All device use to provide interface between microprocessor and I/O device to input and output data either with controlled signals or without control signal. 

 All interfacing device transfer ( input or output ) data to or from I/O device without controlled signal is called general purpose supporting device . 

 All device used to provide interface to I/O device to transfer ( I/P or O/P ) data with control signal between I/O port is called programmable support device.

  The 8255 programmable peripheral interface consist of (3*6) bit three I/O port , port A ,port B, port C. the port C is divided into two parts lower four bit are used to provide handshake signal to port A and upper four bits are used to provide handshaking signal port B
Operation:
  The CS pin is enabled through address decoder circuits.  The address line A0 and A1 are used to select port A0 A1; 00=port A, 01=port B, 10=port C, 11=controlword. When the address select the specified part. The RD signal is
used to read data from I/O device to system and WR signal is used to write data from system to I/O device. When data transfer is completed the CS line by system through address decoder.
Programming 8255 PPI device:
  The 8255 PPI device can be programmed in three modes
Mode 0: in mode 0 all ports are used as general purpose support device part A, part B and part C as input or output.
Mode 1: in mode 1 port A and port B can be programmed as input or output on the base of handshaking principal. When port C is used to provide control signal to port A and port B
Mode 2: in the mode 2 the port A is programmed as input/output port on handshaking principal. The part C is used to provide controlling signal during operation.