Minicomputers

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Minicomputers

Minicomputers (sometimes called the "mini") are defined primarily in terms of price and size. Minicomputers generally have a word size of 8–18 bits (register size); a memory size of 32,000–64,000 16-bit words, or 16,000–32,000 32-bit words; a processing speed of 200–300 kilo-instructions per second (KIPS); and a price in the range of $3,000 to $50,000. The large processors of the 1970s and 1980s cost from $50,000–$100,000 to several million dollars. The mini was an economical solution to low-end, smaller computing.

The mini was relatively small, less than 0.6-meters (two-feet) wide, and mounted in a rack. Its advent also spurred the development of peripherals that had to be developed at a price consistent with that of the mini while providing satisfactory performance.

Some applications of the minicomputer were data acquisition, process control, time-sharing, and terminal and peripheral communication control. Process control systems involve data acquisition and feedback to control the process, with or without human intervention. The machine-machine system involves the use of a minicomputer as a front end communications processor or a peripheral control unit. This offloads the tasks of error checking, polling, hand-shaking, line buffering, or other formatting from the large central processor. Minicomputers were also used in stand-alone or single-user mode to do human research such as reaction time studies. The stand-alone mode was necessary to provide timing independent of other processes.

The minicomputer was capable of performing under normal environmental conditions and did not require the extensive power and air-conditioning of larger but more delicate systems. It could also be made rugged ("rugged-ized") to perform in adverse environments such as combat fronts. It was a more durable tool than many larger systems and more easily transportable. It could also be used in factories for process control, inventory and manufacturing control, or as a satellite to a larger computer, feeding data to it or acting as a peripheral device.

The growth of the minicomputer market was relatively rapid. It began in the 1960s and expanded in the early 1970s. The key to its growth was the development of large-scale integrated circuitry (LSI). Integrated circuits (ICs) were developed in the 1960s, initially with a single function on a chip, a logic gate or flip-flop (memory element); then with medium-scale integration (MSI), with a dozen or more functions on a chip; and, finally, large-scale integration (LSI), with more than 100 functions on a chip; and later, with very large-scale integration (VLSI). The availability of large-scale integration lowered the costs of developing a computer and also aided in the replicability of the manufacturing process. This enabled batch processing , as well as modular construction and the interchangeability of parts.

Minicomputer peripherals included cassette tape units, minidisks, cartridge disks, and the cathode ray tube (CRT) . However, there was initially a discrepancy between the cost of the central processing unit (CPU) and the cost of the peripherals because of declining costs in the batch processing of CPU and memory microchips.

The minicomputer system was designed to balance the needs of input/output and storage with the computing needs in a cost-effective way. The software for a mini usually consisted of an assembler, editor, several compilers , and utility programs. The operating systems were of various types, including paging systems. Some minicomputers were application-specific; the hardware and peripherals, as well as the software, or driver program, were tailored to the application. This was especially true of military, airborne, or other special uses of the computer. Minicomputers were used in some of the early American space launches.

It is difficult to compare the minicomputer with the personal computers of today. The personal computer is far larger in terms of memory capacity and peripheral storage capacity, and faster in processing speed. It is also considerably cheaper in absolute cost and in cost relative to performance. However, the minicomputer can be compared to the personal computer as a low-cost alternative to computing with a large system. In certain applications, such as large-scale computing or complex graphical designs, a maxicomputer or large computer—even a supercomputer—is still required.

Some of the compilers available with minicomputers were FORTRAN (FORmula TRANslator) IV, Algol, RPG, Basic (interpreter), and, eventually, C and Unix. C and the associated operating system Unix, and much of the early work at Bell Laboratories on computers and computer software, were done on the PDP-series computers (Digital Equipment's PDP-x family). Other manufacturers were Data General, Varian, Hewlett-Packard, Honeywell, and Texas Instruments.

The number of registers in a mini was limited. Although the PDP-11 and other members of the family had eight registers, others had fewer. Some minicomputers were single-address machines, with a single register or accumulator. Others, such as the PDP-11 and Interdata 70, were two-address machines, indicating several registers—one of which is addressed in the instruction, along with a memory location. The I/O was sometimes under program control, tying up the CPU or central processor, while others offered direct memory access (DMA), stealing cycles from the CPU, but otherwise operating independently of the CPU, through a separate I/O controller or channel.

Disk operating systems were developed for minicomputers because the amount of main memory was limited. If the operating system took up between 12 kilobytes (K) and 20K of memory and only 32K was available, a limited amount was left for user programs. The operating system was divided into resident and non-resident portions, and the non-resident portions of the operating system as well as user programs were rolled, or swapped, in and out as necessary. One machine of the PDP series implemented time-sharing by swapping whole programs in and out of main memory, with one user program resident at a time.

The minicomputer led to an unexpected development. The PDPseries of computer was expanded in 1975 to the Virtual Address eXtension (VAX) series of computers. The VAX was a 32-bit machine that was comparable to a mainframe, though not in terms of the large cost of some of the mainframes of the day (several million dollars). As of the year 2002, the VAXseries of computers was still in use in universities and elsewhere. It is a phenomenon in the fast-moving computer market.

The development of the VAXfamily of computers coincided with the development of the personal computer. The introduction of the IBM personal computer (8086/8088) in 1981 started another revolution that continues to the present day.

see also Mainframes; Microcomputers; Supercomputers.

Roger R. Flynn