说明：  A1,A0:地址选择线,用来选择8255的PA口,PB口,PC口和控制寄存器. 当A1=0,A0=0时,PA口被选择 当A1=0,A0=1时,PB口被选择 当A1=1,A0=0时,PC口被选择 当A1=1.A0=1时,控制寄存器被选择
(A1, A0: Address line used to port 8255 PA, PB port, PC port and a control register when A1 = 0, A0 = 0 时, PA port is selected when A1 = 0, A0 = 1 when. The PB is selected when A1 = 1, A0 = 0 时, PC port is selected when A1 = 1.A0 = 1, the control register is selected)