Some computer systems use two separate buses,
one to communicate with memory and the other with I/O interfaces
I/O Bus
Communication between CPU and all interface units is via a common I/O Bus
An interface connected to a peripheral device may have a number of data registers , a control register, and a status register
A command is passed to the peripheral by sending to the appropriate interface register
Function code and sense lines are not needed (Transfer of data, control, and status information is always via the common I/O Bus)