Determining Free Memory in Windows Enhanced Mode

ID: Q74941

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:

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

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
Version : WINDOWS:3.0,3.1
Platform : WINDOWS
Issue type :