A comparision of superscalar and decoupled access/execute architectures

Even with a very accurate dynamic branch predictor, a superscalar processor must predict instruction fetch addresses no later than the first pipeline stage to avoid suffering pipeline bubbles every time a branch is taken. Unfortunately, branch addresses generally are not known prior to instruction decode. Therefore, some indirect technique is required to identify a branch instruction and enable branch prediction while the branch instruction is being fetched. This is the branch identification problem. Intel Pentium adopts a scheme that solves this problem; however, its scheme assumes an issue rate of two instructions per cycle. An aggressive superscalar processor, issuing more than two instructions per cycle, cannot effectively use that scheme. The authors propose and compare two viable schemes for solving the branch identification problem for wide-issue superscalar processors.>