diff --git a/snes-core/src/cpu/instructions.rs b/snes-core/src/cpu/instructions.rs
index c2089c1..95a5093 100644
--- a/snes-core/src/cpu/instructions.rs
+++ b/snes-core/src/cpu/instructions.rs
@@ -38,7 +38,7 @@ impl CPU {
                     true => alu::adc8bcd(target as u8, value, carry_flag),
                     false => alu::adc8bin(target as u8, value, carry_flag),
                 };
-                self.registers.a = result as u16;
+                self.registers.a = (self.registers.a & 0x00FF) | (result as u16);
                 self.registers.set_carry_flag(is_carry);
                 self.registers.set_negative_flag(is_negative);
                 self.registers.set_zero_flag(is_zero);
@@ -99,4 +99,21 @@ mod cpu_instructions_tests {
         assert_eq!(cpu.registers.a, 0x40);
         assert!(!cpu.registers.get_carry_flag());
     }
+
+    #[test]
+    fn test_dec() {
+        /*
+        let mut cpu = CPU::new();
+        let mut bus = Bus::new();
+        cpu.registers.a   = 0x0000;
+        cpu.registers.pbr = 0x00;
+        cpu.registers.pc  = 0x0000;
+        cpu.registers.set_memory_select_flag(true);
+        bus.write(0x000001, 1);
+        cpu.dec(&bus, AddressingMode::Immediate);
+        assert_eq!(cpu.registers.a, 1);
+        assert!(!cpu.registers.get_carry_flag());
+        assert!(!cpu.registers.get_zero_flag());
+        */
+    }
 }
diff --git a/snes-core/src/utils/alu.rs b/snes-core/src/utils/alu.rs
index 64c890d..00d96a4 100644
--- a/snes-core/src/utils/alu.rs
+++ b/snes-core/src/utils/alu.rs
@@ -1,3 +1,5 @@
+/// TODO: refactor functions to work with generic types (either u8 or 16) to reduce duplication
+
 pub fn adc8bin(target: u8, value: u8, carry: bool) -> (u8, bool, bool, bool) {
     let is_carry = match target.checked_add(value) {
         None => true,
@@ -66,6 +68,72 @@ pub fn adc16bcd(target: u16, value: u16, carry: bool) -> (u16, bool, bool, bool)
     (result, is_carry, is_negative, is_zero)
 }
 
+pub fn dec8bin(target: u8, value: u8, carry: bool) -> (u8, bool, bool, bool) {
+    let is_carry = match target.checked_sub(value) {
+        None => true,
+        Some(res) => match res.checked_sub(carry as u8) {
+            None => true,
+            Some(_) => false,
+        },
+    };
+    let result = target
+        .wrapping_sub(value)
+        .wrapping_sub(carry as u8);
+    let is_negative = (result >> 7) == 1;
+    let is_zero = result == 0;
+    (result, is_carry, is_negative, is_zero)
+}
+
+pub fn dec16bin(target: u16, value: u16, carry: bool) -> (u16, bool, bool, bool) {
+    let is_carry = match target.checked_sub(value) {
+        None => true,
+        Some(res) => match res.checked_sub(carry as u16) {
+            None => true,
+            Some(_) => false,
+        },
+    };
+    let result = target
+        .wrapping_sub(value)
+        .wrapping_sub(carry as u16);
+    let is_negative = (result >> 15) == 1;
+    let is_zero = result == 0;
+    (result, is_carry, is_negative, is_zero)
+}
+
+pub fn dec8bcd(target: u8, value: u8, carry: bool) -> (u8, bool, bool, bool) {
+    let mut is_carry = carry;
+    let value = (!value).wrapping_add(1); // complement of value
+    let six_complement = (!6 as u8).wrapping_add(1);
+    let mut result = (target & 0xF) + (value & 0xF) + (is_carry as u8) + 1;
+    if result > 0xF {
+        result = result.wrapping_add(six_complement);
+    }
+    result = result.wrapping_add(target & 0xF0).wrapping_add(value & 0xF0).wrapping_add(1);
+    is_carry = result < 0xFF;
+    let is_negative = (result >> 7) == 1;
+    let is_zero = result == 0;
+    (result, is_carry, is_negative, is_zero)
+}
+
+pub fn dec16bcd(target: u16, value: u16, carry: bool) -> (u16, bool, bool, bool) {
+    let mut is_carry = carry;
+    let mut result = (target & 0xF) + (value & 0xF) + (is_carry as u16);
+    if result > 9 {
+        result += 6;
+    }
+    result += (target & 0xF0) + (value & 0xF0);
+    if result > 0x9F {
+        result += 0x60;
+    }
+    result += (target & 0xF00) + (value & 0xF00);
+    if result > 0x9FF {
+        result += 0x600;
+    }
+    is_carry = result > 0x9FFF;
+    let is_negative = (result >> 15) == 1;
+    let is_zero = result == 0;
+    (result, is_carry, is_negative, is_zero)
+}
 
 #[cfg(test)]
 mod alu_tests {
@@ -220,4 +288,64 @@ mod alu_tests {
         assert_eq!(negative, false);
         assert_eq!(zero, false);
     }
+
+    #[test]
+    fn test_dec8bin() {
+        let (result, carry, negative, zero) = dec8bin(1, 1, false);
+        assert_eq!(result, 0);
+        assert_eq!(carry, false);
+        assert_eq!(negative, false);
+        assert_eq!(zero, true);
+
+        let (result, carry, negative, zero) = dec8bin(0, 1, false);
+        assert_eq!(result, 0b11111111);
+        assert_eq!(carry, true);
+        assert_eq!(negative, true);
+        assert_eq!(zero, false);
+
+        let (result, carry, negative, zero) = dec8bin(0, 1, true);
+        assert_eq!(result, 0b11111110);
+        assert_eq!(carry, true);
+        assert_eq!(negative, true);
+        assert_eq!(zero, false);
+    }
+
+    #[test]
+    fn test_dec16bin() {
+        let (result, carry, negative, zero) = dec16bin(1, 1, false);
+        assert_eq!(result, 0);
+        assert_eq!(carry, false);
+        assert_eq!(negative, false);
+        assert_eq!(zero, true);
+
+        let (result, carry, negative, zero) = dec16bin(0, 1, false);
+        assert_eq!(result, 0b11111111_11111111);
+        assert_eq!(carry, true);
+        assert_eq!(negative, true);
+        assert_eq!(zero, false);
+
+        let (result, carry, negative, zero) = dec16bin(0, 1, true);
+        assert_eq!(result, 0b11111111_11111110);
+        assert_eq!(carry, true);
+        assert_eq!(negative, true);
+        assert_eq!(zero, false);
+    }
+
+    #[test]
+    fn test_dec8bcd() {
+        let (result, carry, negative, zero) = dec8bcd(0b0100_0111, 0b0010_1000, false);
+        assert_eq!(result, 0b0001_1001);
+        assert_eq!(carry, false);
+        assert_eq!(negative, false);
+        assert_eq!(zero, false);
+    }
+
+    #[test]
+    fn test_dec16bcd() {
+        let (result, carry, negative, zero) = dec16bcd(0b0100_0111, 0b0010_1000, false);
+        assert_eq!(result, 0b0001_1001);
+        assert_eq!(carry, false);
+        assert_eq!(negative, false);
+        assert_eq!(zero, false);
+    }
 }