# Run 2

Creating a basic virtual machine (VM) runtime that includes an operand stack and a context stack for frames is an interesting and complex task. Below, I'll outline a simple way to implement such a VM in Java.

### 1. Operand Stack and Context Stack
- **Operand Stack**: The operand stack is used to evaluate expressions. Each method execution can have its own operand stack.
- **Context Stack (Call Stack)**: The context stack, or call stack, holds the execution context of each method invocation, which includes the local variables, return address, and operand stack.

### 2. Essential Components
- **Instruction Set**: Define a simple instruction set for our VM.
- **Frame**: Represents a single method invocation containing the operand stack and local variables.
- **VM Class**: Manages the context stack and overall execution.

### 3. Example Implementation

```java
import java.util.Stack;

// Enum representing the different operations
enum Instruction {
    PUSH, ADD, SUB, MUL, DIV, PRINT, RETURN
}

// Frame class
class Frame {
    int[] locals; // Local variables
    Stack<Integer> operandStack; // Operand stack for evaluation
    int returnAddress; // Optional, if implementing RETURN

    public Frame(int localSize) {
        locals = new int[localSize];
        operandStack = new Stack<>();
    }
}

// VM Class
class VirtualMachine {
    // This stack represents the call stack (context stack)
    private Stack<Frame> contextStack = new Stack<>();

    public void execute(Frame frame, int[] instructions) {
        contextStack.push(frame);
        int pc = 0; // Program Counter

        while (pc < instructions.length) {
            Instruction instruction = Instruction.values()[instructions[pc]];
            switch (instruction) {
                case PUSH:
                    pc++;
                    int value = instructions[pc];
                    contextStack.peek().operandStack.push(value);
                    break;

                case ADD:
                    // ASSUMING OPERANDS ARE ON STACK
                    int a = contextStack.peek().operandStack.pop();
                    int b = contextStack.peek().operandStack.pop();
                    contextStack.peek().operandStack.push(a + b);
                    break;

                case SUB:
                    int c = contextStack.peek().operandStack.pop();
                    int d = contextStack.peek().operandStack.pop();
                    contextStack.peek().operandStack.push(d - c);
                    break;

                case MUL:
                    int e = contextStack.peek().operandStack.pop();
                    int f = contextStack.peek().operandStack.pop();
                    contextStack.peek().operandStack.push(e * f);
                    break;

                case DIV:
                    int g = contextStack.peek().operandStack.pop();
                    int h = contextStack.peek().operandStack.pop();
                    contextStack.peek().operandStack.push(h / g);
                    break;

                case PRINT:
                    System.out.println("PRINT: " + contextStack.peek().operandStack.pop());
                    break;

                case RETURN:
                    contextStack.pop(); // Remove the current frame
                    break;
            }
            pc++;
        }
    }
}

// Example Runtime Execution
public class SimpleVMTest {
    public static void main(String[] args) {
        // Simple program: 2 + 3
        // PUSH 2, PUSH 3, ADD, PRINT

        int[] instructions = {
                Instruction.PUSH.ordinal(), 2,
                Instruction.PUSH.ordinal(), 3,
                Instruction.ADD.ordinal(),
                Instruction.PRINT.ordinal()
        };

        VirtualMachine vm = new VirtualMachine();
        Frame frame = new Frame(0);

        vm.execute(frame, instructions); // Expected output is 5
    }
}
```

### 4. Explanation
- **Instruction Enum**: This enum represents the possible instructions our simple VM can execute.
- **Frame Class**: Encapsulates the local variables and operand stack for a single invocation. In a more complex implementation, you could also include the return address, method references, etc.
- **VirtualMachine Class**: The virtual machine manages the execution stack (context stack) and executes instructions. It processes each instruction in sequence.
- **SimpleVMTest Class**: Contains a simple example of how to run a program on our VM. This program pushes 2 and 3 onto the operand stack, adds them, and prints the result.

### 5. Potential Expansions
- You could expand this basic VM to support more operations like loops, conditionals, complex data types, or to handle method calls and returns by saving the program counter, and more.
- Implementing a memory model (heap) for object management.
- Implement handling for exceptions and branching instructions.

This basic example gives you the foundation of a stack-based virtual machine—a good starting point for learning how more sophisticated VMs (like the Java Virtual Machine) might work.