File Control Block: Difference between revisions

Revision as of 21:06, 13 August 2015

Revision as of 19:45, 14 August 2015

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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,<ref name="Duncan_1988_MS-DOS_Encyclopedia"/><ref name="Chappell_1994_DOS-Internals"/> use other filesystems<ref name="Duncan_1988_MS-DOS_Encyclopedia"/><ref name="Chappell_1994_DOS-Internals"/> than FAT or to share files<ref name="Chappell_1994_DOS-Internals"/> over networks in the future, FCBs were felt to be too small to handle the extra data required for such features<ref name="Chappell_1994_DOS-Internals"/> and therefore FCBs were seen as inadequate for various future expansion paths.<ref name="Duncan_1988_MS-DOS_Encyclopedia"/> Also, they didn't provide a field to specify sub-directories.<ref name="Duncan_1988_MS-DOS_Encyclopedia"/> Exposing file system related data to user-space was also seen as a security risk.<ref name="Chappell_1994_DOS-Internals"/> FCBs were thus superseded by [[file handle]]s, as used on [[UNIX]] and its derivatives.<ref name="Duncan_1988_MS-DOS_Encyclopedia"/> 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 (CONFIG.SYS directive)|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 23 or later, the FCBsFCB areformat dummiesdepends thaton storeif referencesSHARE.EXE tois loaded and if the actualFCB refers to a local or remote file managementand dataoften inrefer to operatinga systemSFT memoryentry. Because of this, the number of FCBs which can be kept open at once in DOS 23 or higher is limited as well, usually to 4; using the [[FCBS (CONFIG.SYS directive)|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.<ref name="Paul_1997_NWDOSTIP"/>

FCBs were supported in all versions of MS-DOS and [[Microsoft Windows|Windows]] until the introduction of the [[FAT32]] filesystem. [[Windows 95]], [[Windows 98]] and [[Windows Me]] do not support the use of FCBs on FAT32 drives due to its 32-bit cluster numbers,<ref name="Chappell_1994_DOS-Internals"/><!-- While this source predates the introduction of FAT32, it discusses the various real and potential future limitations of the FCB approach and the cumbersome workarounds introduced in DOS 2, DOS 4 and DOS 5 to keep FCBs alive for a bit longer --> except to read the volume label. This caused some old DOS applications, including [[Wordstar]], to fail under these versions of Windows.