diff --git a/src/interpreter/computer.rs b/src/interpreter/computer.rs
index 07042b6..6dbda18 100644
--- a/src/interpreter/computer.rs
+++ b/src/interpreter/computer.rs
@@ -49,7 +49,7 @@ impl Computer {
         let op: fn(Lhs, Rhs) -> ArithmeticResult = |lhs, rhs| lhs + rhs;
         let flag_set: fn(&mut Flags, ArithmeticResult, Lhs, Rhs) = |flags, result, lhs, rhs| {
             flags.cf = result < rhs;
-            flags.of = lhs.msb() && rhs.msb() != result.msb();
+            flags.of = (lhs.msb() && rhs.msb()) && (result.msb() != lhs.msb());
             flags.zf = result.zero();
             flags.sf = result.msb();
             flags.pf = result.parity();
diff --git a/src/interpreter/interpreter.rs b/src/interpreter/interpreter.rs
index 88aa355..bc1aa61 100644
--- a/src/interpreter/interpreter.rs
+++ b/src/interpreter/interpreter.rs
@@ -427,14 +427,21 @@ impl Interpreter {
                 },
                 Mnemonic::CALL_p(_) => todo!(),
                 Mnemonic::CALL_v(offset) => {
-                    // save next instruction onto stack for later RET
                     if let Some(next_instr) = ip.next() {
                         self.computer.push_stack(next_instr.start.into())?;
                     }
-                    // jump to target
                     Self::ip_jump(&self.instructions, &mut ip, offset);
                 }
-                Mnemonic::CALL_Mod(_) => todo!(),
+                Mnemonic::CALL_Mod(target) => {
+                    if let Some(next_instr) = ip.next() {
+                        self.computer.push_stack(next_instr.start.into())?;
+                    }
+                    Self::ip_jump(
+                        &self.instructions,
+                        &mut ip,
+                        self.computer.read_modrm(target).into(),
+                    );
+                }
                 Mnemonic::CALL_Mp(_) => todo!(),
 
                 /*
diff --git a/src/interpreter/register.rs b/src/interpreter/register.rs
index be2f08d..38385e1 100644
--- a/src/interpreter/register.rs
+++ b/src/interpreter/register.rs
@@ -22,7 +22,7 @@ impl Register {
             bx: BX::new(),
             cx: CX::new(),
             dx: DX::new(),
-            sp: 0xffff,
+            sp: 0xffda,
             bp: 0,
             si: 0,
             di: 0,
diff --git a/src/operands.rs b/src/operands.rs
index c3091fe..e506b37 100644
--- a/src/operands.rs
+++ b/src/operands.rs
@@ -151,7 +151,7 @@ impl From<usize> for ImmediateOperand {
 impl Into<Word> for ImmediateOperand {
     fn into(self) -> u16 {
         match self {
-            ImmediateOperand::Byte(_) => self.sign_extend().into(),
+            ImmediateOperand::Byte(b) => b as Word,
             ImmediateOperand::Word(w) => w,
         }
     }