Instruction prefetching device with prediction of a branch destination for each branch count instruction.

摘要

Instructions of a loop are repeatedly prefetched with a branch history table (46) made to memorize a predicted branch direction of "go" to branch for a branch cont instruction of the loop. The lopp is left without renewing the predicted branch direction when a prediction evaluating circuit (66) finds that the predicted branch direction is incorrect. Alternatively, the predicted branch direction is temporarily renewed to "no go" to branch during penultimate execution of the branch count instruction before leave of the loop and then renewed back to "go" to branch during ultimate execution of the branch count instruction on leaving the loop. It is possible in either event to again enter the loop at once. Only when there is no data for a branch count instruction, the predicted branch direction must be stored in the branch history table together with a predicted branch destination address for an instruction which stands foremost in the loop. Renewal of the branch history table is not different in this event from the renewal carried out when the prediction evaluating circuit finds incorrectness of prediction for a branch instruction which is not a branch count instruction.