The first instruction: HLT

Time to implement the first instruction. Remember that everything needs to be encoded as numbers. I have mentioned the “special stop instruction” before. In machine language, each instruction has a memorizable name so you don’t always have to remember its number. The special stop instruction is called HLT.

In our simulation, we will give it the number 118. In machine language, it is common practice to use the hexadecimal (or hex) system, because all 8 bit numbers can be written using only 2 characters. For instance, 118 in decimal is 76 in hex. to distinguish decimal numbers from hex numbers, they are written like this: 0x76. In fact, you can write them exactly like this in Java.

The number associated with an instruction is called its Op Code and the name is called its Mnemonic. Here’s a handy overview for our first instruction:

Look at the following implementation and make sure you understand what it does.

    public boolean decodeAndExecute(int instruction) {
        switch (instruction) {
            case 0x76:
                // HLT
                return false;
            default:
                throw new RuntimeException("Unknown opcode 0x" + String.format("%02x", instruction));
        }
        return true; // Your Java compiler will probably complain because this line can't ever be reached.
    }

If a HLT op code is encountered, the method return false, causing the run loop to stop. In every other case, a RuntimeException is thrown, basically because no other op codes are known to our simulator yet.

Your Java compiler will complain about the line containing return true; because this line can actually never be reached. Let’s fix that quickly by adding one other opcode. The new instruction will be to do nothing at all. It’s mnemonic is NOP literally for “no operation.” It may seem funny, but all CPUs I am aware of have such a NOP instruction.

It’s easily implemented. Can you imagine how?