Preparing Your Application for 64-bit Windows
There are several features that make it easier for you to develop applications that will run on both 32- and 64-bit Windows. Most of these, such as the new data types, are described in Getting Ready for 64-bit Windows.
The 64-bit toolkit that ships with the Windows SDK includes a 64-bit MIDL compiler, Midl.exe, for generating native 64-bit stubs, as well as 32-bit stubs. Use the /env win64 switch to generate 64-bit stubs only. The default is to generate dual stubs that run on both platforms.