diff --git a/Components/DiskII/DiskII.cpp b/Components/DiskII/DiskII.cpp
index f2f78ac4e..21f3daf7c 100644
--- a/Components/DiskII/DiskII.cpp
+++ b/Components/DiskII/DiskII.cpp
@@ -22,7 +22,7 @@ namespace  {
 DiskII::DiskII(int clock_rate) :
 	clock_rate_(clock_rate),
 	inputs_(input_command),
-	drives_{{static_cast<unsigned int>(clock_rate), 300, 1}, {static_cast<unsigned int>(clock_rate), 300, 1}}
+	drives_{{clock_rate, 300, 1}, {clock_rate, 300, 1}}
 {
 	drives_[0].set_clocking_hint_observer(this);
 	drives_[1].set_clocking_hint_observer(this);
diff --git a/Components/DiskII/IWM.hpp b/Components/DiskII/IWM.hpp
index d409b6b00..cdc52e957 100644
--- a/Components/DiskII/IWM.hpp
+++ b/Components/DiskII/IWM.hpp
@@ -16,6 +16,30 @@
 
 namespace Apple {
 
+/*!
+	Defines the drive interface used by the IWM, derived from the external pinout as
+	per e.g. https://old.pinouts.ru/HD/MacExtDrive_pinout.shtml
+
+	These are subclassed of Storage::Disk::Drive, so accept any disk the emulator supports,
+	and provide the usual read/write interface for on-disk data.
+*/
+struct IWMDrive: public Storage::Disk::Drive {
+	IWMDrive(int input_clock_rate, int number_of_heads) : Storage::Disk::Drive(input_clock_rate, number_of_heads) {}
+
+	enum Line: int {
+		CA0		= 1 << 0,
+		CA1		= 1 << 1,
+		CA2		= 1 << 2,
+		LSTRB	= 1 << 3,
+		SEL		= 1 << 4,
+	};
+
+	virtual void set_enabled(bool) = 0;
+	virtual void set_control_lines(int) = 0;
+	virtual bool read() = 0;
+	virtual void write(bool value) = 0;
+};
+
 class IWM:
 	public Storage::Disk::Drive::EventDelegate {
 	public:
diff --git a/Components/DiskII/MacintoshDoubleDensityDrive.cpp b/Components/DiskII/MacintoshDoubleDensityDrive.cpp
new file mode 100644
index 000000000..5301bd9e2
--- /dev/null
+++ b/Components/DiskII/MacintoshDoubleDensityDrive.cpp
@@ -0,0 +1,55 @@
+//
+//  MacintoshDoubleDensityDrive.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 10/07/2019.
+//  Copyright © 2019 Thomas Harte. All rights reserved.
+//
+
+#include "MacintoshDoubleDensityDrive.hpp"
+
+using namespace Apple::Macintosh;
+
+DoubleDensityDrive::DoubleDensityDrive(int input_clock_rate, bool is_800k) :
+	IWMDrive(input_clock_rate, is_800k ? 2 : 1),	// Only 800kb drives are double sided.
+	is_800k_(is_800k) {
+	// Start with a valid rotation speed.
+	if(is_800k) {
+		set_rotation_speed(393.3807f);
+	}
+}
+
+// MARK: - Control input/output.
+
+void DoubleDensityDrive::set_enabled(bool) {
+}
+
+void DoubleDensityDrive::set_control_lines(int lines) {
+}
+
+bool DoubleDensityDrive::read() {
+	return false;
+}
+
+void DoubleDensityDrive::write(bool value) {
+}
+
+// MARK: - Speed Selection
+
+void DoubleDensityDrive::did_step(Storage::Disk::HeadPosition to_position) {
+	// The 800kb drive automatically selects rotation speed as a function of
+	// head position; the 400kb drive doesn't do so.
+	if(is_800k_) {
+		/*
+			Numbers below cribbed from the Kryoflux forums.
+		*/
+		const int zone = to_position.as_int() >> 4;
+		switch(zone) {
+			case 0:		set_rotation_speed(393.3807f);	break;
+			case 1:		set_rotation_speed(429.1723f);	break;
+			case 2:		set_rotation_speed(472.1435f);	break;
+			case 3:		set_rotation_speed(524.5672f);	break;
+			default:	set_rotation_speed(590.1098f);	break;
+		}
+	}
+}
diff --git a/Components/DiskII/MacintoshDoubleDensityDrive.hpp b/Components/DiskII/MacintoshDoubleDensityDrive.hpp
new file mode 100644
index 000000000..cc4f7f1cb
--- /dev/null
+++ b/Components/DiskII/MacintoshDoubleDensityDrive.hpp
@@ -0,0 +1,36 @@
+//
+//  MacintoshDoubleDensityDrive.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 10/07/2019.
+//  Copyright © 2019 Thomas Harte. All rights reserved.
+//
+
+#ifndef MacintoshDoubleDensityDrive_hpp
+#define MacintoshDoubleDensityDrive_hpp
+
+#include "IWM.hpp"
+
+namespace Apple {
+namespace Macintosh {
+
+class DoubleDensityDrive: public IWMDrive {
+	public:
+		DoubleDensityDrive(int input_clock_rate, bool is_800k);
+
+		void set_enabled(bool) override;
+		void set_control_lines(int) override;
+		bool read() override;
+		void write(bool value) override;
+
+	private:
+		// To receive the proper notifications from Storage::Disk::Drive.
+		void did_step(Storage::Disk::HeadPosition to_position) override;
+
+		bool is_800k_;
+};
+
+}
+}
+
+#endif /* MacintoshDoubleDensityDrive_hpp */
diff --git a/Machines/Apple/Macintosh/SonyDrive.cpp b/Machines/Apple/Macintosh/SonyDrive.cpp
index 90a7c245e..28fef911d 100644
--- a/Machines/Apple/Macintosh/SonyDrive.cpp
+++ b/Machines/Apple/Macintosh/SonyDrive.cpp
@@ -11,7 +11,7 @@
 using namespace Apple::Macintosh;
 
 SonyDrive::SonyDrive(int input_clock_rate, bool is_800k) :
-	Storage::Disk::Drive(static_cast<unsigned int>(input_clock_rate), is_800k ? 2 : 1), is_800k_(is_800k) {
+	Storage::Disk::Drive(input_clock_rate, is_800k ? 2 : 1), is_800k_(is_800k) {
 	// Start with a valid rotation speed.
 	if(is_800k) {
 		set_rotation_speed(393.3807f);
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index 71fb17477..7748daaeb 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -639,6 +639,8 @@
 		4BC9DF4F1D04691600F44158 /* 6560.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BC9DF4D1D04691600F44158 /* 6560.cpp */; };
 		4BC9E1EE1D23449A003FCEE4 /* 6502InterruptTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4BC9E1ED1D23449A003FCEE4 /* 6502InterruptTests.swift */; };
 		4BCA6CC81D9DD9F000C2D7B2 /* CommodoreROM.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BCA6CC61D9DD9F000C2D7B2 /* CommodoreROM.cpp */; };
+		4BCD634922D6756400F567F1 /* MacintoshDoubleDensityDrive.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BCD634722D6756400F567F1 /* MacintoshDoubleDensityDrive.cpp */; };
+		4BCD634A22D6756400F567F1 /* MacintoshDoubleDensityDrive.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BCD634722D6756400F567F1 /* MacintoshDoubleDensityDrive.cpp */; };
 		4BCE0051227CE8CA000CA200 /* Video.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BCE004D227CE8CA000CA200 /* Video.cpp */; };
 		4BCE0052227CE8CA000CA200 /* DiskIICard.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BCE004E227CE8CA000CA200 /* DiskIICard.cpp */; };
 		4BCE0053227CE8CA000CA200 /* AppleII.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BCE0050227CE8CA000CA200 /* AppleII.cpp */; };
@@ -1430,6 +1432,8 @@
 		4BCA6CC61D9DD9F000C2D7B2 /* CommodoreROM.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; name = CommodoreROM.cpp; path = Encodings/CommodoreROM.cpp; sourceTree = "<group>"; };
 		4BCA6CC71D9DD9F000C2D7B2 /* CommodoreROM.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; name = CommodoreROM.hpp; path = Encodings/CommodoreROM.hpp; sourceTree = "<group>"; };
 		4BCA98C21D065CA20062F44C /* 6522.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = 6522.hpp; sourceTree = "<group>"; };
+		4BCD634722D6756400F567F1 /* MacintoshDoubleDensityDrive.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = MacintoshDoubleDensityDrive.cpp; sourceTree = "<group>"; };
+		4BCD634822D6756400F567F1 /* MacintoshDoubleDensityDrive.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = MacintoshDoubleDensityDrive.hpp; sourceTree = "<group>"; };
 		4BCE004A227CE8CA000CA200 /* AppleII.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = AppleII.hpp; sourceTree = "<group>"; };
 		4BCE004B227CE8CA000CA200 /* Card.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = Card.hpp; sourceTree = "<group>"; };
 		4BCE004C227CE8CA000CA200 /* DiskIICard.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = DiskIICard.hpp; sourceTree = "<group>"; };
@@ -1823,6 +1827,8 @@
 				4B302183208A550100773308 /* DiskII.cpp */,
 				4BEE1498227FC0EA00133682 /* IWM.cpp */,
 				4BEE1499227FC0EA00133682 /* IWM.hpp */,
+				4BCD634722D6756400F567F1 /* MacintoshDoubleDensityDrive.cpp */,
+				4BCD634822D6756400F567F1 /* MacintoshDoubleDensityDrive.hpp */,
 			);
 			path = DiskII;
 			sourceTree = "<group>";
@@ -3900,6 +3906,7 @@
 				4B055A9D1FAE85DA0060FFFF /* D64.cpp in Sources */,
 				4B055ABB1FAE86170060FFFF /* Oric.cpp in Sources */,
 				4B12C0EE1FCFAD1A005BFD93 /* Keyboard.cpp in Sources */,
+				4BCD634A22D6756400F567F1 /* MacintoshDoubleDensityDrive.cpp in Sources */,
 				4B05401F219D1618001BF69C /* ScanTarget.cpp in Sources */,
 				4B055AE81FAE9B7B0060FFFF /* FIRFilter.cpp in Sources */,
 				4B055A901FAE85A90060FFFF /* TimedEventLoop.cpp in Sources */,
@@ -4121,6 +4128,7 @@
 				4B8334841F5DA0360097E338 /* Z80Storage.cpp in Sources */,
 				4BA61EB01D91515900B3C876 /* NSData+StdVector.mm in Sources */,
 				4BD191F42191180E0042E144 /* ScanTarget.cpp in Sources */,
+				4BCD634922D6756400F567F1 /* MacintoshDoubleDensityDrive.cpp in Sources */,
 				4B0F94FE208C1A1600FE41D9 /* NIB.cpp in Sources */,
 				4B89452A201967B4007DE474 /* File.cpp in Sources */,
 				4B4DC8211D2C2425003C5BF8 /* Vic20.cpp in Sources */,
diff --git a/Storage/Disk/Controller/DiskController.cpp b/Storage/Disk/Controller/DiskController.cpp
index ea4b3c056..547f01812 100644
--- a/Storage/Disk/Controller/DiskController.cpp
+++ b/Storage/Disk/Controller/DiskController.cpp
@@ -15,7 +15,7 @@ using namespace Storage::Disk;
 Controller::Controller(Cycles clock_rate) :
 		clock_rate_multiplier_(128000000 / clock_rate.as_int()),
 		clock_rate_(clock_rate.as_int() * clock_rate_multiplier_),
-		empty_drive_(new Drive(static_cast<unsigned int>(clock_rate.as_int()), 1, 1)) {
+		empty_drive_(new Drive(clock_rate.as_int(), 1, 1)) {
 	// seed this class with a PLL, any PLL, so that it's safe to assume non-nullptr later
 	Time one(1);
 	set_expected_bit_length(one);
diff --git a/Storage/Disk/Drive.cpp b/Storage/Disk/Drive.cpp
index 846c15714..82e332291 100644
--- a/Storage/Disk/Drive.cpp
+++ b/Storage/Disk/Drive.cpp
@@ -18,7 +18,7 @@
 
 using namespace Storage::Disk;
 
-Drive::Drive(unsigned int input_clock_rate, int revolutions_per_minute, int number_of_heads):
+Drive::Drive(int input_clock_rate, int revolutions_per_minute, int number_of_heads):
 	Storage::TimedEventLoop(input_clock_rate),
 	rotational_multiplier_(60.0f / float(revolutions_per_minute)),
 	available_heads_(number_of_heads) {
@@ -35,7 +35,7 @@ Drive::Drive(unsigned int input_clock_rate, int revolutions_per_minute, int numb
 	}
 }
 
-Drive::Drive(unsigned int input_clock_rate, int number_of_heads) : Drive(input_clock_rate, 300, number_of_heads) {}
+Drive::Drive(int input_clock_rate, int number_of_heads) : Drive(input_clock_rate, 300, number_of_heads) {}
 
 void Drive::set_rotation_speed(float revolutions_per_minute) {
 	// TODO: probably I should look into
@@ -166,7 +166,7 @@ void Drive::set_event_delegate(Storage::Disk::Drive::EventDelegate *delegate) {
 }
 
 void Drive::advance(const Cycles cycles) {
-	cycles_since_index_hole_ += static_cast<unsigned int>(cycles.as_int());
+	cycles_since_index_hole_ += cycles.as_int();
 	if(event_delegate_) event_delegate_->advance(cycles);
 }
 
diff --git a/Storage/Disk/Drive.hpp b/Storage/Disk/Drive.hpp
index 62baeb042..2c0eb0f57 100644
--- a/Storage/Disk/Drive.hpp
+++ b/Storage/Disk/Drive.hpp
@@ -24,8 +24,8 @@ namespace Disk {
 
 class Drive: public ClockingHint::Source, public TimedEventLoop {
 	public:
-		Drive(unsigned int input_clock_rate, int revolutions_per_minute, int number_of_heads);
-		Drive(unsigned int input_clock_rate, int number_of_heads);
+		Drive(int input_clock_rate, int revolutions_per_minute, int number_of_heads);
+		Drive(int input_clock_rate, int number_of_heads);
 		~Drive();
 
 		/*!
diff --git a/Storage/Tape/Tape.cpp b/Storage/Tape/Tape.cpp
index 72ece532d..a3d8c2f27 100644
--- a/Storage/Tape/Tape.cpp
+++ b/Storage/Tape/Tape.cpp
@@ -13,7 +13,7 @@ using namespace Storage::Tape;
 
 // MARK: - Lifecycle
 
-TapePlayer::TapePlayer(unsigned int input_clock_rate) :
+TapePlayer::TapePlayer(int input_clock_rate) :
 	TimedEventLoop(input_clock_rate)
 {}
 
@@ -115,7 +115,7 @@ void TapePlayer::process_next_event() {
 
 // MARK: - Binary Player
 
-BinaryTapePlayer::BinaryTapePlayer(unsigned int input_clock_rate) :
+BinaryTapePlayer::BinaryTapePlayer(int input_clock_rate) :
 	TapePlayer(input_clock_rate)
 {}
 
diff --git a/Storage/Tape/Tape.hpp b/Storage/Tape/Tape.hpp
index 329da3e3b..47b7bbd1d 100644
--- a/Storage/Tape/Tape.hpp
+++ b/Storage/Tape/Tape.hpp
@@ -97,7 +97,7 @@ class Tape {
 */
 class TapePlayer: public TimedEventLoop, public ClockingHint::Source {
 	public:
-		TapePlayer(unsigned int input_clock_rate);
+		TapePlayer(int input_clock_rate);
 
 		void set_tape(std::shared_ptr<Storage::Tape::Tape> tape);
 		bool has_tape();
@@ -130,7 +130,7 @@ class TapePlayer: public TimedEventLoop, public ClockingHint::Source {
 */
 class BinaryTapePlayer: public TapePlayer {
 	public:
-		BinaryTapePlayer(unsigned int input_clock_rate);
+		BinaryTapePlayer(int input_clock_rate);
 		void set_motor_control(bool enabled);
 		bool get_motor_control() const;
 
diff --git a/Storage/TimedEventLoop.cpp b/Storage/TimedEventLoop.cpp
index 5541bf935..92f005406 100644
--- a/Storage/TimedEventLoop.cpp
+++ b/Storage/TimedEventLoop.cpp
@@ -15,7 +15,7 @@
 
 using namespace Storage;
 
-TimedEventLoop::TimedEventLoop(unsigned int input_clock_rate) :
+TimedEventLoop::TimedEventLoop(int input_clock_rate) :
 	input_clock_rate_(input_clock_rate) {}
 
 void TimedEventLoop::run_for(const Cycles cycles) {
@@ -46,11 +46,11 @@ void TimedEventLoop::run_for(const Cycles cycles) {
 	assert(cycles_until_event_ > 0);
 }
 
-unsigned int TimedEventLoop::get_cycles_until_next_event() {
-	return static_cast<unsigned int>(std::max(cycles_until_event_, 0));
+int TimedEventLoop::get_cycles_until_next_event() {
+	return std::max(cycles_until_event_, 0);
 }
 
-unsigned int TimedEventLoop::get_input_clock_rate() {
+int TimedEventLoop::get_input_clock_rate() {
 	return input_clock_rate_;
 }
 
diff --git a/Storage/TimedEventLoop.hpp b/Storage/TimedEventLoop.hpp
index c3bd7ec75..367f2dc43 100644
--- a/Storage/TimedEventLoop.hpp
+++ b/Storage/TimedEventLoop.hpp
@@ -42,7 +42,7 @@ namespace Storage {
 			/*!
 				Constructs a timed event loop that will be clocked at @c input_clock_rate.
 			*/
-			TimedEventLoop(unsigned int input_clock_rate);
+			TimedEventLoop(int input_clock_rate);
 
 			/*!
 				Advances the event loop by @c number_of_cycles cycles.
@@ -52,12 +52,12 @@ namespace Storage {
 			/*!
 				@returns the number of whole cycles remaining until the next event is triggered.
 			*/
-			unsigned int get_cycles_until_next_event();
+			int get_cycles_until_next_event();
 
 			/*!
 				@returns the input clock rate.
 			*/
-			unsigned int get_input_clock_rate();
+			int get_input_clock_rate();
 
 		protected:
 			/*!
@@ -101,7 +101,7 @@ namespace Storage {
 			Time get_time_into_next_event();
 
 		private:
-			unsigned int input_clock_rate_ = 0;
+			int input_clock_rate_ = 0;
 			int cycles_until_event_ = 0;
 			float subcycles_until_event_ = 0.0;
 	};