3- PCI protocol:   it is Used in connecting the elements of modern, high performance computer systems,
        where it offers a number of significant advantages like speed, configurability.  PCI is a synchronous bus with clock rate of 33 MHz and nowadays it is extended to support operation at 66 MHz and  implements a 32-bit multiplexed Address and Data bus where it could support a 64-bit data bus through a longer connector slot. The PCI bus speed is  independent of the CPU's speed and expansion of the bus colud be achieved by means of a bridge.