    FCBS=x,y

This line allows DOS to support file control blocks (FCB). Very early versions
of DOS controlled file access with FCBs. Later versions use file handles,
instead. New programs use file handles, but some older ones still use FCBs, so
DOS still supports their use. They are also used to control file sharing in a
network. 

x specifies the number of FCBS, minimum of 1, maximum of 255 (default 4).

y specifies the number of FCBS that CANNOT be closed by DOS unless a program
    running on your computer requests their closure. This prevents a program
    on another computer in a network (enabled by the SHARE command) from
    closing a file you want to remain open.

x should be > than y by 8 or more. If you set x ó y, DOS probably won't run on
a network at all. If an old program using FCBs is run on a computer which uses
DOS 4.0 or later and a disk partition of ò 32 Mbytes (32 million characters),
the SHARE command must be executed before using the program, or it won't work.
