Q77226: Memory Access Methods for Protected Mode Applications

Article: Q77226
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

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The information in this article applies to:

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

Some applications introduce incompatibilities by implementing a private version
of the GlobalLock() function to translate a handle to a selector. While a
private function can be made to work for any one version of Windows, it is not
guaranteed to work in future versions.

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:

  streamlined

Additional query words: 3.00 3.10

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

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