diff --git a/Components/AY38910/AY38910.cpp b/Components/AY38910/AY38910.cpp
index d9e0ae4af..fab7e94b3 100644
--- a/Components/AY38910/AY38910.cpp
+++ b/Components/AY38910/AY38910.cpp
@@ -173,11 +173,15 @@ void AY38910::select_register(uint8_t r) {
 }
 
 void AY38910::set_register_value(uint8_t value) {
+	// There are only 16 registers.
 	if(selected_register_ > 15) return;
-	registers_[selected_register_] = value;
+
+	// If this is a register that affects audio output, enqueue a mutation onto the
+	// audio generation thread.
 	if(selected_register_ < 14) {
-		int selected_register = selected_register_;
+		const int selected_register = selected_register_;
 		task_queue_.defer([=] () {
+			// Perform any register-specific mutation to output generation.
 			uint8_t masked_value = value;
 			switch(selected_register) {
 				case 0: case 2: case 4:
@@ -208,12 +212,34 @@ void AY38910::set_register_value(uint8_t value) {
 					envelope_position_ = 0;
 				break;
 			}
+
+			// Store a copy of the current register within the storage used by the audio generation
+			// thread, and apply any changes to output volume.
 			output_registers_[selected_register] = masked_value;
 			evaluate_output_volume();
 		});
-	} else {
-		if(port_handler_) port_handler_->set_port_output(selected_register_ == 15, value);
 	}
+
+	// Decide which outputs are going to need updating (if any).
+	bool update_port_a = false;
+	bool update_port_b = true;
+	if(port_handler_) {
+		if(selected_register_ == 7) {
+			const uint8_t io_change = registers_[7] ^ value;
+			update_port_b = !!(io_change&0x80);
+			update_port_a = !!(io_change&0x40);
+		} else {
+			update_port_b = selected_register_ == 15;
+			update_port_a = selected_register_ != 15;
+		}
+	}
+
+	// Keep a copy of the new value that is usable from the emulation thread.
+	registers_[selected_register_] = value;
+
+	// Update ports as required.
+	if(update_port_b) set_port_output(true);
+	if(update_port_a) set_port_output(false);
 }
 
 uint8_t AY38910::get_register_value() {
@@ -238,6 +264,8 @@ uint8_t AY38910::get_port_output(bool port_b) {
 
 void AY38910::set_port_handler(PortHandler *handler) {
 	port_handler_ = handler;
+	set_port_output(true);
+	set_port_output(false);
 }
 
 void AY38910::set_data_input(uint8_t r) {
@@ -245,6 +273,16 @@ void AY38910::set_data_input(uint8_t r) {
 	update_bus();
 }
 
+void AY38910::set_port_output(bool port_b) {
+	// Per the data sheet: "each [IO] pin is provided with an on-chip pull-up resistor,
+	// so that when in the "input" mode, all pins will read normally high". Therefore,
+	// report programmer selection of input mode as creating an output of 0xff.
+	if(port_handler_) {
+		const bool is_output = !!(registers_[7] & (port_b ? 0x80 : 0x40));
+		port_handler_->set_port_output(port_b, is_output ? registers_[port_b ? 15 : 14] : 0xff);
+	}
+}
+
 uint8_t AY38910::get_data_output() {
 	if(control_state_ == Read && selected_register_ >= 14 && selected_register_ < 16) {
 		// Per http://cpctech.cpc-live.com/docs/psgnotes.htm if a port is defined as output then the
diff --git a/Components/AY38910/AY38910.hpp b/Components/AY38910/AY38910.hpp
index f3545de19..942e6b957 100644
--- a/Components/AY38910/AY38910.hpp
+++ b/Components/AY38910/AY38910.hpp
@@ -83,7 +83,7 @@ class AY38910: public ::Outputs::Speaker::SampleSource {
 		*/
 		void set_port_handler(PortHandler *);
 
-		// to satisfy ::Outputs::Speaker (included via ::Outputs::Filter; not for public consumption
+		// to satisfy ::Outputs::Speaker (included via ::Outputs::Filter.
 		void get_samples(std::size_t number_of_samples, int16_t *target);
 		bool is_zero_level();
 		void set_sample_volume_range(std::int16_t range);
@@ -128,10 +128,11 @@ class AY38910: public ::Outputs::Speaker::SampleSource {
 		uint8_t data_input_, data_output_;
 
 		int16_t output_volume_;
-		inline void evaluate_output_volume();
+		void evaluate_output_volume();
 
-		inline void update_bus();
+		void update_bus();
 		PortHandler *port_handler_ = nullptr;
+		void set_port_output(bool port_b);
 };
 
 }