Unlocking Large-Model Extra Data Segments (83363)

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.