0 1 ▁ 1 1 ▁ 0 0 ▁ ▁This ▁says ▁that ▁device ▁$ D ▁( 1 1 0 1 ) ▁should ▁talk ▁( 1 1 ) ▁and ▁return ▁the ▁contents ▁of ▁register ▁zero ▁( 0 0 ). ▁To ▁a ▁mouse ▁this ▁means ▁" t ell ▁me ▁the ▁latest ▁position ▁changes ". ▁Register s ▁can ▁contain ▁between ▁two ▁and ▁eight ▁bytes . ▁Register ▁zero ▁is ▁generally ▁the ▁primary ▁communic ations ▁channel . ▁Register s ▁one ▁and ▁two ▁are ▁undefined , ▁and ▁are ▁generally ▁intended ▁to ▁allow ▁ 3 rd ▁party ▁developers ▁to ▁store ▁configuration ▁information . ▁Register ▁three ▁always ▁contains ▁device ▁identification ▁information . ▁ ▁En umer ation ▁and ▁identification ▁The ▁addresses ▁and ▁enumer ation ▁of ▁the ▁devices ▁are ▁set ▁to ▁default ▁values ▁when ▁reset . ▁For ▁instance , ▁all ▁key boards ▁are ▁set ▁to ▁$ 2 , ▁and ▁all ▁m ice ▁to ▁$ 3 . ▁When ▁the ▁machine ▁is ▁first ▁power ed ▁on , ▁the ▁A DB ▁device ▁driver ▁will ▁send ▁out ▁talk ▁commands ▁asking ▁each ▁of ▁these ▁known ▁default ▁addresses , ▁in ▁turn , ▁for ▁the ▁contents ▁of ▁register ▁three . ▁If ▁no ▁response ▁comes ▁from ▁a ▁particular ▁address , ▁the ▁computer ▁marks ▁it ▁dead ▁and ▁doesn ' t ▁bother ▁pol ling ▁it ▁later . ▁ ▁If ▁a ▁device ▁does ▁respond , ▁it ▁does ▁so ▁by ▁saying ▁it ▁is ▁moving ▁to ▁a ▁new ▁randomly ▁selected ▁higher ▁address . ▁The ▁computer ▁then ▁respond s ▁by ▁sending ▁another ▁command ▁to ▁that ▁new ▁address , ▁asking ▁the ▁device ▁to ▁move ▁to ▁yet ▁another ▁new