diff --git a/src/interpreter/computer.rs b/src/interpreter/computer.rs
index 72192c3..494ee5f 100644
--- a/src/interpreter/computer.rs
+++ b/src/interpreter/computer.rs
@@ -140,3 +140,8 @@ impl fmt::Display for Computer {
         write!(f, "{} | {}", self.regs, self.flags)
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+}
diff --git a/src/operands.rs b/src/operands.rs
index 078a392..f557323 100644
--- a/src/operands.rs
+++ b/src/operands.rs
@@ -32,14 +32,11 @@ impl ImmediateOperand {
     pub fn sign_extend(self) -> Self {
         match self {
             Self::Byte(_) => {
-                let sign = self.msb();
-                let byte = self.flip_sign();
-                let word = byte.as_word();
-                if sign {
-                    return word.flip_sign();
+                return if self.msb() {
+                    self.flip_sign().as_word().flip_sign()
                 } else {
-                    return word;
-                }
+                    self.as_word()
+                };
             }
             Self::Word(_) => self,
         }
@@ -391,6 +388,34 @@ impl std::fmt::Display for Pointer32 {
 mod tests {
     use super::*;
 
+    #[test]
+    fn add_byte_byte() {
+        let a = ImmediateOperand::Byte(5);
+        let b = ImmediateOperand::Byte(7);
+        assert_eq!(a + b, ImmediateOperand::Byte(5 + 7))
+    }
+
+    #[test]
+    fn add_word_word() {
+        let a = ImmediateOperand::Word(5);
+        let b = ImmediateOperand::Word(7);
+        assert_eq!(a + b, ImmediateOperand::Word(5 + 7))
+    }
+
+    #[test]
+    fn add_byte_word() {
+        let a = ImmediateOperand::Byte(5);
+        let b = ImmediateOperand::Word(7);
+        assert_eq!(b + a, ImmediateOperand::Word(5 + 7))
+    }
+
+    #[test]
+    fn sub_byte_word() {
+        let a = ImmediateOperand::Byte(5);
+        let b = ImmediateOperand::Word(7);
+        assert_eq!(b - a, ImmediateOperand::Word(7 - 5))
+    }
+
     #[test]
     fn test_msb() {
         let pos = ImmediateOperand::Byte(1 << 4);
@@ -431,6 +456,13 @@ mod tests {
 
     #[test]
     fn test_sign_extend() {
+        let byte = ImmediateOperand::Byte(0b01010101);
+        let word = ImmediateOperand::Word(0b0000000001010101);
+        assert_eq!(byte.sign_extend(), word);
+    }
+
+    #[test]
+    fn test_sign_extend_neg() {
         let byte = ImmediateOperand::Byte(1 << 7);
         let word = ImmediateOperand::Word(1 << 15);
         assert_eq!(byte.sign_extend(), word);