diff --git a/Components/68901/MFP68901.cpp b/Components/68901/MFP68901.cpp
index 5c83da90d..4ba27e068 100644
--- a/Components/68901/MFP68901.cpp
+++ b/Components/68901/MFP68901.cpp
@@ -9,7 +9,7 @@
 #include "MFP68901.hpp"
 
 #define LOG_PREFIX "[MFP] "
-#define NDEBUG
+//#define NDEBUG
 #include "../../Outputs/Log.hpp"
 
 using namespace Motorola::MFP68901;
@@ -17,9 +17,15 @@ using namespace Motorola::MFP68901;
 uint8_t MFP68901::read(int address) {
 	address &= 0x1f;
 	switch(address) {
-		case 0x00:		LOG("Read: general purpose IO");		break;
-		case 0x01:		LOG("Read: active edge");				break;
-		case 0x02:		LOG("Read: data direction");			break;
+		case 0x00:
+			LOG("Read: general purpose IO " << PADHEX(2) << int(gpip_input_ | gpip_direction_));
+		return gpip_input_ | gpip_direction_;
+		case 0x01:
+			LOG("Read: active edge " << PADHEX(2) << int(gpip_active_edge_));
+		return gpip_active_edge_;
+		case 0x02:
+			LOG("Read: data direction "  << PADHEX(2) << int(gpip_direction_));
+		return gpip_direction_;
 		case 0x03:		LOG("Read: interrupt enable A");		break;
 		case 0x04:		LOG("Read: interrupt enable B");		break;
 		case 0x05:		LOG("Read: interrupt pending A");		break;
@@ -46,9 +52,20 @@ uint8_t MFP68901::read(int address) {
 void MFP68901::write(int address, uint8_t value) {
 	address &= 0x1f;
 	switch(address) {
-		case 0x00:		LOG("Write: general purpose IO");		break;
-		case 0x01:		LOG("Write: active edge");				break;
-		case 0x02:		LOG("Write: data direction");			break;
+		case 0x00:
+			LOG("Write: general purpose IO " << PADHEX(2) << int(value));
+			gpip_output_ = value;
+		break;
+		case 0x01:
+			LOG("Write: active edge " << PADHEX(2) << int(value));
+			gpip_active_edge_ = value;
+			reevaluate_gpip_interrupts();
+		break;
+		case 0x02:
+			LOG("Write: data direction " << PADHEX(2) << int(value));
+			gpip_direction_ = value;
+			reevaluate_gpip_interrupts();
+		break;
 		case 0x03:		LOG("Write: interrupt enable A");		break;
 		case 0x04:		LOG("Write: interrupt enable B");		break;
 		case 0x05:		LOG("Write: interrupt pending A");		break;
@@ -173,3 +190,22 @@ void MFP68901::decrement_timer(int timer) {
 		// TODO: interrupt. Reload, possibly.
 	}
 }
+
+// MARK: - GPIP
+void MFP68901::set_port_input(uint8_t input) {
+	gpip_input_ = input;
+	reevaluate_gpip_interrupts();
+}
+
+uint8_t MFP68901::get_port_output() {
+	return 0xff;
+}
+
+void MFP68901::reevaluate_gpip_interrupts() {
+	const uint8_t gpip_state = gpip_input_ ^ gpip_active_edge_;
+	// An interrupt is detected on any falling edge.
+	if((gpip_state ^ gpip_interrupt_state_) & gpip_interrupt_state_) {
+		LOG("Should post GPIP interrupt");
+	}
+	gpip_interrupt_state_ = gpip_state;
+}
diff --git a/Components/68901/MFP68901.hpp b/Components/68901/MFP68901.hpp
index d27839765..f7dcf7978 100644
--- a/Components/68901/MFP68901.hpp
+++ b/Components/68901/MFP68901.hpp
@@ -17,12 +17,7 @@ namespace MFP68901 {
 
 class PortHandler {
 	public:
-		/*!
-			Requests the current input on the GPIP port.
-		*/
-		virtual uint8_t get_port_input() {
-			return 0xff;
-		}
+		// TODO: announce changes in output.
 };
 
 class MFP68901 {
@@ -59,6 +54,15 @@ class MFP68901 {
 		} timers_[4];
 
 		HalfCycles cycles_left_;
+
+		// MARK: - GPIP
+		uint8_t gpip_input_ = 0;
+		uint8_t gpip_output_ = 0;
+		uint8_t gpip_active_edge_ = 0;
+		uint8_t gpip_direction_ = 0;
+		uint8_t gpip_interrupt_state_ = 0;
+
+		void reevaluate_gpip_interrupts();
 };
 
 }
diff --git a/Machines/AtariST/AtariST.cpp b/Machines/AtariST/AtariST.cpp
index 8ac2bc970..256005def 100644
--- a/Machines/AtariST/AtariST.cpp
+++ b/Machines/AtariST/AtariST.cpp
@@ -214,7 +214,8 @@ class ConcreteMachine:
 	public Atari::ST::Machine,
 	public CPU::MC68000::BusHandler,
 	public CRTMachine::Machine,
-	public ClockingHint::Observer {
+	public ClockingHint::Observer,
+	public Motorola::ACIA::ACIA::InterruptDelegate {
 	public:
 		ConcreteMachine(const Target &target, const ROMMachine::ROMFetcher &rom_fetcher) :
 			mc68000_(*this),
@@ -250,9 +251,14 @@ class ConcreteMachine:
 
 			memory_map_[0xff] = BusDevice::IO;
 
+			midi_acia_->set_interrupt_delegate(this);
+			keyboard_acia_->set_interrupt_delegate(this);
+
 			midi_acia_->set_clocking_hint_observer(this);
 			keyboard_acia_->set_clocking_hint_observer(this);
 			ikbd_.set_clocking_hint_observer(this);
+
+			set_gpip_input();
 		}
 
 		~ConcreteMachine() {
@@ -408,19 +414,6 @@ class ConcreteMachine:
 									mfp_->write(int(address), cycle.value->halves.high);
 								}
 							}
-
-							/*
-								Atari ST GPIP bits:
-
-									GPIP 7: monochrome monitor detect
-									GPIP 6: RS-232 ring indicator
-									GPIP 5: FD/HD interrupt
-									GPIP 4: keyboard/MIDI interrupt
-									GPIP 3: unused
-									GPIP 2: RS-232 clear to send
-									GPIP 1: RS-232 carrier detect
-									GPIP 0: centronics busy
-							*/
 						break;
 
 						// Video controls.
@@ -584,6 +577,7 @@ class ConcreteMachine:
 		};
 		BusDevice memory_map_[256];
 
+		// MARK: - Clocking Management.
 		bool may_defer_acias_ = true;
 		bool keyboard_needs_clock_ = false;
 		void set_component_prefers_clocking(ClockingHint::Source *component, ClockingHint::Preference clocking) final {
@@ -594,6 +588,29 @@ class ConcreteMachine:
 				(midi_acia_.last_valid()->preferred_clocking() != ClockingHint::Preference::RealTime);
 			keyboard_needs_clock_ = ikbd_.preferred_clocking() != ClockingHint::Preference::None;
 		}
+
+		// MARK: - GPIP input.
+		void acia6850_did_change_interrupt_status(Motorola::ACIA::ACIA *acia) final {
+			set_gpip_input();
+		}
+		void set_gpip_input() {
+			/*
+				Atari ST GPIP bits:
+
+					GPIP 7: monochrome monitor detect
+					GPIP 6: RS-232 ring indicator
+					GPIP 5: FD/HD interrupt
+					GPIP 4: keyboard/MIDI interrupt
+					GPIP 3: unused
+					GPIP 2: RS-232 clear to send
+					GPIP 1: RS-232 carrier detect
+					GPIP 0: centronics busy
+			*/
+			mfp_->set_port_input(
+				0x80 |
+				((keyboard_acia_->get_interrupt_line() || midi_acia_->get_interrupt_line()) ? 0x10 : 0x00)
+			);
+		}
 };
 
 }