/****************************************************
                VIRTUAL MACHINE
		   VERSION 1.0
                  Kevin Taylor


*****************************************************/
public class VirtualMachine{

private int pc = 0;
private int[] memory = {8,5,
                        8,2,
								11,
								10, 16,
								9, 16,
								21,
								0,
								0,0,0,0,0,0,0,0,0,0,0,0};
private IntStack stack = new IntStack(20);
private int d, x, y, ir;


   public VirtualMachine(int pc, int[] memory, IntStack stack){
	this.pc = pc;
	this.memory = memory;
	this.stack = stack;
	this.x = x;
	this.y = y;
	this.ir = ir;
   this.d = d;











//Switch Statement for handling op code instructions in the VM

              do{
                  ir = memory[pc];
                  pc++;




                       switch(ir){

                        case 8:     // Load Constant

								   stack.push( memory[pc] );
									pc++;
									break;

							   case 9:    //Load Variable from addess

								   d = memory[pc];
									stack.push(d);
									pc++;
									break;


								case 10:   //store variable in address

									d = stack.pop();
									memory[memory[pc]] = d;
									pc++;
									break;

								case 11:   // add two operands and push the result

								   y = stack.pop();
									x = stack.pop();
									d = x + y;
									stack.push(d);
									pc++;
									break;

								case 12:   //subtract top two operands and push the result

								   y = stack.pop();
									x = stack.pop();
									d = x - y;
									stack.push(d);
									pc++;
									break;

								case 13: //multiply top two operands and push the result

								   y = stack.pop();
									x = stack.pop();
									d = x * y;
									stack.push(d);
									pc++;
									break;

								case 14: //Divide top two operands and push the result

								   y = stack.pop();
									x = stack.pop();
									d = x / y;
									stack.push(d);
									pc++;
									break;

								case 15: // Check if equal

								   y = stack.pop();
							   	x = stack.pop();

							if( x==y)
							   stack.push(1);

								else
								   stack.push(0);

							break;

							   case 16:  //Check if less

								   y = stack.pop();
									x = stack.pop();

						    if(x<y)
							    stack.push(1);

								else
								   stack.push(0);


							 break;

							    case 17:  //Check if greater

								   y = stack.pop();
									x = stack.pop();

						    if(x>y)
							    stack.push(1);

								else
								   stack.push(0);


							 break;


								 case 18:   //jump to address pc

								   pc = memory[pc];
									break;

								case 19: //jump if false

								   if(stack.pop() == 0)
									   pc = memory[pc];
										break;

								case 20: //read an int

								    d = memory[pc];
								    stack.push(d);
								    pc++;
								    break;

								 case 21: //write an int

								   System.out.println(stack.pop());
									 pc++;
									 break;

								 case 22:  //call subprogram

								    stack.push(pc + 1);
									 pc = memory[pc];
									 break;

								 case 23: //return from subprogram

								    pc = stack.pop();
									 break;




								case 0: //halt program

								   System.exit(0);
									break;


									} //end do
						}//end Switch
						while(true);



  }// end vm constructer






      public static void main(String[] args){

	    int pc = 0;
		 IntStack stack = new IntStack(20);
		 int[] memory =  {8,5,
                        8,2,
								11,
								10, 16,
								9, 16,
								21,
								0,
								0,0,0,0,0,0,0,0,0,0,0,0};



		VirtualMachine vm = new VirtualMachine(pc, memory, stack );







		}// end main

}//end Class