diff --git a/sources/CPU/CPU.hpp b/sources/CPU/CPU.hpp
index 6c60e09..96930b9 100644
--- a/sources/CPU/CPU.hpp
+++ b/sources/CPU/CPU.hpp
@@ -374,6 +374,10 @@ namespace ComSquare::CPU
 		int JML(uint24_t valueAddr, AddressingMode);
 		//! @brief No OP.
 		int NOP(uint24_t, AddressingMode);
+		//! @brief Decrement the X register.
+		int DEX(uint24_t, AddressingMode);
+		//! @brief Decrement the Y register.
+		int DEY(uint24_t, AddressingMode);
 
 		//! @brief All the instructions of the CPU.
 		//! @info Instructions are indexed by their opcode
@@ -514,7 +518,7 @@ namespace ComSquare::CPU
 			{&CPU::STA, 3, "sta", AddressingMode::DirectPage, 2}, // 85
 			{&CPU::STX, 3, "stx", AddressingMode::DirectPage, 2}, // 86
 			{&CPU::STA, 6, "sta", AddressingMode::DirectPageIndirectLong, 2}, // 87
-			{&CPU::BRK, 7, "dey #-#", AddressingMode::Implied, 2}, // 88
+			{&CPU::DEY, 2, "dey", AddressingMode::Implied, 1}, // 88
 			{&CPU::BRK, 7, "bit #-#", AddressingMode::Implied, 2}, // 89
 			{&CPU::BRK, 7, "txa #-#", AddressingMode::Implied, 2}, // 8A
 			{&CPU::PHB, 3, "phb", AddressingMode::Implied, 1}, // 8B
@@ -580,7 +584,7 @@ namespace ComSquare::CPU
 			{&CPU::BRK, 7, "cmp #-#", AddressingMode::Implied, 2}, // C7
 			{&CPU::INY, 2, "iny", AddressingMode::Implied, 1}, // C8
 			{&CPU::BRK, 7, "cmp #-#", AddressingMode::Implied, 2}, // C9
-			{&CPU::BRK, 7, "dex #-#", AddressingMode::Implied, 2}, // CA
+			{&CPU::DEX, 2, "dex", AddressingMode::Implied, 1}, // CA
 			{&CPU::BRK, 7, "wai #-#", AddressingMode::Implied, 2}, // CB
 			{&CPU::BRK, 7, "cpy #-#", AddressingMode::Implied, 2}, // CC
 			{&CPU::BRK, 7, "cmp #-#", AddressingMode::Implied, 2}, // CD
diff --git a/sources/CPU/Instructions/MathematicalOperations.cpp b/sources/CPU/Instructions/MathematicalOperations.cpp
index 5b6e9e1..2b16056 100644
--- a/sources/CPU/Instructions/MathematicalOperations.cpp
+++ b/sources/CPU/Instructions/MathematicalOperations.cpp
@@ -12,8 +12,8 @@ namespace ComSquare::CPU
 		unsigned value = this->_bus->read(valueAddr) + this->_registers.p.c;
 		if (!this->_registers.p.m)
 			value += this->_bus->read(valueAddr + 1) << 8u;
-		unsigned negativeMask = this->_isEmulationMode ? 0x80u : 0x8000u;
-		unsigned maxValue = this->_isEmulationMode ? UINT8_MAX : UINT16_MAX;
+		unsigned negativeMask = this->_registers.p.m ? 0x80u : 0x8000u;
+		unsigned maxValue = this->_registers.p.m ? UINT8_MAX : UINT16_MAX;
 
 		this->_registers.p.c = static_cast<unsigned>(this->_registers.a) + value > maxValue;
 		if ((this->_registers.a & negativeMask) == (value & negativeMask))
@@ -21,7 +21,7 @@ namespace ComSquare::CPU
 		else
 			this->_registers.p.v = false;
 		this->_registers.a += value;
-		if (this->_isEmulationMode)
+		if (this->_registers.p.m)
 			this->_registers.a %= 0x100;
 		this->_registers.p.z = this->_registers.a == 0;
 		this->_registers.p.n = this->_registers.a & negativeMask;
@@ -51,7 +51,7 @@ namespace ComSquare::CPU
 
 	int CPU::SBC(uint24_t valueAddr, AddressingMode mode)
 	{
-		unsigned negativeMask = this->_isEmulationMode ? 0x80u : 0x8000u;
+		unsigned negativeMask = this->_registers.p.m ? 0x80u : 0x8000u;
 		unsigned value = this->_bus->read(valueAddr);
 		if (!this->_registers.p.m)
 			value += this->_bus->read(valueAddr + 1) << 8u;
@@ -63,7 +63,7 @@ namespace ComSquare::CPU
 		else
 			this->_registers.p.v = false;
 		this->_registers.a += ~value + oldCarry;
-		if (this->_isEmulationMode)
+		if (this->_registers.p.m)
 			this->_registers.a %= 0x100;
 		this->_registers.p.z = this->_registers.a == 0;
 		this->_registers.p.n = this->_registers.a & negativeMask;
@@ -90,4 +90,28 @@ namespace ComSquare::CPU
 		}
 		return cycles;
 	}
+
+	int CPU::DEX(uint24_t, AddressingMode)
+	{
+		unsigned negativeMask = this->_registers.p.x_b ? UINT8_MAX : UINT16_MAX;
+
+		this->_registers.x--;
+		if (this->_registers.p.x_b)
+			this->_registers.xh = 0;
+		this->_registers.p.z = this->_registers.x == 0;
+		this->_registers.p.n = this->_registers.x & negativeMask;
+		return 0;
+	}
+
+	int CPU::DEY(uint24_t, AddressingMode)
+	{
+		unsigned negativeMask = this->_registers.p.x_b ? UINT8_MAX : UINT16_MAX;
+
+		this->_registers.y--;
+		if (this->_registers.p.x_b)
+			this->_registers.yh = 0;
+		this->_registers.p.z = this->_registers.y == 0;
+		this->_registers.p.n = this->_registers.y & negativeMask;
+		return 0;
+	}
 }
\ No newline at end of file
diff --git a/tests/CPU/Math/testADC.cpp b/tests/CPU/Math/testADC.cpp
index 00a5106..3d4f193 100644
--- a/tests/CPU/Math/testADC.cpp
+++ b/tests/CPU/Math/testADC.cpp
@@ -67,7 +67,7 @@ Test(ADC, overflowEmulation)
 Test(ADC, signedOverflow)
 {
 	Init()
-	snes.cpu->_isEmulationMode = false;
+	snes.cpu->_registers.p.m = false;
 	snes.cpu->_registers.a = 0x7FFF;
 	snes.wram->_data[0] = 0x1;
 	snes.cpu->ADC(0x0, ComSquare::CPU::AddressingMode::Implied);
@@ -81,7 +81,7 @@ Test(ADC, signedOverflow)
 Test(ADC, signedOverflowEmulated)
 {
 	Init()
-	snes.cpu->_isEmulationMode = true;
+	snes.cpu->_registers.p.m = true;
 	snes.cpu->_registers.a = 0x007F;
 	snes.wram->_data[0] = 0x1;
 	snes.cpu->ADC(0x0, ComSquare::CPU::AddressingMode::Implied);
@@ -95,7 +95,7 @@ Test(ADC, signedOverflowEmulated)
 Test(ADC, negative)
 {
 	Init()
-	snes.cpu->_isEmulationMode = false;
+	snes.cpu->_registers.p.m = false;
 	snes.cpu->_registers.a = 0x8FFF;
 	snes.wram->_data[0] = 0x1;
 	snes.cpu->ADC(0x0, ComSquare::CPU::AddressingMode::Implied);
diff --git a/tests/CPU/Math/testSBC.cpp b/tests/CPU/Math/testSBC.cpp
index 52b0186..e784481 100644
--- a/tests/CPU/Math/testSBC.cpp
+++ b/tests/CPU/Math/testSBC.cpp
@@ -62,7 +62,7 @@ Test(SBC, overflowEmulation)
 	Init()
 	snes.cpu->_isEmulationMode = true;
 	snes.cpu->_registers.a = 0x1;
-	snes.cpu->_registers.p.m = false;
+	snes.cpu->_registers.p.m = true;
 	snes.cpu->_registers.p.c = false;
 	snes.wram->_data[0] = 0x02;
 	snes.cpu->SBC(0x0, ComSquare::CPU::AddressingMode::Implied);