KnowledgeBase Archive

An Archive of Early Microsoft KnowledgeBase Articles

View on GitHub

Q74941: Determining Free Memory in Windows Enhanced Mode

Article: Q74941
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

-------------------------------------------------------------------------------
The information in this article applies to:

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

SUMMARY
=======

When an application requires memory for a particular purpose, it should request
that amount of memory from the system. If a given request fails, the application
can present an error message to the user, or make a smaller request.

In the MS-DOS (non-Windows) environment, it is customary for an application to
request that the operating system determine how much memory is free and report
that information. The application can then allocate that amount of memory and
scale its capacity limits accordingly. This is acceptable in an environment
where only one application is running at any given time, which has complete
access to all system resources available.

However, in the Windows cooperative multitasking environment, applications must
share system resources with other applications running simultaneously.

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

In enhanced-mode Windows, determining the amount of free system memory is a very
complex problem because Windows uses virtual memory. There are also a number of
different types of memory that are used for specific purposes. The following
list enumerates some of these memory options:

  DDE share
  Discardable
  Fixed
  Movable
  Page locked
  Pageable
  Provided by GlobalDOSAlloc

The presence of any of these attributes will affect the amount of free memory.

It has been suggested that to attempt an allocation and then properly handle
failure by potentially trying another allocation is too slow. However, it is
doubtful that any method of calculating available memory will be any faster (if
such a calculation was even possible). An overriding complication is that any
memory use in another application or in a virtual machine will invalidate any
computed value.

An excellent discussion about dealing with varying amounts of system memory is in
Chapter 18 of "Peter Norton's Windows 3 Power Programming Techniques" (Bantam
Books, 1990) beginning on page 661. Given the caveats above, it is possible to
obtain a very rough estimate of free system memory. Two routines report this
information: the GlobalCompact() API and the MS-DOS Protected Mode Interface
(DPMI) function 0500h (get free memory information).

There are two pools of memory in enhanced-mode Windows:

1. The DPMI memory pool managed by the WIN386 paging memory manager. Use DPMI
  function 0500h to determine the size of this pool.

2. The global heap memory pool(s) managed by KRNL386.EXE (the Windows
  enhanced-mode Kernel. Use the GlobalCompact(-1) function to determine the
  size of this pool.

A rough estimate of available memory can be computed by placing the following
code fragment into an application:

    FreeMemEst = max(GlobalCompact(-1),
               (DPMI_Call_AX_0500h->MaxUnlockedPageAlloc - 1L) * 4096L);
    FreeMemEst = min(FreeMemEst, (16L * 1024L * 1024L) - (64L * 1024L));

The first line of code determines available memory according to both memory
managers. The second line accounts for the limitation imposed by the
GlobalAlloc() API, which sets the maximum size of a segment at (16 megabytes
minus 64 kilobytes). The GlobalCompact() API does require a lot of processing
time, especially in standard (286 protect) mode.

Additional query words: no32bit 3.00 3.10

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

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.