32-Bit and 64-Bit WIA Interoperability

On systems that run Windows 64-Bit Edition for Extended Processors, all WIA components are 64-bit so the WIA infrastructure was changed to allow interoperability between these 64-bit drivers and existing 32-bit applications.

On 64-bit editions of the Windows operating system, the 64-bit WIA minidriver is loaded in the WIA service's 64-bit process. However, WIA minidriver UI extensions are loaded in the application's process space. A Microsoft Win32 application's unmodified 32-bit process that runs on an x64-based machine would not be able to load the 64-bit UI extension.

To mitigate the 32-bit to 64-bit problem, Microsoft provides a 64-bit extension host, wiawow64.exe. This host ensures transparent interoperability between 32-bit applications and 64-bit WIA UI extensions. The wiawow64.exe extension host will be available in Windows Server 2003 64-Bit Edition for Extended Processors, Windows XP 64-Bit Edition for Extended Processors, Windows Vista, and later operating system versions.

The WIA service will determine where UI extensions get physically loaded, depending on whether the application is 64-bit or 32-bit:

• 64-bit application. The 64-bit WIA minidriver UI extension is loaded directly into the process space of the application. This is similar to what happens when you run a 32-bit application on 32-bit versions of the Windows operating system.

• 32-bit application. WIA launches the wiawow64.exe extension host that UI extensions will be loaded into. A separate instance of wiawow64.exe is created and launched each time a call to any of the interface methods comes in from a 32-bit application. The wiawow64.exe host runs in the same context as the application and communicates with the application through the existing COM interfaces.

Even though wiawow64.exe is completely transparent to both WIA application writers and WIA driver developers, driver developers have to debug the wiawow64.exe process rather than the 32-bit application to debug 64-bit UI extensions.