Source : Free On-Line Dictionary of Computing

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
     
        	FCBS=x,y
     
        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.
     
        (1995-03-21)