PRB: Avoiding "Insufficient Memory to Run" Errors

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

SYMPTOMS

During the process of loading an application, Microsoft Windows displays an application execution error message box with the following message:

Insufficient memory to run this application; close one or more Windows applications and try again.

CAUSE

One of the reasons this message is displayed is that the system has no memory available with an address less than 1 megabyte (MB). When Windows loads an application, it calls the GlobalDosAlloc() API to allocate memory in the address space below 1 MB for the application's task database. If the GlobalDosAlloc() call fails, Windows displays the application execution error message.

RESOLUTION

Use as little memory below 1 MB as possible.

MORE INFORMATION

A common situation that leads to an insufficient memory error regards an application developed using a large memory model that includes many extra data segments and/or very large extra data segments. When Windows loads the application, it allocates fixed memory to hold the extra data segments. In Windows 3.0, these fixed memory blocks are also page locked, which prevents the memory manager from moving the blocks to disk as memory fills.

Fixed memory is allocated from the bottom of the global heap, which starts at the lowest possible memory address. If all the memory below 1 MB is filled with page locked memory blocks, Windows cannot move blocks in memory or swap blocks to disk to free any memory. When Windows cannot allocate a task database for a new task, it displays the error message box discussed above. An application can determine whether all memory addressed below 1 MB is in use by attempting to allocate a small block of memory with the GlobalDosAlloc function. (Windows 3.0 requires 512 bytes of low memory for the task database of each application.) If the allocation fails, Windows will not be able to start another program. Even if the allocation is successful, the failure of another memory allocation required by the application may prevent it from loading.

One way to address this problem is to remove the page lock from an application's extra data segments. However, memory accessed by an interrupt service routine must be page locked to keep its data available at all times. An application can use the services of the tool helper dynamic-link library (TOOLHELP.DLL) and a few Windows functions to modify the flags on the extra data segments. After modification, the data segments are movable and not page locked.

Use the GlobalFirst() and GlobalNext() functions provided by the tool helper library to walk the global heap looking for memory blocks owned by the application. If a memory block is page locked (the wcPageLock field of the GLOBALENTRY data structure is not zero), call the GlobalPageUnlock function to change the lock count for the memory block. Calling GlobalRealloc() to change the block from fixed to movable memory might also be desirable.

For more information on the tool helper library, see Chapter 8 of the "Microsoft Windows Software Development Kit: Programmer's Reference, Volume 1: Overview" version 3.1 manual.

Another method to unlock data segments uses the GlobalPageUnlock() and GlobalUnfix() functions. For more information on this technique, please see the following article(s) in the Microsoft Knowledge Base:

Q83363 Sample Code Unlocks Large-Model Extra Data Segments

Additional query words:

Keywords : kbprg kb16bitonly KrToolHlp
Version : WINDOWS:3.0,3.1
Platform : WINDOWS
Issue type : kbprb