serial: dont fake a connection if it's not there

src/processor/memory/mmio/serial.rs

@@ -35,11 +35,15 @@ pub struct Serial {
     output_byte: u8,
     bits_remaining: u8,
     control: SerialControl,
+    is_connected: bool,
 }
 
 impl Serial {
     pub fn tick(&mut self, steps: usize) -> bool {
         let mut will_interrupt = false;
+        if !self.is_connected {
+            return false;
+        }
         for _ in 0..steps {
             if self.control.transfer_in_progress {
                 self.output_byte = self.output_byte << 1 | self.byte >> 7;
@@ -98,6 +102,7 @@ impl Default for Serial {
             output_byte: 0,
             bits_remaining: 7,
             control: SerialControl::default(),
+            is_connected: false,
         }
     }
 }