diff --git a/src/interpreter/interpreter.rs b/src/interpreter/interpreter.rs
index 348d9f8..b69ec7b 100644
--- a/src/interpreter/interpreter.rs
+++ b/src/interpreter/interpreter.rs
@@ -576,15 +576,19 @@ impl Interpreter {
                  * Sign extensions
                  */
                 Mnemonic::CBW => {
+                    // extend sign bit into AX, i.e. all bits set
                     if ImmediateOperand::Byte(self.computer.regs.ax.lower).msb() {
-                        // extend sign bit into AX, i.e. all bits set
                         self.computer.regs.ax.upper = 0xff;
+                    } else {
+                        self.computer.regs.ax.upper = 0x00;
                     }
                 }
                 Mnemonic::CWD => {
+                    // extend sign bit into DX, i.e. all bits set
                     if ImmediateOperand::Word(self.computer.regs.ax.read()).msb() {
-                        // extend sign bit into DX, i.e. all bits set
                         self.computer.regs.dx.write(0xffff);
+                    } else {
+                        self.computer.regs.dx.write(0x0000);
                     }
                 }