WOW64
The WOW64 subsystem provides a high-performance 32-bit Windows environment that enables x64 Windows to take full advantage of the more than 10,000 existing 32-bit Windows applications available today. Because of the underlying hardware compatibility of the x64 architecture, 32-bit applications are able to run at full speed in the WOW64 subsystem. Many applications will actually run faster in WOW64 than they would run in 32-bit Windows because of the larger available memory address space and the greater efficiencies of the x64 processor architecture.
WOW64 isolates 32-bit applications from 64-bit applications, but it provides for interoperability and data exchange across the boundary through COM and remote procedure call (RPC) and through transparent cut and paste. WOW64 runs 32-bit applications seamlessly while preventing file and registry collisions between 32-bit and 64-bit versions of an application.
The .NET Framework is supported under WOW64 as a native 32-bit application.
WOW64 supports Windows graphical applications, console applications, and services. It does not support 16-bit applications, with the exception of some 16-bit installers in which it can seamlessly substitute the 32-bit version.
An important WOW64 limitation is that 32-bit applications cannot load 64-bit DLLs and 64-bit applications cannot load 32-bit DLLs. This means that 32-bit ActiveX controls, for example, cannot be run in the 64-bit version of Microsoft Internet Explorer (IE) and that 32-bit DLLs that provide context-sensitive menu extensions to Windows Explorer will not work and will need to be rewritten to run natively in x64. Window XP Professional x64 Edition includes the 32-bit version of IE 6.0 and uses it by default to provide a seamless and transparent experience when browsing the Internet.
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