Thread: My VM project - mysterious instruction

I've started writing a virtual machine as a pet project (pet project #3 actually) and, without boring you with the details, I'll just say that the instructions are stored in a vector of CInstruction. CInstruction is a very basic class:

Code:

class CInstruction {
public:
    byte opcode;
    byte operands[2];
};

In the virtual machine class the Run method iterates through this vector and "does the right thing" based on the instructions opcode. However at the end of each run, I have the instruction 253, with the operands 253, 253, appear out of nowhere. I assume it's a random value in the vector but I'm not sure how it got there.

Here's the run method:

Code:

int CVirtualMachine::Run()
{
    CInstruction inst = nextInstruction();

    while (inst.opcode != 0x05) {
#ifdef DEBUG
        std::cout << "\nDEBUG: ip : " << (unsigned) inst.opcode << " " << (unsigned) inst.operands[0] <<
                     ", " << (unsigned) inst.operands[1] << endl;
#endif
        switch (inst.opcode) {
            case 0x00:  // nop
                break;
            case 0x05:  // end
                endProgram(inst.operands);
                break;
            case 0x06:  // movl l, r
                movl(inst.operands);
                break;

            default:
                throw BadInstruction();
        }

        inst = nextInstruction();
    }

    return 0;
}

The vector is simply defined in the class as

Code:

vector<CInstruction> instructions;

Then it isn't touched until my main driver program calls LoadDebug:

Code:

bool CVirtualMachine::LoadDebug(vector<CInstruction> program)
{
    for (unsigned i = 0; i < program.size(); i++)
        instructions.push_back(program[i]);

    return true;
}

LoadDebug is just there so I can feed it "raw instructions" until I start to write an assembler.

Oh, here's nextInstruction:

Code:

CInstruction CVirtualMachine::nextInstruction()
{
    return instructions[ip++];
}

I think that's all that could affect the instructions vector.
Help!

Oh yeah in case you were wondering it's all coming along nicely otherwise. Here's a sample output for your pleasure:

Code:

DEBUG: ip : 6 10, 1

DEBUG: Registers:
        r0: 10
        r1: 0
        r2: 0
        r3: 0

DEBUG: ip : 6 255, 3

DEBUG: Registers:
        r0: 10
        r1: 0
        r2: 255
        r3: 0

DEBUG: ip : 253 253, 253
*** ERROR ***
        Bad Instruction!
Press any key to continue

Whoop just solved it. I was never giving it the end instruction (0x05) so I guess the while loop went wild.

Yep I've just spent the last few hours working on it and I started using enums when I got to about the 10th instruction.

It's in a state now where some simple programs can be written but it's a hassle to feed it CInstructions manually so I've just started working on an assembler. This should be fun