file control block
(FCB) An MS-DOS data structure that stores information about an open file. The number of FCBs is configured in CONFIG.SYS with a command
where x (between 1 and 255 inclusive, default 4) specifies the number of file control blocks to allocate and therefore the number of files that MS-DOS can have open at one time. y (not needed from DOS 5.0 onward) specifies the number of files to be closed automatically if all x are in use.