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Q74696: Using Memory Below 1 Megabyte

Article: Q74696
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
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SUMMARY
=======


Memory allocated with a base address below 1 megabyte (low memory) is useful for
communicating between applications developed for the Microsoft Windows graphical
environment and MS-DOS terminate-and-stay- resident (TSR) programs and device
drivers.

The only way a Windows-based application can directly allocate memory guaranteed
to be below 1 MB is to use the GlobalDosAlloc function. However, this memory is
a limited resource and should be used with care.

MORE INFORMATION
================

The following clients use memory below 1 MB:

- MS-DOS

- MS-DOS device drivers

- TSRs

- Parts of the Windows kernel

- Windows enhanced mode virtual drivers (VxDs)

- Windows-based applications that call GlobalDosAlloc

- Windows-based applications that call GlobalAlloc and receive low memory by
  chance

- The task database for each active Windows-based application (This small block
  of low memory holds data used by MS-DOS.)

The first four clients on the list allocate their memory before any Windows-based
applications are run, therefore an application cannot prevent this consumption
of the low memory resource. (The user can modify the CONFIG.SYS and AUTOEXEC.BAT
files to reduce the number of devices and TSRs.)

Virtual devices can allocate (or map) memory below 1 MB to communicate with
various hardware devices and MS-DOS device drivers. VxDs such as the virtual
NetBIOS driver and various virtual display drivers map memory below the 1 MB
line, reducing the amount of low memory available to Windows.

The last three clients of low memory listed can progressively consume more of the
resource as the system runs, therefore an application can increase the chance
that its low memory allocations will succeed by performing them as early as
possible during system initialization. Two methods for doing this are:

1. Load the application or dynamic-link library (DLL) from the "run=" or "load="
  line in the WIN.INI file or from the StartUp group provided by Windows 3.1,
  or

2. Create a Windows device driver that performs the allocation when it is first
  loaded.

Beware of using too much low memory because other applications that need low
memory may begin to fail. The worst outcome of allocating too much low memory is
that Windows will be unable to allocate the task database required to start an
additional application.

In Windows enhanced mode, the lower memory that Windows-based applications
allocate is local to the system virtual machine (VM). Other virtual machines (or
MS-DOS compatibility boxes) cannot see the memory that the GlobalDosAlloc
function allocates. Allocating "global" low memory (seen by all virtual
machines) requires a virtual device, or the memory must be allocated before
Windows is loaded.

Additional query words: 3.00 3.10 meg

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

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