ZXSpectrum48K/Core/Cpu/Z80.cs

using System;
using Core.Interfaces;

namespace Core.Cpu
{
    public partial class Z80
    {
        //T-State counter
        public long TotalTStates { get; set; }

        // Interrupt Flip-Flops
        public bool IFF1;
        public bool IFF2;

        // Main Register Set
        public RegisterPair AF;
        public RegisterPair BC;
        public RegisterPair DE;
        public RegisterPair HL;

        // Alternate Register Set
        public RegisterPair AF_Prime;
        public RegisterPair BC_Prime;
        public RegisterPair DE_Prime;
        public RegisterPair HL_Prime;

        // Index Registers
        public RegisterPair IX;
        public RegisterPair IY;

        // Special Purpose Registers
        public ushort PC; // Program Counter
        public ushort SP; // Stack Pointer
        public byte I;    // Interrupt Vector
        public byte R;    // Memory Refresh

        // The Memory Bus
        private readonly IMemory _memory;
        private readonly IIoBus _ioBus;

        public Z80(IMemory memory, IIoBus ioBus)
        {
            _memory = memory;
            _ioBus = ioBus;
            Reset();            
        }

        public void Reset()
        {
            PC = 0x0000;
            // The Z80 initializes SP to 0xFFFF on boot
            SP = 0xFFFF;

            AF.Word = 0;
            BC.Word = 0;
            DE.Word = 0;
            HL.Word = 0;
        }

        public int Step()
        {
            // Fetch the next opcode and increment the Program Counter
            byte opcode = _memory.Read(PC++);
            int tStates = ExecuteOpcode(opcode);
            TotalTStates += tStates;

            // Decode and execute
            return tStates;
        }

        // Reads a 16-bit word from the current PC (Little-Endian) and advances PC by 2
        private ushort FetchWord()
        {
            byte low = _memory.Read(PC++);
            byte high = _memory.Read(PC++);
            return (ushort)((high << 8) | low);
        }

        private byte FetchByte()
        {
            return _memory.Read(PC++);
        }

        public string GetFlagsString()
        {
            byte f = AF.Low;
            return $"S:{(f >> 7) & 1} " +
                   $"Z:{(f >> 6) & 1} " +
                   $"Y:{(f >> 5) & 1} " + // Undocumented flag
                   $"H:{(f >> 4) & 1} " +
                   $"X:{(f >> 3) & 1} " + // Undocumented flag
                   $"P/V:{(f >> 2) & 1} " +
                   $"N:{(f >> 1) & 1} " +
                   $"C:{f & 1}";
        }

        private void Sbc(byte value)
        {
            byte a = AF.High;
            byte carry = (byte)(AF.Low & 0x01); // Get the current Carry flag (Bit 0)

            // Calculate the raw integer result to check for borrows/underflows
            int result = a - value - carry;

            // Update the Accumulator
            AF.High = (byte)result;

            // --- Update Flags (F Register) ---
            AF.Low = 0; // Clear all flags

            // Sign Flag (Bit 7)
            if ((result & 0x80) != 0) AF.Low |= 0x80;

            // Zero Flag (Bit 6)
            if ((byte)result == 0) AF.Low |= 0x40;

            // Half-Carry Flag (Bit 4) - Set if borrow from bit 4
            if (((a & 0x0F) - (value & 0x0F) - carry) < 0) AF.Low |= 0x10;

            // Overflow Flag (Bit 2) - Set if operands have different signs and result sign changes
            if ((((a ^ value) & 0x80) != 0) && (((a ^ result) & 0x80) != 0)) AF.Low |= 0x04;

            // Subtract Flag (Bit 1) - ALWAYS set for subtraction
            AF.Low |= 0x02;

            // Carry Flag (Bit 0) - Set if the overall result dropped below 0
            if (result < 0) AF.Low |= 0x01;
        }

        private int ExecuteOpcode(byte opcode)
        {
            switch (opcode)
            {
                case 0x00: // NOP
                    return 4;
                case 0x11: //LD DE, nn
                    DE.Word = FetchWord();
                    return 10;
                case 0x2B: // DEC HL
                    HL.Word--;
                    return 6;
                case 0x36: // LD (HL), n
                    byte nValue = FetchByte();
                    _memory.Write(HL.Word, nValue);
                    return 10;
                case 0x3E: //LD A, n
                    AF.High = FetchByte();
                    return 7;
                case 0x47: // LD B, A
                    BC.High = AF.High;
                    return 4;
                case 0x62: // LD H, D
                    HL.High = DE.High;
                    return 4;
                case 0x6B: // LD L, E
                    HL.Low = DE.Low;
                    return 4;
                case 0xC3:
                    PC = FetchWord();
                    return 10;
                case 0xD3: // OUT (n), A
                    byte portOffset = FetchByte();

                    // The Z80 puts 'A' on the top 8 bits, and 'n' on the bottom 8 bits of the port address
                    ushort portAddress = (ushort)((AF.High << 8) | portOffset);

                    _ioBus.Write(portAddress, AF.High);

                    return 11; // Takes 11 T-States
                case 0xDE: // SBC A, n
                    Sbc(FetchByte());
                    return 7;
                case 0xED:
                    return ExecuteExtendedPrefix();
                case 0xF3: // DI (Disable Interrupts)
                    IFF1 = false;
                    IFF2 = false;
                    return 4;
                case 0xAF: // XOR A
                    AF.High = 0;
                    AF.Low = 0x44;
                    return 4;

                default:
                    throw new NotImplementedException($"Opcode 0x{opcode:X2} at PC 0x{(PC - 1):X4} is not implemented.");
            }
        }
        private int ExecuteExtendedPrefix()
        {
            // Fetch the actual extended instruction
            byte extendedOpcode = _memory.Read(PC++);

            switch (extendedOpcode)
            {
                case 0x47: // LD I, A
                    I = AF.High;
                    return 9;
                default:
                    throw new NotImplementedException($"Extended ED Opcode 0x{extendedOpcode:X2} at PC 0x{(PC - 1):X4} is not implemented.");
            }
        }
    }
}