Expanded Memory for DOS Applications under MS Windows


SOURCE: Customer Support Center/Atlanta USA
The following information was obtained from Microsoft Online services.

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======================================================================
                 EXPANDED MEMORY FOR DOS APPLICATIONS
======================================================================


UNDER REAL MODE WINDOWS 3.00
============================


Real mode Windows 3.00 and DOS applications can both use the same
external Expanded Memory Manager (EMM). Expanded memory can be
provided either by a physical expanded memory board, or by a 386
Expanded Memory Manager. Common expanded memory boards include the
Intel Above Board and AST RAMpage. Common 386 Expanded Memory Managers
include Microsoft EMM386.SYS (included with the Windows product),
COMPAQ CEMM.EXE (included with COMPAQ 386 machines), Qualitas 386 to
the Max (386MAX.SYS), and Quarterdeck QEMM.SYS.

If a physical expanded memory board is used, you will get the best
performance from real mode Windows by backfilling to provide large
page frame LIM 4.0 support. Some boards do not have the hardware
register support to provide large page frame LIM 4.0 -- only small
page frame (see the "Expanded Memory" section of this application
note). All 386 Expanded Memory Managers provide large page frame LIM
4.0 support.

Remember that DOS applications require LIM 3.2 (a 64K contiguous page
frame) to use expanded memory, but real mode Windows requires LIM 4.0
(does not require a 64K contiguous page frame). If you see expanded
memory in Windows but not in your DOS applications, you probably do
not have a 64K contiguous page frame (LIM 3.2), and you may need to
rearrange adapter locations. Refer to the "Expanded Memory" section of
this application note for more information.

UNDER STANDARD MODE WINDOWS 3.00
================================

As noted in the standard mode section of "Windows 3.00 Modes and
Memory Usage" of this application note, Windows 3.00 standard mode
does not use expanded memory. DOS applications running under standard
mode can use expanded memory only with a physical expanded memory card
such as an Intel Above Board or AST RAMPage. 386 Expanded Memory
Managers cannot be used to provide expanded memory support for DOS
applications running from standard mode. Still, 386-compatible EMMs
can be loaded to provide DOS application expanded memory support
outside of standard mode.

UNDER ENHANCED MODE WINDOWS 3.00
================================

Expanded memory emulation is provided internally for DOS applications
running under enhanced mode. The only requirement is the presence of a
64K contiguous page frame. See the "Enhanced Mode and the 384K
Reserved I/O Address Space" section of this application note for more
information on page frame placement. Expanded memory for DOS
applications can be allocated and/or limited via PIF parameters.

EXTERNAL 386 EXPANDED MEMORY MANAGERS
=====================================

Some 386 Expanded Memory Managers have a special feature that allows
standard or enhanced mode Windows to turn them off when Windows is
run. Memory managers with this capability are EMM386.SYS and CEMM.EXE.
Windows can turn off EMM386.SYS even if expanded memory is in use at
the time. CEMM.EXE requires that no expanded memory be in use when
standard or enhanced mode Windows is run. Other memory managers such
as 386MAX and QEMM cannot be loaded with standard or enhanced mode
Windows.

Some 386 Expanded Memory Managers provide the capability to load DOS
device drivers into free areas of the 384K reserved I/O address space.
This process works correctly under real mode Windows, and if you must
run real mode, this is recommended for best performance. EMM386.SYS
and CEMM.EXE do not provide this capability, but 386MAX.SYS and
QEMM.SYS do.

If you plan to switch back and forth between real mode and standard or
enhanced mode Windows 3.00, we recommend using EMM386.SYS. Neither
standard nor enhanced mode will run with 386MAX or QEMM. Using one of
these memory managers requires you to reboot with a different
CONFIG.SYS and AUTOEXEC.BAT to switch between Windows 3.00 operating
modes. Using EMM386.SYS allows you to run whichever mode you want.








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