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, 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, led 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: The 20-byte-long field starting at offset 0x0C contained fields which (among others) provided further information about the file: Further values were used by newer versions of DOS until new information could no longer fit in these 20 bytes. Some preceding "negative offset" bytes were squeezed from reserved spaces in CP/M Zero Page and DOS Program Segment Prefix for storing file attributes. Usage In CP/M, 86-DOS and PC DOS 1.x/MS-DOS 1.xx, the FCB was the only method of accessing files. Under DOS a few INT 21h subfunctions provided the interface to operate on files using the FCB. 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 FCBs were thus superseded by file handles, as used on UNIX and its derivatives.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