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Harmonization of speed and cache data exchange When data is always a challenge to harmonize speed. For example, if one user process produces some of the data and transmit them to another user through the process of RAM, in general, the speed of data generation and reading do not match. Harmonization speed is usually achieved by buffering data in RAM and synchronization of processes to access the buffer. In the I / O subsystem to coordinate the exchange rate is also commonly used data buffering in memory. In those specialized operating systems, which provide high-speed I / O is a priority (Real-time services network file services, etc.), much of RAM are not under the code of applications, and by buffering data. But buffering only on the basis of RAM in the I / O subsystem is not is the difference between the exchange rate and operational memory, which processes data processing facilities, and the speed of external devices often too large to a temporary buffer could be operational memory-volume may simply not enough. For such cases should be special measures, and is often used as a buffer disk file, also called spul- file (from the spool - spool, the same buffer, only to train). A typical example of an organization spulinga output to the printer. To print documents in dozens of megabytes is not unusual, therefore, for temporary storage (and print any document takes a few minutes to tens of minutes) of RAM is not enough. Another solution to this problem is to use a large buffer memory controllers external devices. This approach is especially useful when placing data on disk too slow exchange (or the data displayed on the disc itself). For example, the graphics display controllers used buffer memory, commensurate with the volume of operational, and it has accelerated withdrawal schedules on the screen. Buffering data not only agree on the speed of the processor and external devices, but also to other tasks, reduce the number of actual I / O operations through data caching. The disk cache is a feature I / O subsystems virtually all operating systems, significantly reducing time access to the stored data. Sharing devices and data between processes I / O devices can be processes in a monopoly, and the joint use of (shared). This OS should provide access control by the same means as in access to other resources of computer systems by testing a user or group of users on whose behalf a process to carry out an operation of the device. For example, a particular group of users allowed to capture the serial port of monopoly ownership, and other users is prohibited. The operating system can control access not only to the device as a whole, but to individual portion of data stored or displayed this device. The disc is a typical example of the device for which it is important to control access to the device is not as a whole and to individual directories and files. In finding information on graphic display windows of the screen are also a resource, which must be a kind of ongoing access to the system processes. It is for each portion of the device or data may be put to the right, not directly connected with access to the device as a whole. For example, the file system, usually for each directory and file can be set individual access rights. Obviously, joint access to parts of the device or the data stored on it, a prerequisite for the job is a shared device as a whole. The same device in different time periods can be used as shared, and monopoly regimes. Nevertheless, there are devices, which are usually characterized by one of these regimes, such as sequential alphanumeric ports and terminals are used in the monopoly regime, and drive-line sharing. The operating system should provide the devices in both modes, providing tracking procedures capture and release devices used monopoly, in the case of sharing of optimizing the sequence of I / O operations for the various processes to improve overall productivity, if possible. For example, data from several processes disc can be streamlined so the sequence of operations that unproductive time on the movement of heads significantly reduced (although for some of the processes may slow I / O operations). In the division between devices may be needed in distinguishing portion of the two processes from each other. Normally such a need arises to share the so-called serial devices, data which, unlike direct access devices not addressed. Typical representatives of such devices is a printer that is not allocated in the possession of monopoly power, and at the same time each document will be published as a coherent set of pages. For these devices is organized mainly by finding jobs, each job is a portion of data that can break, such as paper for printing. To keep jobs queue used spul- file, which both agree on the speed of the printer and memory and provides logical segmentation data portion. As spul- file is shared direct access devices, the processes can simultaneously output to the printer, placing data in its section spul- file. Providing convenient logical interface between devices and the rest of the system Variety of I / O devices made especially urgent as a masking OS logical interface between peripheral devices and applications. Virtually all modern operating systems support as a basis for such an interface file model of peripheral devices when any device looks for a consistent set of application programmer bytes, which is working with the unified system calls (for example, read and write), asking the name of the device and the debug output log file shifting from the start byte. To sustain such an interface I / O subsystem lot of work to be done, given the difference in data operations, such as the hard disk and graphics terminal. The attractiveness of the model debug output log file device is its simplicity and uniformity to the devices of any kind, but in many cases for programming some of Input / Output devices, it is too poor. Therefore, the model is often used as the only basis on which I / O subsystem builds more content model device type. I / O subsystem provides, as a rule, a specific interface to display graphical information on the screen or printer, for programming operations networking, etc. This specific interface developer can always build on the existing base interface.
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