Basic Computer Organization

Even tough the size,shape,performance,reliability and cost of computers have been changing over the years the basic logical structure (based on the stored program concept) by Von Neumann,has not changed.No matter what shape and size of computer we are talking about,all computer systems perform the following five basic operations,for converting raw input data into information which is useful to their users.

1-Inputting:

The process of entering data and instructions into computer system.

2-Storing:

Saving data and instructions to make them readily available for initial or additional processing as and when required.

3-Processing:

Performing arithmetic operations (add,subtract,multiply,divide etc) or logical operations (comparisons like equal to,less Than,greater than etc) on data to convert them into useful information.

4-Outputting:

The process of producing useful information or results for the user,such as a printed report or visual display.

5-Controlling:

Directing the manner and sequence in which all of the above operations are performed.

The goal of this article is to to familiarise you with the computer system units,which perform these functions.It will provide you with an overview of computer systems,as they are viewed by computer systems architects.

The internal architecture of computers differs from one system model to another.However,the basic organisation remains the same for all computer systems.A block diagram of the basic computer organisation is shown in below figure.In this figure,the solid lines indicate the flow of instruction and data and dotted lines represent the control exercised by the control unit.It displays the five major building blocks ((functional units) of a digital computer system.These five units correspond to the fie basic operations performed by all computer systems.The function of each of these units are described below:

Basic Computer Organization

1-Input Unit:

Data and instructions must enter the computer system,before any computation can be performed on the supplied data.The task is performed by the input unit,which links the external environment with the computer system.Data and Instructions enter input units in forms,which depend upon the particular device used.For Example data are entered from a keyboard in a manner similar to typing and this differs from the way in which  data are entered through a scanner,which is another type of  input device.However,regardless of the form in which they receive their inputs,all input devices must transform the input data into the binary codes which the primary memory of a computer is designed to accept.This transformation is accomplished by units called input interface.Input devices are designed to match the unique physical or electrical characteristics of input devices, to the requirements of the computer system.

In short,the following functions are performed by an input unit:

1-It accepts (or reads) the instructions and data from the outside world.
2-It converts these instructions and data in computer acceptable form.
3-It supplies the converted instructions and data to the computer system for further processing.

Output Unit:

The job of an output is just the reverse of that of an input unit.It supplies the information obtained from data processing to the outside world.Hence,it links the computer with the external computer.As computer work with binary code,the results produced are also in the binary form.Hence,before supplying the results to the outside world they must be converted to human acceptable (readable) form.This task accomplished by units called output Interface.Output Interfaces are designed to match the unique physical or electrical characteristics of output devices (terminals,printers etc) to the requirements of the external environment.

In short,the following functions are performed by an output unit:

1-It accepts the results produced by the computer which are in coded form and hence cannot be easily understood by us.

2-It converts these coded results to human acceptable (readable) form.

3-It supplies the converted results to the outside world.

Storage Unit:

The Data and Instructions,which are entered into the computer system through input units,have to be stored inside the computer before the actual processing starts.Similarly,the results produced by the computer after processing,must also be kept somewhere inside the computer system before being passed on to the output units.Moreover,the intermediate results produced by the computer must also be preserved of ongoing processing.The storage computer unit of a computer system is designed to cater to all these needs.It provides space for storing data and instructions ,space for intermediate results and space for the final results.

In short,the functions of the storage unit are to hold (store):

1-The Data and Instructions required for processing.(received from input devices)

2-Intermediate results of processing

3-Final results of processing before these results are released to an output device.

The storage unit of all computers  is comprised of the following two types of storage:

1-Primary Storage:

The primary storage also known as main memory is used to hold pieces of program instructions and data,intermediate of processing and recently produced results of processing of the job(s),which computer system is currently working on.These pieces of information are represented electronically  in the main memory chips circuitry,and while it remains in the main memory,the central processing unit can access it directly at a very fast speed.However,the primary storage can hold information only while the computer system is on.As soon as the computer system is switched off or reset,the information held in the primary storage disappears.Moreover,the primary storage normally has limited storage capacity,because it is very expensive.The primary storage of modern computer systems is made up of semiconductor devices.

2-Secondary Storage:

The secondary storage also known as auxiliary storage, is used to take care of the limitations of the primary storage.That is,it is used supplement the limited  storage capacity and the volatile characteristics of primary storage.This is because secondary storage is much cheaper than primary storage and it can retain information even when the computer system is switched off or reset.The secondary Storage is normally used to hold the program instructions,data and information of these jobs on which the computer system is not working on currently,but needs to hold them for processing later.The most commonly used secondary storage is the magnetic disk.

Arithmetic Logic Unit:

The arithmetic logic unit (ALU) of a computer system is the place,where the actual execution of the instructions takes place during the processing operation.To be more precise,calculations are performed and all the comparison (decisions) are made in the ALU.The data and instructions,stored in the primary storage before processing are transferred as and when needed to the ALU are temporarily transferred back to the primary storage until needed later.Hence,data may move from primary storage to ALU and back again to storage,many times before the processing is over.

The type and number of arithmetic and logic operations,which a computer can perform,is determined by the engineering design of the ALU.However,almost all ALUs are designed to perform the four basic arithmetic operations (add,subtract,multiply and divide) and logic operations or comparisons,such as less than equal to and greater than.

Control Unit:

How does the input device know that it is time for it to feed data into the storage unit?How does the ALU know what should be done with the data and not the intermediate results?All this is possible due to the control unit of the computer system.Although,it does not perform any actual processing on the data,the control unit acts as a central nervous system for the other component of the computer system.It manages and coordinates the entire computer system.It obtains instructions from the program stored in main memory interprets the information's and issues signal which cause other unit of the system to execute them.

Central Processing Unit:

The control unit and the arithmetic logic unit of a computer system are jointly known as the  Central Processing Unit (CPU).The CPU is the brain of a computer system.In Human Body,all major decisions are taken by the brain and the other parts of the body function as directed by the brain.Similarly,in a computer system,all major calculations and comparison are made inside the CPU,and the CPU is responsible for activating and controlling the operations of other units of the computer system.

The System Concept:

You might have observed that we have been referring to a computer as a system (Computer System).What can be the reason behind this?To know the answer let us first understand the definition of a system

A system is a group of integrated parts,which have the common purpose of achieving some objectives.Hence,the following three characteristics are key to a system:

1-A system has more than one element.

2-All the elements of a system are logically related.

3-All the elements of a system are controlled in a manner to achieve the system goal.

Since a computer is made up of integrated components (input,output,storage and CPU).which work together to perform the steps called for in the program being executed,it is a system.The input or output units cannot functions,until they receive signals from the CPU.Similarly,the Storage Unit of the CPU alone is of no use.Hence,the usefulness of each unit depends on other units, and can be realised only when all units are put together (integrated) to form a system.

Points To Remember:

1-All Computer System perform the following five basic operations for converting raw input data into useful information-inputting,storing,processing,outputting and controlling.

2-The main components of a computer system are shown in figure above.

3-The input unit allows and instructions to be fed to he computer system from the outside world,in computer acceptable form.

4-The Input Interface transform,the input data and instructions fed to the computer,through its input devices,into the binary codes,which are acceptable to the computer.

5-The Output Unit allows the computer system to supply the information,obtained  from data processing to the outside world, in human acceptable (reliable) form.

6-The Output Interfaces transform the information obtained from data processing,from binary form to human acceptable (readable) form.

7-The Storage Unit of a computer system, holds the data and instructions to be processed and the intermediate and final results of processing.The two types of storage are - primary and secondary storage is slower in operation larger in capacity,cheaper in price and can retain information even when the computer system is switched off or reset.

8-During Data processing,the actual execution of the instructions,takes place in the Arithmetic Logic Unit (ALU) of a computer system.

9-The Control Unit of a computer system manages and coordinates the operation of all the other components of the computer system.

10-The control unit and the arithmetic logic unit of a computer system are jointly known as the Central Processing Unit (CPU),which serves as the brain of the computer system,and is responsible for controlling the operations of all other units of the system.

11-A Computer is often referred to as a computer system because it is made up of integrated components (Input,Output,Storage and CPU) which work together to perform the steps called for in the program being executed.