Explanation of the DOS command. It is CONFIG.SYS.

This command will cause part of MS-DOS to be loaded into
the high-memory-area (HMA) and to possibly keep a link between
upper and lower memory.

Switches:    umb/noumb  Umb causes a link between upper and lower 
                        memory. Noumb disconnects this link.
             high/low   this will cause DOS to load into HMA or 
                        not, respectively.
Example:
       To load DOS into HMA and to keep a link betwen upper and lower
       memory you include this line in your CONFIG.SYS file:
             dos=high, umb