As an alternative to commercial hardware description languages, AMD has developed an RTL language for microprocessor designs that is simple enough to admit a clear semantic definition, providing a basis for formal verification. We describe a mechanical proof system for designs represented in this language, consisting of a translator to the ACL2 logical programming language and a methodology for verifying properties of the resulting programs using the ACL2 prover. As an illustration, we present a proof of IEEE compliance of the floating-point adder of the AMD Athlon processor.