versions and 32 vs 64-bit code

[Nathan Coe]

Is there a way to change what is compiled in based on whether the -m32 or -m64
option is chosen? I can see that there are predefined versions (for X86 and
X86_64 e.g.), but I don't know if this is based on the compile options, or the
platform the compilation is being performed on. As an example, if I am
compiling on a 64-bit intel machine, is the version X86 or X86_64 set by 
default?
Thanks.