A Fully Reversible Asymptotically Zero Energy Microprocessor

Reversibility is the only way to compute with asymptotically zero power, and is a novel approach to low power, low energy computing. Recent implementations of reversible and adi-abatic 15, 7] logic in standard cmos silicon processes have motivated further research into reversible computing. The application of reversible computing techniques to reduce energy dissipation of current generation cmos circuits has so far been found to be limited, but the techniques used to design reversible computers are interesting in and of themselves, and other technologies, such as Josephson Junctions and quantum computers as well as future cmos technologies , may require fully reversible logic. This paper discusses the design of a fully reversible microprocessor architecture. Computing with reversible logic is the only way to avoid dissipating the energy associated with bit erasure. Low energy techniques such as voltage scaling lower the cost of erasing information. Techniques such as clock gating eeectively reduce the number of bits erased. Reversible techniques have already been used to lower the cost of bit erasure for nodes that have a high cost of erasure, but this work is directed at saving every bit, computing fully reversibly. The goal is to convert a conventional risc processor to completely reversible operation. This investigation indicates where bit erasure happens in a conventional machine and the varying diiculty across datapath modules of computing without erasing bits. The initial motivation for reversible computing research came from an investigation of fundamental limits of energy dissipation during computation 8]. The link between entropy in the information science sense and entropy in the thermodynamics sense, exhibited by Maxwell's demon 9], requires a minimum energy dissipation of k B T ln 2, where k B is Boltzmann's constant , when a bit is erased. Erasing a bit is a logically irreversible operation with a physically irreversible eeect. A reversible computer avoids bit erasure. Judicious application of reversibility in adiabatic circuits has already proven its usefulness in reducing energy dissipation 2]. This paper examines the complexity and diiculty in avoiding bit erasure entirely and discusses a set of techniques for designing reversible systems.