COMPUTER TECHNOLOGY Minicomputers JOSEPH B. SIDOWSKI, UNIVERSITY OF I/O buses. Multiply/divide, read-only and then develop and sell applied systems. SOUTH FLORIDA, Tampa, Florida33620 memory (ROM), special instructions, and At the same time, increased competition other features are standard with some and the introduction of new modular The characteristics and costs of machines, but optional with many others. components have sent the prices of minis minicomputers are discussed along with A real-time clock is generally an additional down so that less than $3,000 will peripherals, time-sharing, and turnkey cost. purchase a machine with 4k memory,8-bit systems. The article concludes with a Although alike in many respects, word length, and a l-microsec cycle time. survey of over 100 small computer minicomputers differ in regard to memory The newest addition to the Digital systems. cycle and access times, input/output Equipment Corporation (DEC) line, the methods, interrupt systems, addressing, PDP-8/E, is priced at $4,990 for the basic For the purpose of this paper, a survey software, options, and so on. Perhaps the package and $6,500 with ASR-33 was made of binary computersystemswith most common characteristics of the great teletypewriter. The DEC PDP-I, basic system costs of less than $30,000. majority of these machines, in addition to introduced in 1958, sold for approximately Perhaps this is misleading as a price low cost, is dedication to a particulartask. $120,000. Its more capable grandson,the definition of minicomputers, sincesomeof Large-scale computers are often used for a PDP-8/L, was introduced 10 years later at the machines are expandable to such a variety of purposes and handle large $8,500. degree that a fully instrumented system problems; they cost more, generally, Although prices are down, it is with maximum memory and attached because of greater memory size, word important to note that the basic units options can run well over $100,000. Some lengths, and attachments, such as disks, offered by many manufacturers are manufacturers design their machines so tapes, printers,and so on. stripped-down models offering little that the basic processor can be sold at The potential buyer, of course, should flexibility. Some companies offer a minicomputer prices and then expanded know what he wants out of the machinein relatively inexpensive basic package, but modularly into a system to meet most terms of task performance(s), future accelerate the prices of optional, but needs. So a more appropriate definition demands or plans for system expansion, necessary, units. If a teletypewriter (the might have limited the basicsystemcost to and how much he is willing to pay. In basic minicomputerI/O device) is optional, about $18,000 or less. The $30,000 limit laboratories, the minicomputer has the added cost may range from $800 to was exercised because it provides the functioned as a monitor, a controller, a $2,000. Expanding the metnory of a basic reader with a broader comparative view data-acquisition unit, and as a data reducer $8,000 4k system, including and incorporates. systems which are and analyzer. If the buyer's interest is teletypewriter, to 8k could cost $6,000 presently in use in academic and research merely to use the machine as an oversize merely for the additional 4k. Yet 8k may facilities. Opinions differ over what really calculator or a mini number cruncher,he is be needed for the use of FORTRAN, constitutes a minicomputer. An oft-quoted better advised to travel another route. The which the user may require. (Typically, figure is $25,000 or less. Other differences real laboratory value of the small computer memory costs represent about 50% of the relate to capacityand function. is .in its capacity for on-line real-time total processor cost for minis.) A 4,096-word memory can be functioning. A realistic feel for the Fortunately, as prices drop, it may be considered a minicomputer basic, with necessary equipment and the costs of possible to trade off between .program most memories expandable at extra cost to on-line real-time laboratory computer efficiency and memory size. As the prices 16k or 32k. Several are expandable to 65k, systems can be obtained from the reports of additional memory units go down, the and a few basic processors are cited at l k by Haber (1968), Huesmann (1969), languages designed for on-line operations or 2k (a few companies offer central Mayzner (1968), Moise and Jarrard (1969), willbe usedmore. processors with no memory). Memory and Restle and Brown (1969). Of course, Technological trends are toward more cycle times range from 775 nsec (one the machines can be connected to a cost-effective minicomputer processors and machine lists .5/30 microsec) to commonmassdata base,e.g., a magtape or peripherals. The costs of bipolar and MOS 8.0 microsec. Computer word lengths are disk. They can also be tied to larger (metal oxide semiconductors) digital logic 8, 12, 16, 18, 20, and 24 bits, with the computers and to a common data base so electronics are decreasing while speeds 16-bit-word machines being the most that the smaller machine can call for improve. And thin film and semiconductor popular. The 16-bit outnumber the assistance when its capabilities are strained. memories should challenge core memories. 8-bit-word computers by a factor of 3 or 4 In a data communication system, the In projecting 106 -capacity memory trends, to 1~ The other word lengths make up minicomputer can be utilized as a terminal Zirnbel! cites core memory costs over the about 10% of the total. and as a preprocessor of data before next 5 years as 2¢-4¢/bit (0.25- to Minicomputer central processors operate sending it on to a largercomputer. l-microsec cycle time), magnetic thin film in a manner similar to that or the larger Whatever the need, there are many costs at l¢-3¢/bit (0.10- to 0.5-microsec computers--they are not, however, companies to choose from. The number of cycle time), and semiconductor memories considered in the electronicdesk calculator minicomputers on the market has increased at ~d-2¢/bit (0.025- to 0.2-microsec cycle category, although the machines can fulfill tremendously over the last 2-3 years. New time). Plated wire memories provide most of the functions. The processors are manufacturers have appeared, older firms nondestructure readout, and stored data typically fixed-point systems with single have introduced new models or altered old can be modified easily. They alsoconsume address instructions which contain core ones, and middlemen have emerged who little power or space. An all-MOS central memory, registers, an arithmetic unit, and take computers manufactured by others processor might have memory-centered Behav. Res. Meth.& Instru., 1970,Vol. 2 (6) 267 logic, erasable and ROM stores, and more time sharing, and a survey of over 100 faster memory cycle time may have a complex interrupt logic. small computer systems. Obviously, this longer run time than Computer B. Industry projections made in 1969 information is not new. Minicomputer Memory parity checks and memory indicated an annual growth of 350/0-50% in hardware and software characteristics are protection features are included in some the minicomputer market. The most recent cited in company brochures and reviewed machines, but generally are optional. Parity concensus (September 1970) is that the in a number of technical publications. check systems are used to facilitate error mark will be missed by a substantial Many articles in this journal and elsewhere detection. Memory protect requires that amount. Some of the smaller firms are describe computer-controlled laboratory special instructions be used to write in finding sales outlets drying up, and a few systems; Utall's volume (1968) provides certain regions of memory; otherwise, the firms have decided to postpone public information on real-time computer protect prevents writing in that region. stock issues until market conditions techniques in psychology. And the small Lower- and upper-bound registers improve. computer handbooks supplied by frequently define the restricted core area. Of course, the most substantial manufacturers (e.g., Digital Equipment Paging techniques are used by most minicomputer manufacturer has been the Corporation , Varian, Data General, minicomputers to address core. The size of Digital Equipment Corporation. Thousands Raytheon, and others provide a wealth of the page is determined by the number of of the DEC machines have been sold, and information. adjacent core locations that are specified their service network is worldwide. But by the address field. Although page sizes other companies tum out very fine FACTORSINFLUENCING range from 256 to 2,048 words, the most machines also, and their products warrant MINICOMPUTER OPERATIONS common is 256. So the memory might be investigation. For the behavioral scientist, Several of the important factors thought of as being divided into sections of the DEC Users Society (DECUS) offers an influencing minicomputer operations and 256 words each. advantage because of the opportunity for comparisons are listed below. As noted Instruction sets and speeds must be information exchange between users of earlier, the most critical requirements are considered also. For data and instructions similar machines; selected DECUS that the computer perform the task to be stored interchangeably in memory, Conference Proceedings are reported in the required at the necessary speeds, and that data word length and instruction word September 1969 and March 1970 issuesof the price is right. Comparisons of hardware length should be the same. Some 8-bit this journal. Several other companies have and software capacities are important only machines use two words per instructions, recently organized users' societies (e.g., insofar as they satisfy the critical operating like the 16-bit processors.