Bus Nt1310

A bus is a shared link between the different components of a computer. A computer may contain different number of buses. Only one device can communicate over the bus at one time. It actually interconnects the processor with the memory unit and the I/O unit. Buses include certain set if wires or lines which are used for communication between the components. These set of wires are three set of buses- Address bus, Control bus and Data bus. They all are involved for the transfer of information. The data bus carries the data to be transferred. The address bus carries the address of the data. The control bus is used to carry the control signals between multiple devices.

4.2.1 Address Bus

• Address Bus is used by the processor to specify

For example, a system which has 32-bit address bus can directly access 232=4GB of memory space.

• Hence the size of the bus specifies the number of address locations that a processor can access from the main memory.

How address bus works?
Suppose processor want to load the contents at address M to any register. It issues the address M on the address bus and then the address bus communicates with memory and finds the location M which the processor wants to access.

4.2.2 Data Bus • The data bus is used to transfer the data between the various functional units attached to the system bus. The data and instructions both are transferred using the data bus.
• It is a bi-directional bus because the data and instructions are transferred from CPU to memory and I/O unit and vice-versa. Therefore the processor can write or read the data to and from the memory and I/O devices.
• When the processor sends out the address on the address bus, the data is fetched from the memory by data bus.
• The width of the data bus is the word length of the processor. By word length, we mean the number of bits that the CPU can process in a single

How data bus works?
Consider the example taken in case of address bus. When the address bus carries the address M of the instruction to the memory and finds its location, the data stored at that memory address is fetched by the data bus to the CPU.

4.2.3 Control Bus

• The control bus is used to carry the control signals which monitors and regulates the activities of the subsystems. Therefore the signals are used to synchronize the interconnected systems.
• It is a bidirectional bus because the processor sends and receives the control and timing signals to control the communication between the components.
• There are various types of control signals that are generated in the system:
 Memory Read or Write signal is generated when processor want to read or write the data to and from the memory.
 I/O Read or Write signal is generated when processor want to read or write the data to and from the I/O.
 Bus Request is a control signal used to make request for bus.
 Bus Grant is used to grant the bus to the processor if it is free.
 Interrupt Request and Interrupt Acknowledge is issued to interrupt the current working of the