Compatibility: 5.0+
Purpose: On 286- and 386-based and better computers, this command provides
a way to tell DOS to load itself in the High Memory Area ,
outside of the 640K of conventional  memory.  On 386-based and
better computers, it also provides a way to enable loading of
device drivers and TSRs into UMBs .

Uses: Maximize the amount of conventional memory available to
applications.

Syntax: DOS=HIGH[,UMB or NOUMB]
or: DOS=LOW[,UMB or NOUMB]
or: DOS=UMB or NOUMB

  Parameters  

HIGH sets DOS to use the 64K HMA  to hold most of its code and data
areas.  This option works on 286-based or better computers.  LOW
is the default.

UMB lets DOS load TSRs and device drivers into UMB  memory via the
DEVICEHIGH= and Loadhigh commands and makes upper memory
available for applications which know how to use it.  This option
works on 386-based or better computers.  NOUMB is the default.

  TECH Notes  

  When DOS=UMB is used, DOS uses fn 5803H (set upper-memory link state)
to enable access to UMBs.  In order to load programs into UMBs or
allocate UMBs for your programs, you may also need to use fn 5801H (set
memory allocation strategy).

  See Accessing Upper Memory for info on allocating memory in UMBs.

  It is okay to have DOS= twice in CONFIG.SYS (once or UMB and once for
HIGH).  The Memmaker command does that just to confuse you.

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DOS= (CONFIG.SYS Command)