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Q83008: XMS Calls Under Windows 3.1

Article: Q83008
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS: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) 3.1 
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SUMMARY
=======

When enhanced mode Windows is running, the WIN386 module answers all extended
memory specification (XMS) calls. The standard mode MS-DOS Extender (DOSX)
answers all calls only in protected mode.

The standard mode task swapper answers memory allocation calls to arbitrate the
use of extended memory between Windows and MS-DOS applications, and to
facilitate task swapping.

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

Windows enhanced mode provides its own XMS services, without regard to the XMS
driver that was installed before Windows started up. The amount of XMS memory
available in an MS-DOS window under enhanced mode is determined by the PIF
(program information file) settings, and not by the amount of memory actually
available in the system. The enhanced mode XMS driver returns failure for all
XMS Lock Region calls, because enhanced mode XMS uses virtual memory rather than
physical memory.

In standard mode, the task swapper hooks the XMS driver, allowing it to arbitrate
XMS use between Windows-based and MS-DOS-based applications. Most XMS calls are
passed through to the original XMS driver. However, calls that manage extended
memory are affected by the PIF settings. Because it hooks the XMS driver, the
standard mode task swapper can use extended memory allocated to Windows, but not
currently in use, for swapping. This feature is disabled if a "limulator"
[Lotus/Intel/Microsoft (LIM) standard expanded memory system driver] is
present.

The standard mode MS-DOS extender hooks XMS detection in protected mode. The INT
2Fh call that retrieves the address of the XMS driver's control function
(AX=4310h) returns a protected mode address that an application can call to
perform XMS control functions. This is intended for the use by application
installation programs that must determine the version number of an installed XMS
driver. Applications in the Windows environment are discouraged from making
other XMS calls in protected mode. In particular, attempting to lock XMS memory
in standard mode may result in a page fault or fatal system crash.

Additional query words: 3.10 no32bit switcher

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

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