Acia Tx return nothing when there is no new data

acia6551.go

@@ -95,10 +95,14 @@ func (a *Acia6551) statusRegister() byte {
 // Implements io.Reader, for external programs to read TX'ed data from
 // the serial output.
 func (a *Acia6551) Read(p []byte) (n int, err error) {
-	a.txEmpty = true
-	copy(p, []byte{a.tx})
-	// TODO: Handle txInterrupt
-	return 1, nil
+	if a.txEmpty {
+		return 0, nil
+	} else {
+		a.txEmpty = true
+		copy(p, []byte{a.tx})
+		// TODO: Handle txInterrupt
+		return 1, nil
+	}
 }
 
 // Implements io.Writer, for external programs to write to the

acia6551_test.go

@@ -36,6 +36,18 @@ func TestAciaReset(t *testing.T) {
 	assert.Equal(t, 0, a.controlData)
 }
 
+func TestAciaReaderWithTxEmpty(t *testing.T) {
+	a := AciaSubject()
+
+	// Nothing to read
+	assert.True(t, a.txEmpty)
+
+	value := make([]byte, 1)
+	bytesRead, _ := a.Read(value)
+
+	assert.Equal(t, 0, bytesRead)
+}
+
 func TestAciaWriteByteAndReader(t *testing.T) {
 	a := AciaSubject()