implement overrun flag for 6850 ACIA

src/main/java/com/loomcom/symon/devices/Acia6850.java

@@ -86,6 +86,20 @@ public class Acia6850 extends Acia {
                 throw new MemoryAccessException("No register.");
         }
     }
+    
+    @Override
+    public synchronized int rxRead() {
+        overrun = false;
+        return super.rxRead();
+    }
+    
+    @Override
+    public synchronized void rxWrite(int data) {
+        if(rxFull) {
+            overrun = true;
+        }
+        super.rxWrite(data);
+    }
 
 
     private void setCommandRegister(int data) {

src/test/java/com/loomcom/symon/AciaTest6850.java

@@ -125,6 +125,24 @@ public class AciaTest6850 {
 
         assertEquals(0x01, acia.read(CMD_STAT_REG) & 0x03);
     }
+    
+    @Test
+    public void aciaShouldOverrunAndReadShouldReset()
+            throws Exception {
+        
+        Acia acia = newAcia();
+        
+        // overrun ACIA
+        acia.rxWrite('a');
+        acia.rxWrite('b');
+        
+        assertEquals(0x20, acia.read(CMD_STAT_REG) & 0x20);
+        
+        // read should reset
+        acia.rxRead();
+        assertEquals(0x00, acia.read(CMD_STAT_REG) & 0x20);
+        
+    }
 
     @Test
     public void readingBuffersShouldResetStatus()