diff --git a/pia.cpp b/pia.cpp
index 6f4eb8e..584d14e 100644
--- a/pia.cpp
+++ b/pia.cpp
@@ -17,6 +17,8 @@ inline bool c2_low_to_high(uint8_t cr) { return cr & 0x10; }
 
 inline bool c2_high_to_low(uint8_t cr) { return !c2_low_to_high(cr); }
 
+inline bool output_selected(uint8_t cr) { return cr & 0x04; }
+
 void PIA::write(Memory::address a, uint8_t b) {
 #if defined(DEBUGGING)
 	Serial.print(millis());
@@ -27,13 +29,13 @@ void PIA::write(Memory::address a, uint8_t b) {
 #endif
 	switch(a % 4) {
 	case 0:
-		write_porta(b);
+		output_selected(porta_cr)? write_porta(b): write_ddra(b);
 		break;
 	case 1:
 		write_porta_cr(b);
 		break;
 	case 2:
-		write_portb(b);
+		output_selected(portb_cr)? write_portb(b): write_ddrb(b);
 		break;
 	case 3:
 		write_portb_cr(b);
@@ -49,11 +51,11 @@ uint8_t PIA::read(Memory::address a) {
 #endif
 	switch (a % 4) {
 	case 0:
-		return read_porta();
+		return output_selected(porta_cr)? read_porta(): read_ddra();
 	case 1:
 		return read_porta_cr();
 	case 2:
-		return read_portb();
+		return output_selected(portb_cr)? read_portb(): read_ddrb();
 	case 3:
 		return read_portb_cr();
 	}
@@ -63,8 +65,10 @@ uint8_t PIA::read(Memory::address a) {
 void PIA::checkpoint(Stream &s) {
 	s.write(portb_cr);
 	s.write(portb);
+	s.write(ddrb);
 	s.write(porta_cr);
 	s.write(porta);
+	s.write(ddra);
 	s.write(irq_b1);
 	s.write(irq_b2);
 	s.write(irq_a1);
@@ -78,8 +82,10 @@ void PIA::checkpoint(Stream &s) {
 void PIA::restore(Stream &s) {
 	portb_cr = s.read();
 	portb = s.read();
+	ddrb = s.read();
 	porta_cr = s.read();
 	porta = s.read();
+	ddra = s.read();
 	irq_b1 = s.read();
 	irq_b2 = s.read();
 	irq_a1 = s.read();
diff --git a/pia.h b/pia.h
index 0916194..7aa9946 100644
--- a/pia.h
+++ b/pia.h
@@ -1,15 +1,16 @@
 #ifndef __PIA_H__
 #define __PIA_H__
 
+// Motorola 6820 / 6821 PIA
 // https://en.wikipedia.org/wiki/Peripheral_Interface_Adapter
 class PIA {
 public:
-	PIA(): portb(0), portb_cr(0), porta(0), porta_cr(0),
+	PIA(): portb(0), portb_cr(0), porta(0), porta_cr(0), ddrb(0), ddra(0),
 		ca1(false), ca2(false), cb1(false), cb2(false),
 		irq_a1(false), irq_a2(false), irq_b1(false), irq_b2(false) {}
 
 	virtual void reset() {
-		portb = portb_cr = porta = porta_cr = 0;
+		portb = portb_cr = ddrb = porta = porta_cr = ddra = 0;
 		irq_a1 = irq_a2 = irq_b1 = irq_b2 = ca1 = ca2 = cb1 = cb2 = false;
 	}
 
@@ -28,19 +29,23 @@ public:
 	static const uint8_t IRQ2 = 0x40;
 
 protected:
-	// "device-side" operations (called from memory interface)
+	virtual uint8_t read_ddra() { return ddra; }
 	virtual uint8_t read_porta() { irq_a1 = irq_a2 = false; return porta; }
 	virtual uint8_t read_porta_cr();
+	virtual uint8_t read_ddrb() { return ddrb; }
 	virtual uint8_t read_portb() { irq_b1 = irq_b2 = false; return portb; }
 	virtual uint8_t read_portb_cr();
 
+	virtual void write_ddra(uint8_t b) { ddra = b; }
 	virtual void write_porta(uint8_t b) { porta = b; }
 	virtual void write_porta_cr(uint8_t b) { porta_cr = (b & 0x3f); }
+	virtual void write_ddrb(uint8_t b) { ddrb = b; }
 	virtual void write_portb(uint8_t b) { portb = b; }
 	virtual void write_portb_cr(uint8_t b) { portb_cr = (b & 0x3f); }
 
 private:
-	uint8_t portb_cr, portb, porta_cr, porta;
+	uint8_t porta_cr, porta, ddra;
+	uint8_t portb_cr, portb, ddrb;
 	bool ca1, ca2, irq_a1, irq_a2;
 	bool cb1, cb2, irq_b1, irq_b2;
 };