21. Computer

A computer is a machine for manipulating data according to a list of instructions. Computers take numerous physical forms. Early electronic computers were the size of a large room, consuming as much power as several hundred modern personal computers. Today, computers can be made small enough to fit into a wrist watch and be powered from a watch battery.

Society has come to recognize personal computers and their portable equivalent, the laptop computer, as icons of the information age; they are what most people think of as "a computer". However, the most common form of computer in use today is by far the embedded computer. Embedded computers are small, simple devices that are often used to control other devices—for example; they may be found in machines ranging from fighter aircraft to industrial robots, digital cameras, and even children's toys.

Every computer has a memory by virtue of which it stores data. All data is in form of numbers. A computer stores a number into a specific location in its memory and later retrieves the value. Most computer memories store data in binary number form. In the binary number system, numbers are represented by sequences of the two binary digits 0 and 1, which are called bits. In a computer, the two possible values of a bit correspond to the on and off states of the computer's electronic circuitry. The bits are grouped together in the memory in order for them to represent larger values. Eight bits make up a byte and this collection of bits can represent decimal numbers ranging from 0-255.

Memory is measured in term of capacity instead of speed. It is expressed in terms of kilobytes, megabytes, and gigabytes. Although the prefixes kilo-, mega-, and giga-, are taken from the metric system, they have a slightly different meaning when applied to computer memories. The word computer memory is usually used for the data storage devises either the data storage in volatile form or in the non-volatile form. The data storage can be of many types like optical discs, hard drive, and there is also storage medium i.e. RAM.

The RAM computer memory is different in this aspect that it is not a permanent storage medium, like the others storage mediums like hard drive or other magnetic storage mediums. The unit of speed of the primary storage medium is measured in nanosecond and for the secondary storage medium, the unit of measurement is millisecond and for the tertiary storage medium the unit of measurement is second.