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Block diagram of digital computer

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Block diagram of digital computer and the functioning of its blocks

A digital computer is considered to be a calculating device that can perform arithmetic operations at enormous speed. It is
defined as a device that operates upon information/data. To be able to process data the computer is made of various functional units to perform its specified task.

1. Input Unit: The data and instructions are inputted into the computer using keyboard which is one of the most commonly used input devices. Other commonly used input devices are mouse, floppy disk drive, magnetic tape etc. Thus, we can conclude that, all the input devices accepts the data and instruction from outside world, convert it to a form that the computer can understand, supply the converted data to the computer system for further processing.

2. Storage Unit: The storage unit of a computer holds data and instructions that are entered
through the input unit, before they are processed. It stores programs, data
as well as intermediate results and results for output. Its main function
is to store information.

The various storage devices can be divided into two main categories:

(a) Primary Storage (Main Memory):
This memory is generally used to hold the program being currently executed in the computer, the data being received from input device, the intermediate and final results of a program. The primary memory is temporary in nature. The data is lost when the computer is switched off. The data is electronically represented in the main memory chip’s circuitry and while it remains in the main memory, central processing unit (CPU) can access it directly at a very fast speed. Primary storage has limited storage capacity because it is very expensive and generally made up of semiconductor devices.

(b) Secondary Storage (Auxiliary Memory):
It is used like an archive. It stores operating system, data files, compilers, assemblers, application programs etc. The Program and data if needed by central processing unit (CPU) are first transferred from the secondary memory to main memory. The results are also stored in secondary memory to main memory. It is a mass storage memory, slower but cheaper. It is non-volatile in nature i.e. data is not lost even if the power supply is switched off. Some of the most commonly used secondary storage devices are Hard Disk, Compact Disk etc. Their access time is in milliseconds.

3. Central Processing Unit (CPU) :
The control unit and arithmetic logic unit of computer are together known as central processing unit (CPU). The CPU is like brain and performs following functions: It performs all calculations, it takes all decisions, and it controls all units of a computer.
A personal computer may have CPU-IC such as Intel 8088, 80386, 80486, Celeron, Pentium, Pentium Pro, Pentium II, III, IV, Dual Core, AMD etc.z

(a) Control Unit: It controls all other units in a computer. The control unit instructs the input unit where to store the data after receiving it from user. It controls the flow of data and instructions from the storage unit to Arithmetic logic unit (ALU), it does not perform any actual processing of data. It manages and coordinates the entire computer system and synchronizes its working, thus referred to as “Central Nervous System” or “Brain of the Computer”.

(b) Arithmetic and Logic Unit:
The function of an Arithmetic logic unit (ALU) is to perform arithmetic and logical operations such as addition, subtraction, multiplication, division,AND, OR, NOT, Exclusive OR etc. It also performs increment, decrements, shift and clear operations. It is the place where the actual execution of instructions takes place during processing operation. To be precise, all calculations and comparisons are made in the Arithmetic and Logical Unit (ALU).

4. Output Unit: 
An output unit performs the reverse operation of that of an input unit. As computers, work with binary code the results produced are also in binary form. So before applying results to the outside world it must be converted to human acceptable (readable) form. So, it supplies information obtained from processing to outside world. Units called output interfaces accomplish this task.

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