UNR Sim: A Simulated Computerfor Computer Engineering Education

Andrew M. Olson Dwight D. Egbert Frederick C. Harris, Jr.

Computer Science and EngineeringUniversity of Nevada, Reno

Reno, NV, USAegbert@cse.unr.edu

Abstract

UNR Sim is a decimal based Von Neumannarchitecture computing device simulator.Chronologically precedent simulators did not includefactors we considered necessary or desirable, eachin and of itself. Actualized by these inadvertentomissions, we decided to integrate all the desirablebehaviors instantiated in these simulators while eitherneutralizing or negating their undesirable aspects.This design inspiration resulted in a robust computingdevice simulator suitable for pedagogical functionalityat the introductory level with extensions available formore advanced students to continue their explorationof fundamental Von Neumann architecture computingsystems.

Keywords: Computer Architecture, InteractiveTeaching Tools, Simulation.

1 Introduction

“The very idea of a computer science program thatdid not provide students with an insight into the com-puter would be strange in a university that purports toeducate students rather than to merely train them.”[8]

Electrical engineering students are taught the fun-damentals of semiconductor behavior in order to un-derstand how integrated circuits function. In a sim-ilar manner, computer engineering students need tounderstand the fundamentals of computing machin-ery behavior in order to understand how computersfunction. Concurrently and in conjunction with theunderstanding of fundamentals, is the preference foreducation over training. This implies a need for such abinitio internalization of the manner in which computingdevices function.

The fundamental facet addressed by UNR Sim is thesimplest form of the Von Neumann cycle, as shownbelow in Figure 1. Each element in this sequence isdemonstrated through illumination of the conductivepath connecting the two components in each step, innomine, fetch, increment and execute.

From our beginning, the discussion initiates with anexamination of historical and paradigmatic pedagogicalcomputer simulations in Section 2. Following ourdiscussion of previous implementations of pedagogicaleducational paradigms, we have a design discussion ofUNR Sim in Section 3. Following this, we consider thedetails of the implementation of UNR Sim in Section 4.The paper concludes with a discussion of conclusionsand future development work in Section 5.

Figure 1: The von Neumann Cycle.

2 Background

Four distinct phyla make up the the taxonomy ofcomputer simulators preceding UNR Sim. The mostsignificant of these, as it is the precursor and mostcorrelative to UNR Sim, is Simulated Computer II. Themost primitive level of computer simulation, or in ourcontext, most basic taxonomic phylum, consists of gate

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and logic level simulators, in this document representedby Logisim. On a similar conceptual level, focusingon the Von Neumann cycle instead of simple hardwareelements, is the the Little Man Computer. Occupyingthe contrasting position, of representing primarily thecomputing hardware conceptualizations using hexadec-imal notation, is Easy 4, and its succeeding upgrade,Easy 8.

The primary consequence of these distinctly variantapproaches is a paucity of integration across theseaspects, most decidedly the three points of decimalnotation, focus on data movement in accordance withthe Von Neumann cycle and finally, and not trivially,the ability to run the simulator on a wide range ofmodern computers without secondary emulation. UNRSim addresses these issues both individually and intoto. Regarding the first, UNR Sim primarily functionsin decimal mode, analogous to Simulated Computer IIand the Little Man Computer, although unlike those,it is possible to display and input data in hexadecimalmode as well to show the relationship between thetwo. The Von Neumann cycle is displayed in UNRSim by highlighting the wire bus connection betweenthe two data involved in the particular step of theVon Neumann cycle, unlike Simulated Computer IIwhich implies a more directional movement of thedata rather than the data occupying a connectionspecified by addressing. Lastly, although important,arguably least critical, UNR Sim can be run on allthree major platforms, Windows, Linux and OS Xwithout specialized knowledge of emulators of obsoletehardware.

2.1 Simulated Computer II

In 1982, Carousel Software released Simulated Com-puter II for the Atari 800 and Commodore 64.[6]During this chronological period, software development,particularly game development, was wildly divergent,with numerous small companies creating a broad rangeof offerings. So while Simulated Computer II didwin the award of “Best Microcomputer Software of1982” from Learning Magazine, amid so many softwareofferings on the market of all quality levels, it was notsurprising to find it did not find wide spread adoption asevidenced by the rarity of copies of Simulated ComputerII.[6]

Notwithstanding such factors, Simulated ComputerII is notable for its elegant implementation of theconcepts underpinning the Little Man Computer anddemonstrating how those concepts apply in the mostfundamental level to a non-specific Von Neumannarchitecture computing device as show in Figure 2.

Figure 2: Simulated Computer II.[6]

Unfortunately, while Simulated Computer II is anexcellent pedagogical tool, it is not as fully actualizedas could be implemented today. The primary difficultyposed by Simulated Computer II is the fact it waswritten for what is now obsolete hardware, requiringfunctional knowledge of emulation software. However,as has already been shown, due to its obvious strengths,something similar to Simulated Computer II is certainlya desirable tool and so it is pedagogically valuable tocreate a computing device simulator with the positivequalities of Simulated Computer II without these neg-ative factors.

2.2 Logisim

No discussion of simulation would be truly completewithout examining at least one simulator of the mostbasic kind, id est a component level simulator likeLogisim, shown in Figure 3. As stated by the creator ofLogisim, Carl Burch, “Logisim can be used (and is used)to design and simulate entire CPUs for educationalpurposes.”[1] Acknowledging this as true, however, doeslittle to ameliorate the fact such an endeavor can bean entire class, not merely a single session or modulein a class focused on computational device design andarchitecture in general.

This extensive knowledge base required to applyLogisim to the pedagogical challenge of the fundamen-tals of computer architecture design, in particular thefunction of the Von Neumann cycle concomitant withsimplified decimal interface, renders Logisim unsuitablefor our purposes. In addition to this challenge, the factthat the input is all wires and signals places Logisim asperhaps a way to create a simulator at the conceptuallevel of UNR Sim, it is not inherently at that level byitself.

Figure 3: Logisim.[1]

2.3 Little Man Computer

The Little Man Computer, a conceptual computingdevice simulator was created in 1965 by Stuart Madnickof MIT.[10] The primary focus of the Little ManComputer is the fetch and execute cycle, ultimately anintegral part of the basic Von Neumann cycle, albeit noteven all the necessary components, as implemented byUNR Sim. Typical implementations of the Little ManComputer are instantiated in decimal, not hexadecimal,correlative with the minimalist ideal of this simulator.The Little Man Computer, shown in Figure 4 is theparadigmatic representation of the most minimal char-acteristics of the Von Neumann cycle.

In contrapositive effect, Little Man Computer isnot a single implemented computing device simula-tor, but rather a conceptual exercise which has beeninstantiated in different ways creating inconsistenciesin ultimate effectiveness for pedagogical application.Concurrently, Little Man Computer also obfuscatescertain details inherent to the internal functioning ofa typical central processing unit for the prototypicalVon Neumann architecture computing device by meansof oversimplification.

Figure 4: Little Man Computer.[10]

2.4 Easy4 Computer

In increasing order of complexity from SimulatedComputer II and UNR Sim is Easy4 Computer, aMicrosoft Windows based application included in PCArchitecture from Assembly Language to C.[9] Thisapplication, as shown in Figure 5, is restricted toan accessible memory domain of sixteen locations dueto its functional range of sixteen inputs for Easy 4.architecture.

Easy 4 and Easy 8 both use hexadecimal notationfor all memory and functional behaviors. This is anadditional complexity often seen to impede studentassimilation of more critical concepts forming the fun-damental basis of computational machine behavior.

Figure 5: Fundamental EasyX Architecture.[9]

3 UNR Sim Components

The cardinal aspect of computer architecture to betaught by UNR Sim is the von Neumann cycle, withthe use of decimal notation, connection highlighting andaccessibility substantive to this pedagogical focus, as inFigure 6.

The first elements of UNR Sim were based on thebehavior and appearance of Simulated Computer II,

Figure 6: Fundamentals Contributing to the VonNeumann Cycle.

with its components from Little Man Computer andreplicated later in Easy 4. Visual characteristics wereupdated for current student expected environmentalexperiences and also kept simple, to reflect the funda-mental nature of the lessons under study. Immediatelyaccessible controls and codes were left as similar tothe original Simulated Computer II, however a fewwere discarded as excess to requirements and manywere added although not included in the primarydocumentation. The animation of data movement waschanged from the disingenuous slowly traveling light toa highlight of the entire connection, showing the natureof wire exclusivity. Lastly, a sound generator wasadapted from Simulated Computer II to give studentssomething more responsive to interact with.

3.1 Visual Characteristics

UNR Sim in its basic configuration is shown below inFigure 7. At the top left of the display is the screen.At the bottom left of the display is the CPU. At thetop right of the display are the control buttons. Atthe bottom right of the display are the memory banks.The lines connecting the screen, CPU and memoryare representative of the wire connecting these threesimulated components of an entire computing device.

The first thing we removed was the teletype output,replacing it with a single screen for input and output asfound on modern computing systems. Students oftenfound the dual system confusing, as many had neverexperienced a teletype machine. The colors were chosento accentuate the three primary interface means, innomine the screen, the CPU and the memory. Thecontrol buttons were drawn in black so as to be thesame as the wires, in that they are both representa-tions of concepts more than exact representations ofcomponents in the machine.

3.2 Commands, Mnemonics and Errors

Arithmetic systems are rarely integrated fluidly intomathematical education to allow naive computer sci-ence and engineering students to intellectually assimi-late material such as the binary and hexadecimal sys-

Figure 7: Fundamental Mode of UNR Sim.

tems used in computer science and engineering withoutdifficulty. This dual assimilation issue, that of thedifficulty in comprehension of both the von Neumanncycle and hexadecimal notation concurrently, is avoidedby the primary numerical system used in UNR Sim (asit had been in Simulated Computer II) is decimal. Thisis a continuation of the decimal arithmetical systemused by the Little Man Computer.[10] In addition, thetop left button in Figure 7 toggles the display and inputbetween hexadecimal and decimal.

The commands selected for UNR Sim, as shown inTable 1, are very similar to those used by SimulatedComputer II. The fundamental commands to load andrun code are unchanged. However, the ability to resetthe simulation such that it starts with a cleared CPUbut the same memory values was added as well asthe ability to clear the memory completely. We did,however, remove the command input to change betweenoperation codes and mnemonics, replacing it with abutton located above the memory.

Command Command EffectRUN Starts Program Running at 00RUNxx Starts Program Running at xxRNSxx Starts Program Running at Speed xxNEW Clears All MemoryRST Resets the CPULOAD Starts Loading at Location 00LOADxx Starts Loading at Location xx

Table 1: UNR Sim Control Commands

All the mnemonics were continued from SimulatedComputer II and many were added. However, theequivalent operation codes were maintained betweenthe earlier program and the current program, with theadded mnemonics added sequentially after the originalmnemonics. A table of operation codes and mnemonicsthat are shared between UNR Sim and Simulatedcomputer are shown in Table 2

Error messaging was updated due to the loss of theoutput teletype as well as to reflect the more typical

OpCode Mnemonic Function001 LDAxx LoaD ACC with xx002 STAxx STore ACC in xx003 ADDxx ADD xx to ACC009 JMPxx JuMP to xx010 SKZ Skip Next if ACC = 0011 SNZ Skip Next if ACC != 0

Table 2: Example OpCodes and Mnemonics

Error Code ErrorOVF OverflowUDF Undefined LocationINV Invalid InstructionDVZ Divide by ZeroNAS Not a Sound

Table 3: UNR Sim Error Messages

Figure 8: Bus Highlighting for LoaD Accumulator

behavior of modern computer systems to respond witherror codes at the time of failed execution of a com-mand. Table 3 displays the formally documented errorcodes.

3.3 Wire and Bus Displays

The primary display mechanism used by UNR Sim isto highlight the entire wire and bus connection betweenthe two points of interest for the particular stage ofthe Von Neumann cycle in particular. The threefundamental steps displayed are when the simulatedcomputer fetches the instruction. The second stepdisplayed is when the computer increments the programcounter. The final step under examination is when thecomputer executes the function, see Figure 7. Each ofthese steps is shown as a change in color of the wholeconnection.

The practice of highlighting the entire wire and busconnection was implemented to echo the step functionof the Von Neumann cycle. For each step of the cycle,one connection is highlighted.

3.4 Audio Generator

The original Simulated Computer II had a simplesound generator capable of sounding discrete tonesa single time. UNR Sim generates a sine wave ofa single frequency with programmable duration andvolume. Unlike normal functions, when the tone is done

playing, the simulator progresses to the next step in theprogram. This was done so that students could programthe simulator to reproduce a melody, by setting therun time to maximum speed to minimize delay betweentones.

The purpose of the audio generator is to give studentsan application upon which to apply their programmingskills. In a similar manner to Simulated ComputerII, the audio generator in UNR Sim can be used forprogramming assignments.

4 Implementation

The primary tool used to design UNR Sim wasPython 2.7.9[5] and the Pygame 1.9.1[4] library. Thesewere chosen because both Python and Pygame aretransportable across all major operating systems,Python is the sixth most popular programminglanguage by normalized rating,[3] and is normallypackaged with the code in plain text so it is easilymodifiable by end users.

The visual layout was organized by percent of fullscale as opposed to on a per pixel basis. This way all ofthe individual components can be referenced by a staticspatial orientation and location scheme. Using thismethodology, every location on the screen is referredto on a scale from zero to one hundred. Pygame wasused to display all the graphic and text elements in anindependent window.

Commands and mnemonics are implemented withindividual logic statements, to allow for ease of replace-ment or modification. No command or mnemonic isdependent on any other, to prevent the possibility ofcascading errors causing a system wide failure. Thecommands were selected from the original SimulatedComputer II commands. The basic mnemonics andopcodes were transferred from Simulated ComputerII, however the advanced mnemonics were adaptedfrom the Motorola 68000 opcodes.[2] Error codes wereselected from Simulated Computer II and observationof common user errors.

Wire highlighting was accomplished by locating allnodes of the wire and bus combination used to carrythe signal between the two points and then overwritingthat wire and bus combination with another color. Thisremains highlighted until the next step of the VonNeumann cycle is taken. This was done to explicate thenature of a wire connection, that being it is intact nomatter where the signal is on its transmission betweentwo points.

The tone for the sound generator is created from adiscrete array representation of a sine wave at a specificfrequency. These discrete values are then sent through

the Pygame sound generator to the sound system onthe device UNR Sim is currently run on.

All the keyboard handling was implemented inPygame as well. In addition, mouse location andstatus was implemented in Pygame, particularly forthe control buttons.

5 Conclusion and Future Work

5.1 Conclusion

It has long been a proverb that one is unable toconstruct a building descendant from the rafters. Inthe context of computers, this inherently implies afundamental applicable knowledge and understandingof the foundation and seminal basis for computerfunctionality. For von Neumann architecture com-puting devices, this mechanism is the von Neumanncycle. UNR Sim explicates this cycle after excising allpossible extraneous distracting complications, such ashexadecimal numeration and recondite user experienceabstractions.

5.2 Future Work

Future work for the Simulated Computer 2015 projectincludes the addition of non-core features; exempligratia, the implementation of a turtle graphics moduleto allow students to draw pictures or methods to saveand load external code.

Other types of future work include the migration ontoa more general purpose platform such as Unity,[7] givingaccess to smart phones, tablets, consoles, web platformsand other operating systems and hardware.

References

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[2] Motorola 68000 Family Programmer’s ReferenceManual. http://www.freescale.com/files/

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[3] Programming Language Popularity. http://

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[4] Pygame; Modules for Writing Games in Python.http://pygame.org/news.html. Accessed: 2015-04-28.

[5] Python programming language. https://www.

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[6] Simulated Computer II atari mania.http://www.atarimania.com/game-atari-400

-800-xl-xe-simulated-computer-ii_4681.

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[7] Unity. https://unity3d.com/. Accessed: 2015-04-27.

[8] Why Do We Teach Computer Architecture? http:

//alanclements.org/teachingarchitecture.

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[9] D. Hergert and N. Thibeault. PC Architecturefrom Assembly Language to C. Prentice Hall, 1998.

[10] W. Yurcik and H. Osborne. A crowd of little mancomputers: visual computer simulator teachingtools. In Simulation Conference, 2001. Proceedingsof the Winter, volume 2, pages 1632–1639 vol.2,2001.