Using Memory Below 1 Megabyte

 Last reviewed: July 22, 1997
Article ID: Q74696
3.00 3.10 WINDOWS kbprg

The information in this article applies to:

  • Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

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 reference words: 3.00 3.10 meg
KBCategory: kbprg
KBSubcategory: KrMm
Keywords : kb16bitonly

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Last reviewed: July 22, 1997
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