Reversible computer engineering and architecture

There exists a set of reversible primitive modules that may be combined according to a set of simple rules to create larger systems, which are themselves reversible primitive modules. These modules may be combined to create a fully reversible, universal computer. I describe techniques for creating reversible primitive modules from simple switches, such as conventional CMOS transistors, and the rules for combining these modules to maintain reversibility. As an example of a complex system that can be constructed from these modules, a fully reversible microprocessor, Pendulum, is presented. This universal microprocessor has been fabricated in a 0.5 pm CMOS process and tested. Pendulum has eighteen instructions, including memory access, branching instructions, and direction changing instructions. The datapath supports twelve-bit wide data and address values. The motivation for reversible computing research comes from an investigation of fundamental limits of energy dissipation during computation. A minimum energy dissipation of kBT ln 2, where kB is Boltzmann's constant, is required when a bit is erased. Erasing a bit is a logically irreversible operation with a physically irreversible effect. A reversible computer avoids bit erasure. Fully reversible in the case of the Pendulum processor means reversible at the instruction set architecture (ISA) level and the circuit level. The primary contributions of this thesis are, first, the discoveries about how to design and build reversible switching circuits, including memory and control structures, and notational inventions that ease reversible system design, and second, the architectural innovations brought about by the development of the Pendulum reversible processor. Thesis Supervisor: Thomas F. Knight, Jr. Title: Senior Research Scientist, MIT Al Lab