diff --git a/MC6809/inc/mc6809.h b/MC6809/inc/mc6809.h
index 41de2a4..23e8821 100644
--- a/MC6809/inc/mc6809.h
+++ b/MC6809/inc/mc6809.h
@@ -206,11 +206,11 @@ namespace EightBit {
 		void adjustBorrow(register16_t datum) { adjustBorrow(datum.word); }
 
 		void adjustOverflow(uint8_t before, uint8_t data, uint8_t after) {
-			setFlag(CC(), VF, !!((before ^ data ^ after ^ (after >> 1)) & Bit7));
+			setFlag(CC(), VF, ~(before ^ data) & (before ^ after) & Bit7);
 		}
 
 		void adjustOverflow(uint16_t before, uint16_t data, uint16_t after) {
-			setFlag(CC(), VF, !!((before ^ data ^ after ^ (after >> 1)) & Bit15));
+			setFlag(CC(), VF, ~(before ^ data) & (before ^ after) & Bit15);
 		}
 
 		void adjustOverflow(register16_t before, register16_t data, register16_t after) {
@@ -243,14 +243,14 @@ namespace EightBit {
 		void adjustSubtraction(uint8_t before, uint8_t data, register16_t after) {
 			const auto result = after.low;
 			adjustNZ(result);
-			adjustBorrow(after);
-			adjustOverflow(before, data, result);
+			adjustCarry(after);
+			setFlag(CC(), VF, (before ^ data ^ result ^ (after.high << 7)) & Bit7);
 		}
 
 		void adjustSubtraction(uint16_t before, uint16_t data, uint32_t after) {
 			const register16_t result = after & Mask16;
 			adjustNZ(result);
-			adjustBorrow(after);
+			adjustCarry(after);
 			adjustOverflow(before, data, result);
 		}
 
@@ -300,7 +300,7 @@ namespace EightBit {
 		// Instruction implementations
 
 		uint8_t adc(uint8_t operand, uint8_t data);
-		uint8_t add(uint8_t operand, uint8_t data, int carry = 0);
+		uint8_t add(uint8_t operand, uint8_t data, uint8_t carry = 0);
 		register16_t add(register16_t operand, register16_t data);
 		uint8_t andr(uint8_t operand, uint8_t data);
 		uint8_t asl(uint8_t operand);
@@ -329,7 +329,7 @@ namespace EightBit {
 		void rti();
 		void rts();
 		uint8_t sbc(uint8_t operand, uint8_t data);
-		uint8_t sub(uint8_t operand, uint8_t data, int carry = 0);
+		uint8_t sub(uint8_t operand, uint8_t data, uint8_t carry = 0);
 		register16_t sub(register16_t operand, register16_t data);
 		uint8_t sex(uint8_t from);
 		void swi();
diff --git a/MC6809/src/mc6809.cpp b/MC6809/src/mc6809.cpp
index 421c01c..5f0ea10 100644
--- a/MC6809/src/mc6809.cpp
+++ b/MC6809/src/mc6809.cpp
@@ -756,7 +756,7 @@ uint8_t EightBit::mc6809::adc(const uint8_t operand, const uint8_t data) {
 	return add(operand, data, carry());
 }
 
-uint8_t EightBit::mc6809::add(const uint8_t operand, const uint8_t data, const int carry) {
+uint8_t EightBit::mc6809::add(const uint8_t operand, const uint8_t data, const uint8_t carry) {
 	const register16_t addition = operand + data + carry;
 	adjustAddition(operand, data, addition);
 	return addition.low;
@@ -837,10 +837,7 @@ uint8_t EightBit::mc6809::da(uint8_t operand) {
 }
 
 uint8_t EightBit::mc6809::dec(const uint8_t operand) {
-	const uint8_t result = operand - 1;
-	adjustNZ(result);
-	adjustOverflow(operand, 1, result);
-	return result;
+	return sub(operand, 1);
 }
 
 uint8_t EightBit::mc6809::eor(uint8_t operand, const uint8_t data) {
@@ -898,10 +895,7 @@ void EightBit::mc6809::exg(const uint8_t data) {
 }
 
 uint8_t EightBit::mc6809::inc(uint8_t operand) {
-	const uint8_t result = operand + 1;
-	adjustNZ(result);
-	adjustOverflow(operand, 1, result);
-	return result;
+	return add(operand, 1);
 }
 
 void EightBit::mc6809::jsr(const register16_t address) {
@@ -1109,7 +1103,7 @@ uint8_t EightBit::mc6809::sbc(const uint8_t operand, const uint8_t data) {
 	return sub(operand, data, carry());
 }
 
-uint8_t EightBit::mc6809::sub(const uint8_t operand, const uint8_t data, const int carry) {
+uint8_t EightBit::mc6809::sub(const uint8_t operand, const uint8_t data, const uint8_t carry) {
 	const register16_t subtraction = operand - data - carry;
 	adjustSubtraction(operand, data, subtraction);
 	return subtraction.low;