DOS--Notes

Must install HIMEM.SYS for DOS=UMB or DOS=HIGH

You must install the HIMEM.SYS device driver or another extended memory     
manager before you specify either DOS=UMB or DOS=HIGH.

Using the UMB parameter

You must specify the DOS=UMB command in order to load programs and device
drivers into the upper memory area. Using the upper memory area frees more
space in conventional memory for programs. In addition to using this
command, you must install an upper-memory-block (UMB) provider. If your
computer has an 80386 or 80486 processor, you can use <EMM386.EXE> for your
UMB provider.

If you specify DOS=UMB and no UMB provider is installed, MS-DOS will not
display an error message.

Using the HIGH parameter

If you specify the HIGH parameter, MS-DOS attempts to load part of itself
into the HMA. Loading part of MS-DOS into the HMA frees conventional memory
for programs. If you specify DOS=HIGH and MS-DOS is unable to use the HMA,
the following message will appear:

    HMA not available
    Loading DOS low

Combining parameters

You can include more than one parameter on a single DOS command line, using
commas to separate them. For example, the following command lines are
valid:

    dos=umb,low

    dos=high,umb

You can place the DOS command anywhere in your CONFIG.SYS file.