diff --git a/M6502/inc/mos6502.h b/M6502/inc/mos6502.h
index 851d1df..7d3371e 100644
--- a/M6502/inc/mos6502.h
+++ b/M6502/inc/mos6502.h
@@ -56,6 +56,8 @@ namespace EightBit {
 		void busWrite() noexcept final;
 		[[nodiscard]] uint8_t busRead() noexcept final;
 
+		// Instructions with BCD effects
+
 		[[nodiscard]] virtual uint8_t sub(uint8_t operand, uint8_t data, int borrow = 0) noexcept;
 		[[nodiscard]] uint8_t sbc(uint8_t operand, uint8_t data) noexcept;
 		[[nodiscard]] uint8_t sub_b(uint8_t operand, uint8_t data, int borrow = 0) noexcept;
@@ -66,6 +68,12 @@ namespace EightBit {
 		[[nodiscard]] uint8_t add_b(uint8_t operand, uint8_t data, int carry) noexcept;
 		[[nodiscard]] uint8_t add_d(uint8_t operand, uint8_t data, int carry) noexcept;
 
+		// Undocumented compound instructions (with BCD effects)
+
+		virtual void arr(uint8_t value) noexcept;
+		virtual void arr_b(uint8_t value) noexcept;
+		virtual void arr_d(uint8_t value) noexcept;
+
 	private:
 		const uint8_t IRQvector = 0xfe;		// IRQ vector
 		const uint8_t RSTvector = 0xfc;		// RST vector
@@ -174,7 +182,6 @@ namespace EightBit {
 		// Undocumented compound instructions
 
 		void anc(uint8_t value) noexcept;
-		void arr(uint8_t value) noexcept;
 		void asr(uint8_t value) noexcept;
 		void axs(uint8_t value) noexcept;
 		void dcp(uint8_t value) noexcept;
diff --git a/M6502/src/mos6502.cpp b/M6502/src/mos6502.cpp
index b707d52..69ce799 100644
--- a/M6502/src/mos6502.cpp
+++ b/M6502/src/mos6502.cpp
@@ -698,10 +698,33 @@ void EightBit::MOS6502::anc(const uint8_t value) noexcept {
 }
 
 void EightBit::MOS6502::arr(const uint8_t value) noexcept {
-	A() = andr(A(), value);
-	A() = ror(A());
+	decimal() ? arr_d(value) : arr_b(value);
+}
+
+void EightBit::MOS6502::arr_d(const uint8_t value) noexcept {
+
+	// With thanks to https://github.com/TomHarte/CLK
+	// What a very strange instruction ARR is...
+
+	A() &= value;
+	auto unshiftedA = A();
+	A() = through((A() >> 1) | (carry() << 7));
+	P() = setBit(P(), VF, (A() ^ (A() << 1)) & VF);
+
+	if (lowerNibble(unshiftedA) + (unshiftedA & 0x1) > 5)
+		A() = lowerNibble(A() + 6) | higherNibble(A());
+
+	P() = setBit(P(), CF, higherNibble(unshiftedA) + (unshiftedA & 0x10) > 0x50);
+	
+	if (carry())
+		A() += 0x60;
+}
+
+void EightBit::MOS6502::arr_b(const uint8_t value) noexcept {
+	A() &= value;
+	A() = through((A() >> 1) | (carry() << 7));
 	P() = setBit(P(), CF, A() & Bit6);
-	P() = setBit(P(), VF, ((A() & Bit6) >> 6) ^((A() & Bit5) >> 5));
+	P() = setBit(P(), VF, (A() ^ (A() << 1)) & VF);
 }
 
 void EightBit::MOS6502::asr(const uint8_t value) noexcept {