665 lines
23 KiB
C#
665 lines
23 KiB
C#
using System;
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using Core.Interfaces;
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namespace Core.Cpu
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{
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public partial class Z80
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{
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//T-State counter
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public long TotalTStates { get; set; }
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public int InterruptMode { get; private set; } = 0; // Defaults to 0 on power-up
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// Interrupt Flip-Flops
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public bool IFF1 { get; private set; } = false;
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public bool IFF2 { get; private set; } = false;
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// Main Register Set
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public RegisterPair AF;
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public RegisterPair BC;
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public RegisterPair DE;
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public RegisterPair HL;
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// Alternate Register Set
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public RegisterPair AF_Prime;
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public RegisterPair BC_Prime;
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public RegisterPair DE_Prime;
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public RegisterPair HL_Prime;
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// Index Registers
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public RegisterPair IX;
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public RegisterPair IY;
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// Special Purpose Registers
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public ushort PC; // Program Counter
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public ushort SP; // Stack Pointer
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public byte I; // Interrupt Vector
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public byte R; // Memory Refresh
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// The Memory Bus
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private readonly IMemory _memory;
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private readonly IIoBus _ioBus;
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public Z80(IMemory memory, IIoBus ioBus)
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{
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_memory = memory;
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_ioBus = ioBus;
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Reset();
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}
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public void Reset()
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{
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PC = 0x0000;
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// The Z80 initializes SP to 0xFFFF on boot
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SP = 0xFFFF;
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AF.Word = 0;
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BC.Word = 0;
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DE.Word = 0;
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HL.Word = 0;
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}
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public int Step()
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{
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// Fetch the next opcode and increment the Program Counter
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byte opcode = _memory.Read(PC++);
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int tStates = ExecuteOpcode(opcode);
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TotalTStates += tStates;
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// Decode and execute
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return tStates;
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}
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// Reads a 16-bit word from the current PC (Little-Endian) and advances PC by 2
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private ushort FetchWord()
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{
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byte low = _memory.Read(PC++);
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byte high = _memory.Read(PC++);
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return (ushort)((high << 8) | low);
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}
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private byte FetchByte()
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{
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return _memory.Read(PC++);
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}
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public string GetFlagsString()
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{
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byte f = AF.Low;
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return $"S:{(f >> 7) & 1} " +
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$"Z:{(f >> 6) & 1} " +
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$"Y:{(f >> 5) & 1} " + // Undocumented flag
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$"H:{(f >> 4) & 1} " +
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$"X:{(f >> 3) & 1} " + // Undocumented flag
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$"P/V:{(f >> 2) & 1} " +
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$"N:{(f >> 1) & 1} " +
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$"C:{f & 1}";
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}
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private void Sbc(byte value)
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{
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byte a = AF.High;
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byte carry = (byte)(AF.Low & 0x01); // Get the current Carry flag (Bit 0)
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// Calculate the raw integer result to check for borrows/underflows
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int result = a - value - carry;
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// Update the Accumulator
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AF.High = (byte)result;
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// --- Update Flags (F Register) ---
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AF.Low = 0; // Clear all flags
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// Sign Flag (Bit 7)
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if ((result & 0x80) != 0) AF.Low |= 0x80;
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// Zero Flag (Bit 6)
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if ((byte)result == 0) AF.Low |= 0x40;
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// Half-Carry Flag (Bit 4) - Set if borrow from bit 4
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if (((a & 0x0F) - (value & 0x0F) - carry) < 0) AF.Low |= 0x10;
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// Overflow Flag (Bit 2) - Set if operands have different signs and result sign changes
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if ((((a ^ value) & 0x80) != 0) && (((a ^ result) & 0x80) != 0)) AF.Low |= 0x04;
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// Subtract Flag (Bit 1) - ALWAYS set for subtraction
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AF.Low |= 0x02;
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// Carry Flag (Bit 0) - Set if the overall result dropped below 0
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if (result < 0) AF.Low |= 0x01;
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}
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private void Sbc16(ushort value)
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{
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int hl = HL.Word;
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int carry = AF.Low & 0x01;
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// Calculate the raw integer result to check for underflows
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int result = hl - value - carry;
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// Update the HL register
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HL.Word = (ushort)result;
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// --- Update Flags (F Register) ---
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AF.Low = 0; // Clear all flags
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// Sign Flag (Bit 7) - Set if the 16-bit result is negative (bit 15 is 1)
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if ((result & 0x8000) != 0) AF.Low |= 0x80;
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// Zero Flag (Bit 6) - Set if the entire 16-bit result is exactly 0
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if ((ushort)result == 0) AF.Low |= 0x40;
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// Half-Carry Flag (Bit 4) - Set if borrow from bit 11
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if (((hl & 0x0FFF) - (value & 0x0FFF) - carry) < 0) AF.Low |= 0x10;
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// Overflow Flag (Bit 2) - Set if operands have different signs and result sign changes
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if ((((hl ^ value) & 0x8000) != 0) && (((hl ^ result) & 0x8000) != 0)) AF.Low |= 0x04;
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// Subtract Flag (Bit 1) - ALWAYS set for subtraction
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AF.Low |= 0x02;
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// Carry Flag (Bit 0) - Set if the overall 16-bit result dropped below 0
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if (result < 0) AF.Low |= 0x01;
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}
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private byte Dec8(byte value)
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{
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byte result = (byte)(value - 1);
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// Store the existing Carry flag so we can preserve it
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byte carry = (byte)(AF.Low & 0x01);
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// Clear all flags
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AF.Low = 0;
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// Sign Flag (Bit 7)
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if ((result & 0x80) != 0) AF.Low |= 0x80;
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// Zero Flag (Bit 6)
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if (result == 0) AF.Low |= 0x40;
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// Half-Carry Flag (Bit 4) - Set if borrow from bit 4 (happens if the lower nibble was 0)
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if ((value & 0x0F) == 0) AF.Low |= 0x10;
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// Parity/Overflow Flag (Bit 2) - Set if the original value was 0x80 (maximum negative)
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if (value == 0x80) AF.Low |= 0x04;
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// Subtract Flag (Bit 1) - ALWAYS SET for decrements
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AF.Low |= 0x02;
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// Restore the original Carry Flag (Bit 0)
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AF.Low |= carry;
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return result;
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}
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private byte Inc8(byte value)
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{
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byte result = (byte)(value + 1);
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// Store the existing Carry flag so we can preserve it
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byte carry = (byte)(AF.Low & 0x01);
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// Clear all flags
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AF.Low = 0;
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// Sign Flag (Bit 7)
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if ((result & 0x80) != 0) AF.Low |= 0x80;
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// Zero Flag (Bit 6)
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if (result == 0) AF.Low |= 0x40;
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// Half-Carry Flag (Bit 4) - Set if carry from bit 3 (happens if lower nibble was 0x0F)
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if ((value & 0x0F) == 0x0F) AF.Low |= 0x10;
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// Parity/Overflow Flag (Bit 2) - Set if the original value was 0x7F (maximum positive)
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if (value == 0x7F) AF.Low |= 0x04;
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// Subtract Flag (Bit 1) - ALWAYS 0 for increments (already 0 because we cleared AF.Low)
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// Restore the original Carry Flag (Bit 0)
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AF.Low |= carry;
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return result;
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}
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private void Cp(byte value)
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{
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byte a = AF.High;
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int result = a - value;
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// --- Update Flags (F Register) ---
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AF.Low = 0; // Clear all flags
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// Sign Flag (Bit 7)
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if ((result & 0x80) != 0) AF.Low |= 0x80;
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// Zero Flag (Bit 6)
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if ((byte)result == 0) AF.Low |= 0x40;
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// Half-Carry Flag (Bit 4) - Set if borrow from bit 4
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if (((a & 0x0F) - (value & 0x0F)) < 0) AF.Low |= 0x10;
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// Overflow Flag (Bit 2) - Set if operands have different signs and result sign changes
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if ((((a ^ value) & 0x80) != 0) && (((a ^ result) & 0x80) != 0)) AF.Low |= 0x04;
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// Subtract Flag (Bit 1) - ALWAYS set for CP/SUB
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AF.Low |= 0x02;
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// Carry Flag (Bit 0) - Set if the overall result dropped below 0
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if (result < 0) AF.Low |= 0x01;
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}
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private void And(byte value)
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{
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AF.High = (byte)(AF.High & value);
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// --- Update Flags ---
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AF.Low = 0; // Clear all flags
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// Sign Flag (Bit 7) - Set if the highest bit is 1
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if ((AF.High & 0x80) != 0) AF.Low |= 0x80;
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// Zero Flag (Bit 6) - Set if the result is 0
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if (AF.High == 0) AF.Low |= 0x40;
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// Half-Carry Flag (Bit 4) - ALWAYS SET to 1 for Z80 AND instructions!
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AF.Low |= 0x10;
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// Parity Flag (Bit 2) - Set if the result has an even number of 1 bits
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if (HasEvenParity(AF.High)) AF.Low |= 0x04;
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// Subtract Flag (N) and Carry Flag (C) are ALWAYS 0
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}
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private void Add16(ushort value)
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{
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int hl = HL.Word;
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int result = hl + value;
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// Update the HL register
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HL.Word = (ushort)result;
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AF.Low &= 0xEC;
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// Half-Carry Flag (Bit 4) - Set if there is a carry from bit 11
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if (((hl & 0x0FFF) + (value & 0x0FFF)) > 0x0FFF) AF.Low |= 0x10;
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// Carry Flag (Bit 0) - Set if the result overflows 16 bits
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if (result > 0xFFFF) AF.Low |= 0x01;
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}
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private bool HasEvenParity(byte value)
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{
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int bits = 0;
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for (int i = 0; i < 8; i++)
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{
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if ((value & (1 << i)) != 0) bits++;
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}
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return (bits % 2) == 0;
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}
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private void Push(ushort value)
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{
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// High byte goes first
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SP--;
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_memory.Write(SP, (byte)(value >> 8));
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// Low byte goes second
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SP--;
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_memory.Write(SP, (byte)(value & 0xFF));
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}
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private int ExecuteOpcode(byte opcode)
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{
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sbyte offset = 0;
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switch (opcode)
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{
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case 0x00: // NOP
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return 4;
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case 0x01: // LD BC, nn
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BC.Word = FetchWord();
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return 10; // Takes 10 T-States
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case 0x04: // INC B
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BC.High = Inc8(BC.High);
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return 4;
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case 0x0E: // LD C, n
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BC.Low = FetchByte();
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return 7; // Takes 7 T-States
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case 0x10: // DJNZ d
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sbyte djnzOffset = (sbyte)FetchByte();
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BC.High--; // Decrement the B register
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if (BC.High != 0)
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{
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PC = (ushort)(PC + djnzOffset);
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return 13; // Jump taken
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}
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return 8; // Loop finished, no jump
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case 0x11: //LD DE, nn
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DE.Word = FetchWord();
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return 10;
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case 0x19: // ADD HL, DE
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Add16(DE.Word);
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return 11;
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case 0x20: // JR NZ, e
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offset = (sbyte)FetchByte();
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if ((AF.Low & 0x40) == 0)
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{
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PC = (ushort)(PC + offset);
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return 12;
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}
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return 7;
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case 0x21: // LD HL, nn
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HL.Word = FetchWord();
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return 10;
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case 0x22: // LD (nn), HL
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ushort dest22 = FetchWord();
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_memory.Write(dest22, HL.Low);
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_memory.Write((ushort)(dest22 + 1), HL.High);
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return 16;
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case 0x23: // INC HL
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HL.Word++;
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return 6;
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case 0x28: // JR Z, e
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offset = (sbyte)FetchByte();
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// Check if the Zero Flag (Bit 6) IS set
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if ((AF.Low & 0x40) != 0)
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{
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PC = (ushort)(PC + offset);
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return 12; // Jump taken
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}
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return 7; // Jump not taken
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case 0x2A: // LD HL, (nn)
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{
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ushort srcAddress = FetchWord();
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HL.Low = _memory.Read(srcAddress);
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HL.High = _memory.Read((ushort)(srcAddress + 1));
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return 16; // Takes 16 T-States
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}
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case 0x2B: // DEC HL
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HL.Word--;
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return 6;
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case 0x30: // JR NC, e
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offset = (sbyte)FetchByte();
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// Check if the Carry Flag (Bit 0) is NOT set
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if ((AF.Low & 0x01) == 0)
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{
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PC = (ushort)(PC + offset);
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return 12; // Jump taken
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}
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return 7; // Jump not taken
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case 0x32: // LD (nn), A
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{
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ushort destAddress = FetchWord();
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_memory.Write(destAddress, AF.High);
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return 13;
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}
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case 0x35: // DEC (HL)
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// Read the current byte from memory
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byte memValue = _memory.Read(HL.Word);
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// Decrement it and update flags
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byte decremented = Dec8(memValue);
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// Write the new value back to memory
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_memory.Write(HL.Word, decremented);
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return 11; // Takes 11 T-States
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case 0x36: // LD (HL), n
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byte nValue = FetchByte();
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_memory.Write(HL.Word, nValue);
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return 10;
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case 0x3E: //LD A, n
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AF.High = FetchByte();
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return 7;
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case 0x47: // LD B, A
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BC.High = AF.High;
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return 4;
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case 0x62: // LD H, D
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HL.High = DE.High;
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return 4;
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case 0x6B: // LD L, E
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HL.Low = DE.Low;
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return 4;
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case 0x77: // LD (HL), A
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_memory.Write(HL.Word, AF.High);
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return 7;
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case 0xA7: // AND A
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And(AF.High);
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return 4;
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case 0xAF: // XOR A
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AF.High = 0;
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AF.Low = 0x44;
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return 4;
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case 0xBC: // CP H
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Cp(HL.High);
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return 4;
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case 0xC3:
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PC = FetchWord();
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return 10;
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case 0xCD: // CALL nn
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ushort callAddress = FetchWord();
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Push(PC);
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PC = callAddress;
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return 17;
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case 0xD3: // OUT (n), A
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byte portOffset = FetchByte();
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// The Z80 puts 'A' on the top 8 bits, and 'n' on the bottom 8 bits of the port address
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ushort portAddress = (ushort)((AF.High << 8) | portOffset);
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_ioBus.Write(portAddress, AF.High);
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return 11; // Takes 11 T-States
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case 0xD9: // EXX
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ushort tempBC = BC.Word;
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BC.Word = BC_Prime.Word;
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BC_Prime.Word = tempBC;
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ushort tempDE = DE.Word;
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DE.Word = DE_Prime.Word;
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DE_Prime.Word = tempDE;
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ushort tempHL = HL.Word;
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HL.Word = HL_Prime.Word;
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HL_Prime.Word = tempHL;
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return 4; // Takes 4 T-States
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case 0xDE: // SBC A, n
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Sbc(FetchByte());
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return 7;
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case 0xEB: // EX DE, HL
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ushort tempEx = DE.Word;
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DE.Word = HL.Word;
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HL.Word = tempEx;
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return 4; // Takes 4 T-States
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case 0xED:
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return ExecuteExtendedPrefix();
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case 0xF3: // DI (Disable Interrupts)
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IFF1 = false;
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IFF2 = false;
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return 4;
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case 0xF9: // LD SP, HL
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SP = HL.Word; // (Use SP.Word = HL.Word if you made SP a RegisterPair)
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return 6;
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case 0xFB: // EI
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IFF1 = true;
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IFF2 = true;
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return 4;
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case 0xFD:
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return ExecuteFDPrefix();
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default:
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throw new NotImplementedException($"Opcode 0x{opcode:X2} at PC 0x{(PC - 1):X4} is not implemented.");
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}
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}
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private int ExecuteExtendedPrefix()
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{
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// Fetch the actual extended instruction
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byte extendedOpcode = _memory.Read(PC++);
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byte val = 0;
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switch (extendedOpcode)
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{
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case 0x43: // LD (nn), BC
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ushort dest43 = FetchWord();
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_memory.Write(dest43, BC.Low);
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_memory.Write((ushort)(dest43 + 1), BC.High);
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return 20;
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case 0x47: // LD I, A
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I = AF.High;
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return 9;
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case 0x52: // SBC HL, DE
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Sbc16(DE.Word);
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return 15;
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case 0x53: // LD (nn), DE
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ushort dest53 = FetchWord();
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_memory.Write(dest53, DE.Low);
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_memory.Write((ushort)(dest53 + 1), DE.High);
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return 20;
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case 0x56: // IM 1
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InterruptMode = 1;
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return 8;
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case 0xB0: // LDIR
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// 1. Read byte from (HL)
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val = _memory.Read(HL.Word);
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// 2. Write byte to (DE)
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_memory.Write(DE.Word, val);
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// 3. Increment memory pointers, Decrement byte counter
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HL.Word++;
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DE.Word++;
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BC.Word--;
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// 4. Update Flags
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// Preserve S (0x80), Z (0x40), and C (0x01).
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// H (0x10) and N (0x02) are always reset to 0.
|
|
AF.Low &= 0xC1;
|
|
|
|
// P/V Flag (Bit 2) is set to 1 if BC is not 0
|
|
if (BC.Word != 0)
|
|
{
|
|
AF.Low |= 0x04;
|
|
|
|
// Rewind the PC so the CPU executes this instruction again!
|
|
PC -= 2;
|
|
return 21; // Looping
|
|
}
|
|
return 16;
|
|
case 0xB8: // LDDR
|
|
// 1. Read byte from (HL)
|
|
val = _memory.Read(HL.Word);
|
|
|
|
// 2. Write byte to (DE)
|
|
_memory.Write(DE.Word, val);
|
|
|
|
// 3. Decrement all three pointers
|
|
HL.Word--;
|
|
DE.Word--;
|
|
BC.Word--;
|
|
|
|
// 4. Update Flags
|
|
// Preserve S (0x80), Z (0x40), and C (0x01).
|
|
// H (0x10) and N (0x02) are always reset to 0.
|
|
AF.Low &= 0xC1;
|
|
|
|
// P/V Flag (Bit 2) is set to 1 if BC is not 0
|
|
if (BC.Word != 0)
|
|
{
|
|
AF.Low |= 0x04;
|
|
|
|
// Rewind the PC so the CPU executes this instruction again!
|
|
PC -= 2;
|
|
return 21; // Looping
|
|
}
|
|
|
|
return 16; // Finished!
|
|
default:
|
|
throw new NotImplementedException($"Extended ED Opcode 0x{extendedOpcode:X2} at PC 0x{(PC - 1):X4} is not implemented.");
|
|
}
|
|
}
|
|
|
|
private int ExecuteFDPrefix()
|
|
{
|
|
byte opcode = FetchByte();
|
|
ushort targetAddress = 0;
|
|
byte memVal = 0;
|
|
|
|
switch (opcode)
|
|
{
|
|
case 0x21: // LD IY, nn
|
|
IY.Word = FetchWord();
|
|
return 14;
|
|
case 0x35: // DEC (IY+d)
|
|
sbyte offset = (sbyte)FetchByte();
|
|
targetAddress = (ushort)(IY.Word + offset);
|
|
|
|
// Read, decrement using your existing helper, and write back
|
|
memVal = _memory.Read(targetAddress);
|
|
byte decVal = Dec8(memVal);
|
|
_memory.Write(targetAddress, decVal);
|
|
return 23;
|
|
case 0x75: // LD (IY+d), L
|
|
sbyte offset75 = (sbyte)FetchByte();
|
|
targetAddress = (ushort)(IY.Word + offset75);
|
|
// Write the low byte of HL to memory
|
|
_memory.Write(targetAddress, HL.Low);
|
|
return 19;
|
|
case 0xCB: // The FD CB nested prefix
|
|
{
|
|
sbyte offsetCB = (sbyte)FetchByte(); // This is the '01'
|
|
byte bitOpcode = FetchByte(); // This is the 'CE'
|
|
targetAddress = (ushort)(IY.Word + offsetCB);
|
|
|
|
switch (bitOpcode)
|
|
{
|
|
case 0x4E: // BIT 1, (IY+d)
|
|
{
|
|
byte memValBit = _memory.Read(targetAddress);
|
|
|
|
// Check if bit 1 is 0
|
|
bool bitIsZero = (memValBit & 0x02) == 0;
|
|
|
|
// Preserve the Carry flag (Bit 0), clear everything else
|
|
AF.Low &= 0x01;
|
|
|
|
// Set Half-Carry (Bit 4) - Standard Z80 behavior for BIT
|
|
AF.Low |= 0x10;
|
|
|
|
if (bitIsZero)
|
|
{
|
|
AF.Low |= 0x40; // Set Zero Flag (Bit 6)
|
|
AF.Low |= 0x04; // Set P/V Flag (Bit 2)
|
|
}
|
|
|
|
return 20; // Takes 20 T-States
|
|
}
|
|
case 0x8E: // RES 1, (IY+d)
|
|
byte memValRes = _memory.Read(targetAddress);
|
|
|
|
// 0xFD is Binary 1111 1101.
|
|
// ANDing with this preserves all bits except Bit 1, which becomes 0.
|
|
memValRes &= 0xFD;
|
|
_memory.Write(targetAddress, memValRes);
|
|
return 23;
|
|
case 0xCE: // SET 1, (IY+d)
|
|
memVal = _memory.Read(targetAddress);
|
|
memVal |= 0x02; // 0x02 is Binary 0000 0010 (Bit 1)
|
|
_memory.Write(targetAddress, memVal);
|
|
return 23; // Takes 23 T-States
|
|
|
|
default:
|
|
throw new NotImplementedException($"FD CB opcode {bitOpcode:X2} not implemented!");
|
|
}
|
|
}
|
|
default:
|
|
throw new NotImplementedException($"FD prefix opcode {opcode:X2} not implemented!");
|
|
}
|
|
}
|
|
}
|
|
} |