diff --git a/src/GameBoy.Core/Cpu/Cpu.State.cs b/src/GameBoy.Core/Cpu/Cpu.State.cs
new file mode 100644
index 0000000..08886ee
--- /dev/null
+++ b/src/GameBoy.Core/Cpu/Cpu.State.cs
@@ -0,0 +1,25 @@
+using System.IO;
+
+namespace GameBoy.Cpu
+{
+    public sealed partial class Cpu
+    {
+        public void SaveState(BinaryWriter writer)
+        {
+            writer.Write(A); writer.Write(B); writer.Write(C); writer.Write(D);
+            writer.Write(E); writer.Write(H); writer.Write(L); writer.Write(F);
+            writer.Write(SP); writer.Write(PC);
+            writer.Write(Ime); writer.Write(Halted); writer.Write(Stopped);
+            writer.Write(imeScheduled); writer.Write(haltBug);
+        }
+
+        public void LoadState(BinaryReader reader)
+        {
+            A = reader.ReadByte(); B = reader.ReadByte(); C = reader.ReadByte(); D = reader.ReadByte();
+            E = reader.ReadByte(); H = reader.ReadByte(); L = reader.ReadByte(); F = reader.ReadByte();
+            SP = reader.ReadUInt16(); PC = reader.ReadUInt16();
+            Ime = reader.ReadBoolean(); Halted = reader.ReadBoolean(); Stopped = reader.ReadBoolean();
+            imeScheduled = reader.ReadBoolean(); haltBug = reader.ReadBoolean();
+        }
+    }
+}
diff --git a/src/GameBoy.Core/Cpu/Cpu.cs b/src/GameBoy.Core/Cpu/Cpu.cs
new file mode 100644
index 0000000..3004204
--- /dev/null
+++ b/src/GameBoy.Core/Cpu/Cpu.cs
@@ -0,0 +1,192 @@
+namespace GameBoy.Cpu
+{
+    public sealed partial class Cpu
+    {
+        public const byte FlagZ = 0x80;
+        public const byte FlagN = 0x40;
+        public const byte FlagH = 0x20;
+        public const byte FlagC = 0x10;
+
+        public byte A, B, C, D, E, H, L, F;
+        public ushort SP;
+        public ushort PC;
+
+        public bool Ime;
+        public bool Halted;
+        public bool Stopped;
+
+        private bool imeScheduled;
+        private bool haltBug;
+
+        private readonly ISystemBus bus;
+        private readonly InterruptController interrupts;
+
+        public Cpu(ISystemBus bus)
+        {
+            this.bus = bus;
+            this.interrupts = bus.Interrupts;
+        }
+
+        public void ResetPostBoot()
+        {
+            A = 0x01; F = 0xB0;
+            B = 0x00; C = 0x13;
+            D = 0x00; E = 0xD8;
+            H = 0x01; L = 0x4D;
+            SP = 0xFFFE;
+            PC = 0x0100;
+            Ime = false;
+            Halted = false;
+            Stopped = false;
+            imeScheduled = false;
+            haltBug = false;
+        }
+
+        public ushort AF
+        {
+            get => (ushort)((A << 8) | F);
+            set { A = (byte)(value >> 8); F = (byte)(value & 0xF0); }
+        }
+
+        public ushort BC
+        {
+            get => (ushort)((B << 8) | C);
+            set { B = (byte)(value >> 8); C = (byte)(value & 0xFF); }
+        }
+
+        public ushort DE
+        {
+            get => (ushort)((D << 8) | E);
+            set { D = (byte)(value >> 8); E = (byte)(value & 0xFF); }
+        }
+
+        public ushort HL
+        {
+            get => (ushort)((H << 8) | L);
+            set { H = (byte)(value >> 8); L = (byte)(value & 0xFF); }
+        }
+
+        private bool FlagSet(byte flag) => (F & flag) != 0;
+
+        private void SetFlag(byte flag, bool on)
+        {
+            if (on) F |= flag;
+            else F = (byte)(F & ~flag);
+        }
+
+        private void SetFlags(bool z, bool n, bool h, bool c)
+        {
+            F = 0;
+            if (z) F |= FlagZ;
+            if (n) F |= FlagN;
+            if (h) F |= FlagH;
+            if (c) F |= FlagC;
+        }
+
+        public void Step()
+        {
+            if (HandleInterrupts())
+                return;
+
+            if (Halted)
+            {
+                bus.Tick4();
+                return;
+            }
+
+            bool enableImeAfter = imeScheduled;
+
+            byte opcode = Fetch8();
+            Execute(opcode);
+
+            if (enableImeAfter)
+            {
+                Ime = true;
+                imeScheduled = false;
+            }
+        }
+
+        private bool HandleInterrupts()
+        {
+            byte pending = interrupts.Pending;
+
+            if (pending != 0 && Halted)
+                Halted = false;
+
+            if (pending == 0 || !Ime)
+                return false;
+
+            Ime = false;
+            imeScheduled = false;
+
+            bus.Tick4();
+            bus.Tick4();
+
+            SP--;
+            bus.Write(SP, (byte)(PC >> 8));
+            SP--;
+            bus.Write(SP, (byte)(PC & 0xFF));
+
+            byte live = interrupts.Pending;
+            ushort vector = 0x0000;
+            if (live != 0)
+            {
+                int bit = LowestSetBit(live);
+                vector = (ushort)(0x40 + bit * 8);
+                interrupts.Flag = (byte)(interrupts.Flag & ~(1 << bit));
+            }
+
+            bus.Tick4();
+            PC = vector;
+            return true;
+        }
+
+        private static int LowestSetBit(byte value)
+        {
+            for (int bit = 0; bit < 5; bit++)
+                if ((value & (1 << bit)) != 0)
+                    return bit;
+            return 0;
+        }
+
+        private byte Fetch8()
+        {
+            byte value = bus.Read(PC);
+            if (haltBug)
+                haltBug = false;
+            else
+                PC++;
+            return value;
+        }
+
+        private ushort Fetch16()
+        {
+            byte lo = Fetch8();
+            byte hi = Fetch8();
+            return (ushort)((hi << 8) | lo);
+        }
+
+        private byte Read8(ushort address) => bus.Read(address);
+
+        private void Write8(ushort address, byte value) => bus.Write(address, value);
+
+        private void InternalCycle() => bus.Tick4();
+
+        private void Push16(ushort value)
+        {
+            SP--;
+            Write8(SP, (byte)(value >> 8));
+            SP--;
+            Write8(SP, (byte)(value & 0xFF));
+        }
+
+        private ushort Pop16()
+        {
+            byte lo = Read8(SP);
+            SP++;
+            byte hi = Read8(SP);
+            SP++;
+            return (ushort)((hi << 8) | lo);
+        }
+    }
+}