From 3747d8ce4130388f75a6545a0c81957423a4147f Mon Sep 17 00:00:00 2001
From: Ariejan de Vroom <ariejan@ariejan.net>
Date: Thu, 21 Aug 2014 10:02:04 +0200
Subject: [PATCH] Acia Tx return nothing when there is no new data

---
 acia6551.go      | 12 ++++++++----
 acia6551_test.go | 12 ++++++++++++
 2 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/acia6551.go b/acia6551.go
index a264a9a..5386713 100644
--- a/acia6551.go
+++ b/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
diff --git a/acia6551_test.go b/acia6551_test.go
index 8ce4889..54d5062 100644
--- a/acia6551_test.go
+++ b/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()