A minicomputer (colloquially, mini) is 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). The class at one time formed a distinct group with its own hardware and operating systems, but the contemporary term for this class of system is midrange computer, such as the higher-end SPARC, POWER and Itanium-based systems from Sun Microsystems, IBM and Hewlett-Packard.
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The term evolved in the 1960s to describe the "small" third generation computers that became possible with the use of integrated circuit and core memory technologies. 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. Mini computers were also known as midrange computers. They had relatively high processing power and capacity that mostly fit the needs of mid range organizations. They were used in manufacturing processes or handling email that was sent and received by a company. In the 70's they were the hardware that was used to launch the computer aided design, CAD, industry and other similar industries where a smaller dedicated system was needed.
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.
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. At the launch of the MITS Altair 8800 in 1975, Radio Electronics magazine referred to the system as a "minicomputer", although it would properly be called a microcomputer; as it was the first commercially available personal computer based on the single-chip microprocessor from Intel.
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 68020, 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 computer 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 included server versions that supported 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 second 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 Laboratories, many based in New England also collapsed. DEC was sold to Compaq in 1998.
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.
In the software context, the relatively simple OSs for early microcomputers were usually inspired by minicomputer OSs (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, while Windows NT—the foundation for all current versions of Microsoft Windows—borrowed design ideas liberally from VMS and UNIX). Many of the first generation of PC programmers were educated on minicomputer systems.
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