File Control Block: Difference between revisions

Revision as of 17:57, 24 March 2013

Revision as of 21:42, 17 July 2013

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==Usage==

In CP/M and MS-DOS 1 (which did not include support for [[directory (file systems)|subdirectories]]), the FCB was the only method of accessing files. When subdirectories were introduced in MS-DOS 2, FCBs proved too small to handle the extra data required for that, and thus were superseded by [[file handledescriptors]]s, as used on [[UNIX]] and its derivatives. File handles are simply consecutive integer numbers associated with specific open files.

If a program uses the newer file handle API to open a file, the operating system will manage its internal data structure associated with that file in its own memory area. This has the great advantage that these structures can grow in size in later operating system versions without breaking compatibility with application programs; its disadvantage is that, given the rather simplistic [[memory management]] of MS-DOS and its compatibles, space for as many of these structures as the most "file-hungry" program is likely to use has to be reserved at boot time and cannot be used for any other purpose while the computer is running. Such memory reservation is done using the FILES= command in the [[CONFIG.SYS]] file. This problem does not occur with FCBs in DOS 1 or in CP/M, since the operating system stores all that it needs to know about an open file inside the FCB and thus does not need to use any per-file memory in operating system memory space. When using FCBs in MS-DOS 2 or later, the FCBs are dummies that store references to the actual file management data in operating system memory. Because of this, the number of FCBs which can be kept open at once in DOS 2 or higher is limited as well, usually to 4; using the FCBS= command in the CONFIG.SYS file, it may be increased beyond that number if necessary.