WHAT IS PROGRAMMABLE PERIPHERAL INTERFACE 8255 ?

The 8255 Programmable Peripheral Interface (PPI) is a general purpose interface device which is widely used in microprocessor design. It contains three independent 8 bit ports named Port A, B and C. Port A and B can be programmed as either input or output (all eight line must be same), while port C is split into two 4 bit halves (Port C upper (PC4- PC7) and Port C lower (PC0-PC3)) that can be separately programmed as input or output.

There are four registers that control the operations of the PPI and they are mapped to four address locations in the 8085 Development System as shown below:

      Port                               Address
Port A Register                        80H
Port B Register                        81H
Port C Register                        82H
Control Register                       83H

Now question is what are the work of those resister ?

First three resister ( Port A,Port B,Port C) are the address resister it provide the identity of the Ports. And Control Register it has a main work to control them. It control Which Port is Input and Which is output depending on programer Control Word .