Let’s build a Motorola 6800 (M6800) simulator in C#. We’ll start with a minimal architecture and iterate.

The basic components we’ll need:

  • Memory (64KB)
  • CPU Registers: A, B, X, PC, SP, CCR
  • Instruction decoder and executor
  • Instruction set (partial to start)

✅ Step 1: Basic Structure and CPU Registers

public class Cpu6800
{
    public byte A;         // Accumulator A
    public byte B;         // Accumulator B
    public ushort X;       // Index Register
    public ushort PC;      // Program Counter
    public byte SP;        // Stack Pointer
    public byte CCR;       // Condition Code Register (flags)

    public byte[] Memory = new byte[ushort.MaxValue + 1];

    public void Reset()
    {
        A = 0;
        B = 0;
        X = 0;
        PC = ReadWord(0xFFFE); // Reset Vector
        SP = 0xFF;
        CCR = 0x00;
    }

    public byte ReadByte(ushort addr) => Memory[addr];
    public void WriteByte(ushort addr, byte value) => Memory[addr] = value;
    public ushort ReadWord(ushort addr) => (ushort)((Memory[addr] << 8) | Memory[addr + 1]);
    public void WriteWord(ushort addr, ushort value)
    {
        Memory[addr] = (byte)(value >> 8);
        Memory[addr + 1] = (byte)(value & 0xFF);
    }
}

✅ Step 2: Instruction Execution Skeleton

public void Step()
{
    var opcode = ReadByte(PC++);
    switch (opcode)
    {
        case 0x86: // LDA Immediate
            A = ReadByte(PC++);
            break;
        case 0xB6: // LDA Absolute
            var addr = ReadWord(PC);
            PC += 2;
            A = ReadByte(addr);
            break;
        case 0x20: // BRA
            PC += (sbyte)ReadByte(PC++);
            break;
        default:
            throw new NotImplementedException($"Opcode {opcode:X2} not implemented.");
    }
}

Awesome goal. To hook young software engineers into the world of vintage CPUs like the 6800/68000, we need a scenario that:

  • Has visual/interactive feedback
  • Relates to modern concepts (like games, puzzles, or automation)
  • Feels like they’re controlling hardware, like early embedded systems

🎮 Scenario: “Escape the Byte Dungeon”

Story: You’re inside a forgotten 8-bit machine, trapped in a dungeon built from memory addresses. To escape, you must write code to control a robot moving through the memory grid, collect keys (data bytes), and unlock the exit (a specific memory condition).

💡 Core Mechanics

Mechanic Implementation 6800 Concepts Covered
Memory-as-Map 256-byte memory block (16x16) Memory addressing, indirect access
Robot Movement Write direction codes to a port I/O simulation, memory-mapped I/O
Read Sensors Use memory-mapped addresses for input Polling I/O, branch decisions
Collect Key Check a specific byte in a memory cell Data comparison, flags, branches
Exit Condition Match memory layout or flag to escape Program flow, loop, condition code use

🧠 Learning Objectives

  • Learn assembly control flow
  • Understand memory layout as spatial structure
  • Experience I/O via mapped memory
  • Write tiny programs with constraints and purpose

🛠 Implementation Plan

  1. C# Simulator Enhancements

    • Add a MemoryMap for I/O (robot controls)
    • Add hooks for “rendering” (e.g., Console output of the robot in the grid)

  2. Robot API (6800 Assembly)

    • Movement: 0xF0 memory address — write 0x01=UP, 0x02=DOWN, 0x03=LEFT, 0x04=RIGHT
    • Sensor: 0xF1 — returns 0x01 if key present at current location
    • Status: 0xF2 — set to 0xFF when goal reached

  3. Level Layout

    • Map at 0x0100 - 0x01FF, 16x16 grid
    • Key at 0x0156, Exit at 0x01A3
    • Player starts at 0x0110

  4. Sample Assembly Program

    • Move right → check sensor → collect key → reach goal → write 0xFF to 0xF2

🌟 Bonus

  • Add step-through debug mode in C#
  • Provide visual output of the map
  • Include instruction count limits (teaches optimization)
  • Offer challenge levels

Here are five more engaging and educational scenario ideas tailored for young software engineers exploring the 6800/68000 world for the first time:

Story: You’re controlling a vintage satellite’s onboard 6800 system. It needs to align its antenna toward Earth and send a transmission back before it drifts out of range.

Mechanics:

  • Rotate antenna via memory-mapped commands
  • Read signal strength from memory-mapped sensors
  • Only send if alignment and power levels are correct
  • Must complete in under 200 cycles

Skills Learned:

  • Loops, conditional logic, memory-mapped I/O, cycle budgeting

2. 🎵 Chiptune Synthesizer

Story: Use the 6800 to create a looping melody. Each byte you write to a port triggers a note. You program the CPU like a primitive music sequencer.

Mechanics:

  • Write note values to 0xF0 (sound port)
  • Timing handled via loop counters
  • Compose simple melodies (8-bit style)

Skills Learned:

  • Loops, indexed addressing, timing control, delay loops

3. 🕹️ Simon Says Game Clone

Story: Recreate the classic memory game using memory-mapped buttons and LEDs. The system shows a pattern, the player must input it.

Mechanics:

  • LEDs at 0xF0–0xF3 (output), buttons at 0xF4–0xF7 (input)
  • Generate random pattern in memory
  • Poll input and compare to expected sequence

Skills Learned:

  • Bit manipulation, I/O handling, branching logic, random number (via counters)

4. 📦 Warehouse Sorting Bot

Story: A small autonomous robot must sort boxes in a 2D warehouse using sensors and an actuator. Each box has a type byte. You must move it to the correct slot.

Mechanics:

  • Move via control port (0xF0)
  • Read box type at current location
  • Sort by writing to correct location

Skills Learned:

  • Indexed memory access, control logic, sorting logic, memory map design

5. 🔐 Codebreaker Terminal

Story: You’ve found a locked computer from the past. Its 6800 firmware requires cracking a 3-byte access code through brute-force and logic filters.

Mechanics:

  • Enter code via memory
  • Read result (success/fail) from memory-mapped status
  • Optimize to solve in minimal cycles

Skills Learned:

  • Loops, nested iteration, optimization, memory interaction, brute-force logic