[CPPHTP7 NOTES] CH8. POINTERS(6) Simpletron Computer Simulator

(Exercise 8.18 Machine-Language Programming + Exercise 8.19 Computer Simulator)

Here comes the interesting part! I have created a virtual computer named Simpletron with C++! It is a simple machine that only has 100 memory locations, each can store a four digit signed decimal integers(-9999 to +9999). When running a program, the machine load the program to the start of its memory (location 00) and run the instruction stored in it one by one.

Although it is simple, we can still run program on it! Programs in Simpletron are witten in SML, which stands for Simpletron Machine Language. Each instruction written in SML occupies one memory location of Simpletron, thus, it is a signed four digit integer. The first two digits of each instruction are the operation code that specifies the operation to be performed(shown below). The last two digits are the operand, which are the address of the memory location. (Remember? There are 100 locations, which can be indexed 0 to 99)

Ok, now we have a brief knowledge of SML. We can try writing programs in SML now! (it is miserable, trust me XD)

The books provide some simple programs that can be written in SML. I have written down my programs below. Take a look at them if you are interested. You may also try to write a more complicated program yourself! You will miss C++ once you trying to program in SML though. LOL

Add sum until a negative number is enteredLocationNumberInstruction00+1011Read N01+2011Load N02+4110Branch negative to 1003+2112Store S04+1011Read N05+2011Load N06+4109Branch negative to 0907+3012Add S08+4003Branch to 0309+1112Write S10+4300Halt11+0000Variable N12+0000Variable SAdd sum of seven numbersLocationNumberInstruction00+1011Read N01+2012Load S02+3011Add N03+2112Store S04+2013Load I05+3114Subtract One06+2113Store I07+4209Branch Zero to 0908+4000Branch to 0009+1112Write S10+4300Halt11+0000Variable N12+0000Variable S13+0007Variable I14+0001OneFind the largest number in N numbersLocationNumberInstruction00+1018Read N01+2018Load N02+4217Branch Zero to 1703+1019Read Largest04+2018Load N05+3121Subtract One06+4216Branch zero to 1607+2118Store N08+2019Load Largest09+1020Read X10+3120Subtract X11+4113Branch negative to 1312+4004Branch to 0413+2020Load X14+2119Store Largest15+4004Branch to 0416+1119Write Largest17+4300Halt18+0000Variable N19+0000Variable Largest20+0000Variable X21+0001One

Okay, we have the program now. Wait! How do we run those programs? We don't have the Simpletron machine in reality! Don't be nervous, guys. I am going to show you how I write a Simpletron simulator with C++. Here is how it looks(the testing program is the same with the first one above):

The Simpletron machine has some register units to store neccessary information to run programs. We use an accumulator register to store the result of calculation, an instruction register to store the current instruction and a counter register to indicate the current location of execution. For example, when we need to add the integer at location 10 and the integer at location 11 together, we first load the first integer to the accumulator.(instruction:+2010) Then we add the second integer to the accumulator.(instruction:+3011) The result is now in the accumulator and we may store it at location 12 for later use.(instruction:+2112)  Note that after each instruction, the value in the counter register is increased by 1, which tells the computer to run the instruction at the next memory location.

I created a SimpletronSimulator class to represent the virtual computer. And I used a lot of static const int to represent the operation code in the table above so that I can recognize different operation with a switch easily. Okay, enough talk. The details are all in the sourcecode. I promise that is is a very easy and clear implementation!

// Exercise 8.19 Computer Simulator#include <iostream>#include <iomanip>using namespace std;// to simulate a type of computer called Simpletronclass SimpletronSimulator{public:SimpletronSimulator();void printConsoleMessage();void printErrorMessage();void inputSMLProgram();void executeSMLProgram();void printRegisterAndMemoryDump();private:// memorystatic const int memorySize = 100;int memory[memorySize];bool setMemory( int index ); // false when -99999 sentinel is enteredbool validInput( int input );// registersint accumulator;int instructionCounter;int instructionRegister;int operationCode;int operand;// operation codestatic const int READ = 10;static const int WRITE = 11;static const int LOAD = 20;static const int STORE = 21;static const int ADD = 30;static const int SUBTRACT = 31;static const int DIVIDE = 32;static const int MULTIPLY = 33;static const int BRANCH = 40;static const int BRANCHNEG = 41;static const int BRANCHZERO = 42;static const int HALT = 43;};int main(){SimpletronSimulator simulator;simulator.printConsoleMessage();return 0;}SimpletronSimulator::SimpletronSimulator(){accumulator = 0;instructionCounter = 0;for( int i=0; i<100; i++ ){memory[i] = 0;}}void SimpletronSimulator::printConsoleMessage(){cout << "*** Welcome to Simpletron! ***" << endl;cout << "*** Please enter your program one instruction ***" << endl;cout << "*** (or data word) at a time. I will type the ***" << endl;cout << "*** location number and a question mark (?).  ***" << endl;cout << "*** You then type the word for that location. ***" << endl;cout << "*** Type the sentinel -99999 to stop entering ***" << endl;cout << "*** your program. ***" << endl << endl;inputSMLProgram();cout << endl;cout << "*** Program loading completed ***" << endl;cout << "*** Program execution begins  ***" << endl;executeSMLProgram();cout << "*** Program execution completed  ***" << endl;cout << endl;printRegisterAndMemoryDump();}void SimpletronSimulator::inputSMLProgram(){bool inputContinue = true;for( int counter=0; inputContinue; counter++ )inputContinue = setMemory(counter); // setMemory return false when -99999 is entered}void SimpletronSimulator::executeSMLProgram(){int temp; // for tempory usedo{// load necessary data into registersinstructionRegister = memory[instructionCounter];operationCode = instructionRegister / 100; // first two digits as codeoperand = instructionRegister % 100; // last two digits as locationswitch( operationCode ){case READ:cout << "? ";cin >> temp;memory[operand] = temp;instructionCounter++;break;case WRITE:cout << showpos << memory[operand] << endl;instructionCounter++;break;case LOAD:accumulator = memory[operand];instructionCounter++;break;case STORE:memory[operand] = accumulator;instructionCounter++;break;case ADD:accumulator += memory[operand];instructionCounter++;break;case SUBTRACT:accumulator -= memory[operand];instructionCounter++;break;case DIVIDE:accumulator /= memory[operand];instructionCounter++;break;case MULTIPLY:accumulator *= memory[operand];instructionCounter++;break;case BRANCH:instructionCounter = operand;break;case BRANCHNEG:if( accumulator < 0 ){instructionCounter = operand;}else{instructionCounter++;}break;case BRANCHZERO:if( accumulator == 0 ){instructionCounter = operand;}else{instructionCounter++;}break;case HALT:break;default:break;}}while( operationCode != HALT );}void SimpletronSimulator::printRegisterAndMemoryDump(){cout << setfill('0') << internal; // fill 0 between '+' and numbercout << "REGISTERS:" << endl;cout << "accumulator\t\t\t"<< showpos << setw(5) << accumulator << endl;cout << "instructionCounter\t\t   "<< noshowpos << setw(2) << instructionCounter << endl;cout << "instructionRegister\t\t"<< showpos << setw(5) << instructionRegister << endl;cout << "operationCode\t\t\t   "<< noshowpos << setw(2) << operationCode << endl;cout << "operand\t\t\t\t   " << setw(2) << operand << endl;cout << endl << "MEMORY:" << endl;cout << setfill(' ') << setw(2) <<' ';for( int i=0; i<=9; i++ ) // column numbercout << ' ' << setw(5) << i;cout << endl;for( int i=0; i<=90; i+=10 ) // loop from 0 to 99{cout << noshowpos << setfill(' ') << setw(2) << i; // row numbercout << setfill('0') << showpos; // formatingfor( int j=0; j<=9; j++ ){cout << ' ' << setw(5) << memory[i+j];}cout << endl;}}bool SimpletronSimulator::setMemory( int index ){int input;cout << setfill('0');do // prompt again if not valid{cout << setw(2) << index << " ? ";cin >> input;}while( !validInput(input) );if( input == -99999 ){return false;}else{memory[index] = input;return true;}}bool SimpletronSimulator::validInput( int input ){// valid when input is a 4-digit number or -99999 sentinelreturn ( ( input>=-9999 && input<=9999 ) || input==-99999 );}

If you find any bugs or have any doubts, please feel free to contact me!