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The branch performance problem can be divided into two subproblem. First, prediction of branch direction is needed. Second, for taken branches, the instruction from the target must be available for execution with minimal delay. This project focuses predicting the branch direction on conditional branches. Hardware branch prediction techniques have been studied extensively. The most well known technique which is called as branch address makes a prediction based on the direction the branch went the last few times it was executed. In this technique, the prediction information are kept on the pattern history table which is consist of 2-bit saturated up-down counters. When a conditional branch is being predicted, the content of the counter which is indexed by branch address is used for prediction. This method considers the history of each branch independently and takes advantage of repetitive patterns. Another technique uses the combined history of all recent branches in making a prediction which is called as global history . Each of two branch prediction techniques have distinct advantages. For example, Branch address scheme works well when each branch is strongly biased in a particular direction. The global history technique works well when the direction taken by sequentially executed branches is highly correlated. Therefore, the prediction schemes mentioned above differs how the PHT are indexed.
Figure 1: Four basic Pattern History Table based prediction techniques
In PHT based dynamic branch prediction, the real question is about
which technique that we can use to increase the prediction accuracy.
Next section contains brief information about some basic schemes which
uses different indexing mechanism in prediction.