Bus interconnection

A bus is a shared communication path of one or more wires that carries information between two or more devices. Bus interconnection means connecting the CPU, memory and I/O modules by buses so that they can exchange data, addresses and control signals.

A system bus has three groups of lines:

• Data bus: carries data and instructions. Its width (8, 16, 32, 64 bits) decides how much data moves in one transfer.
• Address bus: carries the address of the memory location or I/O port being used. Its width decides the maximum memory: 2ⁿ locations for n lines.
• Control bus: carries timing and command signals, such as Memory Read/Write, I/O Read/Write, Bus Request/Grant, Interrupt Request/Acknowledge, Clock and Reset.

 +-----+   +--------+   +------+
 | CPU |   | Memory |   | I/O  |
 +--+--+   +---+----+   +--+---+
    |          |           |
 ===+==========+===========+=== Address bus
 ===+==========+===========+=== Data bus
 ===+==========+===========+=== Control bus

Only one device may transmit on the bus at a time; others listen. The advantages are low cost and easy addition of devices. The disadvantage is that the shared bus can become a bottleneck.