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Organization of the file system

on the fixed-size blocks, and can directly access to any cluster (organize direct access to the files). Immediately devices (disks) works part of the OS, called input-output system (see the relevant chapter). System I / O (it consists of a device driver and instances interruption to transfer information between memory and disk system) makes available a very high level of component-OS file system used disk space continuum in the form of a fixed block size. The system I / O is dealing with the physical disk blocks, which are characterized by address, for example, disk 2, cylinder 75, sector 11. File system deals with a logical blocks, each with a number (from 0 or 1 to N). The size of these logical units file the same or a multiple of the block size of the physical disk, and can be set equal to the size of the virtual memory page supported by sensors in conjunction with the computer operating system. The structure of the file management system can be a base subsystem, which is responsible for allocating disk space specific files, and more high-level logical subsystem, which uses a directory tree structure to provide the base module subsystems based on the information it needed from the symbolic name of the file. It is also responsible for the authorization to access files (see Chapter Safety OS). In modern OS taken further divide logical disks drives (it is also a low-level operation), sometimes called the sections (partitions). Sometimes, on the contrary that combine several physical disks into a single logical disk (such as this can be done in Windows NT OS). Each section can have its independent file system. Therefore, we will be presenting to ignore the issue of allocation of physical space for files and consider that each section is a separate disk (virtual). Indeed CD is a hierarchical tree-like structure consisting of a set of files, each of which is the repository of user data, and directories, or directory (ie files, which contain a list of other files belonging to the directory), which are necessary for the storage of information on the files. The standard request for the opening of (open) or the creation of the file (creat) comes from the application of logical subsystem. Logical subsystem using the directory structure, checks the access and is a basic subsystems for accessing blocks file. Since then, the file is considered to be open, is contained in Table open files, application receives at its disposal a descriptor (or handle in the Microsoft) of the file. File descriptor file is a reference to the table in the open files and used in a query application of the read-write the file. Writing in the table shows open files through the allocation of blocks on the disk blocks of the file. If, at the opening of the file is in use by another process that is contained in table open files, then, after checking permissions to the file can be organized shared. This new process also returned descriptor-a reference to the file in the table of open files. Next analyzed in detail in the text of the most important work of system calls.
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