diff --git a/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj b/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
index db6cb5cb44..1223bcc205 100644
--- a/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
+++ b/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj
@@ -673,6 +673,7 @@
     <Compile Include="Consoles\Nintendo\NES\Boards\MMC3_family\Mapper052.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\MMC3_family\Mapper074.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\MMC3_family\Mapper115.cs" />
+    <Compile Include="Consoles\Nintendo\NES\Boards\MMC3_family\Mapper123.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\MMC3_family\Mapper165.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\MMC3_family\Mapper182.cs" />
     <Compile Include="Consoles\Nintendo\NES\Boards\MMC3_family\Mapper189.cs" />
diff --git a/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/MMC3_family/Mapper123.cs b/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/MMC3_family/Mapper123.cs
new file mode 100644
index 0000000000..6894140673
--- /dev/null
+++ b/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/MMC3_family/Mapper123.cs
@@ -0,0 +1,84 @@
+using BizHawk.Common;
+
+namespace BizHawk.Emulation.Cores.Nintendo.NES
+{
+	// Adapted from FCEUX src
+	public sealed class Mapper123 : MMC3Board_Base
+	{
+		private ByteBuffer EXPREGS = new ByteBuffer(8);
+
+		public override bool Configure(NES.EDetectionOrigin origin)
+		{
+			switch (Cart.board_type)
+			{
+				case "MAPPER123": // Nestopia suggests this board is mapper 123, I do not have any ROMs with this ines header info to confirm
+				case "UNIF_UNL-H2288":
+					break;
+				default:
+					return false;
+			}
+
+			BaseSetup();
+			return true;
+		}
+
+		public override void WriteEXP(int addr, byte value)
+		{
+			if (addr >= 0x1000)
+			{
+				if ((addr & 0x800) > 0)
+				{
+					if ((addr & 1) > 0)
+					{
+						EXPREGS[1] = value;
+					}
+					else
+					{
+						EXPREGS[0] = value;
+					}
+				}
+			}
+			else
+			{
+				base.WriteEXP(addr, value);
+			}
+		}
+
+		public override void WritePRG(int addr, byte value)
+		{
+			if (addr < 0x2000)
+			{
+				switch ((addr + 0x8000) & 0x8001)
+				{
+					case 0x8000: break;
+					case 0x8001: break;
+				}
+			}
+			else
+			{
+				base.WritePRG(addr, value);
+			}
+		}
+
+		public override byte ReadPRG(int addr)
+		{
+			if ((EXPREGS[0] & 0x40) > 0)
+			{
+				var bank = (EXPREGS[0] & 5) | ((EXPREGS[0] & 8) >> 2) | ((EXPREGS[0] & 0x20) >> 2);
+				if ((EXPREGS[0] & 2) > 0)
+				{
+					return ROM[((bank >> 1) << 15) + (addr & 0x7FFF)];
+				}
+				else
+				{
+					return ROM[(bank << 14) + (addr & 0x3FFF)];
+				}
+			}
+			else
+			{
+				//return (byte)(base.ReadPRG(addr) & 0x3F);
+				return base.ReadPRG(addr);
+			}
+		}
+	}
+}
diff --git a/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/MMC3_family/Mapper238.cs b/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/MMC3_family/Mapper238.cs
index 3a968dcc5e..11e99cebf5 100644
--- a/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/MMC3_family/Mapper238.cs
+++ b/BizHawk.Emulation.Cores/Consoles/Nintendo/NES/Boards/MMC3_family/Mapper238.cs
@@ -15,6 +15,8 @@ namespace BizHawk.Emulation.Cores.Nintendo.NES
 				case "MAPPER238": // Nestopia suggests this board is mapper 238, I do not have any ROMs with this ines header info to confirm
 				case "UNIF_UNL-603-5052":
 					break;
+				default:
+					return false;
 			}
 
 			BaseSetup();