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Q78433: Detailed Explanation of EMM386.EXE Switches

Article: Q78433
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:5.x,6.0,6.2,6.21,6.22
Operating System(s): 
Keyword(s): 
Last Modified: 17-DEC-2000

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The information in this article applies to:

- Microsoft MS-DOS operating system versions 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 
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SUMMARY
=======

This article explains the following EMM386.EXE switches for MS-DOS 5.0 or
later:

  W=on/off
  Mx
  FRAME=
  /Pmmmm
  Pn=address
  X=
  I=

This article explains these switches in-depth, and describes some of the
differences and limitations associated with the switches. There is also a
separate note regarding the use of X= and I= to maximize available UMB space.

W=on/off
--------

This switch enables or disables support for the Weitek 3167 math coprocessor. If
W=on, MS-DOS 5.0 must be loaded low because the HMA will be used by the
coprocessor.

The Weitek coprocessor and the Intel coprocessor use different methods of passing
instructions and results back and forth to the 80386/80486 chip controlling the
system. The Intel chips do not reside in the memory space of the CPU. The Weitek
chip is memory-mapped; it appears to take up a certain amount of space in
extended memory, but the area of memory it uses is the HMA. Because the HMA is
also the location used to load the MS-DOS 5.0 kernel high, the Weitek processor
cannot be used while MS-DOS 5.0 is loaded high. Therefore, if the W=on switch is
used, the DOS=HIGH command will be unusable and MS-DOS must be loaded low.

Mx/FRAME=/Paddress versus Pn=address
------------------------------------

The Mx, FRAME=address, and /Pmmmm parameters are all essentially equivalent. They
create a 64K LIM 3.2 compatible page frame at the address specified. The syntax
for specifying the address for each of these is on page 606 of the version 5.0
"Microsoft MS-DOS User's Guide and Reference."

The Pn=address parameter allows you to create LIM 4.0 compatible 16K pages. This
allows you to create 16K pages at any available address in the allowable memory
ranges (8000-9C00, which is in conventional memory space, or C000-EC00 which is
part of the upper memory area). Note that multiple uses of this parameter with a
different value for n will create additional 16K pages (up to a maximum of
four). These additional pages do not necessarily need to reside in contiguous
areas of memory with respect to the other pages.

To use LIM expanded memory emulation, a page frame must be created somewhere in
memory to provide a location for the visible pages of expanded memory to appear
as they get bank-switched in and out by the applications using EMS.

LIM versions 3.2 and earlier require a 64K contiguous block for use as the page
frame. LIM 4.0 allows for disjoint page frames of 16K each, and it is not
necessary to create four frames for a total of 64K. An application written for
LIM 4.0 can function with a single page of 16K. However, because most expanded
memory boards in use conform to LIM 3.2 or earlier, very few applications can
take advantage of the reduced "footprint" capability of LIM 4.0. For these
applications, it is still necessary to create a 64K LIM 3.2 compatible page
frame.

X=address and I=address
-----------------------

These switches force EMM386 to include (I=) or exclude (X=) specific areas of
memory between A000-FFFF when EMM386 is determining where to place upper memory
blocks (UMBs) or finding a location for the EMS page frame.

Using I=address can often increase available UMB space by including areas that
EMM386 was not able to automatically detect. Be careful when using this switch
to not include memory areas in use by system adapters or ROMs. If these areas
are included, it may cause the system to hang when booting, or to display other
unpredictable behavior. You can determine which memory areas are free by using
memory scanning utilities from several third party vendors, or MSD from
Microsoft.

The X=address statement is sometimes required to prevent EMM386 from using areas
that it might otherwise attempt to use, such as RAM on certain network adapters,
which is otherwise undetectable.

Additional query words: 6.22 5.00 5.00a 6.00 6.20

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Keywords          :  
Technology        : kbMSDOSSearch kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS500 kbMSDOS500a
Version           : MS-DOS:5.x,6.0,6.2,6.21,6.22

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