SIM-PL: Software for teaching computer hardware at secondary schools in the Netherlands

Transcription

1 SIM-PL: Software for teaching computer hardware at secondary schools in the Netherlands Ben Bruidegom, AMSTEL Instituut Universiteit van Amsterdam Kruislaan 404 NL-1098 SM Amsterdam Wouter Koolen-Wijkstra, CWI (Centrum voor Wiskunde en Informatica) Kruislaan 413, NL-1098 SJ Amsterdam Abstract In August 2007 a new program for secondary schools in the Netherlands will start. The number of hours for the subject Informatics will be extended substantially from that date. Organization and working of digital systems like computers will be an elective topic in the new curriculum for Informatics. We have adapted the computer hardware design and simulation tool, SIM-PL, which was developed for higher education, for use in secondary education. We also developed models of illustrative portions of computer circuits to contribute toward pupils understanding of computer hardware. Pupils can play with basic gates as well as with a very simple processor model and all the levels between these two. SIM-PL is an authoring system by which teachers and pupils can easily construct new circuits from scratch, or visually compose existing circuits. The software is convenient to use in student projects. Keywords Informatics, simulation, digital techniques, computer architecture, authoring system. INTRODUCTION In the Netherlands, secondary schools are split up into two phases. In the first phase (first three years) of the highest level (in Dutch Athenaeums) there is an obligatory subject called information technology (informatiekunde). In the second phase (last three years) there is an elective subject informatics (computer science). In August 2007 there will be a reform of the Dutch school program [UUI2007] and the subject informatics will be extended from 280 to 480 hours. The extra hours will be filled with: more depth to basic subjects like object-oriented programming and databases, a student project (60 hours), free elective modules of 20 or 40 hours with topics like Robotics, Human- Computer Interaction, Bio-informatics, Artificial Intelligence, etc. In this paper we describe some examples of the free elective module How a computer really works. This topic is an interesting object of study, especially for pupils with wide interests in technology. The tool SIM-PL gives pupils the opportunity to obtain knowledge about a lot of machines and devices they use daily. The module is divided into two parts: - Digital Electronics Covers the topics: gates, circuits, number representation, Arithmetic Logic Unit. - Computer Architecture Covers Digital systems, the software hardware interface, assembly programming. Each part takes 20 hours for pupils to learn. Another item is the pupil project. Pupils have to a work on a small research project or do a design assignment for their exams. This is called profielwerkstuk. When

2 the pupils choose this subject they have to spend 80 hours to design and implement a machine or a game with SIM-PL components. EXAMPLES OF THE COURSE Examples of the Digital Electronics part The material is presented bottom up. The bottom level consists of elementary gates, e.g. AND-gates. At the second level, one connects gates to construct circuits that can for example, add three bits (Full adder). The next level is to compose a circuit using Full Adders for adding two 4-bit numbers. Educational knowledge goals: AND-, OR- and XOR-gates; truth tables; combinational logic; circuits to add numbers; time sequence diagrams; binary and hexadecimal codes; Arithmetic Logic Unit. Educational competence goals: making a truth table and a formula of a circuit; converting decimal codes into binary and hexadecimal codes and vice versa. testing and debugging circuits; designing and implementing circuits. Figure 1: State of the circuit after adding numbers 5 and 7 in hexadecimal code. An example of the Computer Architecture part A simple calculator (Figure 2) is the first example of the second part of the module. The structure and working of programmed hardware is elaborated in this part of the

3 course. Knowledge about the subjects in the first part is necessary to understand this part. Educational knowledge goals: which main components the calculator is composed of; the function of these components and the relations between these components; the function of the clock; instruction set and instruction format; the difference between assembly language and machine code; what addresses are; what an operator is and what operands are; What an operate code (opcode) is. Educational competence goals: compile, load and execute (assembly) programs; mark the data path of an instruction; write some assembly programs to understand the machine; determine the operate code and the format of an instruction. Figure 2: State of the machine after execution of the instruction SUB $7, $3, $4 Instruction set For this simple machine, there is a one-to-one correspondence between ALU operations and instructions. The four instructions are represented in Table 1. Syntax instruction Meaning Example Meaning ADD rd, rs, rt Add 2 registers ADD $7, $3, $4 r7 r3 + r4 SUB rd, rs, rt Subtract 2 registers SUB $7, $3, $4 r7 r3 r4 AND rd, rs, rt Bitwise AND 2 registers AND $7, $3, $4 r7 r3 & r4 COPY rd, rt Copy register COPY $7, $4 r7 r4 Table 1 Instruction set of the calculator rd = destination register; rs = first source register, rt = second source register Instruction format Each instruction is composed of 14 bits. The two most significant bits determine the operation of the ALU. The other twelve bits are divided into three fields. Each field contains a 4-bit register address for respectively rs, rt and rd. Example: the assembly instruction ADD $7, $3, $4 is represented by the binary machine code: (0327 hexadecimal).

4 Field ALU S1,S0 First Register operand rs Second Register operand rt Destination Register operand rd Num. of bits Example Meaning add register 3 register 4 register 7 Table 2 Instruction format of an instruction CONSTRUCTING COMPONENTS AND CIRCUITS IN SIM-PL SIM-PL is itself an authoring system; it does not come with a fixed set of components instead but uses a built-in programming language to define components. Our module's course material contains models of the elementary gates of the Digital Electronics section. The module restricts attention to hierarchically creating circuits from earlier ones (see Construction of a circuit). The following description is included for completeness. Construction of a basic component To construct a basic component you have to carry out the next five steps: draw the graphical representation (using standard drawing tools); set down the inputs and outputs (fix the number of bits of each input and output); program the relation between inputs and outputs (see events); if necessary, program a memory function; fix the propagation delay. Events Components are activated by the following events: INIT start condition of the component; INPUT-CHANGE reacts on a change of signal of one of the inputs; CLOCK-RISING reacts on a positive edge of the clock; CLOCK-FALLING reacts on a negative edge of the clock. If one of these events occurs, the corresponding program is executed. These four programs define the functionality of the component. Internal program language: nbit The syntax of the program language equals the common part of C/C++/Java. The basic data type, however, is the n-bit number: an integer of arbitrary, fixed width. Example: the relations between inputs and outputs of a Full Adder in Figure 1 are: { Carry = (a && b) (a && c) (b && c); Sum = a ^ b ^ c; } Construction of a circuit Construction of a circuit consists of doing the next steps: import and configure the basic components of the circuit; set down the inputs and outputs of the whole circuit; draw the connections between all inputs and outputs of the components and the circuit. To show the status of connections during the simulation, you place a probe on these connections.

5 SIMULATION The SIM-PL simulator computes the behavior of a given circuit as a function of time using discrete event simulation, and provides a visualization of the assignment of values to inputs, outputs, wires and memory cells at each desired point in time. The user can supply values to unconnected inputs, and alter memory contents. This allows pupils to test and debug their circuits, especially in the case of failing the specification due to timing issues (propagation-delay). The initial state of the circuit and in particular the content of the instruction memory cells of the processor component can be specified using assembler programs. SIM-PL IN HIGHER EDUCATION SIM-PL is used for lab assignments at courses Architecture and Computer organization and Digital Electronics at the Universiteit van Amsterdam. For these courses components and circuits are built, for example: - Finite State Systems - Single-Cycle and Multicycle machines [HENPAT] - MIPS pipeline processor [HENPAT] - Cache system This year's courseware was produced for courses at Hogeschool van Amsterdam and Fontys Hogeschool voor Informatica in Eindhoven. OTHER SIMULATION TOOLS AND SIM-PL Other simulation tools are MMLogic [HWSIM], SPICE, Elecronic Workbench, LogicWorks and Circuitmaker, most of which are commercial tools with student editions. The unique features of SIM-PL are - SIM-PL is Free Software (GPL). - SIM-PL is an authoring system. - SIM-PL is suitable for Digital Electronics as well as Computer Organization. REFERENCES [UUI2007] Schmidt, Victor (2005) Informatica in de tweede fase na TINFON Journal of education in Information Sciences, 4, [HENPAT] Patterson, D. A. and Hennessy J. L. (2005) Computer Organization & Design Third Edition, Morgan Kaufmann. [HWSIM] Volmar, Kenneth R. (2002) Hardware simulation tools for computer design. CCSC: Central Plain Conference JCSC 17, Home page SIM-PL: Biographies Ben Bruidegom is lecturer in the field of Computer Science and member of the Educational Institute Information Sciences of the Universiteit van Amsterdam. Besides this, Ben is working at the Amsterdam Mathematics, Science and Technology Education Laboratory (AMSTEL) at the same university.

6 Wouter Koolen-Wijkstra obtained his MSc. degree in Logic from the Universiteit van Amsterdam in He currently holds a PhD. position in the group "Quantum computing and Advanced Systems Research" at CWI, the national research institute for mathematics and computer science in the Netherlands. Copyright This work is licenced under the Creative Commons Attribution-NonCommercial- NoDerivs2.5 License. To view a copy of this licence, visit or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.