When writing processor-specific code, the _M_IX86, _M_AMD64 and _M_IA64 can be used for conditional compilation – so far, so good. But sometimes code is not exactly processor-specific but rather specific to the natural machine word length (i.e. 32 bit or 64 bit). Fur such situations, there are defines, too – however there is a little catch: For ancient 16 bit code, there is _WIN16. For 64 bit, the WDK build environment defines _WIN64 by default.