Classification based on memory size: digital computers are also classified in accordance with their memory size as below:
1. Micro Computers
2. Mini Computers
3. Mainframe Computers
4. Super Computers
A micro computer is called micro for two reasons. One because it is miniature in size and another because it uses micro processor. Micro processor is actually the heart cum mine of a computer. The entire CPU of a microcomputer is contained either in a single chip of thumbnail size, or on a few chips. When equipped with memory and input/output control circuitry he micro processor is known as micro computer. Early micros had a word length of 8 bits but currently 16 bit micros are available at work to develop 16 bit micros. The speed of micro computer is of the order of 100 kips. Micro computers have generally a visual display unit, keyboard, one or two floppy disk drives and printer and may be interfaced with some tools like digitizer, joy stick, tracker ball, light pen, etc. they have memory range of 16 to 512 kilobytes. Mini
disk drive may be attached with a capacity of 10 MB if needed. Winchester
The computer designed to be used by a small group 4 to 6 simultaneously is known as a mini computer multi user. Mini computers are at least five times faster than micro computers having CPU speed of approximate 500 kilo instructions per second. Such systems are characterized by their main memory size, which may be 256 KB to 12 MB and word length of 16/32 bits. They use Winchester hard disks drives of 80 MB each, 1 or 2 floppy drives, 7 to 8 printers, 9 on line printers, digitizers, plotters and fixed storage of 160 Mb and magnetic tapes.
A slicing time sharing system is used in this type of computers where a predefined amount of time (few seconds) is allocated to each user (terminal). So, let’s there are about 6 users using the system simultaneously. Then the computer will start attending all the users one by one with a specified interval of time. Say if the time interval is 5 seconds a terminal then the first user will get his time of 5 seconds again after 30 seconds of time.
These are the large size computers which occupy a large air conditioned roomful space and main memory size up to 128 megabyte. Since all its peripherals are mounted in large cabinet type of frames, these computers are also known as main frame computers. 128 users can use such a computer system simultaneously in time sharing mode and employing mini computers as a front end processor. All the first to third generation mode and employing mini computers as a front end processor. All the first to third generation computers, which work with the speed of 5 to 100 million instructions per second, fall under this group. Burroughs 800 UNIVAC 1100/60, ACOS 100 is the popular main frame computers.
Super computer means a computer having main memory of 256 megabytes. This computer is 5 million times faster than the first computer i.e., ENIAC. Since the very high cost of this machine, it is exclusively used in the organizations where the speed is the criteria. With less than 10 installations in
, these computers are employed in weather forecasting, defense etc. India
On the first super computer ILLIAC-IV, 64 different calculations could be made at a time, i.e., it was equivalent of 64 different mainframe computers. It was the first large scale array computer, which became operational in November 1975. Arithmetic can be performed in 8, 32 or 64 bit mode. The ILLIAC can execute instructions at a rate of 300 millions per second.