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Java-IDC-Homework 8 Solved
Introduction to Computer Science, IDC Herzliya
HW 8: Building a Vic Simulator
In this project we build a software implementation of the Vic computer. The result is a simulator that can execute Vic programs. The simulator will have the same functionality as the web-based Vic simulator that you used in HW 5, minus the graphical user interface.
Before working on this project, you must first familiarize yourself with the Vic computer, architecture, and programming. In short, it is assumed that you have done Homework 6.
Usage
> java VicSimulator programFile [inputFile]
Where programFile is a mandatory text file of the form fileName.vic, containing numeric 3-digit Vic commands, and inputFile is an optional text file containing the program’s inputs. Examples:
> java VicSimulator print1To100.vic (Prints the numbers 1 to 100, no input file)
> java VicSimulator sum.vic sum.dat (Prints the sum of the numbers in the input file)
The simulator loads the commands from the program file into the simulated computer, and then executes the program. If the program is expected to process inputs, it reads these inputs from the input file. The simulator prints the program's output to standard output (by default, the screen).
Both the program file and the optional input file are assumed to contain integers between -999 greater than 999.
Implementation
The Vic simulator can be implemented in different ways. This document describes an ObjectOriented implementation, using Java. In order to guide the implementation, we provide a proposed API. In other words, we play the role of the system architect, while you are the developer who has to take our design and make it work. We now turn to provide a bottom-up description of what has to be done, along with proposed implementation stages, unit-testing, and supplied test programs.
Registers
A register is a container that holds a value. This is the basic storage unit of the Vic computer. In this program, registers are implemented as instances (objects) of the Register.java class.
Implementation: a Register object has a single field: an int variable, holding the register's current value. Write the Register.java class, following its supplied API.
Testing: write a testing method that creates two or three registers. Next, test all the Register methods. For example, try something like this:
Register r1 = new Register(10)
System.out.println(r1); // Should print 10 r1.addOne();
System.out.println(r1); // Should print 11
Register r2 = new Register() r2.setValue(r1.getValue())
System.out.println(r2); // Should print 11
Write and test similar command sequences for each one of the Register constructors and methods, printing relevant values before and after invoking each method. Tip: Each Register method can be implemented with one line of code.
Memory
The memory unit is an indexed sequence of registers. In this implementation, the memory is realized as an instance of the Memory.java class. The Vic computer has one memory space, sometimes referred to as the RAM. Therefore, our program will use one Memory object (one instance of the Memory class).
Implementation: a Memory object has one field: an array of Register objects (more accurately, an array of references to Register objects). The class constructor gets the desired length of the memory as a parameter, and proceeds to create an array of so many registers. Write the Memory class, following its supplied API.
Testing: write a tester (testing method) that creates a single Memory object and tests all the Memory methods. The test code should look something like this:
Memory m = new Memory(100);
m.setValue(0, 100); // m[0] = 100
m.setValue(1, 200); // m[1] = 200
m.setValue(2, -17); // m[2] = -17 int v = m.getValue(1); // v = m[1]
System.out.println(v);
System.out.println(m); // Displays the memory's state.
The toString method of the Memory class returns a string that shows the first and last 10 registers in the memory. Here is a typical output of the command System.out.println(m):
0 100
1 200
2 -17
3 0
4 0 5 0
6 0
7 0
8 0
9 0 ...
90 0
91 0
92 0
93 0
94 0
95 0
96 0
97 0
98 0
99 1
That’s the exact output that your program should generate.
Computer
The Vic computer is implemented as an instance of the Computer.java class. We will use only one object of that class.
Stage 1
Start by declaring and initializing the three public constants described in the Computer API. Next, declare 10 constants, for representing the 10 op-codes of the Vic machine language. For example, the first declaration can be: private final static int READ = 8; From now on, constants like READ (rather than the obscure literal "8") should be used to represent op-codes throughout your class code. Next, declare the three fields (private variables) that characterize a Computer object: a memory device (which is a Memory object), a data register (which is a Register object), and a program counter (also a Register object).
Now write the Computer class constructor, the reset method, and the toString methods. Consult the Computer API for details.
Testing: write a tester that creates a Computer object and prints the computer's state (that's two lines of code). The output should look like this:
D = 0
PC = 0
Memory state:
0 0
1 0
2 0
3 0
4 0
5 0
6 0
7 0
8 0
9 0 ...
90 0
91 0
92 0
93 0
94 0
95 0
96 0
97 0
98 0
99 1
Stage 2
We now turn to write the loadProgram method, which loads a program into the computer's memory. This method reads the data from programFile (see the Usage Section at the beginning of this document), line by line, and loads the data – a stream of 3-digit numbers – into the computer's memory, starting at address 0. Implementation tip: Use the services of the stdIn class, both for initializing the program file, and for reading from it.
Testing: Write a testing method that creates a Computer object, and loads a program into it. The code will look something like this:
Computer vic = new Computer(); // or vic.reset(), if the computer already exists.
vic.loadProgram("program1.vic"); System.out.println(vic);
Assuming that the program1.vic file is in your working directory, the output of this code should look like this:
D = 0
PC = 0
Memory state:
0 399
1 900
2 399
3 900
4 0
5 0
6 0
7 0
8 0
9 0 ...
90 0
91 0
92 0
93 0
94 0
95 0
96 0
97 0
98 0
99 1
Stage 3
We now turn to implement the run method - the workhorse of the Vic simulator. Start by reading the run method API. The method simulates the computer's fetch-execute cycle by fetching the current command from memory, parsing the command into an op-code and an address, and storing the parsed values in variables (you may want to handle some of these actions using private methods). If the op-code is STOP, the method prints "Program terminated normally", and terminates. Otherwise, the method executes the command, according to its op-code. There are 9 commands, and thus 9 execution possibilities. We propose implementing the execution of each command by a separate private method. For example, the following method can be used to implement the execution of the LOAD command:
// Assumes that the data register, program counter, and memory
// fields are named dReg, pc, and m, respectively. private void execLoad (int addr) { dReg.setValue(m.getValue(addr)); // dReg = m[addr] pc.addOne(); // pc++ }
Note that the execLoad method advances the program counter. This is very important; if the pc is not incremented properly, in the next cycle (simulated by the next loop iteration) the run method will fail to fetch the correct command from memory. Also note that some commands (like LOAD) simply increment the program counter, while other commands – the branching ones – require a more delicate handling of the program counter. The important thing to remember is that the program counter must be handled as a side-effect of handling the respective commands.
Testing: we recommend implementing and testing the Vic commands in stages, and in the following order:
Implement and test the LOAD and WRITE commands; test your implementation using program1.vic.
Implement and test the ADD command; test your implementation using program2.vic.
Implement and test the SUB command; test your implementation using program3.vic.
Implement that lent and test the STORE command; test your implementation using program4.vic.
Start by writing the main loop of the run method. Next, for each one of the five commands mentioned above, implement its respective private method and test your work using client code like this:
Computer vic = new Computer(); // or vic.reset(), if the computer already exists vic.loadProgram("program1.vic"); // or some other program file vic.run();
System.out.println(vic);
If your test code has loaded program1.vic, the output should look like this:
1
1
Run terminated normally
D register = 1
PC register = 4
Memory state:
0 399
1 900
2 399
3 900
4 0
5 0
6 0
7 0
8 0
9 0 ...
90 0
91 0
92 0
93 0
94 0
95 0
96 0
97 0
98 0
99 1
The first two lines – the two 1's – are the output of the execution of the loaded Vic program (program1.vic). Next, the run method prints "Run terminated normally". The rest of the output is generated by the System.out.println(vic) command.
Stage 4
Now turn to implement the READ command. Before we read any inputs, we must inform the computer where the inputs are coming from. This is done by the Computer’s setInput method.
Going back to the Usage Section at the beginning of this document, note that the inputs are coming from a file whose name is the second (optional) command-line argument. Hint: Implement the setInput method using StdIn. The implementation is one line of code.
Next, implement the READ command. Basically, you need a private execRead method that gets the next line from the input file, and puts it in the data register (don’t forget to advance the program counter).
Testing: To test the setInput and execRead methods, use something like this client code:
Computer vic = new Computer(); // or vic.reset(), if the computer already exists.
vic.loadProgram("program5.vic"); vic.setInput("input1.dat"); vic.run();
System.out.println(vic);
If your working directory includes the files program5.vic and input1.dat, this code should produce the following output:
10
20
30
Run terminated normally
D = 30
PC = 6
Memory state:
0 800
1 900
2 800
3 900
4 800
5 900
6 0
7 0
8 0
9 0 ...
90 0
91 0
92 0
93 0
94 0
95 0
96 0
97 0
98 0
99 1
The first three lines are the output of the Vic program that was executed.
Stage 5
Now turn to implementing the GOTO, GOTOZ, and GOTOP commands. As usual, each command should be implemented by a separate private method. We recommend doing it in stages, as follows:
Implement and test the GOTO and GOTOZ commands; test your implementation using program6.vic and input2.dat.
Implement and test the GOTOP command; test your implementation using max2.vic (a program that prints the maximum of two given numbers) and input3.dat (a file containing two numbers).
VicSimulator
This class is the driver of the Vic Simulator. It consists of a single method, named main. The main method creates a Computer object (we suggest naming it vic), and loads into it the code from the supplied programFile (first command-line argument). This can done by calling the loadProgram method on the vic object. If there is a second command-line argument, main calls the setInput method on vic. Finally, main calls the run method, and then prints the state of the vic computer.
Mazel Tov! you are the proud owner of a Vic computer, built by you from the ground up. You can now test your computer by loading and running any Vic program that comes to your mind. If the program is written in the symbolic Vic language, you must first use the Vic Assembler (available in www.idc.ac.il/vic) to translate it into a corresponding *.vic file. If the program is expected to read one or more inputs, you must put the inputs in a *.dat input file. See the Usage Section at the beginning of this document for more details on executing Vic programs using your Vic simulator.
Documentation
External documentation: There is no need to write Javadoc, since the APIs of all the classes that you have to write are given.
Internal documentation: Use your judgment to write comments that describe significant code segments or highlight anything that you think is worth highlighting.
