Return target address prediction by moving entry pointer to return stack popped at completion to deeper one at return instruction fetch

Abstract

An instruction fetch control apparatus includes an instruction completion notifier, and an entry designation unit predicting a return address of a subroutine during an instruction fetching. The entry designation unit computes a designate entry position in a return address stack by, changing the designate entry to indicate a one-step shallower entry when a call instruction is predicted during the instruction fetching, changing the designate entry independently of a push or pop operation to indicate a one-step deeper entry when a return instruction is predicted during an instruction fetching, and changing the designate entry depending upon a push and a pop operation when a call and return instruction is completed, thereby keeping a position of the designate entry. The entry designation unit designates an entry as predicted return address of a subroutine when the fetched instruction hitsin a branch history and determined as a return instruction.