Computer Buses

Computer Buses

  A computer system consists of different devices.CPU must be able to communicate with all devices. The devices are connected together by a communications channel called bus. A bus consists of a set of communication lines or wires. It is used to move a large amount of bits in the form of electrical pulses from one unit to another. The bus is used to connect the following units:

• Central Processing Unit

1. Control Unit 
2. Arithmetic and Logic Unit

• Main Memory ( RAM, ROM)
• Input / Output Devices

Bus is a common path to transfer data and commands between CPU, memory and input / output devices.It is also used to send or receive data from secondary storage.The capacity of a bus depends on the number of data lines in it.A bus with 16 lines can carry 16 bits or 2 bytes at a time. A bus with 32 lines can carry 32 bits or 4 bytes at a time .

Types Of  Buses


Different types of buses are as follows:

1. Data Bus

2.Address Bus

3.Control Bus 

1.Data Bus:
Data bus is the most common bus. It is used to carry data.It is an electrical path that connects CPU, memory, input / output devices and secondary storage devices.The bus contains parallel set of lines. The number of lines affects the speed of data transfer between different components. A data bus with more lines can carry more data.A data bus with 16 lines can carry 16 bits ( 2 bytes ) of data. A bus with 32 lines can carry 32 bits ( 4 bytes ) of data.

2.Address Bus :
Address bus is used to carry address information. It is a set of wires similar to data bus. It only connects central processing unit and memory. The following steps take place when CPU needs data from the memory:

• CPU places the address of required data on address bus.
• The address is carried to the memory.
• The data from the requested address is fetched and placed on data bus. 
• The data bus carries the data to CPU.

The number of lines in the address bus determines the maximum number of memory addresses.If an address bus has 8 lines, the maximum number of memory locations that can be addressed are 2 = 256. The computers used today have 32 bit address lines .These computers can access 4GB of memory.

3.Control Bus:
Control bus is used to transmit different commands or control signals from one component to another component. Suppose CPU wants to read data from main memory. It will use control bus to send the memory read command to the main memory of computer. The control bus is also used to transmit control signals like ACKS( Acknowledgement  signals) A control signal contains the following:

• Timing Information:It  specifies the time for which a device can use data and address bus.
• Command Signal:It specifies the type of operation to be performed . 

Suppose that CPU gives a command to the main memory to write data . The memory sends acknowledgement signal to CPU after writing the data successfully.CPU receives the signal and then moves to perform some other action.