News – sim6502

Hello 8bit world!

Michael — Mon, 28 Jul 2025 15:05:34 +0000

Welcome to my blog.

So there we are. My blog about my project, the 6502 simulator. And surely you’re wondering why? Why another 6502 simulator?
Just look at the

‘So Why…‘ section

What am I going to write about here?
– about the progress of the project
– new versions and releases
– screenshots
and maybe more. Let’s see.

The progress…

Michael — Mon, 28 Jul 2025 14:57:35 +0000

If you want to see what opcodes are implemented yet, milestones etc. …

Go to this page

Opcode Attribute

Michael — Fri, 11 Jul 2025 14:52:25 +0000

Regarding observing the cycles, I changed my mind. At first, I didn’t attach any importance to it. After some more reading and getting a better understanding of the importance the cycles have, even in a simulation, I will will do some changes to the code.
To do this in a way that keeps the code flexible and maintable, I want to introduce an attribute for the opcode-methods. It will handle the cycles this opcode takes, the adressing mode and probably some more parameters. We’ll see.

[AttributeUsage(AttributeTargets.Method)]
class OpcodeAttribute : Attribute {
    public int Cycles { get; }
    public string AddressingMode { get; }

    public OpcodeAttribute(int cycles, string mode) {
        Cycles = cycles;
        AddressingMode = mode;
    }
}

[AttributeUsage(AttributeTargets.Method)]
class OpcodeAttribute : Attribute {
    public int Cycles { get; }
    public string AddressingMode { get; }

    public OpcodeAttribute(int cycles, string mode) {
        Cycles = cycles;
        AddressingMode = mode;
    }
}

[Opcode(2, "Immediate")]
public void Cmd_A9() {
    // LDA #immediate
    A = FetchByte();
    SetFlags(A);
}


MethodInfo method = this.GetType().GetMethod("Cmd_" + opcode.ToString("X2"));
if (method != null) {
    var attr = method.GetCustomAttribute<OpcodeAttribute>();
    method.Invoke(this, null);
    if (attr != null) {
        CycleCount += attr.Cycles;
        # use attr.AddressingMode to chose the mode for this opcode
    }
}

[Opcode(2, "Immediate")]
public void Cmd_A9() {
    // LDA #immediate
    A = FetchByte();
    SetFlags(A);
}


MethodInfo method = this.GetType().GetMethod("Cmd_" + opcode.ToString("X2"));
if (method != null) {
    var attr = method.GetCustomAttribute<OpcodeAttribute>();
    method.Invoke(this, null);
    if (attr != null) {
        CycleCount += attr.Cycles;
        # use attr.AddressingMode to chose the mode for this opcode
    }
}

Project resume

Michael — Sat, 28 Jun 2025 14:22:14 +0000

After quite a long break, I’m now resuming on this project. At least I’ll try to.
First step is to migrate from .NET Framework to .NET as you can see in “v0.16.1_sdk_alpha“.
The final _sdk_alpha will be merged to master after all tests were successful.
Next step is to introduce one ore more interfaces to facilitate testing and maybe going towards plugins.
So let’s see how it goes….

Unit Tests

Michael — Thu, 23 Mar 2023 16:30:00 +0000

Now that some opcodes have been implemented and are working properly, it becomes more and more important to be able to test the whole system. Especially when new functions have been installed or a refactoring has taken place. That’s why I decided to test with ‘NUnit’. The corresponding tests are written for all methods of the CPU and for all opcodes.

Let’s have a look at how to test one of the opcodes i.e. LDA – 0xA9

        [TestCase(0xff, ExpectedResult = new object[] { false, true, 0xff })]
        [TestCase(0x00, ExpectedResult = new object[] { true, false, 0x00 })]
        public object[] Cmd_A9_Test(byte b)
        {
            object[] result = new object[3];

            cpu.WriteByteToMemory(b, 0x200);
            cpu.SetPC(0x200);
            cpu.Cmd_A9();

            result[0] = cpu.flags["Z"];
            result[1] = cpu.flags["N"];
            result[2] = cpu.A;

            return result;

        }

After writing tests for all existing methods, I will try to move to a ‘test driven development’ TDD. So for each method/function, a test is written first and then the actual method. Let’s see how it will be.

Some opcodes

Michael — Tue, 21 Mar 2023 22:37:39 +0000

Some opcodes are already working.

As of today 21.03.2023:

BRK = 0x00,
***LDA
LDA_IM = 0xA9,
LDA_ZP = 0xA5,
LDA_ZPX = 0xB5,
***LDX
LDX_IM = 0xA2,
LDX_ZP = 0xA6,
LDX_ZPY = 0xB6,
***LDY
LDY_IM = 0xA0,
LDY_ZP = 0xA4,
LDY_ZPX = 0xB4,

***JMP
JMP_ABS = 0x4C,

***Stack
PHA = 0x48,
PLA = 0x68