| File Control Block |
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The FCB originates from CP/M and is also present in all versions of MS-DOS . A full FCB is 36 bytes long; in early versions of CP/M, it was 33 bytes. The meanings of several of the fields in the FCB differ between CP/M and MS-DOS, and also depending on what operation is being performed. The following fields have consistent meanings: Offset Size Contents - 00 Byte Drive number - 0 for default, 1 for A:, 2 for B:,... 01 8 bytes File name } 09 3 bytes File type } together these form an 8.3 name. 0C 20 bytes Implementation dependent - should be initialised to zero before the FCB is opened. 20 1 byte Record number in the current section of the file - used when performing Sequential Access . 21 3 bytes Record number to use when performing Random Access . USAGE In CP/M and MS-DOS 1 (which did not include support for Directories ), the FCB was the only method of accessing files. When directories were introduced in MS-DOS 2, FCBs were superseded by File Handle s. FCBs were supported in all versions of MS-DOS and Windows until the introduction of the FAT32 filesystem.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. Under versions based on Windows NT , FCBs work correctly whatever underlying filesystem is in use. |
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