Q83363: Unlocking Large-Model Extra Data Segments

Article: Q83363
Product(s): Microsoft Windows Software Development Kit
Version(s): 3.0,3.1
Operating System(s): 
Keyword(s): kbfile kbsample kb16bitonly kbOSWin310 kbOSWin300
Last Modified: 23-JUL-2001

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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
=======

An application that is compiled using the large memory model can have multiple
code segments and multiple data segments.

Under Windows 3.0, all the data segments except for the first one (the so called
"extra" data segments) are loaded into memory segments that are fixed and page
locked. Fixed and page locked segments interfere with effective memory
management. They reduce the amount of free memory addressed below 1 megabyte
(MB) in the linear address space, and in some cases, cause out-of-memory errors
as additional applications are run.

Under Windows 3.1, extra data segments are loaded into movable memory and the
above difficulties with fixed and page locked segments do not apply.

A large model application can change the attributes of its extra data segments by
calling the GlobalPageUnlock and GlobalUnfix functions. However, to do this
effectively, the application must be able to enumerate the selectors for its
extra data segments.

Largeapp.exe is a file in the Microsoft Download Center that demonstrates using
the GlobalPageUnlock and GlobalUnfix functions to change the attributes of an
application's extra data segments. Largeapp.exe contains the source code to two
applications: Largeapp.exe, which is a large-model application for the Windows
environment, and SYMSEG, which is an MS-DOS (non-Windows) application.

SYMSEG is a utility that reads a symbol file produced by the Microsoft Linker,
and creates a table of the data segments in an application. LARGEAPP uses the
information from this table to enumerate its own data segments.

MORE INFORMATION
================

The following file is available for download from the Microsoft Download
Center:

  Largeapp.exe

For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.

LARGEAPP determines the version of Windows under which it is running. The segment
attributes must be changed only if the application is running under Windows 3.0.

Additional query words: LARGEAPP

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Keywords          : kbfile kbsample kb16bitonly kbOSWin310 kbOSWin300 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : :3.0,3.1

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