Now that we know what a computer is, we can consider the types of computers used in information systems today. In terms of size (the most common means of classification), computers fit broadly into three types—mainframes, minicomputers, and microcomputers, which are also called personal computers.
A mainframe is a very large, expensive computer (usually selling for well over $1 million) that requires a special support staff and a special physical environment (for example, air conditioning). Mainframes are usually housed in a computer center and are generally used in large business, government, or academic institutions where they support multiple users (usually more than 100 at one time) and can handle multiple processing tasks concurrently. This means that a mainframe can, for example, do a statistical analysis for one user, print a report for a second user, and process student grades concurrently.
Users access a mainframe on PC in terminal mode that interface with the computer. These terminals are usually composed of a keyboard for entering data and instructions and a display screen for viewing the work and any resulting output. They usually do not have their own computing capabilities.
A subset of mainframes is supercomputers, or "monsters," which are the biggest and fastest computers in use today. These very large computers are used almost exclusively for research projects that require extremely high-speed processing and large storage capacities.
At the other extreme in terms of size are personal computers (PCs), which are small, one-user computers. These computers are relatively inexpensive to purchase ($500 to $5,000) and do not require a special environment or special user knowledge. They fit on a desktop and are sometimes referred to as desktop computers. Such personal computers are commonly used by a single user to handle one task at a time. PCs are slower than mainframes and cannot store as much data.
A special type of terminal is a workstation, a high-performance, single-user device that has characteristics of both a stand-alone PC and a terminal. Like PCs, workstations have built-in computing power. However, they are also connected to a minicomputer or a mainframe and to other workstations to take advantage of the increased computing power of the larger machine and to share information. Workstations also differ from PCs in that they can carry out multiple tasks concurrently. The NeXT computer, developed by Steven Jobs—cofounder of Apple Computers—is considered by many to be a workstation because of the capabilities it offers beyond those available on a PC. Hewlett-Packard and Sun are also well known for their workstations.
Currently the dividing line between personal computers and workstations is hazy as personal computers become more powerful and can support multiple tasks. For simplicity's sake, in the remainder of this text we will include workstations with personal computers since both computers have single-user capability. Similarly, we will include minicomputers and supercomputers whenever we refer to mainframes since each can accommodate multiple users.
In addition to classifying computers by size, we may also classify them by their type of logic and their purpose. In terms of type of logic used, a computer can be either a digital computer or an analog computer. A digital computer uses numbers and is therefore a counting machine. An analog computer uses physical relationships and is a measuring machine. Analog computers are often used to work with physical measurements. Today almost all computers are digital, so we will consider only this type of computer.
In terms of its purpose, a computer may be classified as a special-purpose computer or a general-purpose computer. A special-purpose computer is designed for only one purpose. The microprocessor chips installed in many of the smart machines discussed earlier are actually special-purpose computers that can carry out only a designated purpose. Similarly, the computers used for arcade games or for guiding the National Aeronautics and Space Administration's (NASA's) space shuttles are also examples of special-purpose computers that cannot be used for other purposes. A general-purpose computer, on the other hand, can be used for many applications. For example, the same general-purpose computer may be used to play games, to handle payroll computations, to use graphics to design buildings, or to solve complex mathematical problems. In this text, primary attention is given to the general-purpose digital computer, because it is the most commonly encountered type of computer.