ChatGPT Commodore 64 Emulator

C64Emulator.java

import java.util.Arrays;

public class C64Emulator {

    private static final int MEMORY_SIZE = 65536; // 64KB memory
    private byte[] memory = new byte[MEMORY_SIZE];

    // 6502 CPU registers
    private int pc; // Program Counter
    private byte sp; // Stack Pointer
    private byte a, x, y; // Accumulator, X and Y registers
    private byte status; // Processor status

    public C64Emulator() {
        reset();
    }

    private void reset() {
        Arrays.fill(memory, (byte) 0);
        pc = 0x0400; // Start at the BASIC program area for this example
        sp = (byte) 0xFF;
        a = x = y = 0;
        status = 0;
    }

    private byte readMemory(int address) {
        return memory[address & 0xFFFF];
    }

    private void writeMemory(int address, byte value) {
        memory[address & 0xFFFF] = value;
    }

    public void loadProgram(byte[] program, int startAddress) {
        System.arraycopy(program, 0, memory, startAddress, program.length);
        pc = startAddress;
    }

    public void run() {
        while (true) {
            int opcode = readMemory(pc) & 0xFF;
            pc++;
            switch (opcode) {
                case 0xA9: // LDA Immediate
                    a = readMemory(pc);
                    pc++;
                    updateZeroAndNegativeFlags(a);
                    break;
                case 0x00: // BRK
                    return;
                default:
                    throw new IllegalStateException("Unknown opcode: " + opcode);
            }
        }
    }

    private void updateZeroAndNegativeFlags(byte value) {
        if (value == 0) {
            status |= 0x02; // Set Zero flag
        } else {
            status &= ~0x02; // Clear Zero flag
        }
        if (value < 0) {
            status |= 0x80; // Set Negative flag
        } else {
            status &= ~0x80; // Clear Negative flag
        }
    }

    public static void main(String[] args) {
        C64Emulator emulator = new C64Emulator();

        // Simple program to load a value into the accumulator and then halt
        byte[] program = {
            (byte) 0xA9, 0x42, // LDA #$42
            (byte) 0x00        // BRK
        };

        emulator.loadProgram(program, 0x0400);
        emulator.run();

        System.out.println("Program completed. Accumulator value: " + emulator.a);
    }
}