creating a usable computing system. The fundamental goal of a computer system is to execute user programs and solve user problems. While there is no universally agreed upon definition of the concept of an operating system, the following is a reasonable starting point: 9 Operating System : An Overview preparation for the next user. This system was clearly inefficient and dependent on the varying competencies of the individual programmer as operators. 1.4.2 First Generation (1951-1956) The first generation marked the beginning of commercial computing, including the introduction of Eckert and Mauchly’s UNIVAC I in early 1951, and a bit later, the IBM 701 which was also known as the Defence Calculator. The first generation was characterised again by the vacuum tube as the active component technology. Operation continued without the benefit of an operating system for a time. The mode was called “closed shop” and was characterised by the appearance of hired operators who would select the job to be run, initial program load the system, run the user’s program, and then select another job, and so forth. Programs began to be written in higher level, procedure-oriented languages, and thus the operator’s routine expanded. The operator now selected a job, ran the translation program to assemble or compile the source program, and combined the translated object program along with any existing library programs that the program might need for input to the linking program, loaded and ran the composite linked program, and then handled the next job in a similar fashion. Application programs were run one at a time, and were translated with absolute computer addresses that bound them to be loaded and run from these reassigned storage addresses set by the translator, obtaining their data from specific physical I/O device. There was no provision for moving a program to different location in storage for any reason. Similarly, a program bound to specific devices could not be run at all if any of these devices were busy or broken. The inefficiencies inherent in the above methods of operation led to the development of the mono-programmed operating system, which eliminated some of the human intervention in running job and provided programmers with a number of desirable functions. The OS consisted of a permanently resident kernel in main storage, and a job scheduler and a number of utility programs kept in secondary storage. User application programs were preceded by control or specification cards (in those day, computer program were submitted on data cards) which informed the OS of what system resources (software resources such as compilers and loaders; and hardware resources such as tape drives and printer) were needed to run a particular application. The systems were designed to be operated as batch processing system. These systems continued to operate under the control of a human operator who initiated operation by mounting a magnetic tape that contained the operating system executable code onto a “boot device”, and then pushing the IPL (Initial Program Load) or “boot” button to initiate the bootstrap loading of the operating system. Once the system was loaded, the operator entered the date and time, and then initiated the operation of the job scheduler program which read and interpreted the control statements, secured the needed resources, executed the first user program, recorded timing and accounting information, and then went back to begin processing of another user program, and so on, as long as there were programs waiting in the input queue to be executed. The first generation saw the evolution from hands-on operation to closed shop operation to the development of mono-programmed operating systems. At the same time, the development of programming languages was moving away from the basic machine languages; first to assembly language, and later to procedure oriented languages, the most significant being the development of FORTRAN by John W. Backus in 1956. Several problems remained, however, the most obvious was the inefficient use of system resources, which was most evident when the CPU waited while the relatively slower, mechanical I/O devices were reading or writing program data. In addition, system protection was a problem because the operating system kernel was not protected from being overwritten by an erroneous application program. 10 Introduction to Operating Systems and Process Management Moreover, other user programs in the queue were not protected from destruction by executing programs. 1.4.3 Second Generation (1956-1964) The second generation of computer hardware was most notably characterised by transistors replacing vacuum tubes as the hardware component technology. In addition, some very important changes in hardware and software architectures occurred during this period. For the most part, computer systems remained card and tape-oriented systems. Significant use of random access devices, that is, disks, did not appear until towards the end of the second generation. Program processing was, for the most part, provided by large centralised computers operated under mono-programmed batch processing operating systems. The most significant innovations addressed the problem of excessive central processor delay due to waiting for input/output operations. Recall that programs were executed by processing the machine instructions in a strictly sequential order. As a result, the CPU, with its high speed electronic component, was often forced to wait for completion of I/O operations which involved mechanical devices (card readers and tape drives) that were order of magnitude slower. This problem led to the introduction of the data channel, an integral and special-purpose computer with its own instruction set, registers, and control unit designed to process input/output operations separately and asynchronously from the operation of the computer’s main CPU near the end of the first generation, and its widespread adoption in the second generation. The data channel allowed some I/O to be buffered. That is, a program’s input data could be read “ahead” from data cards or tape into a special block of memory called a buffer. Then, when the user’s program came to an input statement, the data could be transferred from the buffer locations at the faster main memory access speed rather than the slower I/O device speed. Similarly, a program’s output could be written another buffer and later moved from the buffer to the printer, tape, or card punch. What made this all work was the data channel’s ability to work asynchronously and concurrently with the main processor. Thus, the slower mechanical I/O could be happening concurrently with main program processing. This process was called I/O overlap. The data channel was controlled by a channel program set up by the operating system I/O control routines and initiated by a special instruction executed by the CPU. Then, the channel independently processed data to or from the buffer. This provided communication from the CPU to the data channel to initiate an I/O operation. It remained for the channel to communicate to the CPU such events as data errors and the completion of a transmission. At first, this communication was handled by polling – the CPU stopped its work periodically and polled the channel to determine if there is any message. Polling was obviously inefficient (imagine stopping your work periodically to go to the post office to see if an expected letter has arrived) and led to another significant innovation of the second generation – the interrupt. The data channel was able to interrupt the CPU with a message – usually “I/O complete.” Infact, the interrupt idea was later extended from I/O to allow signalling of number of exceptional conditions such as arithmetic overflow, division by zero and time-run-out. Of course, interval clocks were added in conjunction with the latter, and thus operating system came to have a way of regaining control from an exceptionally long or indefinitely looping program. These hardware developments led to enhancements of the operating system. I/O and data channel communication and control became functions of the operating system, both to relieve the application programmer from the difficult details of I/O programming and to protect the integrity of the system to provide improved service to users by segmenting jobs and running shorter jobs first (during “prime time”) and 11 Operating System : An Overview relegating longer jobs to lower priority or night time runs. System libraries became more widely available and more comprehensive as new utilities and application software components were available to programmers. In order to further mitigate the I/O wait problem, system were set up to spool the input batch from slower I/O devices such as the card reader to the much higher speed tape drive and similarly, the output from the higher speed tape to the slower printer. In this scenario, the user submitted a job at a window, a batch of jobs was accumulated and spooled from cards to tape “off line,” the tape was moved to the main computer, the jobs were run, and their output was collected on another tape that later was taken to a satellite computer for off line tape-to-printer output. User then picked up their output at the submission windows. Toward the end of this period, as random access devices became available, tape- oriented operating system began to be replaced by disk-oriented systems. With the more sophisticated disk hardware and the operating system supporting a greater portion of the programmer’s work, the computer system that users saw was more and more removed from the actual hardware-users saw a virtual machine. The second generation was a period of intense operating system development. Also it was the period for sequential batch processing. But the sequential processing of one job at a time remained a significant limitation. Thus, there continued to be low CPU utilisation for I/O bound jobs and low I/O device utilisation for CPU bound jobs. This was a major concern, since computers were still very large (room-size) and expensive machines. Researchers began to experiment with multiprogramming and multiprocessing in their computing services called the time-sharing system. A noteworthy example is the Compatible Time Sharing System (CTSS), developed at MIT during the early 1960s. 1.4.4 Third Generation (1964-1979) The third generation officially began in April 1964 with IBM’s announcement of its System/360 family of computers. Hardware technology began to use integrated circuits (ICs) which yielded significant advantages in both speed and economy. Operating system development continued with the introduction and widespread adoption of multiprogramming. This marked first by the appearance of more sophisticated I/O buffering in the form of spooling operating systems, such as the HASP (Houston Automatic Spooling) system that accompanied the IBM OS/360 system. These systems worked by introducing two new systems programs, a system reader to move input jobs from cards to disk, and a system writer to move job output from disk to printer, tape, or cards. Operation of spooling system was, as before, transparent to the computer user who perceived input as coming directly from the cards and output going directly to the printer. The idea of taking fuller advantage of the computer’s data channel I/O capabilities continued to develop. That is, designers recognised that I/O needed only to be initiated by a CPU instruction – the actual I/O data transmission could take place under control of separate and asynchronously operating channel program. Thus, by switching control of the CPU between the currently executing user program, the system reader program, and the system writer program, it was possible to keep the slower mechanical I/O device running and minimizes the amount of time the CPU spent waiting for I/O completion. The net result was an increase in system throughput and resource utilisation, to the benefit of both user and providers of computer services. This concurrent operation of three programs (more properly, apparent concurrent operation, since systems had only one CPU, and could, therefore execute just one instruction at a time) required that additional features and complexity be added to the operating system. First, the fact that the input queue was now on disk, a direct access device, freed the system scheduler from the first-come-first-served policy so that it could select the “best” next job to enter the system (looking for either the shortest job 14 Introduction to Operating Systems and Process Management 1.5.1 Batch Processing Operating System In a batch processing operating system environment users submit jobs to a central place where these jobs are collected into a batch, and subsequently placed on an input queue at the computer where they will be run. In this case, the user has no interaction with the job during its processing, and the computer’s response time is the turnaround time­the time from submission of the job until execution is complete, and the results are ready for return to the person who submitted the job. 1.5.2 Time Sharing Another mode for delivering computing services is provided by time sharing operating systems. In this environment a computer provides computing services to several or many users concurrently on-line. Here, the various users are sharing the central processor, the memory, and other resources of the computer system in a manner facilitated, controlled, and monitored by the operating system. The user, in this environment, has nearly full interaction with the program during its execution, and the computer’s response time may be expected to be no more than a few second. 1.5.3 Real Time Operating System (RTOS) The third class is the real time operating systems, which are designed to service those applications where response time is of the essence in order to prevent error, misrepresentation or even disaster. Examples of real time operating systems are those which handle airlines reservations, machine tool control, and monitoring of a nuclear power station. The systems, in this case, are designed to be interrupted by external signals that require the immediate attention of the computer system. These real time operating systems are used to control machinery, scientific instruments and industrial systems. An RTOS typically has very little user-interface capability, and no end-user utilities. 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 are available may be just as catastrophic as having it not move at all because the system is busy. A number of other definitions are important to gain an understanding of operating systems: 1.5.4 Multiprogramming Operating System A multiprogramming operating system is a system that allows more than one active user program (or part of user program) to be stored in main memory simultaneously. Thus, it is evident that a time-sharing system is a multiprogramming system, but note that a multiprogramming system is not necessarily a time-sharing system. A batch or real time operating system could, and indeed usually does, have more than one active user program simultaneously in main storage. Another important, and all too similar, term is “multiprocessing”. 1.5.5 Multiprocessing System A multiprocessing system is a computer hardware configuration that includes more than one independent processing unit. The term multiprocessing is generally used to refer to large computer hardware complexes found in major scientific or commercial applications. More on multiprocessor system can be studied in Unit-1 of Block-3 of this course. 1.5.6 Networking Operating System A networked computing system is a collection of physical interconnected computers. The operating system of each of the interconnected computers must contain, in addition to its own stand-alone functionality, provisions for handing communication 15 Operating System : An Overview and transfer of program and data among the other computers with which it is connected. Network operating systems are not fundamentally different from single processor operating systems. They obviously need a network interface controller and some low- level software to drive it, as well as programs to achieve remote login and remote files access, but these additions do not change the essential structure of the operating systems. 1.5.7 Distributed Operating System A distributed computing system consists of a number of computers that are connected and managed so that they automatically share the job processing load among the constituent computers, or separate the job load as appropriate particularly configured processors. Such a system requires an operating system which, in addition to the typical stand-alone functionality, provides coordination of the operations and information flow among the component computers. The networked and distributed computing environments and their respective operating systems are designed with more complex functional capabilities. In a network operating system, the users are aware of the existence of multiple computers, and can log in to remote machines and copy files from one machine to another. Each machine runs its own local operating system and has its own user (or users). A distributed operating system, in contrast, is one that appears to its users as a traditional uni-processor system, even though it is actually composed of multiple processors. In a true distributed system, users should not be aware of where their programs are being run or where their files are located; that should all be handled automatically and efficiently by the operating system. True distributed operating systems require more than just adding a little code to a uni- processor operating system, because distributed and centralised systems differ in critical ways. Distributed systems, for example, often allow program to run on several processors at the same time, thus requiring more complex processor scheduling algorithms in order to optimise the amount of parallelism achieved. More on distributed systems can be studied in Unit-2 of Block-3 of this course. 1.5.8 Operating Systems for Embedded Devices As embedded systems (PDAs, cellphones, point-of-sale devices, VCR’s, industrial robot control, or even your toaster) become more complex hardware-wise with every generation, and more features are put into them day-by-day, applications they run require more and more to run on actual operating system code in order to keep the development time reasonable. Some of the popular OS are: • Nexus’s Conix - an embedded operating system for ARM processors. • Sun’s Java OS - a standalone virtual machine not running on top of any other OS; mainly targeted at embedded systems. • Palm Computing’s Palm OS - Currently the leader OS for PDAs, has many applications and supporting companies. • Microsoft’s Windows CE and Windows NT Embedded OS. Let us study various examples of the popular operating systems in the next section. • Please go through the various Views and Structure of operating system from Unit 2 in Block 1 of MCS-022 (Operating System Concepts and Networking Management). 16 Introduction to Operating Systems and Process Management 1.6 DESIRABLE QUALITIES OF OS The desirable qualities of an operating system are in terms of: Usability, Facilities, Cost, and Adaptability. • Usability: • Robustness • Accept all valid inputs and can handle them. • Consistency • Proportionality • Convenience • Powerful with high level facilities. • Facilities: • Sufficient for intended use • Complete • Appropriate. • Costs: • Want low cost and efficient services. • Good algorithms. Make use of space/time tradeoffs, special hardware. • Low overhead. Cost of doing nothing should be low. E.g., idle time at a terminal. • Low maintenance cost. System should not require constant attention. • Adaptability: • Tailored to the environment. Support necessary activities. Do not impose unnecessary restrictions. What are the things people do most -- make them easy. • Changeable over time. Adapt as needs and resources change. e.g., expanding memory and new devices, or new user population. • Extendible-Extensible Adding new facilities and features - which look like the old ones. 1.7 OPERATING SYSTEMS : SOME EXAMPLES In the earlier section we had seen the types of operating systems. In this section we will study some popular operating systems. 1.7.1 DOS DOS (Disk Operating System) was the first widely-installed operating system for personal computers. It is a master control program that is automatically run when you start your personal computer (PC). DOS stays in the computer all the time letting you run a program and manage files. It is a single-user operating system from Microsoft for the PC. It was the first OS for the PC and is the underlying control program for Windows 3.1, 95, 98 and ME. Windows NT, 2000 and XP emulate DOS in order to support existing DOS applications. 1.7.2 UNIX UNIX operating systems are used in widely-sold workstation products from Sun Microsystems, Silicon Graphics, IBM, and a number of other companies. The UNIX 19 Operating System : An Overview The operating system is responsible for the following activities in connection with processes management: • The creation and deletion of both user and system processes • The suspension and resumption of processes. • The provision of mechanisms for process synchronization • The provision of mechanisms for deadlock handling. More details can be studied in the Unit-2 of this course. 1.8.2 Memory Management Memory is the most expensive part in the computer system. Memory is a large array of words or bytes, each with its own address. Interaction is achieved through a sequence of reads or writes of specific memory address. The CPU fetches from and stores in memory. There are various algorithms that depend on the particular situation to manage the memory. Selection of a memory management scheme for a specific system depends upon many factors, but especially upon the hardware design of the system. Each algorithm requires its own hardware support. The operating system is responsible for the following activities in connection with memory management. • Keep track of which parts of memory are currently being used and by whom. • Decide which processes are to be loaded into memory when memory space becomes available. • Allocate and deallocate memory space as needed. Memory management techniques will be discussed in great detail in Unit-1 of Block-2 of this course. 1.8.3 Secondary Storage Management The main purpose of a computer system is to execute programs. These programs, together with the data they access, must be in main memory during execution. Since the main memory is too small to permanently accommodate all data and program, the computer system must provide secondary storage to backup main memory. Most modem computer systems use disks as the primary on-line storage of information, of both programs and data. Most programs, like compilers, assemblers, sort routines, editors, formatters, and so on, are stored on the disk until loaded into memory, and then use the disk as both the source and destination of their processing. Hence the proper management of disk storage is of central importance to a computer system. There are few alternatives. Magnetic tape systems are generally too slow. In addition, they are limited to sequential access. Thus tapes are more suited for storing infrequently used files, where speed is not a primary concern. The operating system is responsible for the following activities in connection with disk management: • Free space management • Storage allocation • Disk scheduling. More details can be studied in Unit-3 of Block-2 of this course. 20 Introduction to Operating Systems and Process Management 1.8.4 I/O Management One of the purposes of an operating system is to hide the peculiarities or specific hardware devices from the user. For example, in UNIX, the peculiarities of I/O devices are hidden from the bulk of the operating system itself by the I/O system. The operating system is responsible for the following activities in connection to I/O management: • A buffer caching system • To activate a general device driver code • To run the driver software for specific hardware devices as and when required. More details can be studied in the Unit-3 of Block-2 of this course. 1.8.5 File Management File management is one of the most visible services of an operating system. Computers can store information in several different physical forms: magnetic tape, disk, and drum are the most common forms. Each of these devices has it own characteristics and physical organisation. For convenient use of the computer system, the operating system provides a uniform logical view of information storage. The operating system abstracts from the physical properties of its storage devices to define a logical storage unit, the file. Files are mapped, by the operating system, onto physical devices. A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data. Data files may be numeric, alphabetic or alphanumeric. Files may be free-form, such as text files, or may be rigidly formatted. In general a files is a sequence of bits, bytes, lines or records whose meaning is defined by its creator and user. It is a very general concept. The operating system implements the abstract concept of the file by managing mass storage device, such as types and disks. Also files are normally organised into directories to ease their use. Finally, when multiple users have access to files, it may be desirable to control by whom and in what ways files may be accessed. The operating system is responsible for the following activities in connection to the file management: • The creation and deletion of files. • The creation and deletion of directory. • The support of primitives for manipulating files and directories. • The mapping of files onto disk storage. • Backup of files on stable (non volatile) storage. • Protection and security of the files. 1.8.6 Protection The various processes in an operating system must be protected from each other’s activities. For that purpose, various mechanisms which can be used to ensure that the files, memory segment, CPU and other resources can be operated on only by those processes that have gained proper authorisation from the operating system. For example, memory addressing hardware ensures that a process can only execute within its own address space. The timer ensures that no process can gain control of the CPU without relinquishing it. Finally, no process is allowed to do its own I/O, to protect the integrity of the various peripheral devices. Protection refers to a mechanism for controlling the access of programs, processes, or users to the resources defined by a computer controls to be imposed, together with some means of enforcement. 21 Operating System : An Overview Protection can improve reliability by detecting latent errors at the interfaces between component subsystems. Early detection of interface errors can often prevent contamination of a healthy subsystem by a subsystem that is malfunctioning. An unprotected resource cannot defend against use (or misuse) by an unauthorised or incompetent user. More on protection and security can be studied in Unit-4 of Block-3. 1.8.7 Networking A distributed system is a collection of processors that do not share memory or a clock. Instead, each processor has its own local memory, and the processors communicate with each other through various communication lines, such as high speed buses or telephone lines. Distributed systems vary in size and function. They may involve microprocessors, workstations, minicomputers, and large general purpose computer systems. The processors in the system are connected through a communication network, which can be configured in the number of different ways. The network may be fully or partially connected. The communication network design must consider routing and connection strategies and the problems of connection and security. A distributed system provides the user with access to the various resources the system maintains. Access to a shared resource allows computation speed-up, data availability, and reliability. 1.8.8 Command Interpretation One of the most important components of an operating system is its command interpreter. The command interpreter is the primary interface between the user and the rest of the system. Many commands are given to the operating system by control statements. When a new job is started in a batch system or when a user logs-in to a time-shared system, a program which reads and interprets control statements is automatically executed. This program is variously called (1) the control card interpreter, (2) the command line interpreter, (3) the shell (in Unix), and so on. Its function is quite simple: get the next command statement, and execute it. The command statements themselves deal with process management, I/O handling, secondary storage management, main memory management, file system access, protection, and networking. The Figure 2 depicts the role of the operating system in coordinating all the functions. Figure 2: Functions Coordinated by the Operating System Secondary Storage Management Memory Management Networking Communication Management Process Management Protection & Security File Management Operating System I/O Management User Interface