UNIT 1 OS PDF

UNIT – I : Basic of Operating System and Its Structures Syllabus: - Introduction: Computer System Organization – Architecture, Structure, Operations. Management: Process, Memory, Storage. Structures: Services-System Interface, System Calls- System Program, Design-structure...

UNIT – I : Basic of Operating System and Its Structures Syllabus: - Introduction: Computer System Organization – Architecture, Structure, Operations. Management: Process, Memory, Storage. Structures: Services-System Interface, System Calls- System Program, Design-structure Introduction of operating systems A computer system can be divided roughly into four components: 1. The hardware The operating system 2. The application programs and 3. The users. The hardware - the CPU, the Memory and the Input Output devices - provides the basic computing resources for the system. Figure 1 Abstract view of computer component The Application program 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. An operating system is a program that manages the computer hardware. It also provides a basis for application programs and acts as an intermediary between the computer user and the computer hardware The users are person who use system to perform task by using all above component of system. To understand fully about the operating system's role, we next explore operating systems from two viewpoints: that of the user view and that of the system view. User View In Case 1: 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. Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 1  In Case 2: a user sits at a terminal connected to a mainframe or minicomputer. Other users are accessing the same computer through other terminals. These users share resources and may exchange information. The operating system in such cases is designed to maximize resource utilization— to assure that all available CPU time, memory, and I/O are used efficiently and that no individual user takes more than her fair share. In Case 3: users sit at workstations connected to networks of other workstations and servers. These users have dedicated resources at their disposal, but they also share resources such as networking and servers—file, compute, and print servers. Therefore, their operating system is designed to compromise between individual usability and resource utilization. Recently, many varieties of handheld computers have come into fashion. Most of these devices are standalone units for individual users. Some are connected to networks, either directly by wire or (more often) through wireless modems and networking. Because of power, speed, and interface limitations, they perform relatively few remote operations. Their operating systems are designed mostly for individual usability, but performance per amount of battery life is important as well. 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. Operating System We have looked at the operating system's role from the views of the user and of the system. Operating systems exist because they offer a reasonable way to solve the problem of creating a usable computing system. The fundamental goal of computer systems is to execute user programs and to make solving user problems easier. Toward this goal, computer hardware is constructed. The common functions a system should have controlling and allocating resources are then brought together into one piece of software: the operating system. A more common definition is that the operating system is the one program running Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 2  at all times on the computer (usually called the kernel), with all else being systems programs and application programs. The 1960’s definition of an operating system is ―the software that controls the hardware‖. Operating Systems is a program that manages the computer hardware. It also provides a basis for application programs and acts as an intermediary between the computer user and the computer hardware. Operating system as the programs that make the hardware useable, in brief, an operating system is the set of programs that controls a computer. Some examples of operating systems are UNIX, Mach, MS-DOS, MS-Windows, Windows/NT, Chicago, OS/2, MacOS etc. In OS we have 3 services and we will differentiate these services. Kernel services, library services, and application-level services, all of which are part of the operating system. Processes run Applications, which are linked together with libraries that perform standard services. The kernel supports the processes by providing a path to the peripheral devices. The core of the operating system is the kernel, a control program that functions in privileged 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. Some operating systems are designed to be convenient, others to be efficient, and others some combination of the two. Types of operating systems Operating System is divided 4 basic types and future 2 more type is added into it Batch Processing Operating system Early computers were physically enormous machines run from a console. The common input devices were card readers and tape drives. The common output devices were line printers, tape drives, and card punches. The user did not interact directly with the computer systems. The user prepared a job - which consisted of the program, the data, and some control information about the nature of the job (control cards) and submitted it to the computer operator. The job was usually in the form of punch cards. At some later time (after minutes, hours, or days), the output appeared. The operating system in these early computers was fairly simple. Its major task was to transfer control automatically from one job to the next. The operating system was always resident in memory (Figure 2). To speed up processing, operators batched together jobs with similar needs and ran them through the computer as a group. Thus, the programmers would leave their programs with the operator. The operator would sort programs into batches with similar requirements and, as the computer became available, would run each batch. Figure 2 Memory layout for a simple batch system. In this execution environment, the CPU is often idle, because the speeds of the mechanical I/O devices are Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 3  intrinsically slower than are those of electronic devices. Even a slow CPU works in the microsecond range, with thousands of instructions executed per second. The introduction of disks resulted in faster I/O devices. The introduction of disk technology allowed the operating system to keep all jobs on a disk, rather than in a serial card reader. With direct access to several jobs, the operating system could perform job scheduling, to use resources and perform tasks efficiently. Multiprogramming Operating system The most important aspect of job scheduling is the ability to multiprogram. A single user cannot, in general, keep either the CPU or the I/0 devices busy at all times. M ultiprogramming increases CPU utilization by organizing jobs so that the CPU always has one to execute. The operating system keeps several jobs in memory simultaneously (Figure 3). This set of jobs is a subset of the jobs kept in the job pool, since the number of jobs that can be kept simultaneously in memory is usually much smaller than the number of jobs that can be in the job pool. The operating system picks and begins to execute one of the jobs in the memory. Eventually, the job may have to wait for some task, such as an I/O operation, to complete. In a non- multiprogrammed system, the CPU would sit idle. Figure 3 Memory layout for a multiprogramming system. In a multiprogramming system, the operating system simply switches to, and executes, another job. When that job needs to wait, the CPU is switched to another job, and so on. Eventually, the first job finishes waiting and gets the CPU back. As long as at least one job needs to execute, the CPU is never idle. Multiprogrammed operating systems are therefore fairly sophisticated. All the jobs that enter the system are kept in the job pool. This pool consists of all processes residing on disk awaiting allocation of main memory. If several jobs are ready to be brought into memory, and if there is not enough room for all of them, then the system must choose among them. Making this decision is job scheduling. When the operating system selects a job from the job pool, it loads that job into memory for execution. Having several programs in memory at the same time requires some form of memory management. The system must choose among them. Making this decision is CPU scheduling. Time-sharing Operating system or Multitasking Multiprogrammed, batched systems provided an environment where the various system resources (for example, CPU, memory, peripheral devices) were utilized effectively, but it did not provide for user interaction with the computer system. Time sharing (or multitasking) is a logical extension of multiprogramming. The CPU executes multiple jobs by switching among them, but the switches occur so frequently that the users can interact with each program while it is running. An interactive computer system provides direct communication between the user and the system. The user gives instructions to the operating system or to a Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 4  program directly, using a keyboard or a mouse, and waits for immediate results. The response time or time slice should be short typically within 1 second or so. A time-shared operating system allows many users to share the computer simultaneously. Since each action or command in a time-shared system tends to be short, only a little CPU time is needed for each user. As the system switches rapidly from one user to the next, each user is given the impression that the entire computer system is dedicated to her use, even though it is being shared among many users. A time-shared operating system uses CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared computer. Each user has at least one separate program in memory. A program loaded into memory and executing is commonly referred to as a process. When a process executes, it typically executes for only a short time before it either finishes or needs to perform I/O. I/O may be interactive; that is, output is to a display for the user and input is from a user keyboard, mouse, or other device. Since interactive I/O typically runs at "people speeds," it may take a long time to complete. Rather than let the CPU sit idle when this interactive input takes place, the operating system will rapidly switch the CPU to the program of some other user. Time-sharing operating systems are even more complex than multiprogrammed operating systems. In both, several jobs must be kept simultaneously in memory, so the system must have memory management and protection. To obtain a reasonable response time, jobs may have to be swapped in and out of main memory to the disk that now serves as a backing store for main memory. A common method for achieving this goal is virtual memory, which is a technique that allows the execution of a job that may not be completely in memory. The main advantage of the virtual-memory scheme is that programs can be larger than physical memory. Further, it abstracts main memory into a large, uniform array of storage, separating logical memory as viewed by the user from physical memory. This arrangement frees programmers from concern over memory- storage limitations. Real time systems Operating system Another form of a special-purpose operating system is the real-time system. A real-time system is used when rigid time requirements have been placed on the operation of a processor or the flow of data; thus, it is often used as a control device in a dedicated application. A real-time system has well-defined, fixed time constraints or Time slice. Processing must be done within the defined constraints, or the system will fail. A real-time system functions correctly only if it returns the correct result within its time constraints. Contrast this requirement to a time-sharing system, where it is desirable but not mandatory to respond quickly. Real-time systems come in two flavours: hard and soft. A hard real-time system guarantees that critical tasks be completed on time. This goal requires that all delays in the system be bounded, from the retrieval of stored data to the time that it takes the operating system to finish any request made of it. Such time constraints dictate the facilities that are available in hard real-time systems. Secondary storage of any sort is usually limited or missing, with data instead being stored in short-term memory or in read-only memory (ROM). ROM is located on nonvolatile storage devices that retain their contents even in the case Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 5  of electric outage; most other types of memory are volatile. Hard real-time systems conflict with the operation of time-sharing systems, and the two cannot be mixed. A less restrictive type of real-time system is a soft real-time system, where a critical real-time task gets priority over other tasks, and retains that priority until it completes. As in hard real-time systems, the operating-system kernel delays need to be bounded. Soft real time is an achievable goal that can be mixed with other types of systems. Soft real-time systems have more limited utility than hard real-time systems. Given their lack of deadline support, they are risky to use for industrial control and robotics. They are useful, however in several areas, including multimedia, virtual reality, and advanced scientific projects. These systems need advanced operating-system features that cannot be supported by hard real-time systems. Because of the expanded uses for soft real- time functionality, it is finding its way into most current operating systems, including major versions of UNIX. Parallel processing System or Multiprocessor System Most systems to date are single-processor systems; that is, they have only one main CPU. Parallel systems (also known as M ultiprocessor systems or tightly coupled systems) are growing in importance. Such systems have more than one processor in close communication, sharing the computer bus, the clock, and sometimes memory and peripheral devices. Multiprocessor systems have three main advantages. 1. Increased throughput. By increasing the number of processors, we hope to get more work done in less time. The speed-up ratio with N processors is not N; rather, it is less than N. When multiple processors cooperate on a task, a certain amount of overhead is incurred in keeping all the parts working correctly plus contention for shared resources. 2. Economy of scale. Multiprocessor systems can save more money than multiple single-processor systems, because they can share peripherals, mass storage, and power supplies. If several programs operate on the same set of data, it is cheaper to store those data on one disk and to have all the processors share them, than to have many computers with local disks and many copies of the data. 3. Increased reliability. If functions can be distributed properly among several processors, then the failure of one processor will not halt the system, only slow it down. If we have ten processors and one fails, then each of the remaining nine processors must pick up a share of the work of the failed processor. Thus, the entire system runs only 10 percent slower, rather than failing altogether. This ability to continue providing service proportional to the level of surviving hardware is called graceful degradation. Systems designed for graceful degradation are also called fault tolerant. The most common multiple-processor systems now use symmetric multiprocessing (SM P), in which each processor runs an identical copy of the operating system, and these copies communicate with one another as needed. Some systems use asymmetric multiprocessing (ASM P), in which each processor is assigned a specific task. A master processor controls the system; the Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 6  other processors either look to the master for instruction or have predefined tasks. This scheme defines a master-slave relationship. The master processor schedules and allocates work to the slave processors. SMP means that all processors are peers; no master-slave relationship exists between processors. Each processor concurrently runs a copy of the operating system. Figure 4 illustrates a typical SMP architecture. An example of the SM P system is Encore's version of UNIX for the M ultimax computer. Figure 4 Symmetric and asummetric multiprocessing architecture. The difference between symmetric and asymmetric multiprocessing may be the result of either hardware or software. Special hardware can differentiate the multiple processors, or the software can be written to allow only one master and multiple slaves. Sun's operating system SunOS Version 4 provides asymmetric multiprocessing, whereas Version 5 (Solaris 2) is symmetric on the same hardware. Distributed System A network, in the simplest terms, is a communication path between two or more systems. Distributed systems depend on networking for their functionality. By being able to communicate, distributed systems are able to share computational tasks, and provide a rich set of features to users. Networks vary by the protocols used, the distances between nodes, and the transport media. TCP/IP is the most common network protocol, although ATM and other protocols are in widespread use. Likewise, operating-system support of protocols varies. Most operating systems support TCP/IP, including the Windows and UNIX operating systems. A local-area network (LAN), exists within a room, a floor, or a building. A wide- area network (W AN), usually exists between buildings, cities, or countries. A global company may have a WAN to connect its offices, worldwide. These networks could run one protocol or several protocols. The continuing advent of Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 7  new technologies brings about new forms of networks. For example, a metropolitan-area network (MAN), could link buildings within a city. BlueTooth devices communicate over a short distance of several feet, in essence creating a small-area network. The media to carry networks are equally varied. They include copper wires, fiber strands, and wireless transmissions between satellites, microwave dishes, and radios. When computing devices are connected to cellular phones, they create a network. Even very short-range infrared communication can be used for networking. These networks also vary by their performance and reliability. 1. Client-Server Systems - As PCs have become faster, more powerful, and cheaper, designers have shifted away from the centralized system architecture. Terminals connected to centralized systems are now being supplanted by PCs. Correspondingly, user-interface functionality that used to be handled directly by the centralized systems is increasingly being handled by the PCs. As a result, centralized systems today act as server systems to satisfy requests generated by client systems. a. Compute-server systems provide an interface to which clients can send requests to perform an action, in response to which they execute the action and send back results to the client. Figure 6 General structure of a client- server system. b. File-server systems provide a file-system interface where clients can create, update, read, and delete files. 2. Peer-to-Peer Systems - The growth of computer networks-especially the Internet and World Wide Web (WWW)-has had a profound influence on the recent development of operating systems. Virtually all modern PCs and workstations are capable of running a web browser for accessing hypertext documents on the Web. Operating systems now also include the system software that enables a computer to access the Internet via a local-area network or telephone connection. Several include the web browser itself, as well as electronic mail, remote login, and file-transfer clients and servers. Computer System Organization How computer systems operate, we need a general knowledge of the structure of a computer system. 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 shown in Figure. Each device controller is in charge of a specific type of device (for example, disk drives, audio devices, and video displays). The CPU and the device controllers can execute concurrently, competing for memory cycles. Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 8  To ensure orderly access to the shared memory, a memory controller is provided whose function is to synchronize access to the memory. Figure 7 A modern computer system For a computer to start running - 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 in read-only memory (ROM ) or electrically erasable programmable read-only memory (EEPROM), known by the general term firmware, within the computer hardware. 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 to start executing that system. To accomplish this goal, the bootstrap program must locate and load into memory the operating system kernel. The operating system then starts executing the first process, such as "init," and waits for some event to occur. 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). Interrupts are an important part of a computer architecture. Each computer design has its own interrupt mechanism, but several functions are common. The interrupt must transfer control to the appropriate interrupt service routine. The straightforward method for handling this transfer would be to invoke a generic routine to examine the interrupt information; the routine, in turn, would call the interrupt-specific handler. However, interrupts must be handled quickly. Figure 8 Interrupt time line for a single process doing output Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 9  2. Storage Structure - Computer programs must be in main memory (also called random-access memory or RAM) to be executed. Main memory is the only large storage area that the processor can access directly. It commonly is implemented in a semiconductor technology called dynamic random- access memory (DRAM), which forms an array of memory words. Each word has its own address. Interaction is achieved through a sequence of load or store instructions to specific memory addresses. The load instruction moves a word from main memory to an internal register within the CPU, whereas the store instruction moves the content of a register to main memory. Figure 9 Storage-device hierarchy A typical instruction-execution cycle, as executed on a system with a von Neumann architecture, first fetches an instruction from memory and stores that instruction in the instruction register. The instruction is then decoded and may cause operands to be fetched from memory and stored in some internal register. After the instruction on the operands has been executed, the result may be stored back in memory. M ain memory is usually too small to store all needed programs and data permanently. Main memory is a volatile storage device that loses its contents when power is turned off or otherwise lost. 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 (web browsers, compilers, word processors, spreadsheets, and so on) are stored on a disk until they are loaded into memory. Many programs then use the disk as both a source and a destination of the information for their processing. The wide variety of storage systems in a computer system can be organized in a hierarchy shown in Figure according to speed and cost. The higher levels are expensive, but they are fast. As we move down the hierarchy, the Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 10  cost per bit generally decreases, whereas the access time generally increases. This trade-off is reasonable; if a given storage system were both faster and less expensive than another—other properties being the same— then there would be no reason to use the slower, more expensive memory. 3. I/O Structure - Storage is only one of many types of I/O devices within a computer. A large portion of operating system code is dedicated to managing I/O, both because of its importance to the reliability and performance of a system and because of the varying nature of the devices. A general-purpose computer system consists of CPUs and multiple device controllers that are connected through a common bus. Each device controller is in charge of a specific type of device. Depending on the controller, there may be more than one attached device. For instance, seven or more devices can be attached to the small computer-systems interface (SCSI) controller. A device controller maintains some local buffer storage and a set of special-purpose registers. The device controller is responsible for moving the data between the peripheral devices that it controls and its local buffer storage. Typically, operating systems have a device driver for each device controller. This device driver understands the device controller and presents a uniform interface to the device to the rest of the operating system. To start an I/O operation, the device driver loads the appropriate registers within the device controller. Figure 10 How a modern computer system works. The device controller, in turn, examines the contents of these registers to determine what action to take (such as "read a character from the keyboard")- The controller starts the transfer of data from the device to its local buffer. Once the transfer of data is complete, the device controller informs the device driver via an interrupt that it has finished its operation. The device driver then returns control to the operating system, possibly Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 11  returning the data or a pointer to the data if the operation was a read. For other operations, the device driver returns status information. This form of interrupt-driven I/O is fine for moving small amounts of data but can produce high overhead when used for bulk data movement such as disk I/O. To solve this problem, direct memory access (DM A) is used. After setting up buffers, pointers, and counters for the I/O device, the device controller transfers an entire block of data directly to or from its own buffer storage to memory, with no intervention by the CPU. Computer System Architecture A computer system may be organized in a number of different ways, which we can categorize roughly according to the number of general-purpose processors used 1. Single-Processor Systems - Most systems use a single processor. On a single-processor system, there is one main CPU capable of executing a general-purpose instruction set, including instructions from user processes. They may come in the form of device-specific processors, such as disk, keyboard, and graphics controllers; or, on mainframes, they may come in the form of more general-purpose processors, such as I/O processors that move data rapidly among the components of the system. All of these special-purpose processors run a limited instruction set and do not run user processes. Sometimes they are managed by the operating system, in that the operating system sends them information about their next task and monitors their status. For example, a disk-controller microprocessor receives a sequence of requests from the main CPU and implements its own disk queue and scheduling algorithm. This arrangement relieves the main CPU of the overhead of disk scheduling. PCs contain a microprocessor in the keyboard to convert the keystrokes into codes to be sent to the CPU. In other systems or circumstances, special- purpose processors are low-level components built into the hardware. The operating system cannot communicate with these processors; they do their jobs autonomously. The use of special-purpose microprocessors is common and does not turn a single-processor system into a multiprocessor. 2. M ultiprocessor Systems - Although single-processor systems are most common, multiprocessor systems (also known as parallel systems or tightly coupled systems) are growing in importance. Such systems have two or more processors in close communication, sharing the computer bus and sometimes the clock, memory, and peripheral devices. Multiprocessor systems have three main advantages: I. Increased throughput. By increasing the number of processors, we hope to get more work done in less time. The speed-up ratio with N processors is not N; rather, it is less than N. When multiple processors cooperate on a task, a certain amount of overhead is incurred in keeping all the parts working correctly plus contention for shared resources. Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 12  II. Economy of scale. Multiprocessor systems can save more money than multiple single-processor systems, because they can share peripherals, mass storage, and power supplies. If several programs operate on the same set of data, it is cheaper to store those data on one disk and to have all the processors share them, than to have many computers with local disks and many copies of the data. III. Increased reliability. If functions can be distributed properly among several processors, then the failure of one processor will not halt the system, only slow it down. If we have ten processors and one fails, then each of the remaining nine processors must pick up a share of the work of the failed processor. Thus, the entire system runs only 10 percent slower, rather than failing altogether. Increased reliability of a computer system is crucial in many applications. The ability to continue providing service proportional to the level of surviving hardware is called graceful degradation. Some systems go beyond graceful degradation and are called fault tolerant, because they can suffer a failure of any single component and still continue operation. The multiple-processor systems in use today are of two types. 1. Some systems use asymmetric multiprocessing, in which each processor is assigned a specific task. A master processor controls the system; the other processors either look to the master for instruction or have predefined tasks. This scheme defines a master-slave relationship. The master processor schedules and allocates work to the slave processors. 2. The most common systems use symmetric multiprocessing (SM P), in which each processor performs all tasks within the operating system. SMP means that all processors are peers; no master-slave relationship exists between processors. The benefit of this model is that many processes can run simultaneously—N processes can run if there are N CPUs—without causing a significant deterioration of performance. Figure 11 symmetric multiprocessing system A multiprocessor system of this form will allow processes and resources—such as memory—to be shared dynamically among the various processors and can lower the variance among the processors. The difference between symmetric and asymmetric multiprocessing may result from either hardware or software. Special hardware can differentiate the multiple processors, or the software can be written to allow only one master and multiple slaves. A recent trend in CPU design is to include multiple compute cores on a single chip. In essence, these are multiprocessor chips. Two-way chips are Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 13  becoming mainstream, while N-way chips are going to be common in high- end systems. 3. Clustered Systems - Another type of multiple-CPU system is the clustered system. Like multiprocessor systems, clustered systems gather together multiple CPUs to accomplish computational work. Clustered systems differ from multiprocessor systems, however, in that they are composed of two or more individual systems coupled together. The definition of the term clustered is not concrete; many commercial packages wrestle with what a clustered system is and why one form is better than another. The generally accepted definition is that clustered computers share storage and are closely linked via a local-area network (LAN) or a faster interconnect such as InfiniBand. Clustering is usually used to provide high-availability service; that is, service will continue even if one or more systems in the cluster fail. High availability is generally obtained by adding a level of redundancy in the system. A layer of cluster software runs on the cluster nodes. Each node can monitor one or more of the others (over the LAN). If the monitored machine fails, the monitoring machine can take ownership of its storage and restart the applications that were running on the failed machine. Figure 12 Cluster System Clustering can be structured asymmetrically or symmetrically. In asymmetric clustering, one machine is in hot-standby mode while the other is running the applications. The hot-standby host machine does nothing but monitor the active server. If that server fails, the hot-standby host becomes the active server. In symmetric mode, two or more hosts are running applications, and are monitoring each other. This mode is obviously more efficient, as it uses all of the available hardware. Other forms of clusters include parallel clusters and clustering over a wide-area network (W AN). Parallel clusters allow multiple hosts to access the same data on the shared storage. Because most operating systems lack support for simultaneous data access by multiple hosts, parallel clusters are usually accomplished by use of special versions of software and special releases of applications Cluster technology is changing rapidly. Some cluster products support dozens of systems in a cluster, as well as clustered nodes that are separated by miles. Many of these improvements are made possible by storage-area networks (SANs), If the applications and their data are stored on the SAN, then the cluster software can assign the application to run on any host that is attached to the SAN. If the host fails, then any other host can take over Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 14  Operating System Structure A system as large and complex as a modern operating system must be engineered carefully if it is to function properly and be modified easily. A common approach is to partition the task into small components, rather than have one monolithic system. Each of these modules should be a well-defined portion of the system, with carefully defined inputs, outputs, and function. 1. Simple Structure approach Many commercial systems do not have a well-defined structure. Frequently, such operating systems started as small, simple, and limited systems, and then grew beyond their original scope. MS-DOS is an example of such a system. It was originally designed and implemented by a few people who had no idea that it would become so popular. It was written to provide the most functionality in the least space, so it was not divided into modules carefully. Figure 13 shows its structure. Figure 13 MS-DOS layer structures. UNIX is another system that was initially limited by hardware functionality. It consists of two separable parts: the kernel and the system programs. The kernel is further separated into a series of interfaces and device drivers, which were added and expanded over the years as UNIX evolved. The traditional UNIX operating system is shown in Figure 14. Everything below the system-call interface and above the physical hardware is the kernel. The kernel provides the file system, CPU scheduling, memory management, and other operating-system functions through system calls. Taken in sum, that is an enormous amount of functionality to be combined into one level. This makes UNIX difficult to enhance, as changes in one section could adversely affect other areas. Figure 14 UNIX system structure. System calls define the API to UNIX; the set of system programs commonly available defines the user interface. The programmer and user interfaces define the context Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 15  that the kernel must support. New versions of UNIX are designed to use more advanced hardware. Given proper hardware support, operating systems may be broken into pieces that are smaller and more appropriate than are those allowed by the original MS-DOS or UNIX systems. The operating system can then retain much greater control over the computer and over the applications that make use of that computer. Under the top down approach, the overall functionality and features are determined and separated into components. This separation allows programmers to hide information; they are therefore free to implement the low- level routines as they see fit. 2. Layered Approach Figure 15 a), b) An operating-system layer. The modularization of a system can be done in many ways. One method is the layered approach, in which the operating system is broken up into a number of layers (or levels), each built on top of lower layers. The bottom layer (layer 0) is the hardware; the highest (layer N) is the user interface. An operating-system layer is an implementation of an abstract object that is the encapsulation of data, and of the operations that can manipulate those data. A typical operating-system layer-say layer M-is depicted in Figure 15. Layer consists of data structures and a set of routines that can be invoked by higher-level layers. Layer M, in turn, can invoke operations on lower-level layers. The main advantage of the layered approach is modularity. The layers are selected such that each uses functions and services of only lower-level layers. This approach simplifies debugging and system verification. The first layer can be debugged without concern for the rest of the system, because, it uses only the basic hardware to implement its functions. Once the first layer is debugged, its correct functioning can be assumed while the second layer is debugged, and so on. If an error is found during the debugging of a particular layer, the error must be on that layer, because the layers below it are already debugged. Thus, the design and implementation of the system are simplified when the system is broken down into layers. Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 16  Each layer is implemented with only those operations provided by lower level layers. A layer does not need to know how these operations are implemented; it needs to know only what these operations do. The major difficulty with the layered approach involves the careful definition of the layers, because a layer can use only those layers below it. For example, the device driver for the disk space used by virtual-memory algorithms must be at a level lower than that of the memory-management routines, because memory management requires the ability to use the disk space. A final problem with layered implementations is that they tend to be less efficient than other types. For instance, when a user program executes an I/O operation, it executes a system call that is trapped to the I/O layer, which calls the memory-management layer, which in turn calls the CPU-scheduling layer, which is then passed to the hardware. At each layer, the parameters may be modified; data may need to be passed, and so on. These limitations have caused a small backlash against layering in recent years. Fewer layers with more functionality are being designed, providing most of the advantages of modularized code while avoiding the difficult problems of layer definition and interaction. Figure 16 OS/2 layer structure. 3. Microkernels Approach As the UNIX operating system expanded, the kernel became large and difficult to manage. In the mid-1980s, researchers at Carnegie Mellon University developed an operating system called M ach that modularizes the kernel using the microkernel approach. This method structures the operating system by removing all nonessential components from the kernel, and implementing them as system-level and user-level programs. The result is a smaller kernel. There is little consensus regarding which services should remain in the kernel and which should be implemented in user space. The main function of the microkernel is to provide a communication facility between the client program and the various services that are also running in user space. Communication is provided by message passing. For example, if the client program wishes to access a file, it must interact with the file server. The client program and the service never interact directly. Rather, they communicate indirectly by exchanging messages with the microkernel. The benefits of the microkernel approach include the ease of extending the operating system. All new services are added to user space and consequently do not require modification of the kernel. When the kernel does have to be modified, the changes tend to be fewer, because the microkernel is a smaller kernel. The resulting operating system is easier to port from one hardware design to another. The microkernel also provides more security and reliability, since Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 17  most services are running as user rather than kernel-processes. If a service fails, the rest of the operating system remains untouched. Several contemporary operating systems have used the microkernel approach. Tru64 UNIX provides a UNIX interface to the user, but it is implemented with a M ach kernel. The M ach kernel maps UNIX system calls into messages to the appropriate user-level services. W indows NT uses a hybrid structure. Windows NT is designed to run various applications, including Win32, 0S/2, and POSIX. It provides a server that runs in user space for each application type. Client programs for each application type also run in user space. The kernel coordinates the message passing between client applications and application servers. The client-server structure of Windows NT is depicted in Figure 17. Figure 17 Windows NT client-server structure. Operating System Operations Modern operating systems are interrupt driven. If there are no processes to execute, no I/O devices to service, and no users to whom to respond, an operating system will sit quietly, waiting for something to happen. Events are almost always signaled by the occurrence of an interrupt or a trap. A trap (or an exception) is a software-generated interrupt caused either by an error (for example, division by zero or invalid memory access) or by a specific request from a user program that an operating-system service be performed. The interrupt-driven nature of an operating system defines that system's general structure. For each type of interrupt, separate segments of code in the operating system determine what action should be taken. An interrupt service routine is provided that is responsible for dealing with the interrupt. 1. Dual-M ode Operation - In order to ensure the proper execution of the operating system, we must be able to distinguish between the execution of operating-system code and user defined code. The approach taken by most computer systems is to provide hardware support that allows us to differentiate among various modes of execution. Figure 17 Dual Mode operation At the very least, we need two separate modes of operation: user mode and kernel mode. A bit, called the mode bit, is added to the hardware of the computer to indicate the current mode: kernel (0) or Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 18  user (1). With the mode bit, we are able to distinguish between a task that is executed on behalf of the operating system and one that is executed on behalf of the user. When the computer system is executing on behalf of a user application, the system is in user mode. However, when a user application requests a service from the operating system (via a system call), it must transition from user to kernel mode to fulfill the request. At system boot time, the hardware starts in kernel mode. The operating system is then loaded and starts user applications in user mode. Whenever a trap or interrupt occurs, the hardware switches from user mode to kernel mode. The dual mode of operation provides us with the means for protecting the operating system from errant users from one another. The instruction to switch to user mode is an example of a privileged instruction. Some other examples include I/O control, timer management, and interrupt management. 2. Timer - We must ensure that the operating system maintains control over the CPU. We must prevent a user program from getting stuck in an infinite loop or not calling system services and never returning control to the operating system. To accomplish this goal, we can use a timer. A timer can be set to interrupt the computer after a specified period. The period may be fixed or variable. A variable timer is generally implemented by a fixed-rate clock and a counter. The operating system sets the counter. Every time the clock ticks, the counter is decremented. When the counter reaches 0, an interrupt occurs. Before turning over control to the user, the operating system ensures that the timer is set to interrupt. If the timer interrupts, control transfers automatically to the operating system, which may treat the interrupt as a fatal error or may give the program more time. We can use the timer to prevent a user program from running too long. A simple technique is to initialize a counter with the amount of time that a program is allowed to run. Every second, the timer interrupts and the counter is decremented by 1. As long as the counter is positive, control is returned to the user program. When the counter becomes negative, the operating system terminates the program for exceeding the assigned time limit. Process Management A program does nothing unless its instructions are executed by a CPU. A program in execution, as mentioned, is a process. A time-shared user program such as a compiler is a process. A word-processing program being run by an individual user on a PC is a process. A system task, such as sending output to a printer, can also be a process. A process needs certain resources—including CPU time, memory, files, and I/O devices—to accomplish its task. These resources are either given to the process when it is created or allocated to it while it is running. In addition to the various physical and logical resources that a process obtains when it is created, various initialization data (input) may be passed along. We emphasize that a program by itself is not a process; a program is a passive entity, such as the contents of a file stored on disk, whereas a process is an active entity. A single-threaded process has one program counter specifying the next instruction to execute. Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 19  The execution of such a process must be sequential. The CPU executes one instruction of the process after another, until the process completes. Further, at any time, one instruction at most is executed on behalf of the process. Thus, although two processes may be associated with the same program, they are nevertheless considered two separate execution sequences. A multithreaded process has multiple program counters, each pointing to the next instruction to execute for a given thread. A process is the unit of work in a system. Such a system consists of a collection of processes, some of which are operating-system processes and the rest of which are user processes. All these processes can potentially execute concurrently by multiplexing the CPU among them on a single CPU. The operating system is responsible for the following activities in connection with process management: 1) Creating and deleting both user and system processes 2) Suspending and resuming processes 3) Providing mechanisms for process synchronization 4) Providing mechanisms for process communication 5) Providing mechanisms for deadlock handling Memory Management The main memory is central to the operation of a modern computer system. Main memory is a large array of words or bytes ranging in size from hundreds of thousands to billions. Each word or byte has its own address. Main memory is a repository of quickly accessible data shared by the CPU and I/O devices. The central processor reads instructions from main memory during the instruction- fetch cycle and both reads and writes data from main memory during the data- fetch cycle. The main memory is generally the only large storage device that the CPU is able to address and access directly. For example, for the CPU to process data from disk, those data must first be transferred to main memory by CPU-generated I/O calls. In the same way, instructions must be in memory for the CPU to execute them. For a program to be executed, it must be mapped to absolute addresses and loaded into memory. As the program executes, it accesses program instructions and data from memory by generating these absolute addresses. Eventually, the program terminates, its memory space is declared available, and the next program can be loaded and executed. To improve both the utilization of the CPU and the speed of the computer's response to its users, general-purpose computers must keep several programs in memory, creating a need for memory management. Many different memory management schemes are used. These schemes reflect various approaches, and the effectiveness of any given algorithm depends on the situation. In selecting a memory-management scheme for a specific system, we must take into account many factors—especially on 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:  Keeping track of which parts of memory are currently being used and by whom. Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 20   Deciding which processes (or parts thereof) and data to move into and out of memory.  Allocating and deallocating memory space as needed. Storage Management To make the computer system convenient for users, 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. The operating system maps files onto physical media and accesses these files via the storage devices. 1. File-System M anagement - File management is one of the most visible components of an operating system. Computers can store information on several different types of physical media. M agnetic disk, optical disk, and magnetic tape are the most common. Each of these media has its own characteristics and physical organization. Each medium is controlled by a device, such as a disk drive or tape drive, that also has its own unique characteristics. These properties include access speed, capacity', data- transfer rate, and access method (sequential or random). 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, alphanumeric, or binary. Files may be free-form (for example, text files), or they may be formatted rigidly (for example, fixed fields). The operating system implements the abstract concept of a file by managing mass storage media, such as tapes and disks, and the devices that control them. Also, files are normally organized into directories to make them easier to use. Multiple users have access to files, it may be desirable to control by whom and in what ways (for example, read, write, append) files may be accessed. The operating system is responsible for the following activities  Creating and deleting files  Creating and deleting directories to organize files  Supporting primitives for manipulating files and directories  Mapping files onto secondary storage  Backing up files on stable (nonvolatile) storage media. 2. M ass-Storage M anagement - Main memory is too small to accommodate all data and programs, and because the data that it holds are lost when power is lost, the computer system must provide secondary storage to back up main memory. Most modern computer systems use disks as the principal on-line storage medium for both programs and data. Most programs includes compilers, assemblers, word processors, editors, and formatters—are stored on a disk until loaded into memory and then use the disk as both the source and destination of their processing. The operating system is responsible for the following activities  Free-space management  Storage allocation  Disk scheduling Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 21  Because secondary storage is used frequently, it must be used efficiently. The entire speed of operation of a computer may hinge on the speeds of the disk subsystem and of the algorithms that manipulate that subsystem. Backups of disk data, seldom-used data, and long-term archival storage are some examples. Some of the functions that operating systems can provide include mounting and unmounting media in devices, allocating and freeing the devices for exclusive use by processes, and migrating data from secondary to tertiary storage. 3. Caching - Caching is an important principle of computer systems. Information is normally kept in some storage system (such as main memory). As it is used, it is copied into a faster storage system—the cache—on a temporary basis. When we need a particular piece of information, we first check whether it is in the cache. If it is, we use the information directly from the cache; if it is not, we use the information from the source, putting a copy in the cache under the assumption that we will need it again soon. In addition, internal programmable registers, such as index registers, provide a high-speed cache for main memory. The programmer (or compiler) implements the register-allocation and register-replacement algorithms to decide which information to keep in registers and which to keep in main memory. W ithout this cache, the CPU would have to wait several cycles while an instruction was fetched from main memory. For similar reasons, most systems have one or more high-speed data caches in the memory hierarchy. Because caches have limited size, cache management is an important design problem. Careful selection of the cache size and of a replacement policy can result in greatly increased performance. Main memory can be viewed as a fast cache for secondary storage, since data in secondary storage must be copied into main memory for use, and data must be in main memory before being moved to secondary storage for safekeeping. The movement of information between levels of a storage hierarchy may be either explicit or implicit, depending on the hardware design and the controlling operating-system software. Data transfer from cache to CPU and registers is usually a hardware function, with no operating-system intervention. In contrast, transfer of data from disk to memory is usually controlled by the operating system. The situation becomes more complicated in a multiprocessor environment where, in addition to maintaining internal registers, each of the CPUs also contains a local cache. Since the various CPUs can all execute concurrently, we must make sure that an update to the value of A in one cache is immediately reflected in all other caches where A resides. This situation is called cache coherency, and it is usually a hardware problem. 4. I/O Systems - One of the purposes of an operating system is to hide the peculiarities of specific hardware devices from the user. For example, in UNIX, the peculiarities of I/O devices are hidden from the bulk of the Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 22  operating system itself by the I/O subsystem. The I/O subsystem consists of several components:  A memory-management component that includes buffering, caching, and spooling.  A general device-driver interface.  Drivers for specific hardware devices Only the device driver knows the peculiarities of the specific device to which it is assigned. Structures: System Services An operating system provides an environment for the execution of programs. It provides certain services to programs and to the users of those programs. The specific services provided, of course, differ from one operating system to another, but we can identify common classes. These operating-system services are provided for the convenience of the programmer, to make the programming task easier. 1. User interface - Almost all operating systems have a user interface (UI). This interface can take several forms. One is a command-line interface (CLI), which uses text commands and a method for entering them (say, a program to allow entering and editing of commands). Another is a batch interface, in which commands and directives to control those commands are entered into files, and those files are executed. Most commonly a graphical user interface (GUI) is used. Here, the interface is a window system with a pointing device to direct I/O, choose from menus, and make selections and a keyboard to enter text. Some systems provide two or all three of these variations. 2. Program execution - The system must be able to load a program into memory and to run that program. The program must be able to end its execution, either normally or abnormally (indicating error). 3. I/O operations - A running program may require I/O, which may involve a file or an I/O device. For specific devices, special functions may be desired (such as recording to a CD or DVD drive or blanking a CRT screen). For efficiency and protection, users usually cannot control I/O devices directly. Therefore, the operating system must provide a means to do I/O. 4. File-system manipulation - The file system is of particular interest. Obviously, programs need to read and write files and directories. They also need to create and delete them by name, search for a given file, and list file information. Finally, some programs include permissions management to allow or deny access to files or directories based on file ownership. 5. Communications - There are many circumstances in which one process needs to exchange information with another process. Such communication may occur between processes that are executing on the same computer or between processes that are executing on different computer systems tied together by a computer network. Communications may be implemented via Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 23  shared memory or through message passing, in which packets of information are moved between processes by the operating system. 6. Error detection - The operating system needs to be constantly aware of possible errors. Errors may occur in the CPU and memory hardware (such as a memory error or a power failure), in I/O devices (such as a parity error on tape, a connection failure on a network, or lack of paper in the printer), and in the user program (such as an arithmetic overflow, an attempt to access an illegal memory location, or a too-great use of CPU time). For each type of error, the operating system should take the appropriate action to ensure correct and consistent computing. Debugging facilities can greatly enhance the user's and programmer's abilities to use the system efficiently. 7. Resource allocation - When there are multiple users or multiple jobs running at the same time, resources must be allocated to each of {hem. Many different types of resources are managed by the operating system. Some (such as CPU cycles, main memory, and file storage) may have special allocation code, whereas others (such as I/O devices) may have much more general request and release code. 8. Accounting - We want to keep track of which users use how much and what kinds of computer resources. This record keeping may be used for accounting (so that users can be billed) or simply for accumulating usage statistics. Usage statistics may be a valuable tool for researchers who wish to reconfigure the system to improve computing services. 9. Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information. When several separate processes execute concurrently, it should not be possible for one process to interfere with the others or with the operating system itself. Protection involves ensuring that all access to system resources is controlled. Security of the system from outsiders is also important. Such security starts with requiring each user to authenticate himself or herself to the system, usually by means of a password, to gain access to system resources.. Interface There are two fundamental approaches for users to interface with the operating system. One technique is to provide a command-line interface or command interpreter that allows users to directly enter commands that are to be performed by the operating system. The second approach allows the user to interface with the operating system via a graphical user interface or GUI. 1. Command Interpreter - Some operating systems include the command interpreter in the kernel. Others, such as Windows XP and UNIX, treat the command interpreter as a special program that is running when a job is initiated or when a user first logs on (on interactive systems). On systems with multiple command interpreters to choose from, the interpreters are known as shells. For example, on UNIX and Linux systems, there are several different shells a user may choose from Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 24  including the Bourne shell, C shell, Bourne-Again shell, the Korn shell, etc. The main function of the command interpreter is to get and execute the next user-specified command. Many of the commands given at this level manipulate files: create, delete, list, print, copy, execute, and so on. The MS-DOS and UNIX shells operate in this way. There are two general ways in which these commands can be implemented. a. In one approach, the command interpreter itself contains the code to execute the command. For example, a command to delete a file may cause the command interpreter to jump to a section of its code that sets up the parameters and makes the appropriate system call. b. An alternative approach—used by UNIX, among other operating systems —implements most commands through system programs. In this case, the command interpreter does not understand the command in any way; it merely uses the command to identify a file to be loaded into memory and executed. 2. Graphical User Interfaces - A second strategy for interfacing with the operating system is through a userfriendly graphical user interface or GUI. Rather than having users directly enter commands via a command- line interface, a GUI allows provides a mouse-based window-and-menu system as an interface. A GUI provides a desktop metaphor where the mouse is moved to position its pointer on images, or icons, on the screen (the desktop) that represent programs, files, directories, and system functions. Depending on the mouse pointer's location, clicking a button on the mouse can invoke a program, select a file or directory—known as a folder— or pull down a menu that contains commands. Graphical user interfaces first appeared due in part to research taking place in the early 1970s at Xerox PARC research facility. The first GUI appeared on the Xerox Alto computer in 1973. The choice of whether to use a command-line or GUI interface is mostly one of personal preference. As a very general rule, many UNIX users prefer a command-line interface as they often provide powerful shell interfaces. Alternatively, most Windows users are pleased to use the Windows GUI environment and almost never use the MS-DOS shell interface. System Calls System calls provide the interface between a process and the operating system. These calls are generally available as assembly-language instructions, and they are usually listed in the various manuals used by the asse mbly- language programmers. Certain systems allow system calls to be made directly from a higher level language program, in this case the calls normally resemble predefined function or subroutine calls. They may generate a call to a special run-time routine that makes the system call. Several languages-such as C, C++, and Perl-have been defined to replace assembly language for systems programming. These languages allow system calls to be made directly. System calls for modern M icrosoft W indows Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 25  platforms are part of the Win32 Application Programmer Interface (API), which is available for use by all the compilers written for Microsoft Windows. The run- time support system (the set of functions built into libraries included with a compiler) for most programming languages provides a much simpler interface. Most of the details of the operating system interface are hidden from the programmer by the compiler and by the run-time support package. System calls occur in different ways, depending on the computer in use. Often, more information is required than simply the identity of the desired system call. The exact type and amount of information vary according to the particular operating system and call. For example, to get input, we may need to specify the file or device to use as the source, and the address and length of user program the memory buffer into which the input should be read. Of course, the device or file and length may be implicit in the call. Three general methods are used to pass parameters to the operating system. The first & simplest approach is to pass the parameters in registers. In the second approach where, there may be more parameters than registers. In these cases, the parameters are generally stored in a block or table in memory and the address of the block is passed as a parameter in a register. In the third approach parameters can also be placed, or pushed, onto the stack by the program, and popped of the stack by the operating system. Some operating systems prefer the block or stack methods, because those approaches do not limit the number or length of parameters being passed. System calls can be grouped roughly into five major categories: Process Control, File Management, Device Management, Information Maintenance, And Communications. 1. Process Control 1. end, abort 2. load, execute 3. create process, terminate process 4. get process attributes, set process attributes 5. wait for time 6. wait event, signal event 7. allocate and free memory A running program needs to be able to halt its execution either normally (end) or abnormally (abort). If a system call is made to terminate the currently running program abnormally, or if the program runs into a problem and causes an error trap, a dump of memory is sometimes taken and an error message generated. The dump is written to disk and may be examined by a debugger to determine the cause of the problem. Under either normal or abnormal circumstances, the operating system must transfer control to the command interpreter. The command interpreter then reads the next command. In an interactive system, the command interpreter simply continues with the next command; it is assumed that the user will issue an appropriate command to respond to any error. In a batch system, the command interpreter usually terminates the entire job and continues with the next job. A process or job executing one program may want to load and execute another program. This feature allows the command interpreter to execute a program as directed by a user command. If we create a new job or process, or even a set of jobs or processes (create process), we should be able to control its execution. This control requires the ability to determine and reset the attributes of a job or Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 26  process, including the job's priority, its maximum allowable execution time, and so on (get process attributes and set process attributes). We may also want to terminate a job or process that we created (terminate process) if we find that it is incorrect or is no longer needed. When new jobs or processes were created, we may need to wait for them to finish their execution. We may want to wait for a certain amount of time (wait time); more likely, we may want to wait for a specific event to occur (W ait event). The jobs or processes should then signal when that event has occurred (signal event). System calls of this type, dealing with the coordination of concurrent processes. 2. File management 1. create file, delete file 2. open, close 3. read, write, reposition 4. get file attributes, set file attributes First need is to create and delete files. Either system call requires the name of the file and perhaps some of the file's attributes. Once the file is created, we need to open it and to use it. We may also read, write, or reposition (rewind or skip to the end of the file, for example). Finally, we need to close the file, indicating that we are no longer using it. We may need these same sets of operations for directories if we have a directory structure for organizing files in the file system. In addition, for either files or directories, we need to be able to de termine the values of various attributes, and perhaps to reset them if necessary. File attributes include the file name, a file type, protection codes, accounting information, and so on. At least two system calls, get file attribute and set file attribute, are required for this function. 3. Device management 1. request device, release device 2. read, write, reposition 3. get device attributes, set device attributes 4. logically attach or detach devices A program, as it is running, may need additional resources to proceed. Additional resources may be more memory, tape drives, access to files, and so on. If the resources are available, they can be granted, and control can be returned to the user program; otherwise, the program will have to wait until sufficient resources are available. Files can be thought of as abstract or virtual devices. Thus, many of the system calls for files are also needed for devices. If the system has multiple users, we must first request the device, to ensure exclusive use of it. After we are finished with the device, we must release it. These functions are similar to the open and close system calls for files. Once the device has been requested and allocated to us, we can read, write, and reposition the device, just as ordinary files. In fact, the similarity between I/O devices and files is so great that many operating systems, including UNIX and MS-DOS, merge the two into a combined file-device structure. 4. Information maintenance 1. get time or date, set time or date 2. get system data, set system data Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 27  3. get process, file, or device attributes 4. set process, file, or device attributes Many system calls exist simply for the purpose of transferring information between the user program and the operating system. For example, most systems have a system call to return the current time and date. Other system calls may return information about the system data, such as the number of current users, the version number of the operating system, the amount of free memory or disk space, and so on. In addition, the operating system keeps information about all its processes, and there are system calls to access this information. There are also system calls to reset the process information (get process attributes and set process attributes). 5. Communications 1. create, delete communication connection 2. send, receive messages 3. transfer status information 4. attach or detach remote devices There are two common models of communication. Both of these methods are common in operating systems, and some systems even implement both. M essage passing is useful when smaller numbers of data need to be exchanged, because no conflicts need to be avoided. It is also easier to implement shared memory for inter-computer communication. Shared memory allows maximum speed and convenience of communication, as it can be done at memory speeds when within a computer. Problems exist, in the areas of protection and synchronization. The two communications models are contrasted in Figure. Figure 11. Communications models. (a) Message passing. (b) Shared memory. In the message-passing model, information is exchanged through an inter- process-communication facility provided by the operating system. Before communication can take place, a connection must be opened. The name of the other communicator must be known and another process should be on the same CPU, or a process on another computer connected by a communications network. Each computer in a network has a host name, such as an IP name, by which it is commonly known. Similarly, each process has a process name, which is translated into an equivalent identifier by the operating system using get hostid and get processid system calls. These identifiers are then passed to the general- purpose open and close calls provided by the file system, or to specific open connection and close connection system calls. The recipient process usually must give its permission for communication to take place with an accept connection call. They execute a wait for connection call and are awakened when a connection is made. The source of the communication, known as the client, and the receiver known as a server, then exchange messages by read message and Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 28  write message system calls. The close connection call terminates the Communication. In the shared-memory model, processes use map memory system calls to gain access to regions of memory owned by other processes. Normally, the operating system tries to prevent one process from accessing another process memory. Shared memory requires that several processes agree to remove this restriction. They may then exchange information by reading and writing data in these shared areas. The form of the data and the location are determined by these processes and are not under the operating system's control. The processes are also responsible for ensuring that they are not writing to the same location simultaneously. System Program Another aspect of a modern system is the collection of system programs, used at the lowest level is hardware. Next to the operating system is the system programs, and finally the application programs. System programs provide a convenient environment for program development and execution. Some of them are simply user interfaces to system calls; others are considerably more complex. They can be divided into these categories: 1. File management: These programs create, delete, copy, rename, print, dump, list, and generally manipulate files and directories. 2. Status information: Some programs simply ask the system for the date, time, amount of available memory or disk space, number of users, or similar status information. That information is then formatted, and is printed to the terminal or other output device or file. 3. File modification: Several text editors may be available to create and modify the content of files stored on disk or tape. 4. Programming-language support: Compilers, assemblers, and interpreters for common programming languages (such as C, C++, Java, Visual Basic, and PERL) are often provided to the user with the operating system. Some of these programs are now priced and provided separately. 5. Program loading and execution: Once a program is assembled or compiled, it must be loaded into memory to be executed. The system may provide absolute loaders, relocatable loaders, linkage editors, and overlay loaders. Debugging systems for either higher-level languages or machine language are needed also. 6. Communications: These programs provide the mechanism for creating virtual connections among processes, users, and different computer systems. They allow users to send messages to one another's screens, to browse web pages, to send electronic-mail messages, to log in remotely, or to transfer files from one machine to another. Most operating systems are supplied with programs that solve common problems, or perform common operations. Such programs include web browsers, word Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 29  processors and text formatters, spreadsheets, database systems, compiler compilers, plotting and statistical-analysis packages, and games. These programs are known as system utilities or application programs. Perhaps the most important system program for an operating system is the command interpreter, the main function of which is to get and execute the next user- specified command. Many of the commands given at this level manipulate files: create, delete, list, print, copy, execute, and so on. These commands can be implemented in two general ways. In one approach, the command interpreter itself contains the code to execute the command. For example, a command to delete a file may cause the command interpreter to jump to a section of its code that sets up the parameters and makes the appropriate system call. An alternative approach used by UNIX, among other operating systems -implements most commands by system programs. In this case, the command interpreter does not understand the command in any way; it merely uses the command to identify a file to be loaded into memory and executed. Approach to the design of a command interpreter has problems. First, because the code to execute a command is a separate system program, the operating system must provide a mechanism for passing parameters from the command interpreter to the system program. Because the command interpreter and the system program may not be in memory at the same time, and the parameter list can be extensive. Also, it is slower to load a program and to execute it than simply to jump to another section of code within the current program. Another problem is that the interpretation of the parameters is left up to the programmer of the system program. Thus, parameters may be provided inconsistently across programs that appear similar to the user, but were written at different times by different programmers. Design-structure Design Goals - The first problem in designing a system is to define goals and specifications. At the highest level, the design of the system will be affected by the choice of hardware and the type of system: batch, time shared, single user, multiuser, distributed, real time, or general purpose. Beyond this highest design level, the requirements may be much harder to specify. The requirements can, however, be divided into two basic groups: user goals and system goals. a. Users desire certain obvious properties in a system: The system should be convenient to use, easy to learn and to use, reliable, safe, and fast. Of course, these specifications are not particularly useful in the system design, since there is no general agreement on how to achieve them. b. A similar set of requirements can be defined by those people who must design, create, maintain, and operate the system: The system should be easy to design, implement, and maintain; it should be flexible, reliable, error free, and efficient. Again, these requirements are vague and may be interpreted in various ways There is, in short, no unique solution to the problem of defining the requirements for an operating system. The wide range of systems in existence Prepared By: Prof. Dheresh Soni – VIT Bhopal University Page 30  shows that different requirements can result in a large variety of solutions for different environments. For example, the requirements for VxWorks, a realtime operating system for embedded systems, must have been substantially different from those for MVS, a large multiuser, multi-access operating system for IBM mainframes. Specifying and designing an operating system is a highly creative task. General principles have been developed in the field of software engineering, and we turn now to a discussion of some of these principles. M echanisms and Policies - One important principle is the separation of policy from mechanism. Mechanisms determine how to do something; policies determine what will be done. For example, the timer construct is a mechanism for ensuring CPU protection, but deciding how long the timer is to be set for a particular user is a policy decision The separation of policy and mechanism is important for flexibility. Policies are likely to change across places or over time. In the worst case, each change in policy would require a change in the underlying mechanism. A general mechanism insensitive to changes in policy would be more desirable. A change in policy would then require redefinition of only certain parameters of the system. For instance, consider a mechanism for giving priority to certain types of programs over others. If the mechanism is properly separated from policy, it can be used to support a policy decision that I/O-intensive programs should have priority over CPU-intensive ones or to support the opposite policy. For example - Microkernel-based operating systems take the separation of mechanism and policy to one extreme by implementing a basic set of primitive building blocks. Implementation - Once an operating system is designed, it must be implemented. Traditionally, operating systems have been written in assembly language. Now, however, they are most commonly written in higher-level languages such as C or C++. The first system that was not written in assembly language was probably the M aster Control Program (MCP) for Burroughs computers. MCP was written in a variant of ALGOL. The Linux and W indows XP operating systems are written mostly in C, although there are some small sections of assembly code for device drivers and for saving and restoring the state of registers. The advantages of using a higher-level language 1. Implementing operating systems are the same as those accrued when the language is used for application programs. 2. The code can be written faster 3. More compact