Overview of Operating System PDF

SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS Chapter 1 Marks 10 Overview of Operating System 1.0 INTRODUCTION Computer s...

SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS Chapter 1 Marks 10 Overview of Operating System 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 utilization. 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 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, organizing data for secure and rapid access, and handling network communications. What Operating Systems Do We begin our discussion by looking at the operating system’s role in the overall computer system. A computer system can be divided roughly into four components: the hardware, the operating system, the application programs, and the users (Figure 1.1). Mr. P R Sali 1 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS Figure 1.1 Abstract view of the components of a computer system. The hardware—the central processing unit (CPU), the memory, and the input/output (I/O) devices—provides the basic computing resources for the system. The application programs—such as word processors, spreadsheets, compilers, and Web browsers—define the ways in which these resources are used to solve users’ computing problems. The operating system controls the hardware and coordinates its use among the various application programs for the various users. We can also view a computer system as consisting of hardware, software, and data. The operating system provides the means for proper use of these resources in the operation of the computer system. An operating system is similar to a government. Like a government, it performs no useful function by itself. It simply provides an environment within which other programs can do useful work. To understand more fully the operating system’s role, we next explore operating systems from two viewpoints: that of the user and that of the system. 1.1.1 User View The user’s view of the computer varies according to the interface being used. Most computer users sit in front of a PC, consisting of a monitor, keyboard, mouse, and system unit. Such a system is designed for one user to monopolize its resources. The goal is to maximize the work (or play) that the user is performing. In this case, the operating system is designed mostly for ease of use, with some attention paid to performance and none paid to resource utilization—how various hardware and software resources are shared. Performance is, of course, important to the user; but such systems are optimized for the single-user experience rather than the requirements of multiple users. Mr. P R Sali 2 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS 1.1.2 System View From the computer’s point of view, the operating system is the program most intimately involved with the hardware. In this context, we can view an operating system as a resource allocator. A computer system has many resources that may be required to solve a problem: CPU time, memory space, file-storage space, I/O devices, and so on. The operating system acts as the manager of these resources. Facing numerous and possibly conflicting requests for resources, the operating system must decide how to allocate them to specific programs and users so that it can operate the computer system efficiently and fairly. As we have seen, resource allocation is especially important where many users access the same mainframe or minicomputer. A slightly different view of an operating system emphasizes the need to control the various I/O devices and user programs. An operating system is a control program. A control program manages the execution of user programs to prevent errors and improper use of the computer. It is especially concerned with the operation and control of I/O devices. 1.2.1 Computer-System Operation A modern general-purpose computer system consists of one or more CPUs and a number of device controllers connected through a common bus that provides access to shared memory (Figure 1.2). Each device controller is in charge of a specific type of device (for example, disk drives, audio devices, or video displays). The CPU and the device controllers can execute in parallel, competing for memory cycles. To ensure orderly access to the shared memory, a memory controller synchronizes access to the memory. For a computer to start running—for instance, when it is powered up or rebooted—it needs to have an initial program to run. This initial program, or bootstrap program, tends to be simple. Typically, it is stored within the computer hardware in read-only memory (ROM) or electrically erasable programmable read-only memory (EEPROM), known by the general term firmware. It initializes all aspects of the system, from CPU registers to device controllers to memory contents. The bootstrap program must know how to load the operating system and how to start executing that system. To accomplish this goal, the bootstrap program must locate the operating-system kernel and load it into memory. Once the kernel is loaded and executing, it can start providing services to the system and its users. Some services are provided outside of the kernel, by system programs that are loaded into memory at boot time to become system processes, or system daemons that run the entire time the kernel is running. Mr. P R Sali 3 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS Figure 1.2 A modern computer system. The occurrence of an event is usually signaled by an interrupt from either the hardware or the software. Hardware may trigger an interrupt at any time by sending a signal to the CPU, usually by way of the system bus. Software may trigger an interrupt by executing a special operation called a system call (also called a monitor call). 1.2.2 Storage Structure The CPU can load instructions only from memory, so any programs to run must be stored there. General-purpose computers run most of their programs from rewritable memory, called main memory (also called random-access memory, or RAM). Main memory commonly is implemented in a semiconductor technology called dynamic random-access memory (DRAM). Computers use other forms of memory as well. We have already mentioned read-only memory, ROM) and electrically erasable programmable read-only memory, EEPROM). Because ROM cannot be changed, only static programs, such as the bootstrap program described earlier, are stored there. The immutability of ROM is of use in game cartridges. EEPROM can be changed but cannot be changed frequently and so contains mostly static programs. For example, smartphones have EEPROM to store their factory-installed programs. Ideally, we want the programs and data to reside in main memory permanently. This arrangement usually is not possible for the following two reasons: 1. Main memory is usually too small to store all needed programs and data permanently. 2. Main memory is a volatile storage device that loses its contents when power is turned off or otherwise lost. Mr. P R Sali 4 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS Thus, most computer systems provide secondary storage as an extension of main memory. The main requirement for secondary storage is that it be able to hold large quantities of data permanently. The most common secondary-storage device is a magnetic disk, which provides storage for both programs and data. Most programs (system and application) are stored on a disk until they are loaded into memory. Many programs then use the disk as both the source and the destination of their processing. Hence, the proper management of disk storage is of central importance to a computer system, Figure 1.3 Storage-device hierarchy. 1.8 FUNCTIONS OF OS The main functions of an operating system are as follows: 1. Process Management 2. Memory Management 3. Secondary Storage Management 4. I/O Management 5. File Management 6. Protection 7. Networking Management 8. Command Interpretation. 1.8.1 Process Management The CPU executes a large number of programs. While its main concern is the execution of user programs, the CPU is also needed for other system activities. These activities are called Mr. P R Sali 5 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS processes. A process is a program in execution. Typically, a batch job is a process. A time- shared user program is a process. A system task, such as spooling, is also a process. For now, a process may be considered as a job or a time- shared program, but the concept is actually more general. The operating system is responsible for the following activities in connection with processes management: 1. The creation and deletion of both user and system processes 2. The suspension and resumption of processes. 3. The provision of mechanisms for process synchronization 4. 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 asequence 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. 1. Keep track of which parts of memory are currently being used and by whom. 2. Decide which processes are to be loaded into memory when memory space becomes available. 3. Allocate and deallocate memory space as needed. Memory management techniques will be discussed in great detail in Unit 3. 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: Mr. P R Sali 6 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS 1. Free space management 2. Storage allocation 3. Disk scheduling. More details can be studied in Unit-4 of this course. 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: 1. A buffer caching system 2. To activate a general device driver code 3. 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 its own characteristics and physical organization. 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 file 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 organized 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: 1. The creation and deletion of files. 2. The creation and deletion of directory. 3. The support of primitives for manipulating files and directories. 4. The mapping of files onto disk storage. 5. Backup of files on stable (non volatile) storage. Mr. P R Sali 7 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS 6. 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 authorization 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. 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 unauthorized or incompetent user. 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. Mr. P R Sali 8 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS The command statements themselves deal with process management, I/O handling, secondary storage management, main memory management, file system access, protection, and networking. Figure depicts the role of the operating system in coordinating all the functions. Types of Operating Systems An Operating System performs all the basic tasks like managing file, process, and memory. Thus operating system acts as manager of all the resources, i.e. resource manager. Thus operating system becomes an interface between user and machine. Types of Operating Systems: Some of the widely used operating systems are as follows- 1. Batch OS – This type of operating system does not interact with the computer directly. The user has to submit a job (written on cards or tape) to a computer operator. There is an operator which takes similar jobs having same requirement and group them into batches. It is the responsibility of operator to sort the jobs with similar needs. Then computer operator places a batch of several jobs on an input device. Jobs are batched together by type of languages and requirement. Then a special program, the monitor, manages the execution of each program in the batch. The monitor is always in the main memory and available for execution. Mr. P R Sali 9 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS 2. Multi programming – In a modern computing system, there are usually several concurrent application processes which want to execute. Now it is the responsibility of the Operating System to manage all the processes effectively and efficiently. One of the most important aspects of an Operating System is to multi program. In a computer system, there are multiple processes waiting to be executed, i.e. they are waiting when the CPU will be allocated to them and they begin their execution. These processes are also known as jobs. Now the main memory is too small to accommodate all of these processes or jobs into it. Thus, these processes are initially kept in an area called job pool. This job pool consists of all those processes awaiting allocation of main memory and CPU. CPU selects one job out of all these waiting jobs, brings it from the job pool to main memory and starts executing it. The processor executes one job until it is interrupted by some external factor or it goes for an I/O task. Non-multi programmed system’s working –  In a non multi programmed system, As soon as one job leaves the CPU and goes for some other task (say I/O ), the CPU becomes idle. The CPU keeps waiting and waiting until this job (which was executing earlier) comes back and resumes its execution with the CPU. So CPU remains free for all this while.  Now it has a drawback that the CPU remains idle for a very long period of time. Also, other jobs which are waiting to be executed might not get a chance to execute because the CPU is still allocated to the earlier job. This poses a very serious problem that even though other jobs are ready to execute, CPU is not allocated to them as the CPU is allocated to a job which is not even utilizing it (as it is busy in I/O tasks).  It cannot happen that one job is using the CPU for say 1 hour while the others have been waiting in the queue for 5 hours. To avoid situations like this and come up with efficient utilization of CPU, the concept of multi programming came up. Mr. P R Sali 10 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS The main idea of multi programming is to maximize the CPU time. Multi programmed system’s working –  In a multi-programmed system, as soon as one job goes for an I/O task, the Operating System interrupts that job, chooses another job from the job pool (waiting queue), gives CPU to this new job and starts its execution. The previous job keeps doing its I/O operation while this new job does CPU bound tasks. Now say the second job also goes for an I/O task, the CPU chooses a third job and starts executing it. As soon as a job completes its I/O operation and comes back for CPU tasks, the CPU is allocated to it.  In this way, no CPU time is wasted by the system waiting for the I/O task to be completed. Therefore, the ultimate goal of multi programming is to keep the CPU busy as long as there are processes ready to execute. This way, multiple programs can be executed on a single processor by executing a part of a program at one time, a part of another program after this, then a part of another program and so on, hence executing multiple programs. Hence, the CPU never remains idle. In the image below, program A runs for some time and then goes to waiting state. In the mean time program B begins its execution. So the CPU does not waste its resources and gives program B an opportunity to run. 3. Multitasking – As the name itself suggests, multi tasking refers to execution of multiple tasks (say processes, programs, threads etc.) at a time. In the modern operating systems, we are able to play MP3 music, edit documents in Microsoft Word, surf the Google Chrome all simultaneously, this is accomplished by means of multi tasking. Multitasking is a logical extension of multi programming. The major way in which multitasking differs from multi programming is that multi programming works solely on the concept of context switching whereas multitasking is based on time sharing alongside the concept of context switching. Mr. P R Sali 11 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS Multi tasking system’s working –  In a time sharing system, each process is assigned some specific quantum of time for which a process is meant to execute. Say there are 3 processes P1, P2, P3 ready to execute. So each of them are assigned some time quantum for which they will execute e.g time quantum of 2 nanoseconds (2 ns). As one process begins execution (say P1), it executes for that quantum of time (2 ns). After 2 ns the CPU starts the execution of the other process (say P2) for the specified quantum of time.  Thus the CPU makes the processes to share time slices between them and execute accordingly. As soon as time quantum of one process expires, another process begins its execution.  Here also basically a context switch is occurring but it is occurring so fast that the user is able to interact with each program separately while it is running. This way, the user is given the illusion that multiple processes/ tasks are executing simultaneously. But actually only one process/ task is executing at a particular instant of time. In multitasking, time sharing is best manifested because each running process takes only a fair quantum of the CPU time. In a more general sense, multitasking refers to having multiple programs, processes, tasks, threads running at the same time. This term is used in modern operating systems when multiple tasks share a common processing resource (e.g., CPU and Memory).  As depicted in the above image, At any time the CPU is executing only one task while other tasks are waiting for their turn. The illusion of parallelism is achieved when the CPU is reassigned to another task. i.e all the three tasks A, B and C are appearing to occur simultaneously because of time sharing.  So for multitasking to take place, firstly there should be multiprogramming i.e. presence of multiple programs ready for execution. And secondly the concept of time sharing. 4. Multiprocessing – In a uni-processor system, only one process executes at a time. Multiprocessing is the use of two or more CPUs (processors) within a single Computer system. The term also refers to the ability of a system to support more than one processor Mr. P R Sali 12 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS within a single computer system. Now since there are multiple processors available, multiple processes can be executed at a time. These multi processors share the computer bus, sometimes the clock, memory and peripheral devices also. Multi processing system’s working –  With the help of multiprocessing, many processes can be executed simultaneously. Say processes P1, P2, P3 and P4 are waiting for execution. Now in a single processor system, firstly one process will execute, then the other, then the other and so on.  But with multiprocessing, each process can be assigned to a different processor for its execution. If its a dual-core processor (2 processors), two processes can be executed simultaneously and thus will be two times faster, similarly a quad core processor will be four times as fast as a single processor. Why use multi processing –  The main advantage of multiprocessor system is to get more work done in a shorter period of time. These types of systems are used when very high speed is required to process a large volume of data. Multi processing systems can save money in comparison to single processor systems because the processors can share peripherals and power supplies.  It also provides increased reliability in the sense that if one processor fails, the work does not halt, it only slows down. e.g. if we have 10 processors and 1 fails, then the work does not halt, rather the remaining 9 processors can share the work of the 10th processor. Thus the whole system runs only 10 percent slower, rather than failing altogether. Multiprocessing refers to the hardware (i.e., the CPU units) rather than the software (i.e., running processes). If the underlying hardware provides more than one processor then that is multiprocessing. It is the ability of the system to leverage multiple processors’ computing power. Difference between Multi programming and Multi processing –  A System can be both multi programmed by having multiple programs running at the same time and multiprocessing by having more than one physical processor. The difference between multiprocessing and multi programming is that Multiprocessing is Mr. P R Sali 13 OSY (22516) SNJB’s SHHJB Polytechnic, Chandwad CH.1 Overview of OS basically executing multiple processes at the same time on multiple processors, whereas multi programming is keeping several programs in main memory and executing them concurrently using a single CPU only.  Multiprocessing occurs by means of parallel processing whereas Multi programming occurs by switching from one process to other (phenomenon called as context switching). Mr. P R Sali 14 OSY (22516)