The 8255 is a general purpose programmable I/O device designed for use with microprocessors. It has 24 I/O pins which may be individually programmed in 2 groups of 12 and used in 3 major modes of operation. In the first mode (MODE 0), each groups of 12 I/O pins may be programed in sets of 4 to be input or output. In MODE 1, The second mode, each group may be programed to have 8 lines for a bidirectional bus, and 5 lines, borrowing one from the other group. for handshaking.
