The energy complexity of register files

Register files (RF) represent a substantial portion of the energy budget in modern processors, and are growing rapidly with the trend towards wider instruction issue. The actual access energy costs depend greatly on the register file circuitry used. This paper compares various RF circuitry techniques for their energy efficiencies, as a function of architectural parameters such as the number of registers and the number of ports. The port priority selection technique was found to be the most energy efficient. The dependence of register file access energy upon technology scaling is also studied. However, as this paper shows, it appears that none of these will be enough to prevent centralized register files from becoming the dominant power component of next-generation superscalar computers, and alternative methods for inter-instruction communication need to be developed. Split register file architecture is analyzed as a possible alternative.