implement overrun status flag for the 6551 ACIA

src/main/java/com/loomcom/symon/devices/Acia6551.java

@@ -181,7 +181,7 @@ public class Acia6551 extends Acia {
      */
     @Override
     public int statusReg() {
-        // TODO: Overrun, Parity Error, Framing Error, DTR, DSR, and Interrupt flags.
+        // TODO: Parity Error, Framing Error, DTR, DSR, and Interrupt flags.
         int stat = 0;
         if (rxFull && System.nanoTime() >= (lastRxRead + baudRateDelay)) {
             stat |= 0x08;
@@ -189,6 +189,9 @@ public class Acia6551 extends Acia {
         if (txEmpty && System.nanoTime() >= (lastTxWrite + baudRateDelay)) {
             stat |= 0x10;
         }
+        if (overrun) {
+            stat |= 0x04;
+        }
         return stat;
     }