Computers are devices that perform calculations or store and retrieve information according to instructions. They can be divided into two main categories: digital and analog. Digital computers use the binary number system, which consists of two digits (0 and 1). Analog computers use a continuous range of values.
The first electronic computer was the ENIAC, which was invented in the United States in 1945. It used vacuum tubes and could perform 5,000 operations per second. The first commercial computer, called UNIVAC I, was delivered to the United States Census Bureau in 1951.
In 1964, IBM introduced the System/360 family of computers, which included models with 8-bit, 16-bit, 32-bit, and 64-bit word sizes. The System/360 was very successful and led to the development of IBM’s current line of mainframe computers. In 1969, ARPANET (the precursor to the Internet) was established using Interface Message Processors (IMPs), which were early versions of what we now call routers.
The first personal computer (PC), called the Altair 8800, was introduced in 1975 by MITS Corporation. It used an Intel 8080 microprocessor chip and had 256 bytes of memory (which is less than 1 kilobyte). The Apple II PC was introduced in 1977 by Apple Computer Company; it had 4 kilobytes of memory and used a MOS Technology 6502 microprocessor chip. In 1981 IBM introduced its first PC compatible machine; this machine became very popular due largely to its use of MS-DOS (Microsoft Disk Operating System).