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Systems of real time are applied to management of various technical objects, such, for example, as the machine, the satellite, scientific experimental installation or technological processes, such as the galvanic line, blast-furnace process, etc. In all these cases there is maximum permissible time during which this or that program operating object otherwise has to be executed there can be an accident: the satellite will leave a visibility range, the experimental data arriving from sensors will be lost, thickness of electroplated coating will not meet standard. Thus, criterion of efficiency for systems of real time is their ability to maintain in advance set time intervals between start of the program and receiving result (the operating influence). This time is called as time of reaction of system, and the corresponding property of system - reactivity. For these systems multiprogram mix represents the fixed set of in advance developed programs, and the program choice on performance is carried out proceeding from current state of object or according to the schedule of planned works.

Limited means for use as an application server: has no means of virtual memory and the forcing-out multitasking, and support of symmetric multiprocessing was absent till the most recent time. There are no API of the main operational environments used for applications programming - UNIX, Windows, OS/2

Such network OS as Banyan Vines, Novell NetWare x, IBM LAN Server, Sun NFS, Microsoft LAN Manager and Windows NT Server, can serve as an operating system of the enterprise while OS NetWare x, Personal Ware, Artisoft LANtastic are suitable for the small working groups more.

Network OS can be divided into two groups: scale of department and scale of the enterprise. OS for departments or the working groups provide a set of network services, including division of files, appendices and printers. They also have to provide properties of fault tolerance, for example, to work with RAID massifs, to support cluster architecture. Network OS of departments are usually simpler in installation and management in comparison with network OS of the enterprise, they have less functional properties, they protect data less and have weaker opportunities for interaction with other types of networks, and also the worst productivity.

Operating systems can differ with features of realization of internal algorithms of management of the main resources of the computer (processors, memory, devices), features of the used design methods, types of hardware platforms, areas of use and many other properties.

Hardware on which it is focused have direct impact on properties of an operating system. As the equipment distinguish operating systems of personal computers, minicomputers, mainframes, clusters and the COMPUTERS networks. Among the listed types of computers can meet both single-processor options, and the multiprocessor. Anyway specifics of hardware are, as a rule, reflected in specifics of operating systems.

Important property of operating systems is possibility of parallelization of calculations within one task. Mnogonitevy OS divides processor time not between tasks, and between their separate branches (threads).

Today there is standard an introduction to OS of functions of support of the multiprocessor data processing. Such functions are available in the Solaris x operating systems of firm Sun, Open Server x of the company Santa Crus Operations, OS/2 of firm IBM, Windows NT of Microsoft and NetWare 1 firm of Novell firm.

A big variety of types of the computers used in computer networks involves a variety of operating systems: for workstations, for servers of networks of level of department and servers of level of the enterprise in general. Various requirements for productivity and functionality can be imposed to them, it is desirable that they possessed property of compatibility which would allow to ensure collaboration of various OS.

Some operating systems can combine in themselves properties of systems of different types, for example, the part of tasks can be carried out in the mode of batch operation, and part - in real time or in the mode of division of time. In such cases the mode of batch operation is called often the background mode.