Usage
In CP/M and MS-DOS 1 (which did not include support for 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were introduced in MS-DOS 2, FCBs proved too small to handle the extra data required for that, and thus were superseded by file handles, as used on UNIX and its derivatives.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 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.
FCBs were supported in all versions of MS-DOS and 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, except to read the volume label. This caused some old DOS applications, including Wordstar, to fail under these versions of Windows.
The FCB interface does not work properly on Windows NT, 2000, etc. either - WordStar does not function properly on these operating systems. DOS emulators DOSEMU and DOSBox implement the FCB interface properly, thus they are a way to run older DOS programs that need FCBs on modern operating systems.
Read more about this topic: File Control Block
Famous quotes containing the word usage:
“...Often the accurate answer to a usage question begins, “It depends.” And what it depends on most often is where you are, who you are, who your listeners or readers are, and what your purpose in speaking or writing is.”
—Kenneth G. Wilson (b. 1923)
“Pythagoras, Locke, Socrates—but pages
Might be filled up, as vainly as before,
With the sad usage of all sorts of sages,
Who in his life-time, each was deemed a bore!
The loftiest minds outrun their tardy ages.”
—George Gordon Noel Byron (1788–1824)
“Girls who put out are tramps. Girls who don’t are ladies. This is, however, a rather archaic usage of the word. Should one of you boys happen upon a girl who doesn’t put out, do not jump to the conclusion that you have found a lady. What you have probably found is a lesbian.”
—Fran Lebowitz (b. 1951)