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;
     }
 

src/test/java/com/loomcom/symon/AciaTest.java

@@ -113,6 +113,25 @@ public class AciaTest {
         assertEquals(0x08, acia.read(0x0001));
     }
     
+    @Test
+    public void aciaShouldOverrunAndReadShouldReset()
+            throws Exception {
+        
+        Acia acia = new Acia6551(0x0000);
+        
+        // overrun ACIA
+        acia.rxWrite('a');
+        acia.rxWrite('b');
+        
+        assertEquals(0x04, acia.read(0x0001) & 0x04);
+        
+        // read should reset
+        acia.rxRead();
+        assertEquals(0x00, acia.read(0x0001) & 0x04);
+        
+    }
+
+
     @Test
     public void readingBuffersShouldResetStatus()
             throws Exception {