Block Diagram of the Computer

Computer System

Internal architecture  of computers  differs  from one system  model to another.  However,  basic organization  remains the  same  for  all  computer   systems. In this figure,  solid  lines  indicate  flow  ofinstruction   and  data,  and  dotted  lines  represent   control  exercised   by control unit.  It displays  the  five  major  building  blocks  (functional   units)  of a digital  computer  system.  These  five  units correspond  to the five basic  operations   performed  by all computer  systems.  Functions   of each  of these  units  are described below:

1. Input Unit

The  input  unit  links  a computer   with  its external  environment. An input unit accepts instruction and data from the user and communicates them to the computer. Data  and instructions   enter  a  computer   through  an  input unit in  a  form  that  depends  upon   the  input  device  used.  For example,  data can  be entered  using  a keyboard   in a manner  similar  to  typing  and  this  differs  from  the  way  in which  data is entered through  a scanner another type of input device.  However,  a computer’s   memory  is designed to  accept  input in binary code  and hence, all  input  devices  must  transform   input  signals  to  binary  codes.  Units called  input interfaces accomplish   this  transformation.   Input  interfaces  match  the  unique  physical  or  electrical characteristics   of iput devices  to the requirements   of a computer  system.

Some of the input devices are keyboard, mouse, light pen and so on.

2. Output Unit

Just as humans communicate with computers through input devices, the computer can communicate with human beings using output devices. Hence, it links a computer with its external environment. A computers work with binary code, results produced are also in binary form. Therefore, before supplying the results to outside world, the system must convert them to human acceptable form. Units called output interfaces accomplish this task.

Some commonly used output devices are printer, monitor and plotter.

3. Storage Unit

Data and instructions entered into computer system through input units have to be stored inside the computer before actual processing starts. Similarly, results produced by a computer after processing to be kept somewhere inside the computer system before being passed on to an output unit. Moreover, a computer must also preserve intermediate results for ongoing processing. Storage unit of a computer system caters to all these needs. It provides space for storing data and instructions, intermediate results, and results for output.

Storage unit of all computers is comprised of following two types of storage:

i. Primary Storage

• Also known as main memory.
• Is used to hold pieces of program instructions and data, intermediate results of processing, and recently produced results of these job(s) on which the computer is currently working.
• Primary storage can hold information only while computer is on. A soon as the computer switches off or resets, the information held in primary storage is erased.
• Primary storage has limited storage capacity and is expensive.
• Primary storage of modern computer systems is made of semiconductor devices.

ii. Secondary storage

• Secondary storage of a computer is also known as its auxiliary storage.
• Secondary storage can retain information even when computer switches off or resets.
• Secondary storage holds the program instructions, data and information of those jobs on which the computer system is currently not working but needs to hold them for processing later.
• Magnetic disk is the most commonly used secondary storage medium.

4. Arithmetic Logic Unit

Arithmetic logic unit of a computer system is the place where actual execution of instructions takes place during processing operation. To be more precise, calculations are performed and all comparisons are made in ALU. Data and instructions stored in primary storage before processing are transferred as and when needed to the ALU where processing takes place. Intermediate results generated in the ALU are temporarily transferred back to primary storage until needed later. Hence, data may move from primary storage to ALU and back again to storage many times before processing is over. However, almost all ALU’s 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.

5. Control Unit

This unit checks the correctness of sequence of operations. It fetches the program instruction from the primary storage unit, interprets them, ensures correct execution of the program. It also controls the input/output devices and directs the overall functioning of the other units of the computer.

Central Processing Unit

Control Unit (CU) and arithmetic logic Unit (ALU) of a computer system are together known as the Central Processing Unit (CPU). The CPU is the “brain” of a computer system. It is the “administrative section” of the computer system.  In a human body, the brain takes all major decisions and do other parts of the body function as directed by the brain. Similarly, in a computer system all major calculations and comparisons take place inside the CPU and CPU is responsible for activating and controlling the operations of other units of the computer system.