Categories of Commercial Computers

Computers can be classified into following categories:

Classification according to the logic used.

Classification according to size.

Classification according to the logic used (based on operating principal):

Analog Computers

These computers recognize data as a continuous measurement of a physical property (e.g. Rotaional or displacement). Their output is usually in the form of readings on dials or graphs. Voltage, pressure, speed and temperature are some physical properties that can be measured in this way. Example:

A service station gasoline pump contains an analog processor that connects fuel flow measurements into quantity and price values.

Automobile speedometer, etc.

The features of analog computer 

Operates by measuring voltage and currents.

•         The accuracy of this computer is poor.
•        Single purpose machine, hence low cost and easily programmable.
•        It has limited memory space.
•        It is not versatile.

      

Digital Computers

These are high-speed programmable, electronic devices that perform mathematical calculations, compare values and store the results; they recognize data by counting discrete signals representing either a high ("on") or low ("off") voltage stale of electricity. Numbers, alphabets and special symbols can all be reduced to representation by 1's and 0's. Example: Personal computers. Scoreboard that directly counts discrete values such as the time left to play and the score of each team. 

Features of Digital Computer

•   It works by counting numbers.
•   The accuracy of this machine is very high.
• It has large memory capacity.
• It is a multipurpose machine, hence high cost and difficult to program. 
• It is versatile in nature.

                                         

Digital computers are further grouped into two categories:

a. General-purpose digital computers

The digital computers, which can theoretically be used for any type of application, from scientific to commercial file processing, are called general-purpose digital computers. For instance, the computers that are used for payroll, graphs, analysis, account, banking system etc.

b. Special purpose digital computers

These are those digital computers, which are made and used for specific job or narrow range of task. The set of instructions required for that task is permanently stored in the computer memory.  For instance, the computer installed in automobiles to control fuel, braking systems.

Hybrid Computers

Hybrid computer is a data processing device, which combines the best features of both analog and digital computers. It helps the user to process both continuous and discrete data. They have the speed of analog computers and the accuracy of digital computers. In the hybrid computer, a converter is fixed to convert the analog signals into digital signals and digital signals into analog signals. They are special purpose machines. These machines are generally used in scientific applications, aeroplanes and industrial control processes.

Example: A computer used in hospitals, where analog section measures patients' temperature or heart function and the digital section records and prints out in number.

Classification according to size:

There are both large and small computers.

Large computer systems

They have been traditionally divided into three main categories:

A. Super Computers

A super computer is generally characterized as being the fastest, most powerful and most expensive computer. Supercomputers recognize the largest word lengths of 64 bits or more. They have large memories with high processing speed. They can process up to a billion operations in a second. These computers have multiple Central Processing Units that can process multiple instructions at a time, known as parallel processing.  They can take input from over 10000 individual workstations. Supercomputers are widely used in scientific applications such as aerodynamic design and simulation, processing of geological data, processing of data regarding genetic coding and collecting and processing weather data.

Ex: CRAY-1, CRAY XMP-14, NEC-500

B. Mainframe Computers      

A mainframe computer is usually slower, less powerful and less expensive than Super computers. It is general-purpose computer system designed for large-scale data processing. Mainframes process data at several million instructions per second (MIPS). More than 1000 remote workstations can be accommodated by a typical mainframe computer. A technique that allows many people at terminals to access the same computer at one time is called time-sharing, Mainframes are used by banks and many businesses to update inventory etc.

Ex:       IBM 1401 Hp 900 model 3705/300

            IBM 4300 Hp 900 model 8705/400

C. Mini Computers

 Mini computers are general-purpose computers, smaller than mainframe and give computing power without adding the prohibitive expenses associated with larger systems. The mini computer's size prevents it from being portable but it can be moved more easily than a mainframe. Mini computer integrates commercial and technical operations better than the more powerful computers. It is generally easier to use. Mini computers are well adapted for functions such as accounting, word processing, database management, Statistical packages for social sciences, Computer Aided Design (CAD) and Numerical Analysis etc.  

Ex: IBM     AS/400/B60

HP  900           VAX 8842

Small Computer Systems (Micro Computer)

A microcomputer is the smallest, least expensive of all computers. The prefix "micro" refers mainly to the physical size of the computer and its circuitry rather than its capabilities. The essential differences between microcomputers and mainframe or mini computers are that microcomputers have smallest memory and less power, are physically smaller and permit fewer peripherals to be attached. Microcomputers are also called “Personal computers.”