Emulator: Add support for negative numbers to DAA opcode

src/cpu.cpp

@@ -527,35 +527,44 @@ void CPU::daa()
 		// bits) of a byte (a byte, since it is 8 bits has two nibbles) to store
 		// a decimal digit.
 		// Example: 64 split on each number gives a 6 and a 4, so 0110 0100 as a BCD
-		uint8_t higher_nibble = m_a & 0xf0;
-		uint8_t lower_nibble = m_a & 0xf;
 
-		// Step 1: Check lower nibble of the BCD stored in A.
-		//         If it is greater than decimal 9 or half carry flag is set
-		//         (meaning that the lower nibble value is > 15),
-		//         add decimal 6 to the lower nibble to make it wrap around
-		if (lower_nibble > 9 || m_hf) {
-			lower_nibble += 6;
-			higher_nibble += isCarry(m_a, 6, 0x10);
-		}
-
-		// Step 2: Check higher nibble (after addition of the carry from the lower nibble).
+		// Step 1: Check upper nibble of the BCD stored in A.
 		//         If it is greater than decimal 9 or the carry flag is set
 		//         (meaning that the upper nibble value is > 15),
 		//         add decimal 6 to the upper nibble to make it wrap around and set carry flag to 1
-		uint32_t new_carry = 0;
-		if (higher_nibble > 9 || m_cf) {
-			higher_nibble += 6;
-			new_carry = 1;
+
+		// Step 2: Check lower nibble of the BCD stored in A.
+		//         If it is greater than decimal 9 or half carry flag is set
+		//         (meaning that the lower nibble value is > 15),
+		//         add decimal 6 to the lower nibble to make it wrap around
+
+		if (!m_nf) {
+			if (m_cf || m_a > 0x99) {
+				m_a += 0x60;
+			}
+
+			if (m_hf || (m_a & 0xf) > 0x9) {
+				m_a += 0x6;
+			}
+		}
+		else {
+			if (m_cf) {
+				m_a -= 0x60;
+			}
+
+			if (m_hf) {
+				m_a -= 0x6;
+			}
 		}
 
-		// Set Accumulator to the correct BCD representation
-		m_a = higher_nibble | lower_nibble;
+		// Carry flag
+		m_cf = (m_a & 0x100) == 0x100;
+
+		m_a = m_a & 0xff;
 
 		// Set flags
-		m_zf = m_a == 0;
+		m_zf = (m_a == 0);
 		m_hf = 0;
-		m_cf = new_carry;
 		break;
 	}
 	default:

src/cpu.h

@@ -32,6 +32,7 @@ public:
 	void add8();
 	void and8();
 	void cp();
+	void daa();
 	void dec8();
 	void inc8();
 	void or8();