Allocates portions of the high memory area (HMA) and Upper Memory Blocks (UMBs) to hold parts of the operating system. The UMBs are unused parts of upper memory (between 640K and 1MB) and HMA is the first 64K of extended memory (above 1MB). The DOS commands establish the necessary links between and the programs/device drivers held in these areas.

The DOS command can only be invoked from Config.sys.


DOS=[high|low] [,umb|,noumb] [,auto|,noauto]
umb|noumbumb specifies that DOS should manage UMBs, if they exist. noumb (default) specifies that DOS should not manage UMBs.
high|lowhigh forces MS-DOS to load a part of itself into the HMA if space is available. low (default) specifies that all MS-DOS is to be kept in conventional memory.
auto|noautoauto (default) causes DOS to automatically load Himem.sys, Ifshlp.sys, Dblbuff.sys, and Setver.exe unless they are otherwise explicitly loaded in Config.sys. AUTO also forces BUFFERS, FILES, FCBS, LASTDRIVE, and STACKS to load high (if space is available).
SingleBoot to MS-DOS mode.


  1. Whilst DOS is used to manage UMBs, these have to be first defined using a UMB provider such as Emm386.exe.

  2. Using DOS to load parts of the operating system, device drivers, and so on into UMBs and HMA, frees space in conventional memory for programs that require it - especially the more complex games.

  3. If Config.sys contain a DOS=SINGLE statement, the computer will display the following on start-up:

    You are currently running in MS-DOS mode. Do you want to
    return to normal mode, to run Windows applications again [Enter=Y, Esc=N]?

    Pressing "Enter" or "Y" causes a reboot which brings you back to the same message.

    Pressing "ESC" or "N" results in the message:

    You are still running in MS-DOS mode. To return to normal
    mode, exit the application you are running, or type WIN again.

    You can now work in DOS, but if you type WIN, the first message repeats.



This page last revised:
November 13, 2000.