How can I programmatically determine my processor type?

Question

How can I determine programmatically whether my machine is an x86, x64 or an IA64?

Answer 1

What's usually more important than the underlying processor is what mode the OS is running in, in ADDITION to the processor that's installed on the host.

Examine the output of "uname -p" (or uname(2))

Intel adopted AMD's extensions for 64-bit instructions so a value of "x86_64" means you're running either an Intel or AMD 64-bit processor, otherwise you're running the regular x86 ISA.