diff --git a/Intel8080/test/Board.h b/Intel8080/test/Board.h
index d101cf0..c6f2152 100644
--- a/Intel8080/test/Board.h
+++ b/Intel8080/test/Board.h
@@ -21,7 +21,7 @@ public:
 
 protected:
 	virtual EightBit::MemoryMapping mapping(uint16_t address) final {
-		return { m_ram, 0x0000, EightBit::MemoryMapping::ReadWrite };
+		return { m_ram, 0x0000, 0xffff, EightBit::MemoryMapping::ReadWrite };
 	}
 
 private:
diff --git a/LR35902/fusetest_LR35902/FuseTestRunner.h b/LR35902/fusetest_LR35902/FuseTestRunner.h
index 428dc51..e4b1634 100644
--- a/LR35902/fusetest_LR35902/FuseTestRunner.h
+++ b/LR35902/fusetest_LR35902/FuseTestRunner.h
@@ -38,7 +38,7 @@ namespace Fuse {
 
 	protected:
 		virtual EightBit::MemoryMapping mapping(uint16_t address) final {
-			return { m_ram, 0x0000, EightBit::MemoryMapping::ReadWrite };
+			return { m_ram, 0x0000, 0xffff, EightBit::MemoryMapping::ReadWrite };
 		}
 
 	public:
diff --git a/LR35902/src/GameBoyBus.cpp b/LR35902/src/GameBoyBus.cpp
index 1a38a12..d2e424e 100644
--- a/LR35902/src/GameBoyBus.cpp
+++ b/LR35902/src/GameBoyBus.cpp
@@ -164,31 +164,31 @@ void EightBit::GameBoy::Bus::validateCartridgeType() {
 EightBit::MemoryMapping EightBit::GameBoy::Bus::mapping(uint16_t address) {
 
 	if ((address < 0x100) && IO().bootRomEnabled())
-		return { m_bootRom, 0x0000, MemoryMapping::ReadOnly };
+		return { m_bootRom, 0x0000, 0xffff, MemoryMapping::ReadOnly };
 	if ((address < 0x4000) && gameRomEnabled())
-		return { m_gameRomBanks[0], 0x0000, MemoryMapping::ReadOnly };
+		return { m_gameRomBanks[0], 0x0000, 0xffff, MemoryMapping::ReadOnly };
 	if ((address < 0x8000) && gameRomEnabled())
-		return { m_gameRomBanks[m_romBank], 0x4000, MemoryMapping::ReadOnly };
+		return { m_gameRomBanks[m_romBank], 0x4000, 0xffff, MemoryMapping::ReadOnly };
 
 	if (address < 0xa000)
-		return { VRAM(), 0x8000, MemoryMapping::ReadWrite };
+		return { VRAM(), 0x8000, 0xffff, MemoryMapping::ReadWrite };
 	if (address < 0xc000) {
 		if (m_ramBanks.size() == 0)
-			return { m_unmapped2000, 0xa000, MemoryMapping::ReadOnly };
+			return { m_unmapped2000, 0xa000, 0xffff, MemoryMapping::ReadOnly };
 		else
-			return { m_ramBanks[m_ramBank], 0xa000, MemoryMapping::ReadWrite };
+			return { m_ramBanks[m_ramBank], 0xa000, 0xffff, MemoryMapping::ReadWrite };
 	}
 	if (address < 0xe000)
-		return { m_lowInternalRam, 0xc000, MemoryMapping::ReadWrite };
+		return { m_lowInternalRam, 0xc000, 0xffff, MemoryMapping::ReadWrite };
 	if (address < 0xfe00)
-		return { m_lowInternalRam, 0xe000, MemoryMapping::ReadWrite };	// Low internal RAM mirror
+		return { m_lowInternalRam, 0xe000, 0xffff, MemoryMapping::ReadWrite };	// Low internal RAM mirror
 	if (address < 0xfea0)
-		return { OAMRAM(), 0xfe00, MemoryMapping::ReadWrite };
+		return { OAMRAM(), 0xfe00, 0xffff, MemoryMapping::ReadWrite };
 	if (address < IoRegisters::BASE)
-		return { m_unmapped60, 0xfea0, MemoryMapping::ReadOnly };
+		return { m_unmapped60, 0xfea0, 0xffff, MemoryMapping::ReadOnly };
 	if (address < 0xff80)
-		return { IO(), IoRegisters::BASE, MemoryMapping::ReadWrite };
-	return { m_highInternalRam, 0xff80, MemoryMapping::ReadWrite };
+		return { IO(), IoRegisters::BASE, 0xffff, MemoryMapping::ReadWrite };
+	return { m_highInternalRam, 0xff80, 0xffff, MemoryMapping::ReadWrite };
 }
 
 int EightBit::GameBoy::Bus::runRasterLines() {
diff --git a/M6502/test/Board.h b/M6502/test/Board.h
index d14cf86..fd1f7ba 100644
--- a/M6502/test/Board.h
+++ b/M6502/test/Board.h
@@ -21,7 +21,7 @@ public:
 
 protected:
 	virtual EightBit::MemoryMapping mapping(uint16_t address) final {
-		return { m_ram, 0x0000, EightBit::MemoryMapping::ReadWrite };
+		return { m_ram, 0x0000, 0xffff, EightBit::MemoryMapping::ReadWrite };
 	}
 
 private:
diff --git a/MC6809/test/Board.cpp b/MC6809/test/Board.cpp
index 46b094b..56856fa 100644
--- a/MC6809/test/Board.cpp
+++ b/MC6809/test/Board.cpp
@@ -60,15 +60,15 @@ void Board::Cpu_ExecutedInstruction_Debug(EightBit::mc6809&) {
 EightBit::MemoryMapping Board::mapping(uint16_t address) {
 
 	if (address < 0x8000)
-		return { m_ram, 0x0000, EightBit::MemoryMapping::ReadWrite };
+		return { m_ram, 0x0000, 0xffff, EightBit::MemoryMapping::ReadWrite };
 
 	if (address < 0xa000)
-		return { m_unused2000, 0x8000, EightBit::MemoryMapping::ReadOnly };
+		return { m_unused2000, 0x8000, 0xffff, EightBit::MemoryMapping::ReadOnly };
 
 	if (address < 0xc000)
-		return { m_io, 0xa000, EightBit::MemoryMapping::ReadWrite };
+		return { m_io, 0xa000, 0xffff, EightBit::MemoryMapping::ReadWrite };
 
-	return { m_rom, 0xc000, EightBit::MemoryMapping::ReadOnly };
+	return { m_rom, 0xc000, 0xffff, EightBit::MemoryMapping::ReadOnly };
 }
 
 void Board::Bus_WrittenByte_Acia(EightBit::EventArgs&) {
diff --git a/MC6809/unittest/Board.cpp b/MC6809/unittest/Board.cpp
index bd814cd..efb4514 100644
--- a/MC6809/unittest/Board.cpp
+++ b/MC6809/unittest/Board.cpp
@@ -13,5 +13,5 @@ void Board::initialise() {
 }
 
 EightBit::MemoryMapping Board::mapping(uint16_t) {
-	return { m_ram, 0x0000, EightBit::MemoryMapping::ReadWrite };
+	return { m_ram, 0x0000, 0xffff, EightBit::MemoryMapping::ReadWrite };
 }
diff --git a/Z80/fusetest_Z80/FuseTestRunner.h b/Z80/fusetest_Z80/FuseTestRunner.h
index cdfdd25..f49a36e 100644
--- a/Z80/fusetest_Z80/FuseTestRunner.h
+++ b/Z80/fusetest_Z80/FuseTestRunner.h
@@ -37,7 +37,7 @@ namespace Fuse {
 
 	protected:
 		virtual EightBit::MemoryMapping mapping(uint16_t address) final {
-			return { m_ram, 0x0000, EightBit::MemoryMapping::ReadWrite };
+			return { m_ram, 0x0000, 0xffff, EightBit::MemoryMapping::ReadWrite };
 		}
 
 	public:
diff --git a/Z80/test/Board.h b/Z80/test/Board.h
index 1b1b8d2..21ee4b7 100644
--- a/Z80/test/Board.h
+++ b/Z80/test/Board.h
@@ -22,7 +22,7 @@ public:
 
 protected:
 	virtual EightBit::MemoryMapping mapping(uint16_t address) final {
-		return { m_ram, 0x0000, EightBit::MemoryMapping::ReadWrite };
+		return { m_ram, 0x0000, 0xffff, EightBit::MemoryMapping::ReadWrite };
 	}
 
 private:
diff --git a/inc/MemoryMapping.h b/inc/MemoryMapping.h
index 1e2ab29..faa060f 100644
--- a/inc/MemoryMapping.h
+++ b/inc/MemoryMapping.h
@@ -11,6 +11,7 @@ namespace EightBit {
 
 		Memory& memory;
 		uint16_t begin = 0xffff;
+		uint16_t mask = 0U;
 		AccessLevel access = Unknown;
 	};
 }
diff --git a/src/Bus.cpp b/src/Bus.cpp
index 99dfd72..f9c8be5 100644
--- a/src/Bus.cpp
+++ b/src/Bus.cpp
@@ -90,7 +90,7 @@ std::map<uint16_t, std::vector<uint8_t>> EightBit::Bus::parseHexFile(const std::
 
 uint8_t& EightBit::Bus::reference(const uint16_t address) {
 	const auto mapped = mapping(address);
-	const uint16_t offset = address - mapped.begin;
+	const uint16_t offset = (address - mapped.begin) & mapped.mask;
 	if (mapped.access == MemoryMapping::ReadOnly)
 		return DATA() = mapped.memory.peek(offset);
 	Ram& ram = (Ram&)(mapped.memory);