XMS Calls Under Windows 3.1

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1

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.