Even though this capability does not exist in WinRT directly, it is possible to determine the processor architecture at runtime through the Win32 API, GetNativeSystemInfo. This API is included in the list of Win32 APIs that can be called from Windows Store apps (see the system page specifically). You can call this directly from a C++ app, of course, and from C# or Visual Basic you can use PInvoke as described on this MSDN forum thread.
In pch.h, add this line:
In the component's header (.h) file, define a method like this:
public ref class Info sealed
static Platform::String^ GetProcessorArchitecture();
Here's the implementation of the class in the .cpp file:
using namespace SysInfo;
using namespace Platform;
str = "x64"; //AMD or Intel
str = "ARM";
str = "IA64";
str = "x86";
str = "Unknown";
var arch = SysInfo.Info.getProcessorArchitecture();
You could also return numeric codes from the C++ component, of course…your choice.