Computer Bus

Computer Bus

Data Bus | Address Bus | Control Bus

Bus Architecture | Bus Width | Bus Speed

The Computer Bus is a communication link  used in a computer system to send Data , Addresses , Control Signals And power to various components within a computer system.

The computer buses connect the various hardware components that are part of the computer system.

The computer system makes use of different types of buses such as data bus , address bus and control bus. In this article we are going to study in detail computer system bus architecture , the types , features and functions of the computer buses.

Computer Bus

Table Of Contents

What Is Computer Bus ?

The  computer system  consist of  number  of  internal  and  external components . These components  are physically  interconnected  and communicate with each other ,   through  a network of  wires running  across the computer system  . These  wires are referred as buses and  are essential  to the functioning  of  the computer  system .

The computer  buses can be in the form of wired cables  or  wires embedded in the  computer motherboard PCB  ( Printed Circuit Board )  visible on the rear side of  motherboard .

It is important for computer science professional to study the computer system bus architecture , technical features of these buses  such as bus width  and bus speed  and its overall impact  on  the  system  performance of a computer system.

Computer System
Computer Components

A  bus is a common communication  pathway  used in a computer system through which information flows from  one computer component to another.  

 The  computer bus system  is  a  network of   buses  which  physically  connect  all the  components  with  wires  (  actual  bus wires  OR  circuit  wires on the motherboard  ) .

The bus system consist of different types of buses depending upon  the  components being connected  and  function  assigned  to the  bus .

A  bus  can  consist  of  set  of wires  grouped  together as  connection wire or a printed circuit boards  which  carry  the  data  and  other commands ( instructions ) from  the  CPU to the  memory  and  to  various  other  components connected to the system. 

 The  bus  performance  is  an important  parameter  to  access  the  computer system  performance . The bus width and the bus speed affects the system performance .

Computer Bus

Types Of Computer Bus

The computer bus system makes use of different types of buses depending upon the purpose and the function  of the bus . 

The computer system buses can be classified on the basis of type of number of factors . These factors include :

  • Components being connected . ( CPU , RAM , Input And  Output Devices  ).
  • Type of  Data being Transmitted  (  Data , Address , Control Signals ) .
  • Location of the  components (  Internal  And External  ).
  • Connectivity with the CPU  (  Through  Northbridge  Or  Through Southbridge ) 

Bus Type On Data Being Transmitted

The computer system buses can be classified on the basis of type of the data being transmitted as :

1. Data Bus ,    2. Address Bus  ,  3. Control Bus.

Computer Bus System

Bus Type On Components Being Connected

The computer system buses can be classified on the basis of type of the components being connected as :

1. System Bus ,    2. Expansion Bus  ,  3. Input And Output Bus.

Bus Type On Location Of Components

The computer system buses can be classified on the basis of location of the component being connected as :

1. Internal Bus ,    2. External Bus  

Computer Bus

Data Bus

In computer architecture , the data bus is  a wired connection dedicated for transmitting the data between the CPU and other components . The data bus is a part of the system  bus  in  addition  to  address bus and control bus .

 A  data bus has many different features , but one of the most important feature  is the bus width . The width of a data bus refers  to the number of bits ( electrical wires ) that  the  bus  is can carry .

The Common  data bus widths include  8 bit , 16 bit ,  32 bit  and 64 bit . The wider the bus width ,  faster would be  the  data  flow   on the  data bus and thus better system performance.

System Bus Architecture
Computer Bus Width

Computer Bus

Control Bus

The  CPU ( Microprocessor ) contains a control unit  which controls  the functioning of all  other components connected to the computer system . The control  bus  is  used  to transfer  the control signals  from  one  component to  another  component  .

 A  control bus is a computer bus that is used by the CPU to communicate with devices that are connected to the computer system. These devices are connected with the help of   cables   and  printed circuits  board such as motherboard.

The Control Bus is a part of System Bus in addition to Data Bus and Address Bus.

Control Bus

The Central Processing Unit ( CPU ) transmits different types of control signals to the system components. The devices also communicate with CPU by transmitting the control signals  using  the  control bus.

 The control bus   is a  bidirectional  and assists the  CPU in synchronizing control signals to the internal components and the external devices connected to the system. 

The control bus transmit control signals such as device interrupt signal , byte enable signal , memory read or write signals  and  status signals.

Control Bus

How CPU Works ?

Address Bus

The computer program consist of number of program instructions. These instructions direct the CPU to perform desired operation. 

The  CPU  executes the program instructions one-by-one by  fetching  the  instructions  from  the  main  memory  RAM  .

 In order to perform the memory read or write  operation  from the main memory RAM  , the  CPU  sends  either  read  or  write  control  signal on  the  control  bus  and  address  of  the memory location  along  the “Address Bus”  from where  the  operation is to be performed . The address  bus  is  a  part  of the “System Bus” .

How CPU Works

System Bus

A  System Bus is a main bus which contains Data Bus , Address Bus And Control Bus. 

The System bus in computer system  connects  number of  internal components which  mainly include  CPU , motherboard , Internal  add on cards  such as  Graphic card , Sound card  , Network card ,  RAM ( Main Memory ) and internal  hard disk  .   

 A  system  bus  is  a  set  of  parallel wires  which connects the  two or more  independent major internal components of a  computer system. System bus  transfers   data , memory   addresses  and  device control  instructions.

Computer Bus

Computer Architecture

Computer Bus Functions

The computer bus system makes use of different types of buses . Each of these bus is assigned to carry specific type of signal and data depending upon its function. 

  1. Data Sharing .
  2. Addressing
  3. Control  Signals
  4. Providing Power to Components .
  5. Sharing System Time .
Computer Bus Types And Functions

Computer Bus And System Performance

Computer Bus Width And Bus Speed

A  bus  is a information highway over which information flows and wider  the bus , the more information can flow over the channel .

And therefore , a  compatible  bus width  and  bus speed  is  important  for  the  optimal  performance of the two most vital system components which includes Central Processing Unit ( CPU )  and  main system memory RAM.

This  is  similar to  a   multi lane  wider highway   that  can carry  more  cars due to  more  number of  lanes available for traffic .  whereas , a single  lane road can carry less number of cars  as compared  to a  multi lane road .

The computer system at the hardware level understands only binary 0 ( zero ) and 1 ( one ) . And therefore , all computer programs are compiled to convert into machine code instructions in binary which computer CPU can decode and execute.      

The  bus  consist  of  group of cables  and each  of these cable  can  carry  1 BIT  ( Binary  0  OR 1  )  at a time  . Therefore ,  a  bus  consist of  a  group  of cables  so that  a  group of  bits  can be sent at a time  through  these  buses .

Why Computer Bus Use Binary ?

Bus Width

The size of a bus is measured in terms number of Bits it can transmit at a time . Each wire can transmit one bit thus more number of wires in the bus can transmit more bits at a time . This number of wires in bus is referred as Bus Width.

The Bus width is an  important measure because it determines how much data can be transmitted at one time. For example, a 16 Bits bus can transmit 16 bits of data and a 32 Bit Bus  can transmit 32 bits of data at a time.

The  bus  consist  of  group of cables  and each  of these cable  can  carry  1 BIT  ( Binary  0  OR 1  )  at a time  . Therefore  a  bus  consist of  a  group  of cables  so that  a  group of  bits  can be sent  through  the  bus  .

This  is  similar to  a   multi lane  wider highway   that  can carry  more  cars due to  more  number of  lanes available for traffic .​

Bus Speed

What Is Bus Speed ?

The Bus performance is important for optimal CPU performance . The Bus performance is measured on two factors ( Bus Width And Bus Speed ) . 

The bus speed is another important parameter for the  bus performance . The bus speed is defined by its frequency expressed in Hertz .

The  bus frequency  is  the  number  of data packets  sent or received per second. Each time that data  is   sent   or  received , It  is  called  as  one cycle. 

The  bus speed  is generally referred  to the FSB – Front Side Bus  speed . The Front Side Bus connects  the  CPU  to  the memory controller  chip  North-bridge .

What Is Bus Bandwidth ?

Let us summarize the bus width and bus speed using the highway analogy. If the bus width is the number of lanes available for traffic and the bus speed is how fast the vehicles  are moving on each of these lanes .

The bandwidth is the product of Bus Width And Bus Speed  and reflects the amount of traffic  that  the  channel  can convey per second.

Motherboard Bus Architecture

The CPU is connected to the internal system components ( RAM , Graphics Card Network card ) and external peripheral devices ( Monitor , Printer , Mouse , Keyboard ) by using device controller circuits placed on the motherboard .

 All the device controller chips are now integrated  into only  two  controller chips called chip-set . The chip-set consist of two prominently visible IC Chips called  North-bridge and South-bridge placed  on  the  motherboard .      

Computer North Bridge And South Bridge

The memory controller chip North-bridge and input / output   controller chip South-bridge circuits  are placed on the motherboard.

The internal components   are connected through North Bridge and other peripheral devices are connected through South Bridge. All these components are connected  by  using  a system bus wires  which  carries  three types  of  information  

1. Memory Addresses , 2. Control Instructions And  3. Data.


Expansion Bus

What Is Expansion Bus?

The  performance features  and  functionality of a computer system can be extended  by  adding   an    additional    cards    such  as  graphics  card   Or   sound card. 

The  expansion slots  are  the ports located  on the motherboard  of  a  computer  system   in which an expansion cards  can be installed .  The  user  can use these  slots  to  insert  additional  expansion cards   as per the  functional  requirements .

An expansion bus is a group of wires OR PCB  used to connect with the expansion slots on the motherboard. These expansion slots are used for  installing  expansion cards .

Motherboard Expansion Slots

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