 An alternating step generator comprises three LFSRs, which we will call LFSR0, LFSR1 and LFSR2 for convenience.The output of one of the registers decides which of the other two is to be used; for instance if LFSR2 outputs a 0, LFSR0 is clocked, and if it outputs a 1, LFSR1 is clocked instead.The output is the exclusive OR of the last bit produced by LFSR0 and LFSR1.