: Recent advances in the field of hardware verification have raised some fresh (and some familiar) issues to do with the scope and limitations of formal proof. In this note, some of these are considered in the context of the Viper verification project. Viper is a microprocessor designed by W. J. Cullyer, C. Pygott and J. Kershaw, of the Royal Signals and Radar Establishment of the U.K. Ministry of Defense, for use in safety-critical applications. Much to their credit, the designers intended from the start that Viper be formally verified; they presented Viper's more abstract specifications in a language suitable for formal reasoning, and they placed the design in the public domain. Viper microprocessors are currently being marketed as verified chips. The formal proof aspects of the verification work have been carried out at the Computer Laboratory of the University of Cambridge. To date, some important properties of a register-transfer level model of Viper, relative to a more abstract ...