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Minicomputer (colloquially, mini) is a largely obsolete term for a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems (mainframe computers) and the smallest single-user systems (microcomputers or personal computers). Formerly this class formed a distinct group with its own hardware and operating systems. While the distinction between mainframe computers and smaller computers remains fairly clear, contemporary middle-range computers are not well differentiated from personal computers, being typically just a more powerful but still compatible version of a personal computer. More modern terms for minicomputer-type machines include midrange systems (IBM parlance), workstations (Sun Microsystems and general UNIX/Linux parlance), and servers.

The term "mini computer" evolved in the 1960s to describe the “small” third generation computers that became possible with the use of transistor and core memory technologies. The term came in fashion about the same time as the miniskirt and mini cars. They usually took up one or a few cabinets the size of a large refrigerator or two, compared with mainframes that would usually fill a room. The first successful minicomputer was Digital Equipment Corporation’s 12-bit PDP-8, which cost from US$16,000 upwards when launched in 1964. The important precursors of the PDP-8 include the PDP-5, LINC, the TX-0, the TX-2, and the PDP-1. Digital Equipment gave rise to a number of minicomputer companies along Massachusetts Route 128, including Data General, Wang Laboratories, Apollo Computer, and Prime Computer.

The 7400 series of TTL integrated circuits started appearing in minicomputers in the late 1960s. The 74181 arithmetic logic unit (ALU) was commonly used in the CPU data paths. Each 74181 had a bus width of four bits, hence the popularity of bit-slice architecture. The 7400 series offered data-selectors, multiplexers, three-state buffers, memories, etc. in dual in-line packages with one-tenth inch spacing, making major system components and architecture evident to the naked eye. (Starting in the 1980s, many minicomputers used VLSI circuits (Very Large Scale Integration), often making the hardware organization much less apparent.)

As microcomputers developed in the 1970s and 80s, minicomputers filled the mid-range area between low powered microcomputers and high capacity mainframes. At the time microcomputers were single-user, relatively simple machines running simple program-launcher operating systems like CP/M or MS-DOS, while minis were much more powerful systems that ran full multi-user, multitasking operating systems like VMS and Unix, often with timesharing versions of BASIC for application development (MAI Basic Four systems being very popular in that regard). The classical mini was a 16-bit computer, while the emerging higher performance 32-bit minis were often referred to as superminis.

The decline of the minis happened due to the lower cost of microprocessor based hardware, the emergence of inexpensive and easily deployable local area network systems, the emergence of the 80286 and the 80386 microprocessors, and the desire of end-users to be less reliant on inflexible minicomputer manufacturers and IT departments/“data centers”—with the result that minicomputers and dumb terminals were replaced by networked workstations and servers and PCs in the latter half of the 1980s.

During the 1990s the change from minicomputers to inexpensive PC networks was cemented by the development of several versions of Unix to run on the Intel x86 microprocessor architecture, including Solaris, FreeBSD, NetBSD and OpenBSD. Also, the Microsoft Windows series of operating systems, beginning with Windows NT, now includes server versions that support preemptive multitasking and other features required for servers.

As microprocessors have become more powerful, CPUs built up from multiple components—once the distinguishing feature differentiating mainframes and midrange systems from microcomputers—have become increasingly obsolete, even in the largest mainframe computers.

Digital Equipment Corporation was the leading minicomputer manufacturer, at one time the 2nd largest computer company after IBM. But as the minicomputer declined in the face of generic UNIX servers and Intel based PCs, not only DEC, but almost every other minicomputer company including Data General, Prime, Computervision, Honeywell and Wang Computer, many based in New England also collapsed. DEC was sold to Compaq in 1998.

Several pioneering computer companies first built minicomputers, such as Digital, Data General, and Hewlett-Packard (HP) (who now refers to its HP3000 minicomputers as “servers” rather than “minicomputers”). And although today’s PCs and servers are clearly microcomputers physically, architecturally their CPUs and operating systems have evolved largely by integrating features from minicomputers.

In the software context, the relatively simple OSes for early microcomputers were usually inspired by minicomputer OSes (such as CP/M's similarity to Digital's RSTS) and multiuser OSs of today are often either inspired by or directly descended from minicomputer OSs (UNIX was originally a minicomputer OS)

• Control Data’s CDC 160A and CDC 1700
• DEC PDP and VAX series
• Data General Nova
• Hewlett-Packard HP3000 series
• Honeywell-Bull Level 6/DPS 6/DPS 6000 series
• IBM midrange computers
• Norsk Data Nord-1, Nord-10, and Nord-100
• Prime Computer Prime 50 series
• SDS SDS-92
• Wang Laboratories 2200 and VS series

• The Soul of a New Machine - about the development of Data General’s Eclipse/MV minicomputers in the early 1980s
• Charles Babbage Institute
• History of computing hardware (1960s-present)
• Supermini

• Early mini computers, still runnable in a German computer museum