Any of the following macros can be used to identify Alpha as the processor for which the compiler is generating code:
_ _M_ALPHA
_ _ALPHA_
_ _alpha
_ _ALPHA
The preferred macro is _ _M_ALPHA because it is consistent with the x86 version of Visual C++. The Alpha edition of Visual C++ predefines these macros, but the x86 edition does not. Therefore, you can use them with #ifdef to conditionally compile code for the Alpha edition of Visual C++ only. See Working with Data Alignment Issues in Chapter 3 for an example.
In makefiles, use the PROCESSOR_ARCHITECTURE environment variable to conditionally compile code. For more information, see Testing the Processor Type in Makefiles in Chapter 4.