You can't without a license from Intel, for various patents etc. Well it has been tried see http://jolt.law.harvard.edu/digest/patent/intel-and-the-x86-architec

You can't without a license from Intel, for various patents etc. Well it has been tried see http://jolt.law.harvard.edu/digest/patent/intel-and-the-x86-architecture-a-legal-perspective-2 http://jolt.law.harvard.edu/digest/patent/intel-and-the-x86-... for a brief history, but it is pretty hard, most of the people who used to make x86 chips (Transmeta, NEC, ...) have dropped out.