Behavior Research Methods & Instrumentation1975, Vol. 7 (2), 183-186

On microprocessor-based computers

E. JAMES KEHOE, LLOYD J. FREI, ROBERT W. TAIT, and I. GORMEZANOUniversity of Iowa, Iowa City, Iowa 52242

Economical microprocessor-based computers have been recently developed by Motorola and ControlLogic, Inc. Apparently, in response to the price competition, DEC has introduced th.e PDP-8/A,incorporating the memory characteristics of the microprocessor-based computers. Relative to olderminicomputers, the microprocessor-based systems and the PDP-8/A "miniprocessor" require more timeto execute an experimentally useful task. However, their real-time operating characteristics would appearto be sufficient for on-line use in behavioral control and data acquisition. The PDP-8/A's compatibilitywith older models of the PDP·8 series enables it to use previously developed software, but, for purposesof software development, the microprocessor systems offer a more powerful architecture and instructionset.

The introduction of microprocessors would appear tooffer a relatively inexpensive means for applyingcomputer technology to the acquisition and control ofbehavioral data. However, as noted by McLean (1974),microprocessors, since they are only central processingunits (CPUs), require considerable additional circuitry tofulfill memory, timing, and I/O functions to interactwith the experimental environment. Yet, in the briefperiod since McLean's observation, at least twomicroprocessor-based systems, with the necessarysupport logic and memory, have become available.Motorola incorporated its M6800 microprocessor into acomplete system, the Motorola Exorciser, while Intel's8080 microprocessor has been incorporated into acomputer system by Control Logic, Inc. To meet thiscompetition, DEC has introduced the PDP-8/A, their"rniniprocessor," as a completely packaged systemwhich can be directly interfaced to peripheral devices.As yet, there has not been an assessment of the relativemerits of these systems in behavioral applications. Toprovide a preliminary basis for such assessment, thesesystems were examined on the basis of the followingcriteria: cost of a minimal system, availability ofsoftware support, ease of software programming, andcapability of real-time operation. Through so consideringthe merits of these three systems, the present paper isintended to contribute to an understanding of theapplicability of microprocessors to behavioralinvestigations.

Two distinct characteristics have emerged in thedevelopment of microprocessor computer systems. First,in contrast to the 12- and l6-bit word lengths ofcurrently employed minicomputers, Motorola's M6800and Intel's 8080 microprocessors employ 8-bit wordlengths. Although DEC markets an 8-bit microprocessor(M7341), the PDP-8/A employs a 12-bit processor,constructed of TTL logic chips, to maintaincompatibility with the PDP-8 minicomputer series.

The preparation of this report was supported by NSF GrantGB-41531.

However, the 8-bit word length found in Motorola's8600 and Intel's 8080 microprocessors, would not placea serious limitation on the use of microprocessor systemsfor control and data acquisition, since a large number ofperipheral devices employ 8-bit codes. Moreover,principally by the use of multiple-word instructions,8-bit microprocessors achieve substantial flexibility inmemory addressing. Second, in contrast to theconditions prevailing at the time of McLean'sobservations in 1973, advances in large scale integrated(LSI) circuitry have permitted the construction ofmicroprocessor-based computer systems with less than24 chips, thus markedly reducing maintenancerequirements. Furthermore, the level of technicalcompetence required for such maintenance would notappear to exceed that possessed by users of solid statelogic modules (e.g., BRS).

In designing these computers, the manufacturers haveprovided the potential for the user to tailor the memoryconfiguration to the minimum required for his task, andthus minimize his cost. As constructed, the units havethe potential of extensively utilizing less expensiveread-only memory (ROM) in lieu of random-accessmemory (RAM). By analogy with the core memorytypical of minicomputers, ROM is permanentlyprogrammed memory employed for the storage ofrelatively invariant programs (e.g., subroutines for theservicing of peripheral devices). Since the contents ofROM cannot be modified in the course of experimentalexecution, variables used by the operating programs arestored in RAM. By the use of ROM, the size of RAM. canbe smaller than generally found in minicomputers.Furthermore, the major use of these systems, asenvisaged here, is for control and data acquisition inbehavioral investigations with statistical analyses beingbest left to larger computer facilities. Accordingly, thetotal RAM requirements would be relatively small, sincethe data would be rapidly transmitted to mass storagedevices (e.g., punched paper tape, digital magnetic tape,or disk).

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184 KEHOE, FREI, TAIT, AND GORMEZANO

COST OF A MINIMAL SYSTEM

Each of the three computer systems underconsideration can be purchased from their respectivemanufacturers with a minimal configuration containingthe following in common: an enclosure with a busoriented structure, console card, CPU, solid statememory, power supplies, TTY interface, and basicsoftware. However, they differ in memory configuration,bus capacity, number of I/O lines, and presence of anROM field programmer. DEC's PDP-8/A offers 4K ofRAM, 12 I/O lines, an 8-s10t bus, but no ROM fieldprogrammer. Whereas, Motorola's Exorciser contains.75K of RAM and 3K of ROM, 16 I/O lines, a 14-slotbus, but no ROM field programmer. In turn, ControlLogic's system provides 4K of RAM, 12 I/O lines, a16-slot bus, and an ROM field programmer. These basicsystems may be purchased from the Digital EquipmentCorporation (Maynard, Massachusetts 01754), Motorola(Semiconductor Products, Box 20912, Phoenix, Arizona85036) and Control Logic Inc. (9 Tech Circle, Natick,Massachusetts 01760) for $2,600, $2,700, and $3,475,respectively. For the specified price, one obtainsturn-key systems requiring only the addition ofinterfaces (e.g., relay drivers) to the peripheral devices.

Substantial cost differences materialize in the furtherexpansion of these minimal systems. Consider, forexample, the expansion of each of these systems by theaddition of basic units of RAM and I/O lines. For 2K ofRAM, DEC, Motorola, and Control Logic ask $960,$650, and $650, respectively; whereas, per I/O line,DEC, Motorola, and Control Logic ask $42, $15, and$15, respectively. Presently, ROM can be expanded onlyin the systems offered by DEC and Control Logic. For2K of reprogrammable ROM, Control Logic enjoys asubstantial price advantage, charging only $520, whileDEC charges $960. In addition, DEC's configurationrequires the purchase, for $500, of a memory extensionboard for any expansion beyond the basic 4K ofmemory. Furthermore, Control Logic's system,containing an ROM field programmer, allows greaterflexibility and speed in the alteration of reprogrammableROMs. In the absence of an ROM field programmer unit,reprogramming must be accomplished by returning theROM to the manufacturer or distributor.

EVALUATION OF THE CPUS

Ease of Software ProgrammingBasic software necessary for loading, editing,

assembling, and debugging programs for RAM and ROMcan be purchased from all three firms. Beyond thesefundamental programs, the PDP-8fA has available to itthe vast body of programs developed for previousmodels of DEC's PDP-8 series. However, toaccommodate these programs, the memory capacityrequired would normally exceed the 4K of the basic

system. Accordingly, to operate within the constraintsof 4K of memory, a substantial investment in softwaredevelopment may be required, for example, SKED (cf.Snapper, 1973). Although Motorola and Intel haveannounced formation of users groups, there is presentlyno readily available software appropriate for behavioralresearch. For users willing to invest the time necessary todevelop their own software, Motorola's M6800architecture and instruction set would appear to offerthe greatest ease of programming, followed by Intel's8080, and trailing a distant third, the processor in DEC'sPDP-8/A.

The programming capabilities of Motorola's 8-bitM6800 CPU approximate those found in l e-bit CPUs(e.g., Data General's NOVA, DEC's PDP-II). Theinstruction set of the M6800 contains many instructionsusing two and three words of memory, thus producing16- and 24-bit instructions, respectively. The three-wordinstructions permit absolute addressing of 65,000locations in memory whereas, the two-word instructionspermit relative addressing of 125 locations on either sideof any given location. Further economy and flexibilityin addressing is achieved in the M6800 by the availabilityof a 16·bit index register, which permits a two-wordinstruction to reference any location whose addressvalue is up to 256 greater than the value contained in theindex register. In addition to the index register, otherarchitectural features of the M6800 include twoaccumulators, aiding manipulation of data, and aself-adjusting "stack" pointer, aiding the sequentialstorage and retrieval of data lists. Since many peripheraldevices employ BCD codes, the Motorola M6800provides instructions for arithmetic manipulation of two4·bit BCD codes without converting the codes to binaryform. For ease of decision making, condition-coderegisters enable two instructions to execute a conditionalbranch (i.e., jump or jump-to-subroutine) on the basis ofa comparison between the contents of an accumulatorand a memory location, without altering the contents ofthe accumulator. In addition to the programmingfeatures of the M6800 per se, a further aid todevelopment of software is provided by standardroutines contained within the 3K ROM of Motorola'sExorciser. This set of routines performs tasks commonlyrequired in all laboratory control systems (e.g., TTYservicing, code conversion, and interrupt servicing).

In contrast to the programming features of Motorola'sM6800, Intel's 8080 CPU provides less flexible andefficient addressing capabilities and fewer specialregisters. Although its instruction set contains two- andthree-word instructions, a relative addressing capabilityis not available. Accordingly, the dominant mode forreferencing memory locations would appear to beabsolute addressing, thus requiring space consumingthree-word instructions. To provide some flexibility inaddressing, a l6-bit "index" register is available in Intel's8080. However, it would appear to be somewhat

cumbersome to employ since its contents, in addition tobeing fixed by specific instructions, are altered by therelative value designated in any instruction using theindex register. For purposes of data manipulation,storage, and retrieval, Inters 8080 possesses a singleaccumulator, a "stack" pointer, and BCD arithmeticcapabilities. Comparable to the decision makingcapabilities of Motorola's M6800, the instruction set ofIntel's 8080 enables a comparison and a conditionalbranch to be made with two instructions."

The programming capabilities of Intel's 8080 and,clearly, Motorola's M6800, far exceed those of the CPUfound in the PDP·8/A. As best as can be discerned, theprogramming features of the PDp·8/A are identical tothose of the PDP-8/E. Specifically, the PDP·8/A'sinstruction set contains only single-word l2-bitinstructions, and the addressing scheme is limited toDEC's singular blend of direct and indirect addressing asconstrained by the division of memory into "pages" of128 locations. Specifically, it is possible for a singleinstruction to directly address only the 128 locations onthe page containing the particular instruction and thefirst 128 locations in memory, referred to as Page "0."Hence, indirect addressing, involving two words ofmemory, is required to reference any other location inmemory. For manipulation, storage, and retrieval ofdata, the PDP-8/A offers a single accumulator and"autoindex' registers, which automatically incrementwhenever indirectly addressed. In contrast to thetwo-instruction compare-and-branch operations availablein the Motorola M6800 and Intel 8080, 7 to 10instructions would be required to perform a similaroperation in the PDP-8/ A. DEC appears to have done anadmirable job in developing as powerful a programmingcapability as can be realized with a single word, 12-bitinstruction length. However, the 8-bit microprocessorsunder consideration, through their use of multiple-wordinstructions, have been able to mimic the power ofprogramming inherent in processors employing 16- or24-bit word lengths.

Real-Time OperationsThe feasibility of using the systems under

consideration for real-time control and data acquisitiondepends on the time necessary to perform anexperimentally useful task, which is a product ofexecution time for a single instruction and the numberof executed instructions. To provide a basis for generalassessment, as well as for comparison among thesesystems, two routines that would generally be found inexperimental control programs were written for each ofthese systems. These routines were designed to servebasic stimulus control and response counting functionsin a clock-driven operating system. Specifically, the firstroutine was designed to count response inputs on any orall of eight response input lines from an experimentalstation. The second routine was designed to control the

\I\CROPROCESSORCo\lPCTERS 185

presentation and duration of a state-dependent sequenceof stimulus patterns transmitted on eight separateoutput lines. For each routine, the three systems werecompared on the basis of the time required to service asingle station with eight input and eight output lines. Inwriting these routines, an earnest attempt was made tooptimize their efficiency However, we realize that amore efficient algorithm may have been overlooked.

For purposes of comparison among systems, it wasass urned that response or clock-pulse countingrequirements would not exceed 4,095. To attain thiscapabilitiy with Motorola's M6800 and Intel's 8080microprocessors, it was necessary to provide adouble-precision algorithm for both theresponse-counter and stimulus-control routines. Whilethe algorithm applied to these microprocessor routinesacquired the capacity to accommodate counts ofapproximately 65,000, the utilization ofdouble-precision did not affect the relative standing ofeach of the CPUs in execution time. In calculatingexecution times for the response-counter routine, theextreme condition was assumed in which data appearedsimultaneously on all eight input lines. Similarly, for thestimulus-control routine, it was assumed that a change instimulus pattern would occur on all eight output lines.Under these extreme conditions, all three CPUs usingRAM would be able to execute both response-counterand stimulus-control routines for one station in less thanI msec (PDP-8/A = 342 microsec,M6800 = 422 microsec, and 8080 = 484 microsec). Ofcourse, servicing each comparable additional stationrequires the addition of the respective single-stationservice times.

DISCUSSION

While the application of minicomputers to behavioralresearch has substantially increased the capabilities forcomplex real-time control and data acquisition, the"rniniprocessor" and microprocessor computer systemsdo not appear to offer any major advances along theselines. Rather, they appear to represent primarily acontribution to the economy of establishing acomputer-based laboratory. At the present time, twoeconomies would appear realizable. First, the cost ofinitial systems is substantially less than that ofcomparable minicomputers. For example, the PDP-8/Asystem. as previously described, costs approximately50';~ less than a PDP-8/E of comparable configuration.However, while the initial cost of microprocessor-basedsystems is similar to that of the PDP-8/A, their furtherexpansion through the use of LSI components is lessexpensive than comparable expansion of the PDP-8/ Awith its older integrated circuitry. Second, inmicroprocessor systems, the appreciably smaller numberof chips employed relative to minicomputers, shoulddecease maintenance costs through reducing the

186 KEHOE, FREI, TAIT, AND GORMEZANO

probability of failure and time necessary to identifyfaulty components.

In exchange for the above economies, the systemsconsidered have sacrificed some speed and certainmemory characteristics. The PDP-8/E, for example,requires at least 40% less time than the PDP-8/A ormicroprocessor systems to execute the routines used toevaluate their respective real-time capabilities. Withregard to memory characteristics, the contents ofcurrently available RAM are "volatile" in that power tomemory must be maintained in order to retain programsand data in storage. On the other hand, the contents ofROM may be retained without power, but at a cost ofbeing relatively inflexible. However, in the foreseeablefuture, these limitations of speed and memory appearrectifiable. Currently, the technology exists for themating of microprocessors and memory chips whichwould yield microprocessor-based computer systemswith speed capabilities exceeding those of presentminicomputers. Prototypes of "nonvolatile" RAM, thatmimic magnetic core memory, are currently being testedin engineering laboratories (e.g., Nitron Corp., 10420Bubb Road, Cupertino, California 95014). Accordingly,realistic prospects exist for the appearance ofmicroprocessor systems with the speed and memorycharacteristics of minicomputers, but with the sameeconomies found in current microprocessor systems.

Conceivably, further reduction in the cost ofestablishing a computer controlled laboratory could berealized by facilities with sufficient technical personnelto design the relatively simple circuit boards necessary toaccommodate the less than two dozen chips found incurrent microprocessor systems. After assembly, suchcircuit boards would be installed in prewired busoriented enclosures that are available from presentmanufacturers of microprocessor-based systems.Accordingly, for laboratories or entire departments, thefabrication of microprocessor systems in this fashionwould permit the economical development of multiple,but individually tailored, microprocessor systemscontaining varying quantities of ROM, RAM, and I/Ooptions.

REFERENCESMcLean, R. S. Microcomputers for experimental psychology.

Behavior Research Methods & Instrumentation, 1974, 6,155-158.

Snapper. A. G. Use of a notation system for digital control andrecording. Behavior Research Methods & Instrumentation,1973, 5, 124-128.

NOTE1. For further information on Intel's 8080, write to Intel

Corporation, 3065 Bowers Avenue, Santa Clara, California95051.