Memory Access Methods for Protected Mode Applications

The information in this article applies to:

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

SUMMARY

Applications that will run solely in one of Windows protected modes (standard mode or enhanced mode) can take advantage of the attributes of protected mode to reduce the size and improve the speed of the application.

These techniques do not apply to applications that can be run in Windows real mode.

MORE INFORMATION

To assure compatibility with future versions of Windows, an application should not make any assumptions about the protection ring of memory selectors the system provides for memory allocations.

Using global memory can be more straightforward when an application is developed for the protected-mode environment. The following four steps provide a procedure for using memory:

1. Use the GlobalAlloc() function to allocate memory. Applications should ALWAYS use the GMEM_MOVEABLE attribute, which signifies that the linear address of the memory block can be changed by the system. The selector or handle to this block will not change unless the application calls GlobalRealloc() to modify the handle, and changes the number of 64K blocks required to satisfy the request. For example, the selector can change if a 60K block is increased to 70K or if a 70K block is reduced to 60K.

2. Use the GlobalLock() function to obtain the corresponding selector. In protected mode, there is no need to bracket each use of an object with GlobalLock/GlobalUnlock calls. These calls are required in real mode because the Windows memory management algorithm cannot move locked objects in memory. In protected mode, locked objects can be moved without changing the selector value that is used to refer to the object.

3. Use the selector and the range of offsets (from zero to the size of the block) to access the memory.

4. When the memory block is no longer required, unlock the memory block using the GlobalUnlock() function, and free the selector using the GlobalFree() function.

The four steps above may be used for discardable memory. However, Windows cannot discard the memory while it is locked. Therefore, even in protected mode, applications that use discardable memory objects should unlock each object when it is not in use. This makes these memory blocks candidates for discarding should the system run out of memory.

Selectors should not be shared between applications unless the rules outlined in the dynamic data exchange (DDE) specification are followed. In particular, the GMEM_DDESHARE or GMEM_SHARE flag must be specified in memory allocation requests. Future versions of Windows may implement separate address spaces; any applications that improperly share memory will not function properly in any such release.

An application that uses any of these techniques should specify the Resource Compiler -T switch when resources are added to the application. This will prevent the application from running in real mode.

Additional techniques to decrease the size and increase the speed of an application developed for protected mode can be found by searching on the words in this Knowledge Base:

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