MCA Part II Paper-XIII Topic: Introduction to Operating System Prepared by: Dr. Kiran Pandey (School of Computer Science) Email-id: [email protected] INTRODUCTION An Operating System (OS) is an interface between a computer user and computer hardware. An operating system is a software which performs all the basic tasks like file management, memory management, process management, handling input and output, and controlling peripheral devices such as disk drives and printers. 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. Operating system Concepts An operating system is a program that acts as an interface between the user and the computer hardware and controls the execution of all kinds of programs. Figure 1: Components of a computer system In a computer system as shown in figure 1, 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: A computer’s operating system is a group of programs designed to serve two basic purposes: ❖ To control the allocation and use of the computing system’s resources among the various users and tasks, and ❖ To provide an interface between the computer hardware and the programmer that simplifies and makes feasible the creation, coding, debugging, and maintenance of application programs. An effective operating system should accomplish the following functions: ❖ Should act as a command interpreter by providing a user friendly environment. ❖ Should facilitate communication with other users. ❖ Facilitate the directory/file creation along with the security option. ❖ Provide routines that handle the intricate details of I/O programming. ❖ Provide access to compilers to translate programs from high-level languages to machine language ❖ Provide a loader program to move the compiled program code to the computer’s memory for execution. ❖ Assure that when there are several active processes in the computer, each will get fair and non-interfering access to the central processing unit for execution. ❖ Take care of storage and device allocation. ❖ Provide for long term storage of user information in the form of files. ❖ Permit system resources to be shared among users when appropriate, and be protected from unauthorized or mischievous intervention as necessary. Though systems programs such as editors and translators and the various utility programs (such as sort and file transfer program) are not usually considered part of the operating system, the operating system is responsible for providing access to these system resources. TYPES OF OPERATING SYSTEM Batch Operating System Batch processing is a technique in which an Operating System collects the programs and data together in a batch before processing starts. An operating system does the following activities related to batch processing − The OS defines a job which has predefined sequence of commands, programs and data as a single unit. Batch processing is the execution of a series of jobs in a program on a computer without manual intervention (non-interactive). Strictly speaking, it is a processing mode: the execution of a series of programs each on a set or "batch" of inputs, rather than a single input (which would instead be a custom job). The users of a batch operating system do not interact with the computer directly. Each user prepares his job on an off-line device like punch cards and submits it to the computer operator. To speed up processing, jobs with similar needs are batched together and run as a group. The problems with Batch Systems are as follows − • Lack of interaction between the user and the job. • CPU is often idle, because the speed of the mechanical I/O devices is slower than the CPU. • Difficult to provide the desired priority. An example of batch processing is the way that credit card companies process billing. The customer does not receive a bill for each separate credit card purchase but one monthly bill for all of that month s purchases. Time Sharing Operating System Time sharing operating system allows many users to share the computer resources simultaneously. In other words, time sharing refers to the allocation of computer resources in time slots to several programs simultaneously. For example a mainframe computer that has many users logged on to it. Each user uses the resources of the mainframe -i.e. memory, CPU etc. The users feel that they are exclusive user of the CPU, even though this is not possible with one CPU i.e. shared among different users. The time sharing systems were developed to provide an interactive use of the computer system. A time shared system uses CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared computer. It allows many users to share the computer resources simultaneously. As the system switches rapidly from one user to the other, a short time slot is given to each user for their executions. The time sharing system provides the direct access to a large number of users where CPU time is divided among all the users on scheduled basis. The OS allocates a set of time to each user. When this time is expired, it passes control to the next user on the system. The time allowed is extremely small and the users are given the impression that they each have their own CPU and they are the sole owner of the CPU. This short period of time during that a user gets attention of the CPU; is known as a time slice or a quantum. The Multics & UNIX operating systems are time sharing Operating Systems. Advantages of Timesharing operating systems are as follows − • Provides the advantage of quick response. • Avoids duplication of software. • Reduces CPU idle time. Disadvantages of Time-sharing operating systems are as follows − • Problem of reliability. • Question of security and integrity of user programs and data. • Problem of data communication. Real Time Operating System A real-time system is defined as a data processing system in which the time interval required to process and respond to inputs is so small that it controls the environment. The time taken by the system to respond to an input and display of required updated information is termed as the response time. So in this method, the response time is very less as compared to online processing. Real-time systems are used when there are rigid time requirements on the operation of a processor or the flow of data and real-time systems can be used as a control device in a dedicated application. A real-time operating system must have well-defined, fixed time constraints, otherwise the system will fail. For example, scientific experiments, medical imaging systems, industrial control systems, weapon systems, robots, air traffic control systems, etc. There are two types of real-time operating systems. Hard real-time systems Hard real-time systems guarantee that critical tasks complete on time. In hard real- time systems, secondary storage is limited or missing and the data is stored in ROM. In these systems, virtual memory is almost never found. Soft real-time systems Soft real-time systems are less restrictive. A critical real-time task gets priority over other tasks and retains the priority until it completes. Soft real-time systems have limited utility than hard real-time systems. For example, multimedia, virtual reality, Advanced Scientific Projects like undersea exploration and planetary rovers, etc. Multiprogramming Operating System In a multiprogramming system there are one or more programs loaded in main memory which are ready to execute. Only one program at a time is able to get the CPU for executing its instructions (i.e., there is at most one process running on the system) while all the others are waiting their turn. The main idea of multiprogramming is to maximize the use of CPU time. Indeed, suppose the currently running process is performing an I/O task (which, by definition, does not need the CPU to be accomplished).