Given that the only two opcodes out simulator understand are “do nothing” and “stop doing anything,” your first program will be a bit boring. But it will get better soon. Let’s first look at how things work in principle.

As usual in Java, we need some class with a main method to run things.

public class Simulation {
    
    public static void main(String[] args) {
        Memory mem = new Memory();
        System.out.println("Loading program into memory.");
        mem.load(0, new int[]{0x00, 0x00, 0x76});
        Cpu cpu = new Cpu();
        System.out.println("Attaching memory to the CPU");
        cpu.attachMemory(mem);
        System.out.println("Running the program");
        cpu.run();
        System.out.println("Finished");
    }
}

Do you see how the Memory.load method is used to load the program into memory? The program consists of the numbers 0x00, 0x00 and 0x76. Using the mnemonics, you can read that as:

NOP
NOP
HLT

Try to run this with java Simulation. It will print out the various steps of the simulation but apart from this, it does … nothing. No surprise, with the somewhat boring opcodes we’ve implemented so far.

At least we know two things:

  • The program finished (the technical term is terminated), as opposed to running forever.
  • The main execution loop works, including incrementing the program counter. We know this because the HLT instruction is only the third one, and it has been reached or else the program would run forever.

Yay, this was your first machine language program! Time to throw a party.

Let’s move on to something more interesting.