1.1 Computer types
A  computer can be  defined as  a fast electronic calculating machine that accepts  the
(data)   digitized input   information   process it as per the list of internally stored instructions and produces the resulting information.  
List  of  instructions are called programs & internal storage  is called computer memory. The different types of computers are

1.Personal computers:  This is the most common   type  found  in  homes, schools,  Business  offices etc.,  It is the most common type of desk top computers with processing and storage units along with various in put and out put devices.
2.Note book computers: These are compact and portable versions of PC
3.Work  stations:  - These have high resolution input/output (I/O) graphics capability, but  with same dimensions as that of desktop computer.
These are used in engineering applications of interactive design work.

4.Enterprise systems: -
 These  are used for business  data  processing in  medium  to large corporations that require much more computing power and storage
 capacity than work stations. Internet associated with servers have become a dominant worldwide source of all types of information.
5.Super computers:  - These   are used   for large   scale numerical   calculations required in the applications like weather forecasting etc.,
1.2 Functional unit
A computer consists of five functionally independent main parts input,  memory,  arithmetic logic unit (ALU), output and control unit. 

 

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Fig a : Functional units of computer   
Input device accepts  the  coded  information as  source  program  i.e. high level language.

This is  either stored in  the  memory  or immediately  used  by the processor to perform the desired operations. The program stored in the memory determines the processing steps. Basically the computer converts one source program to an object program. i.e. into  machine language. Finally the results are sent to the outside world through output device. All  of  these actions are coordinated by the control unit.  Input un it: The  source  program/high  level  language  program/coded  information/simply data is fed to a computer through input devices keyboard is a mos t  common type. Whenever a key is pressed, one corresponding word or number is translated into its equivalent binary  code over a cable & fed either to memory or process  or.  Joysticks, trackballs, mouse, scanners etc are other input devices.  Memory unit: Its function into store programs and data.
It is basically to two types  
1.Primary memory
2.Secondary memory


1. Primary memory:
 - Is the one exclusively associated with the processor and operates at  the  electronics  speeds programs must  be  stored in  this  memory while  they are being executed. The memory contains a large  number  of  semiconductors storage  cells.Each ALU Processor Control Unit capable of storing one bit of information. These are processed in a group of fixed site called word.  To  provide easy access to a word in memory, a distinct address is associated with each word location. Addresses are numbers that
 i dentify memory location.  Number  of bits  in  each  word is  called  word length  of  the computer. Programs must reside in the memory during execution. Instructions and data can be written into the memory or read out under the control of processor.  Memory in  which  any  location can  be reached  in  a  short  and fixed  amount of time after specifying its   address is called random-access memory (RAM).  The time  required    to  access  one word    in  called  memory access time. Memory  which  is  only  readable  by  the user  and contents  of which    can’t  be altered    is  called  read  only memory (ROM) it   contains operating sys  tem.  Caches  are  the small  fast RAM  units, which  are  coupled with   the  processor  and  are aften  contained on the same IC  chip to  achieve high  p erformance. Although primary  storage is essential it   tends   to be   expensive.


2. Secondary   memory:  -Is  used  where large  amounts  of  data  &  programs  have  to  be  stored, particularly information that is accessed infrequently. 

Examples:Magnetic disks & tapes, optical disks (ie CD-ROM’s), floppies etc., Arithmetic logic unit (ALU): Most of the computer operators are executed in  ALU of   the processor   like  addition, subtraction, division, multiplication, etc. the operands are brought into the ALU from  memory  and  stored in high speed storage  elements called  register.Then  according to the instructions the operation is performed in the required sequence. The control and the ALU are may times faster than other devices connected to a computer system. This enables a single processor to control a number of external devices such as keyboards, displays, magnetic and optical disks, sensors and other mechanical controllers. Output unit: - These actually are the  counterparts of input unit.  Its  basic  function  is  to  send  the  processed results to the outside world.
 
Examples:-
 Printer, speakers, monitor etc. Control  unit:-It effectively is the nerve center that sends signals to other units and senses their states. The actual timing signals  that govern the transfer of data between input unit,  processor, memory and output unit are generated by the control unit.  Source: https://elearningatria.files.wordpress.com/2013/10/cse-iv-computer-organization-10cs46-notes.pdf

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