An evaluation of functional unit lengths for single-chip processors

When designing a pipelined single-chip processor (SCP) with pipelined functional units of varying length, the processor issue logic must deal with scheduling of the result bus. In order to prevent serious performance degradation due to result bus conflicts, some pipeline scheduling techniques developed in the 1970's may need to be incorporated into the issue logic. Since this is a non-trivial complication of the issue logic, a set of simulations were performed in order to evaluate the effectiveness of the combination of multiple length functional units and scheduling techniques. Analysis of the simulation results indicates that providing relatively short multiple length functional units is not worthwhile. Multiple length functional unit configurations employing result bus scheduling do perform slightly better than uniform length configurations, but the difference is often less than 1%. Thus, the SCP designer should not waste valuable time improving the performance of each functional unit, but rather should produce a good design for the most complicated unit and design all other units to match it.