Minicomputer

Minicomputer is a largely obsolete term for a class of multi-user computers which make up 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). More modern terms for such machines include midrange systems (common in IBM parlance) workstations (common in Sun Microsystems and general UNIX/Linux parlance), and servers.

History

The term evolved in the 1960s to describe the "small" third generation computers that became possible with the use of transistor and core memory technologies. They usually took up one or a few cabinets, 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.

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 DOS, while minis were much more powerful systems that ran full multi-user, multitasking operating systems like VMS and Unix. The classical mini was a 16-bit computer, while the emerging higher performance 32-bit minis were often referred to as superminis.

The 7400 series of TTL integrated circuits started appearing in minicomputers in the late 1960s. The 74181 Arithmetic and Logic Unit 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 inline 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 (Large and Very Large scale Integration), often making the hardware organization much less apparent.

Today at the turn of the millennium few minicomputers are still in use, having been overtaken by Fourth Generation computers built using a more robust version of the microprocessor technology that is used in personal computers. These are referred to as "servers", taking the name from the server software that they run (typically file server and back-end database software, including email and web server software).

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, 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 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, Linux and FreeBSD/NetBSD. Also, the Microsoft Windows series of operating systems now includes server versions that support preemptive multitasking and other features required for servers, beginning with Windows NT. Significantly, Windows NT was written largely by designers from DEC who were responsible for the DEC VMS operating system, originally developed for the VAX minicomputer range in the 1970s. Also, 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.

Impact

Several pioneering computer companies first built minicomputers, such as DEC, 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.

See also:

• History of computing hardware (1960s-present)
• Supercomputer

List of minicomputers

• 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-92de:Minirechner

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