On the basis of different types of inputs, computers can be classified as:-
1. Digital computers
2. Analog computers
3. Hybrid computers
Classification on basis of technology
1. Analog computers:- ANALOG is a greek word which means establishing similarities between two quantiries. It measures and answers in HOW MUCH. It represents data not as number but as physical analogies. In Analog computers similarities are established on the form of current or voltage signal. Analog computers operate by measuring rather than by counting. In an Analog computer the variable is an electrical signal produces analogous to the variable of physical system. The Arithmatical operations are performed electronically using small signal amplifier blocks. The answers so obtained are then decoded into mathematical form or in forms of graphs. Thus by analog computer it is very easy to get graphical results directly. The speedometer is a good example of analog computation. It receives an input by continuous variable rotational speed of the drive shaft and provides output according to the pointer movement of the dial which in term indicates speed of a car. Hence the speed of the car is measured by analogically converted into circuler motion and measured by counting the member of revolutions. Mathematically speed is derivative of distance travelled with respect to time i.e. v=ds/dt with a differential equation which can be easily represented by an analog computer. It is very fast but the accuracy is poor. These devices are used in electronic industry, power plants and chemical plants.
2. Digital computers:- A digital computer counts and answers in HOW MANY. It represents data as numbers and performs mathematical and logical operations on that data. In digital computers mathematical expressions are finally represented as binary digits '0' and '1'. All operations are done by using these digits at a very high rate. The digital computer basically knows only how to add. Remaining operations like multiplication, division and exponentation are first converted into addition and then calculated. Just like digital watches which directly counts the seconds and minutes in an hour. Digital processors also count discrete values to achieve desired output results. These computers are used in engineering, mathematical, statistical operation, research and accounts.
3. Hybrid computers:- Hybrid computers are combination of all good qualities of both analog and digital computers. In such computers calculations are performed in some portion of computer and some are done in digital portion. Then it utilises the services of devices which convert analog signals into digital signals and digital signals into analog wherever necessary such devices are called as hybrid computers and are used in hospirals intensive care units. Analog devices may measure a patient's heart function, temperature and other vital signs. These measurements then are converted into numbers and supplied to digital section of the system.
Computers based on purpose:- These computers have been designed for general purpose and special purposes:-
A. Special purpose computers:- A computer designed for a specific task can be called as a special purpose computer. These systems are called as "Dedicated systems". To solve navigational problems on board a ship can be termed as special purpose computers. For example computer for controlling precision surgical instruments.
B. General purpose computers:- A computer which can perform a number of tasks is called as a general purpose computer. A personal computer can be used for a variety of applications like Word Processing, mathematical computations, inventory control and sales report etc.
Performance of a computer is also measured in terms of speed of processing, storage capacity, ability to handle a large number of input and output devices and nature of operations performed. Generally the better the performance, the larger the size and higher the cost of computers.
A. Micro computers:- Micro computers are at the lowest end of the computer range. Earlier computers were supported by single user operating systems as their hardware design has been oriented to take care of single user only. But these days they are most commonly micro-computers are personal computer (PC). They are the important gift of fourth generation computers.
Personal computers:- The PC is small in size but it is capable of handling large size applications. It can perform diverse range of functions from keeping track of household accounts to keeping records of bigger organisations. The most common applications of PC's are word processing, speadsheet calculations and database management. The other applications are desktop publishing, accounting and project management etc. They are very popular also for teaching and entertainment.
(a). PC
1. A PC has two floppy disk drives.
2. It has generally 256KB to 640 KB of main memory.
3. It contains one display unit.
4. PC's are single user computers and use MS-DOS as its operating system.
(b). PC-XT
PC's using extended technology are termed as PC-XT. A personal computer that has both a floppy drive and a hard disk drive means it has internal storage capacity in the form of hard disk.
(c). PC-AT
A personal computer using advanced technology is termed as PC-AT. A personal computer that has a floppy and a hard disk drive as well as more powerful microprocessor.
The available PC-AT's are 80286, 80386, 80486, 80586 (pentium). Basically this number shows the microprocessor number they use PC-AT 386 onward can be used as UNIX or XENIX Operating systems.
B. Mini computers:- Mini computers are between Micro computers and Main frames. Mini computers perform better and are larger in size and cost more than micro. They are designed to support more than one user at a time. They posess larger storage capacity and operate at higher speeds and also support fast operating peripheral devices like high speed printers which print hundresds of line per minute. They can also communicate with Main Frame Computers. These computers are used when the volume of processing is large and are used to control and monitor production process to analyze results of experiments in laboratories. Mini computers are also used as LAN servers.
CHARACTERSTICS OF MINI COMPUTERS
1. Mainly use 16 to 32 bit microprocessor as main CPU.
2. Provide atleast 1MB RAM which can be extended upto 8MB.
3. Support multiple terminals from 2 to 64.
4. Support hard disk from 20 MB to GB.
5. Support multiple printers.
Example:- Digital equipment's PDP 11/45, VAX 11, PCL's HALL MARK, Mighty Frame and Magnum.
C. Main frame computers:- Mainframes are more powerful than mini computers. They operate at very high speed, have larger storage capacity and can support hundresds of users. They are used to manage centralised databases. Such databases can be querried by hundreds of users who need to access information from different locations, Main frames are used as controlling node of Wide Area Networks (WAN).
CHARACTERISTICS OF MAIN FRAME COMPUTERS
1. Use 48, 60 and 64 bit addressing.
2. Provide larger amount if RAM upto 64 MB.
3. Multiple terminals are supported ranging from 128 to 256.
Example:- IBM 300 series, Boroughs B 7900, UNIVAC 1180, DEC 10 and EC 1020.
D. Super computers:- Super computers are the highest end of the computer range. They are the fastest and most expensive machines and are considered a national resource. Although initially used for weapon design. There are more than 500 conventional super computers in the world.
One of the methods in which computers are built is by interconnecting hundreds of microprocessors and making them function in parallel. These computers are capable of performing billions of calculations per second.
Some areas where these super computers are used are weather forecasting, biomedical research and aircraft design.
CHARACTERISTICS OF SUPER COMPUTERS
1. Main memory - 16 MB to 25 MB
2. CPU speed- 100 million instructions per seconds.
3. Word Length - 64 bits onwards
4. Types of peripherals -Laser Printers, very high speed disks and high speed tapes.
Examples:- CM-5 from Thinking machines Corporation, Pargon from Intel Corporations, Super computers system division and PARAM from centre for Development of Advanced Computing (C-DAC).
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