Т.А. Волошина, Л.Б. Саратовская - English Reader in Computer Science (1098536), страница 7
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An operating system (commonly abbreviated to either OS or O/S) is an interface between hardware and user. An OS is responsible for the management and coordination of activities and the sharing of the resources of the computer. The operating system acts as a host for computing applications run on the machine. As a host, one of the purposes of an operating system is to handle the details of the operation of the hardware. This relieves application programs from having to manage these details and makes it easier to write applications. Almost all computers (including handheld computers, desktop computers, supercomputers, video game consoles) as well as some robots, domestic appliances (dishwashers, washing machines), and portable media players use an operating system of some type. Some of the oldest models may however use an embedded operating system that may be contained on a compact disk or other data storage device.
The operating system also acts as an interface between an application and the hardware. The user interacts with the hardware from “the other side”. The operating system is a set of services which simplifies the development of applications. Executing a program involves the creation of a process by the operating system. The kernel creates a process by assigning memory and other resources, establishing a priority for the process (in multi-tasking systems), loading program code into memory, and executing the program. The program then interacts with the user and/or other devices and performs its intended function.
Operating systems offer a number of services to application programs and users. Applications access these services through application programming interfaces or system calls. By invoking these interfaces, the application can request a service from the operating system, pass parameters, and receive the results of the application. Users may also interact with the operating system with some kind of software user interface.
For hand-held and desktop computers, the user interface is generally considered part of the operating system. On large multi-user systems the user interface is generally implemented as an application program that runs outside the operating system.
Not all the computers require operating systems. Computers made to perform moderate number of simple tasks do not need to be controlled by the operating system, as they have all the needed mechanisms of communicating with an outer world. For example, a small keyboard and an LCD display is enough for a microwave oven to interface with the user.
The use of operating systems on more complex computational systems is a way to lesser complexity and greater efficiency. All desktop computers are usually equipped with operating systems. The most well-known operating systems are the Windows family and Macintosh operating systems. Special-purpose computer systems use their own unique operating systems.
History
Proprietary operating systems were made to sell the company’s hardware. Without system software (compilers and operating systems), a hardware developer had greater difficulty launching a computer. The availability of operating systems was not tied to a single hardware supplier. The invention of operating systems could easily start producing hardware on which buyers could use standard software. Through the 1950s, many major features were pioneered in the field of operating systems. The development of the IBM System/360 produced a family of mainframe computers for which a single operating system OS/360 was planned.
IBM’s current mainframe operating systems are distant descendants of this original system and applications written for the OS/360 can still be run on modern machines.
In the late 1960s through the late 1970s, several hardware capabilities evolved that allowed similar or ported software to run on more than one system. The operating systems (KRONOS, NOS) developed during the 1970s, supported simultaneous batch and timesharing use.
The enormous investment in software made since 1960s caused most of the original computer manufacturers to continue to develop compatible operating systems along with the hardware.
Functions
An operating system has three main functions:
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Manage the computer’s resources, such as the central processing unit, memory, disk drives, and printers.
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Establish a user interface.
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Execute and provide services for applications software.
It should be taken into account, that much of the work of an operating system is hidden from the user. For example, managing the computer’s resources is done without the user being aware of the details.
Operating system must be able to do two things: manage hardware and software of the computer and provide common interfaces for applications to work with hardware devices. The first task appears, because various applications tend to use the CPU, memory, input/output system and other common hardware at the same time, and operating system must be a judge in solving different conflicts. Performing the second task by providing a special application programming interface is useful when portability is mentioned, for software written without straight dependences on hardware part of a computer system is of a higher level of confidence that it will run on another system.
There are four basic types of operating systems:
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Real-time operating systems are used to control machinery, scientific instruments and industrial systems. A very important part of real-time operating systems is managing the resources of the computer in such a way, that a particular operation is performed similar every other time it occurs, i.e. without dependency on some resources availability.
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Single-user single-task operating systems are used when only one application is needed to be performed at one time. Pocket-sized computers are equipped with such OSes.
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Single-user multi-task operating systems are mostly used on desktop and laptop computer systems. Windows 95, 98, Me and the MacOs are examples of operating systems that will let a single user have several programs in operation at the same time.
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Multi-user operating systems are broadly used in computer networks, allowing many different users to take advantage of the computer’s resources simultaneously. The requirements of the users must be controlled for balance, and one user’s problems should not affect the entire network. Unix and VMS are examples of multi-user OSes.
Single-user OSes with network support and multi-user systems are not the same things. Single-user OS calls the administrator “user”, while other use hardware sharing mechanisms.
Features
Interrupts
Interrupts are central to operating systems, since they provide an efficient way for the operating system to interact with and react to its environment. Interrupt-based programming is directly supported by most CPUs. Interrupts provide a computer with a way of automatically running specific code in response to events. Even very basic computers support hardware interrupts, and allow the programmer to specify code which may be run when that event takes place.
When an interrupt is received, the computer’s hardware automatically suspends whatever program is currently running, saves its status, and runs computer code previously associated with the interrupt. In modern operating systems, interrupts are handled by the operating system’s kernel. Interrupts may come from either the computer’s hardware or from the running program.
Protected Mode and Supervisor Mode
Modern CPUs support something called dual mode operation. CPUs with this capability use two modes: protected mode and supervisor mode, which allow certain CPU functions to be controlled and affected only by the operating system kernel. However, the term is used more generally in operating system theory to refer to all modes which limit the capabilities of programs running in that mode, providing things like virtual memory addressing and limiting access to hardware in a manner determined by a program running in supervisor mode. However, when the operating system passes control to another program, it can place the CPU into protected mode.
In protected mode, programs may have access to a more limited set of the CPUs instructions. In this way the operating system can maintain exclusive control over things like access to hardware and memory.
Memory Management
A multiprogramming operating system kernel is responsible for managing all system memory which is currently in use by programs. This ensures that a program does not interfere with memory already used by another program. Since programs time share, each program must have independent access to memory.
Cooperative memory management, used by many early operating systems assumes that all programs make voluntary use of the kernel’s memory manager, and do not accept their allocated memory. With cooperative memory management it takes only one misbehaved program to crash the system.
Memory protection enables the kernel to limit a process access to the computer’s memory.
Various methods of memory protection exist, including memory segmentation and paging.
Multitasking
Multitasking refers to the running of multiple independent computer programs on the same computer, giving the appearance that it is performing the tasks at the same time. Since most computers can do at most one or two things at one time, this is generally done via time sharing, which means that each program uses a share of the computer’s time to execute.
An operating system kernel contains a piece of software called a scheduler which determines how much time each program will spend executing, and in which order execution control should be passed to programs. Control is passed to a process by the kernel, which allows the program access to the CPU and memory. At a later time control is returned to the kernel through some mechanism, so that another program may be allowed to use the CPU. This, so-called passing of control between the kernel and applications, is called a context switch.
Networking
Currently most operating systems support a variety of networking protocols, hardware, and applications for using them. This means that computers running dissimilar operating systems can participate in a common network for sharing resources such as computing, files, printers, and scanners using either wired or wireless connections. Networks can essentially allow a computer’s operating system to access the resources of a remote computer to support the same functions as it could if those resources were connected directly to the local computer. This includes everything from simple communication to using network file systems or even sharing another computer’s graphics or sound hardware. Many operating systems support one or more vendor-specific or open networking protocols.
Security
A computer being secure depends on a number of technologies working properly. A modern operating system provides access to a number of resources, which are available to software running on the system, and to external devices like networks via the kernel.
The operating system must be capable of distinguishing between requests which should be allowed to be processed, and others which should not be processed. While some systems may simply distinguish between “privileged” and “non-privileged”, systems commonly have a form of requester identity, such as user name. To establish identity there may be a process of authentication. Often a username must be quoted, and each username may have a password. Some other methods of authentication, such as magnetic cards or biometric data, might be used instead.
There are two main types of security - internal and external security. Internal security, or security from an already running program, is only possible if all possibly harmful requests must be carried out through interrupts to the operating system kernel. Internal security is especially relevant for multi-user systems; it allows each user of the system to have private files that the other users cannot tamper with or read.
External security involves a request from outside the computer, such as a login at a connected console or some kind of network connection. External requests are often passed through device drivers to the operating system’s kernel, where they can be passed onto applications, or carried out directly.
Examples of Operating Systems
Microsoft Windows
Microsoft Windows is a family of proprietary operating systems that originated as an add-on to the older MS-DOS operating system for the IBM PC. The most widely used version of the Microsoft Windows family is Windows XP, released in October 2001. Modern versions are based on the newer Windows NT kernel that was originally intended for OS/2.
The first Windows series were not true multi-user operating systems. In addition, they implemented only partial memory protection. They were accordingly widely criticized for the lack of security.
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