The 32-bit Visual C++ compiler automatically defines the preprocessor symbols _WIN32 and _M_IX86. The 64-bit compiler (for AMD/EMT64) automatically defines _WIN32, _WIN64 and _M_AMD64. The Itanium compiler defines _M_IA64 instead of M_AMD64.
ObjectARX bases its 64-bit dependent preprocessor definitions on _WIN64 rather than _M_AMD64.