diff --git a/OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift b/OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift
index 61b403bd0..6ae182abc 100644
--- a/OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift	
+++ b/OSBindings/Mac/Clock Signal/Documents/MachineDocument.swift	
@@ -102,7 +102,7 @@ class MachineDocument:
 		}
 	}
 
-	func machineSpeakerDidChangeInputClock(_ machine: CSMachine!) {
+	func machineSpeakerDidChangeInputClock(_ machine: CSMachine) {
 		setupAudioQueueClockRate()
 	}
 
@@ -147,6 +147,7 @@ class MachineDocument:
 			self.machine = machine
 			self.optionsPanelNibName = analysis.optionsPanelNibName
 			setupMachineOutput()
+			setupActivityDisplay()
 		}
 	}
 
@@ -290,4 +291,43 @@ class MachineDocument:
 		}
 		return super.validateUserInterfaceItem(item)
 	}
+
+	// MARK: Activity display.
+	func setupActivityDisplay() {
+		return
+
+		if machine.leds.count > 0 {
+			let panel = NSPanel()
+			panel.title = "Activity"
+			panel.styleMask = .hudWindow
+			panel.setIsVisible(true)
+
+			for name in machine.leds {
+				let button = NSButton()
+				button.title = name
+				button.setButtonType(.radio)
+				button.translatesAutoresizingMaskIntoConstraints = false
+				button.isEnabled = false
+//				button.color
+				panel.contentView?.addSubview(button)
+
+				let views = ["button": button]
+				let horizontalConstraints =
+					NSLayoutConstraint.constraints(
+						withVisualFormat: "H:|-[button]-|",
+						options: NSLayoutConstraint.FormatOptions(rawValue: 0),
+						metrics: nil,
+						views: views)
+				let verticalConstraints =
+					NSLayoutConstraint.constraints(
+						withVisualFormat: "V:|-[button]-|",
+						options: NSLayoutConstraint.FormatOptions(rawValue: 0),
+						metrics: nil,
+						views: views)
+
+				panel.contentView?.addConstraints(horizontalConstraints)
+				panel.contentView?.addConstraints(verticalConstraints)
+			}
+		}
+	}
 }
diff --git a/OSBindings/Mac/Clock Signal/Machine/CSMachine.h b/OSBindings/Mac/Clock Signal/Machine/CSMachine.h
index 4b0ceddb8..670a446ce 100644
--- a/OSBindings/Mac/Clock Signal/Machine/CSMachine.h	
+++ b/OSBindings/Mac/Clock Signal/Machine/CSMachine.h	
@@ -15,7 +15,7 @@
 
 @class CSMachine;
 @protocol CSMachineDelegate
-- (void)machineSpeakerDidChangeInputClock:(CSMachine *)machine;
+- (void)machineSpeakerDidChangeInputClock:(nonnull CSMachine *)machine;
 @end
 
 typedef NS_ENUM(NSInteger, CSMachineVideoSignal) {
@@ -35,32 +35,32 @@ typedef NS_ENUM(NSInteger, CSMachineKeyboardInputMode) {
 
 @interface CSMachine : NSObject
 
-- (instancetype)init NS_UNAVAILABLE;
+- (nonnull instancetype)init NS_UNAVAILABLE;
 /*!
 	Initialises an instance of CSMachine.
 
 	@param result The CSStaticAnalyser result that describes the machine needed.
 */
-- (instancetype)initWithAnalyser:(CSStaticAnalyser *)result NS_DESIGNATED_INITIALIZER;
+- (nullable instancetype)initWithAnalyser:(nonnull CSStaticAnalyser *)result NS_DESIGNATED_INITIALIZER;
 
 - (void)runForInterval:(NSTimeInterval)interval;
 
 - (float)idealSamplingRateFromRange:(NSRange)range;
 - (void)setAudioSamplingRate:(float)samplingRate bufferSize:(NSUInteger)bufferSize;
 
-- (void)setView:(CSOpenGLView *)view aspectRatio:(float)aspectRatio;
+- (void)setView:(nullable CSOpenGLView *)view aspectRatio:(float)aspectRatio;
 - (void)drawViewForPixelSize:(CGSize)pixelSize onlyIfDirty:(BOOL)onlyIfDirty;
 
-- (void)setKey:(uint16_t)key characters:(NSString *)characters isPressed:(BOOL)isPressed;
+- (void)setKey:(uint16_t)key characters:(nullable NSString *)characters isPressed:(BOOL)isPressed;
 - (void)clearAllKeys;
 
-@property (nonatomic, strong) CSAudioQueue *audioQueue;
-@property (nonatomic, readonly) CSOpenGLView *view;
-@property (nonatomic, weak) id<CSMachineDelegate> delegate;
+@property (nonatomic, strong, nullable) CSAudioQueue *audioQueue;
+@property (nonatomic, readonly, nonnull) CSOpenGLView *view;
+@property (nonatomic, weak, nullable) id<CSMachineDelegate> delegate;
 
-@property (nonatomic, readonly) NSString *userDefaultsPrefix;
+@property (nonatomic, readonly, nonnull) NSString *userDefaultsPrefix;
 
-- (void)paste:(NSString *)string;
+- (void)paste:(nonnull NSString *)string;
 
 @property (nonatomic, assign) BOOL useFastLoadingHack;
 @property (nonatomic, assign) CSMachineVideoSignal videoSignal;
@@ -73,8 +73,11 @@ typedef NS_ENUM(NSInteger, CSMachineKeyboardInputMode) {
 @property (nonatomic, readonly) BOOL hasJoystick;
 @property (nonatomic, assign) CSMachineKeyboardInputMode inputMode;
 
+// LED list.
+@property (nonatomic, readonly, nonnull) NSArray<NSString *> *leds;
+
 // Special-case accessors; undefined behaviour if accessed for a machine not of the corresponding type.
-@property (nonatomic, readonly) CSAtari2600 *atari2600;
-@property (nonatomic, readonly) CSZX8081 *zx8081;
+@property (nonatomic, readonly, nullable) CSAtari2600 *atari2600;
+@property (nonatomic, readonly, nullable) CSZX8081 *zx8081;
 
 @end
diff --git a/OSBindings/Mac/Clock Signal/Machine/CSMachine.mm b/OSBindings/Mac/Clock Signal/Machine/CSMachine.mm
index a0ca43fb1..910773d5a 100644
--- a/OSBindings/Mac/Clock Signal/Machine/CSMachine.mm	
+++ b/OSBindings/Mac/Clock Signal/Machine/CSMachine.mm	
@@ -18,6 +18,7 @@
 #include "MachineForTarget.hpp"
 #include "StandardOptions.hpp"
 #include "Typer.hpp"
+#include "../../../../Activity/Observer.hpp"
 
 #import "CSStaticAnalyser+TargetVector.h"
 #import "NSBundle+DataResource.h"
@@ -28,6 +29,7 @@
 @interface CSMachine() <CSFastLoading>
 - (void)speaker:(Outputs::Speaker::Speaker *)speaker didCompleteSamples:(const int16_t *)samples length:(int)length;
 - (void)speakerDidChangeInputClock:(Outputs::Speaker::Speaker *)speaker;
+- (void)addLED:(NSString *)led;
 @end
 
 struct LockProtectedDelegate {
@@ -50,14 +52,36 @@ struct SpeakerDelegate: public Outputs::Speaker::Speaker::Delegate, public LockP
 	}
 };
 
+struct ActivityObserver: public Activity::Observer {
+	void register_led(const std::string &name) override {
+		[machine addLED:[NSString stringWithUTF8String:name.c_str()]];
+	}
+
+	void register_drive(const std::string &name) override {
+	}
+
+	void set_led_status(const std::string &name, bool lit) override {
+	}
+
+	void announce_drive_event(const std::string &name, DriveEvent event) override {
+	}
+
+	void set_drive_motor_status(const std::string &name, bool is_on) override {
+	}
+
+	__unsafe_unretained CSMachine *machine;
+};
+
 @implementation CSMachine {
 	SpeakerDelegate _speakerDelegate;
+	ActivityObserver _activityObserver;
 	NSLock *_delegateMachineAccessLock;
 
 	CSStaticAnalyser *_analyser;
 	std::unique_ptr<Machine::DynamicMachine> _machine;
 
 	std::bitset<65536> _depressedKeys;
+	NSMutableArray<NSString *> *_leds;
 }
 
 - (instancetype)initWithAnalyser:(CSStaticAnalyser *)result {
@@ -71,6 +95,13 @@ struct SpeakerDelegate: public Outputs::Speaker::Speaker::Delegate, public LockP
 
 		_inputMode = _machine->keyboard_machine() ? CSMachineKeyboardInputModeKeyboard : CSMachineKeyboardInputModeJoystick;
 
+		_leds = [[NSMutableArray alloc] init];
+		Activity::Source *const activity_source = _machine->activity_source();
+		if(activity_source) {
+			_activityObserver.machine = self;
+			activity_source->set_activity_observer(&_activityObserver);
+		}
+
 		_delegateMachineAccessLock = [[NSLock alloc] init];
 
 		_speakerDelegate.machine = self;
@@ -408,4 +439,14 @@ struct SpeakerDelegate: public Outputs::Speaker::Speaker::Delegate, public LockP
 	return !!_machine->keyboard_machine();
 }
 
+#pragma mark - Activity observation
+
+- (void)addLED:(NSString *)led {
+	[_leds addObject:led];
+}
+
+- (NSArray<NSString *> *)leds {
+	return _leds;
+}
+
 @end