▁P IC 1 6 CR 5 4 ▁Micro controller . ▁ ▁Communic ation ▁ ▁The ▁Apple ▁Des ktop ▁Bus ▁system ▁is ▁based ▁around ▁the ▁devices ▁having ▁the ▁ability ▁to ▁decode ▁a ▁single ▁number ▁( the ▁address ) ▁and ▁being ▁able ▁to ▁hold ▁several ▁small ▁bits ▁of ▁data ▁( the ir ▁registers ). ▁All ▁traffic ▁on ▁the ▁bus ▁is ▁driven ▁by ▁the ▁host ▁computer , ▁which ▁sends ▁out ▁commands ▁to ▁read ▁or ▁write ▁data : ▁devices ▁are ▁not ▁allowed ▁to ▁use ▁the ▁bus ▁unless ▁the ▁computer ▁first ▁requests ▁it . ▁ ▁These ▁requests ▁take ▁the ▁form ▁of ▁single - byte ▁strings . ▁The ▁upper ▁four ▁bits ▁contain ▁the ▁address , ▁the ▁ID ▁of ▁one ▁of ▁the ▁devices ▁on ▁the ▁chain . ▁The ▁four ▁bits ▁allow ▁for ▁up ▁to ▁ 1 6 ▁devices ▁on ▁a ▁single ▁bus . ▁The ▁next ▁two ▁bits ▁specify ▁one ▁of ▁four ▁commands , ▁and ▁the ▁final ▁two ▁bits ▁indicate ▁one ▁of ▁four ▁registers . ▁The ▁commands ▁are : ▁▁ ▁talk ▁- ▁tells ▁the ▁selected ▁device ▁to ▁send ▁the ▁contents ▁of ▁a ▁register ▁to ▁the ▁computer ▁ ▁listen ▁- ▁tells ▁the ▁device ▁to ▁set ▁the ▁register ▁to ▁the ▁following ▁value ▁ ▁flush ▁- ▁clear ▁the ▁contents ▁of ▁a ▁selected ▁register ▁ ▁reset ▁- ▁tell ▁all ▁devices ▁on ▁the ▁bus ▁to ▁reset ▁ ▁For ▁instance , ▁if ▁the ▁mouse ▁is ▁known ▁to ▁be ▁at ▁address ▁$ D , ▁the ▁computer ▁will ▁period ically ▁send ▁out ▁a ▁ 1 - byte ▁message ▁on ▁the ▁bus ▁that ▁looks ▁something ▁like : ▁▁ 1 1