Until the mid1960s, digital computers were powerful, physically large and expensive. What was really needed though, were computers of less power, a smaller memory capacity and without such a large array of peripheral equipment. This need was partially satisfied by the rapid improvement in performance of the semiconductor devices (transistors), and their incredible reduction in size, cost and power; all of which led to the development of the minicomputer or mini for short. Although there is no exact definition of a minicomputer, it is generally understood to refer to a computer whose mainframe is physically small, has a fixed word length between 8 and 32 bits and costs less than U.S. $100,000 for the central processor.
Many minicomputers are used merely for a fixed application and run only a single program. This is changed only when necessary either correct errors or when a change in the design of the system is introduced. Since the operating environment for most minis is far less varied and complex than large mainframes, it goes without saying that the software and peripheral requirements differ greatly from those of a computer which runs several hundred ever-changing jobs a day. The operating systems of minis also usually provide system access to either a single user or to a limited number of users at a time. Since many minis are employed in real-time processing, they are usually provided with operating systems that are specialized for this purpose.
For example, most minis have an interrupt feature which allows a program to be interrupted when they receive a special signal indicating that any one of a number of external events, to which they are preprogrammed to respond, has occurred. When the interrupt occurs, the computer stores enough information about the job in process o resume operation after it has responded to the interruption. Because minicomputer systems have been used so often in real-time applications, other aspects of their design have changed; that is, they usually possess the hardware capability to be connected directly to a large variety of measurement instruments, to analog and digital converters, to microprocessors, and ultimately, to an even larger mainframe in order to analyze the collected data.
