feat(core): add system bus interface and interrupt controller

src/GameBoy.Core/Cpu/InterruptController.cs

@@ -0,0 +1,50 @@
+using System.IO;
+
+namespace GameBoy.Cpu
+{
+    public enum Interrupt
+    {
+        VBlank = 0x01,
+        LcdStat = 0x02,
+        Timer = 0x04,
+        Serial = 0x08,
+        Joypad = 0x10
+    }
+
+    public sealed class InterruptController
+    {
+        public byte Enable;
+        public byte Flag;
+
+        public byte ReadFlag()
+        {
+            return (byte)(Flag | 0xE0);
+        }
+
+        public void WriteFlag(byte value)
+        {
+            Flag = (byte)(value & 0x1F);
+        }
+
+        public void Request(Interrupt which)
+        {
+            Flag |= (byte)which;
+        }
+
+        public byte Pending => (byte)(Enable & Flag & 0x1F);
+
+        public bool AnyPending => Pending != 0;
+
+        public void SaveState(BinaryWriter writer)
+        {
+            writer.Write(Enable);
+            writer.Write(Flag);
+        }
+
+        public void LoadState(BinaryReader reader)
+        {
+            Enable = reader.ReadByte();
+            Flag = reader.ReadByte();
+        }
+    }
+}

src/GameBoy.Core/ISystemBus.cs

@@ -0,0 +1,12 @@
+using GameBoy.Cpu;
+
+namespace GameBoy
+{
+    public interface ISystemBus
+    {
+        byte Read(ushort address);
+        void Write(ushort address, byte value);
+        void Tick4();
+        InterruptController Interrupts { get; }
+    }
+}