File control block

A File Control Block (FCB) is a file system structure in which the state of an open file is maintained. A FCB is managed by the operating system, but it resides in the memory of the program that uses the file, not in operating system memory. This allows a process to have as many files open at one time as it wants to, provided it can spare enough memory for an FCB per file.

The FCB originates from CP/M and is also present in most variants of DOS, though only as a backwards compatibility measure in MS-DOS versions 2.0 and later. A full FCB is 36 bytes long; in early versions of CP/M, it was 33 bytes. This fixed size, which could not be increased without breaking application compatibility, lead to the FCB's eventual demise as the standard method of accessing files.

The meanings of several of the fields in the FCB differ between CP/M and DOS, and also depending on what operation is being performed. The following fields have consistent meanings:

In CP/M, 86-DOS and PC DOS 1.x/MS-DOS 1.xx, the FCB was the only method of accessing files. When, with MS-DOS 2, preparations were made to support multiple processes or users, use other filesystems than FAT or to share files over networks in the future, FCBs were felt to be too small to handle the extra data required for such features and therefore FCBs were seen as inadequate for various future expansion paths. Also, they didn't provide a field to specify sub-directories. Exposing file system related data to user-space was also seen as a security risk. FCBs were thus superseded by file handles, as used on UNIX and its derivatives.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 DOS, 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= directive 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 3 or later, the FCB format depends on if SHARE.EXE is loaded and if the FCB refers to a local or remote file and often refer to a SFT entry. Because of this, the number of FCBs which can be kept open at once in DOS 3 or higher is limited as well, usually to 4; using the FCBS= directive in the CONFIG.SYS file, it may be increased beyond that number if necessary. Under DR-DOS, both FILES and FCBS come from the same internal pool of available handles structures and are assigned dynamically as needed.

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