Reducing the Power Dissipation of Register Alias Tables in High–Performance Processors*

Modern microprocessor designs implement register renaming using register alias tables (RATs), which maintain the mapping between architectural and physical registers. Because of the non–trivial power that is dissipated in a disproportionately small area, the power density in the RAT is significantly higher than in some other datapath components. In this paper, we propose mechanisms to reduce the RAT power and the power density by exploiting the fundamental observation that most of the generated register values are used by the instructions in close proximity to the instruction producing a value. Our first technique disables the RAT lookup for a source register if that register is a destination of an earlier instruction dispatched in the same cycle. The second technique eliminates some of the remaining RAT read accesses even if the source register value is produced by an instruction dispatched in an earlier cycle. This is done by buffering a small number of recent register address translations in a set of external latches and satisfying some RAT lookup requests from these latches. The net result of applying both techniques is a 30% reduction in the RAT energy with no performance penalty, little additional complexity and no cycle time degradation.