Minicomputer

A minicomputer, or colloquially mini, is a type of general-purpose computer mostly developed from the mid-1960s, built significantly smaller and sold at a much lower price than mainframe computers. Minicomputers are small relative to earlier and bigger machines.

The class formed a distinct group with its own software architectures and operating systems. Minis were designed for control, instrumentation, human interaction, and communication switching, as distinct from calculation and record keeping. Many were sold indirectly to original equipment manufacturers (OEMs) for final end-use application. During the two-decade lifetime of the minicomputer class (1965–1985), almost 100 minicomputer vendor companies formed. Only a half-dozen remained by the mid-1980s.

When single-chip MOSFET CPU microprocessors appeared in the 1970s, the definition of "minicomputer" subtly shifted: the word came to mean a machine in the middle range of the computing spectrum, between mainframe computers and microcomputers. The easily-misunderstood term "minicomputer" is less often applied to later like systems; a near-synonymous (IBM-adjacent) expert term for this class of system is "midrange computer".

The term "minicomputer" developed in the 1960s to describe the smaller computers that became possible with the use of transistors and core memory technologies, minimal instruction sets and less expensive peripherals such as the ubiquitous Teletype Model 33 ASR. They usually took up one or a few 19-inch rack cabinets, compared with the large mainframes that could fill a room. Later minicomputers tended to be more compact, and while still distinct in terms of architecture and function, some models eventually shrank to a similar size as large microcomputers.

In terms of relative computing power compared to contemporary mainframes, small systems that were similar to minicomputers had been available from the 1950s. In particular, there was an entire class of compact vacuum tube-based drum machines, such as the UNIVAC 1101 (1950), and the Bendix G-15 and LGP-30 (both 1956), all of which shared some features of the minicomputer class. Similar models using magnetic delay-line memory followed in the early 1960s. These machines, however, were essentially designed as small mainframes, using a custom chassis and often supporting only peripherals from the same company. In contrast, the machines that became known as minicomputers were often designed to fit into a standard chassis and deliberately designed to use common devices such as the ASR 33.

Another common difference was that most small machines before the 1970s were not "general purpose", in that they were designed for a specific role such as engineering, process control or accounting. On these machines, programming was generally carried out in their custom machine language, or even hard-coded into a plugboard, although some used a form of BASIC.^{[citation needed][example needed]} DEC wrote, regarding their PDP-5, that it was "the world’s first commercially produced minicomputer". It meets most definitions of "mini" in terms of power and size, but was designed and built to be used as an instrumentation system in labs, not as a general-purpose computer. Many similar examples of small special-purpose machines exist from the early 1960s, including the UK Ferranti Argus and Soviet UM-1NKh.

The CDC 160, circa 1960, is sometimes pointed to as an early example of a minicomputer, as it was small, transistorized and (relatively) inexpensive. However, its basic price of $100,000 (equivalent to $1,062,880 in 2024) and custom desk-like chassis places it within the "small system" or "midrange computer" category as opposed to the more modern use of the term minicomputer. Nevertheless, the CDC 160 remains a strong contender for the term "first minicomputer", provided the earlier drum machines, e.g. SEA CAB 500, are excluded as non-transistorized.

Most computing histories point to the 1964 introduction of Digital Equipment Corporation's (DEC) 12-bit PDP-8 as the first minicomputer. Some of this is no doubt due to DEC's widespread use of the term starting in the mid-1960s. Smaller systems, including those from DEC such as the PDP-5 and LINC, had existed prior to this point, but it was the PDP-8 combination of small size, general purpose orientation and low price that puts it firmly within the modern definition. Its introductory price of $18,500 (equivalent to $184,590 in 2024) places it in an entirely different market segment than earlier examples such as the CDC 160.