From 072ce6b9fc95e67d6961076cdf6cb515f27c8f44 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 3 Apr 2026 22:40:57 -0400
Subject: [PATCH 01/23] Avoid test repetition.

---
 Storage/Tape/Tape.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Storage/Tape/Tape.cpp b/Storage/Tape/Tape.cpp
index bde3d0331..7ab67f684 100644
--- a/Storage/Tape/Tape.cpp
+++ b/Storage/Tape/Tape.cpp
@@ -171,7 +171,7 @@ void BinaryTapePlayer::set_motor_control(const bool enabled) {
 void BinaryTapePlayer::update_observer() {
 	const bool lit = motor_is_running_ && !is_at_end();
 	if(observer_ && lit != observer_lit_) {
-		observer_->set_led_status(LEDName, motor_is_running_ && !is_at_end());
+		observer_->set_led_status(LEDName, lit);
 		observer_lit_ = lit;
 	}
 }

From cef90320b8c58efa56f8b0c05fc0076a8308c8f9 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 3 Apr 2026 22:44:24 -0400
Subject: [PATCH 02/23] Make minor style adjustments.

---
 Storage/Tape/Tape.cpp | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/Storage/Tape/Tape.cpp b/Storage/Tape/Tape.cpp
index 7ab67f684..a8a683dc8 100644
--- a/Storage/Tape/Tape.cpp
+++ b/Storage/Tape/Tape.cpp
@@ -20,7 +20,7 @@ TapeSerialiser::TapeSerialiser(std::unique_ptr<FormatSerialiser> &&serialiser) :
 
 std::unique_ptr<TapeSerialiser> Tape::serialiser(const TargetPlatform::Type platform) const {
 	auto serialiser = format_serialiser();
-	if(auto *recipient = dynamic_cast<TargetPlatform::Recipient *>(serialiser.get())) {
+	if(auto recipient = dynamic_cast<TargetPlatform::Recipient *>(serialiser.get())) {
 		recipient->set_target_platforms(platform);
 	}
 
@@ -57,7 +57,7 @@ void TapeSerialiser::reset() {
 
 Pulse TapeSerialiser::next_pulse() {
 	pulse_ = serialiser_->next_pulse();
-	offset_++;
+	++offset_;
 	return pulse_;
 }
 
@@ -84,7 +84,7 @@ ClockingHint::Preference TapePlayer::preferred_clocking() const {
 	return (!tape_ || serialiser_->is_at_end()) ? ClockingHint::Preference::None : ClockingHint::Preference::JustInTime;
 }
 
-void TapePlayer::set_tape(std::shared_ptr<Storage::Tape::Tape> tape, TargetPlatform::Type platform) {
+void TapePlayer::set_tape(std::shared_ptr<Storage::Tape::Tape> tape, const TargetPlatform::Type platform) {
 	tape_ = tape;
 	serialiser_ = tape->serialiser(platform);
 
@@ -106,7 +106,6 @@ bool TapePlayer::has_tape() const {
 }
 
 void TapePlayer::next_pulse() {
-	// get the new pulse
 	if(tape_) {
 		current_pulse_ = serialiser_->next_pulse();
 		if(serialiser_->is_at_end()) update_clocking_observer();

From e7a40fbf720dcc1affb9ef30ab5c18e69db5a160 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 3 Apr 2026 23:05:01 -0400
Subject: [PATCH 03/23] Correct typo; add CD90-640 ROM.

---
 Machines/Thomson/MO5/MO5.cpp      |  4 ++--
 Machines/Utility/ROMCatalogue.cpp | 13 +++++++++++--
 Machines/Utility/ROMCatalogue.hpp |  6 ++++--
 ROMImages/Thomson/readme.txt      |  3 ++-
 4 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/Machines/Thomson/MO5/MO5.cpp b/Machines/Thomson/MO5/MO5.cpp
index e53b9282c..0cf67ab93 100644
--- a/Machines/Thomson/MO5/MO5.cpp
+++ b/Machines/Thomson/MO5/MO5.cpp
@@ -64,13 +64,13 @@ struct ConcreteMachine:
 		speaker_.set_input_rate(ClockRate);
 		construct_joysticks();
 
-		const auto request = ROM::Request(ROM::Name::ThomasonMO5v11);
+		const auto request = ROM::Request(ROM::Name::ThomsonMO5v11);
 		auto roms = rom_fetcher(request);
 		if(!request.validate(roms)) {
 			throw ROMMachine::Error::MissingROMs;
 		}
 
-		const auto &rom = roms.find(ROM::Name::ThomasonMO5v11)->second;
+		const auto &rom = roms.find(ROM::Name::ThomsonMO5v11)->second;
 		std::copy_n(rom.begin(), rom.size(), rom_.begin());
 
 		Memory::Fuzz(ram_);
diff --git a/Machines/Utility/ROMCatalogue.cpp b/Machines/Utility/ROMCatalogue.cpp
index 251ff5912..81236b015 100644
--- a/Machines/Utility/ROMCatalogue.cpp
+++ b/Machines/Utility/ROMCatalogue.cpp
@@ -1021,7 +1021,7 @@ const std::vector<Description> &Description::all_roms() {
 	// Thomson MO and TO machines.
 	//
 		{
-			ThomasonMO5v1,
+			ThomsonMO5v1,
 			"Thomson",
 			"the Thomson MO5 ROM, version 1",
 			"mo5-v1.0.rom",
@@ -1029,7 +1029,7 @@ const std::vector<Description> &Description::all_roms() {
 			0xf62d2f3au
 		},
 		{
-			ThomasonMO5v11,
+			ThomsonMO5v11,
 			"Thomson",
 			"the Thomson MO5 ROM, version 1.1",
 			"mo5-v1.1.rom",
@@ -1037,6 +1037,15 @@ const std::vector<Description> &Description::all_roms() {
 			0x237c60bfu
 		},
 
+		{
+			ThomsonCD90_640,
+			"Thomson",
+			"the Thomson CD90-640 ROM",
+			"cd90-640.rom",
+			0x7c0,
+			0x5114c0a5u
+		},
+
 	//
 	// Vic-20.
 	//
diff --git a/Machines/Utility/ROMCatalogue.hpp b/Machines/Utility/ROMCatalogue.hpp
index bcb9a6fb8..72b19229e 100644
--- a/Machines/Utility/ROMCatalogue.hpp
+++ b/Machines/Utility/ROMCatalogue.hpp
@@ -176,8 +176,10 @@ enum Name {
 	SinclairQLJS,
 
 	// Thomson MO and TO machines.
-	ThomasonMO5v1,
-	ThomasonMO5v11,
+	ThomsonMO5v1,
+	ThomsonMO5v11,
+
+	ThomsonCD90_640,
 
 	// Vic-20.
 	Vic20BASIC,
diff --git a/ROMImages/Thomson/readme.txt b/ROMImages/Thomson/readme.txt
index a713f4db2..47494f48c 100644
--- a/ROMImages/Thomson/readme.txt
+++ b/ROMImages/Thomson/readme.txt
@@ -3,4 +3,5 @@ ROM files would ordinarily go here; the copyright status of these is uncertain s
 Expected files:
 
 mo5-v1.0.rom — the MO5 ROM, version 1;
-mo5-v1.1.rom — the MO5 ROM, version 1.1.
+mo5-v1.1.rom — the MO5 ROM, version 1.1;
+cd90-640.rom — the CD90-640 floppy disk drive ROM.

From aabb056af44dda3f797d35e074e7043b9605454d Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Fri, 3 Apr 2026 23:20:16 -0400
Subject: [PATCH 04/23] Start sketching out floppy support.

The ROM accesses a7d0, a7d1, a7d3 and a7d8.
---
 Machines/Thomson/MO5/MO5.cpp | 41 +++++++++++++++++++++++-------------
 1 file changed, 26 insertions(+), 15 deletions(-)

diff --git a/Machines/Thomson/MO5/MO5.cpp b/Machines/Thomson/MO5/MO5.cpp
index 0cf67ab93..1ac68caa8 100644
--- a/Machines/Thomson/MO5/MO5.cpp
+++ b/Machines/Thomson/MO5/MO5.cpp
@@ -36,6 +36,7 @@ using Log = Log::Logger<Log::Source::MO5>;
 static constexpr int ClockRate = 1'000'000;
 static constexpr uint8_t MusicExpansionMask = 63;
 
+template <bool has_floppy>
 struct ConcreteMachine:
 	public Activity::Source,
 	public Configurable::Device,
@@ -64,7 +65,11 @@ struct ConcreteMachine:
 		speaker_.set_input_rate(ClockRate);
 		construct_joysticks();
 
-		const auto request = ROM::Request(ROM::Name::ThomsonMO5v11);
+		auto request = ROM::Request(ROM::Name::ThomsonMO5v11);
+		if(has_floppy) {
+			request = request && ROM::Request(ROM::Name::ThomsonCD90_640);
+		}
+
 		auto roms = rom_fetcher(request);
 		if(!request.validate(roms)) {
 			throw ROMMachine::Error::MissingROMs;
@@ -73,6 +78,11 @@ struct ConcreteMachine:
 		const auto &rom = roms.find(ROM::Name::ThomsonMO5v11)->second;
 		std::copy_n(rom.begin(), rom.size(), rom_.begin());
 
+		if(has_floppy) {
+			const auto &floppy_rom = roms.find(ROM::Name::ThomsonCD90_640)->second;
+			std::copy_n(floppy_rom.begin(), floppy_rom.size(), floppy_rom_.begin());
+		}
+
 		Memory::Fuzz(ram_);
 		system_pia_.refresh();
 
@@ -112,7 +122,7 @@ struct ConcreteMachine:
 	) {
 		static constexpr auto duration = CPU::M6809::duration<Cycles>(bus_phase);
 		if(video_ += duration) {
-			system_pia_.set<Motorola::MC6821::Control::CB1>(video_.last_valid()->irq());
+			system_pia_.template set<Motorola::MC6821::Control::CB1>(video_.last_valid()->irq());
 		}
 		time_since_audio_update_ += duration;
 		tape_player_.run_for(duration);
@@ -120,9 +130,7 @@ struct ConcreteMachine:
 			typer_->run_for(duration);
 		}
 
-		if constexpr (read_write == CPU::M6809::ReadWrite::NoData) {
-			return Cycles(0);
-		} else {
+		if constexpr (read_write != CPU::M6809::ReadWrite::NoData) {
 			if(address >= 0xa7c0 && address < 0xa800) {
 				switch(address) {
 					case 0xa7c0:	access<0xa7c0, read_write>(system_pia_, value);				break;
@@ -154,6 +162,8 @@ struct ConcreteMachine:
 				if constexpr (CPU::M6809::is_read(read_write)) {
 					if(address < 0x2000) {
 						value = start_pointer_[address];
+					} else if(address >= 0xa000 && address < 0xa7c0) {
+						value = has_floppy ? floppy_rom_[address - 0xa000] : 0xff;
 					} else if(address >= 0xc000) {
 						value = rom_[address - 0xc000];
 
@@ -174,7 +184,7 @@ struct ConcreteMachine:
 									if(!serialiser) return;
 
 									Storage::Tape::Thomson::MO::Parser parser;
-									const auto dp = m6809_.registers().reg<CPU::M6809::R8::DP>();
+									const auto dp = m6809_.registers().template reg<CPU::M6809::R8::DP>();
 									auto &polarity = ram_[size_t((dp << 8) | 0x44)];
 									auto &data =  ram_[size_t((dp << 8) | 0x45)];
 
@@ -193,7 +203,7 @@ struct ConcreteMachine:
 									if(!*bit) {
 										polarity ^= 0xff;
 									}
-									m6809_.registers().reg<CPU::M6809::R8::A>() = *bit ? 0xff : 0x00;
+									m6809_.registers().template reg<CPU::M6809::R8::A>() = *bit ? 0xff : 0x00;
 
 									// The parser reads up to the end of the bit. The ROM routine ends about two-thirds
 									// of the way through the bit. So 'rewind' the tape a little.
@@ -234,6 +244,7 @@ private:
 
 	std::array<uint8_t, 0x10000 + 0x2000> ram_;
 	std::array<uint8_t, 0x4000> rom_;
+	std::array<uint8_t, 0x7c0> floppy_rom_;
 	uint8_t *start_pointer_ = nullptr;
 
 	uint8_t *video_page(const bool pixels) {
@@ -299,11 +310,11 @@ private:
 		template <Motorola::MC6821::IRQ irq>
 		void set(const bool active) {
 			if constexpr (irq == Motorola::MC6821::IRQ::A) {
-				machine_.m6809_.set<CPU::M6809::Line::FIRQ>(active);
+				machine_.m6809_.template set<CPU::M6809::Line::FIRQ>(active);
 			}
 
 			if constexpr (irq == Motorola::MC6821::IRQ::B) {
-				machine_.m6809_.set<CPU::M6809::Line::IRQ>(active);
+				machine_.m6809_.template set<CPU::M6809::Line::IRQ>(active);
 			}
 		}
 
@@ -419,17 +430,17 @@ private:
 						apply(port_b_, index ? 0x08 : 0x04);
 
 						if(index) {
-							machine_.sound_and_game_pia_.set<Motorola::MC6821::Control::CB1>(is_active);
+							machine_.sound_and_game_pia_.template set<Motorola::MC6821::Control::CB1>(is_active);
 						} else {
-							machine_.sound_and_game_pia_.set<Motorola::MC6821::Control::CA1>(is_active);
+							machine_.sound_and_game_pia_.template set<Motorola::MC6821::Control::CA1>(is_active);
 						}
 					} else {
 						apply(port_b_, index ? 0x80 : 0x40);
 
 						if(index) {
-							machine_.sound_and_game_pia_.set<Motorola::MC6821::Control::CB2>(is_active);
+							machine_.sound_and_game_pia_.template set<Motorola::MC6821::Control::CB2>(is_active);
 						} else {
-							machine_.sound_and_game_pia_.set<Motorola::MC6821::Control::CA2>(is_active);
+							machine_.sound_and_game_pia_.template set<Motorola::MC6821::Control::CA2>(is_active);
 						}
 					}
 				break;
@@ -546,7 +557,7 @@ private:
 	}
 
 	HalfCycles get_typer_delay(const std::string &) const final {
-		if(m6809_.get<CPU::M6809::Line::PowerOnReset>()) {
+		if(m6809_.template get<CPU::M6809::Line::PowerOnReset>()) {
 			return Cycles(1'000'000);
 		} else {
 			return Cycles(0);
@@ -611,5 +622,5 @@ std::unique_ptr<Machine> Machine::ThomsonMO(
 ) {
 	using Target = Analyser::Static::Thomson::MOTarget;
 	const Target *const thomson_target = dynamic_cast<const Target *>(target);
-	return std::make_unique<ConcreteMachine>(*thomson_target, rom_fetcher);
+	return std::make_unique<ConcreteMachine<true>>(*thomson_target, rom_fetcher);
 }

From 646d638cd4764ec9e699e0d09c0d6944c3a53b21 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 09:08:06 -0400
Subject: [PATCH 05/23] Add `const`s.

---
 Machines/Acorn/Electron/Plus3.cpp | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/Machines/Acorn/Electron/Plus3.cpp b/Machines/Acorn/Electron/Plus3.cpp
index 207dd91ff..3ff02de4c 100644
--- a/Machines/Acorn/Electron/Plus3.cpp
+++ b/Machines/Acorn/Electron/Plus3.cpp
@@ -15,7 +15,7 @@ Plus3::Plus3() : WD1770(P1770) {
 	set_control_register(last_control_, 0xff);
 }
 
-void Plus3::set_disk(std::shared_ptr<Storage::Disk::Disk> disk, size_t drive) {
+void Plus3::set_disk(std::shared_ptr<Storage::Disk::Disk> disk, const size_t drive) {
 	get_drive(drive).set_disk(disk);
 }
 
@@ -31,7 +31,7 @@ const Storage::Disk::Disk *Plus3::disk(const std::string &name) {
 }
 
 
-void Plus3::set_control_register(uint8_t control) {
+void Plus3::set_control_register(const uint8_t control) {
 	//	bit 0 => enable or disable drive 1
 	//	bit 1 => enable or disable drive 2
 	//	bit 2 => side select
@@ -43,7 +43,7 @@ void Plus3::set_control_register(uint8_t control) {
 	set_control_register(control, changes);
 }
 
-void Plus3::set_control_register(uint8_t control, uint8_t changes) {
+void Plus3::set_control_register(const uint8_t control, const uint8_t changes) {
 	if(changes&3) {
 		set_drive(control&3);
 	}
@@ -57,13 +57,13 @@ void Plus3::set_control_register(uint8_t control, uint8_t changes) {
 	if(changes & 0x08) set_is_double_density(!(control & 0x08));
 }
 
-void Plus3::set_motor_on(bool on) {
+void Plus3::set_motor_on(const bool on) {
 	// TODO: this status should transfer if the selected drive changes. But the same goes for
 	// writing state, so plenty of work to do in general here.
 	get_drive().set_motor_on(on);
 }
 
-void Plus3::set_activity_observer(Activity::Observer *observer) {
+void Plus3::set_activity_observer(Activity::Observer *const observer) {
 	for_all_drives([observer] (Storage::Disk::Drive &drive, size_t index) {
 		drive.set_activity_observer(observer, "Drive " + std::to_string(index+1), true);
 	});

From 48507726c877a04c0d2bfc0f37f169c00bf1fb33 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 09:12:01 -0400
Subject: [PATCH 06/23] Assert that the CD90-640 is WD based, wire that up.

---
 Machines/Thomson/MO5/CD90-640.cpp             | 30 ++++++++++++
 Machines/Thomson/MO5/CD90-640.hpp             | 49 +++++++++++++++++++
 Machines/Thomson/MO5/MO5.cpp                  | 36 +++++++++++---
 .../Clock Signal.xcodeproj/project.pbxproj    | 10 ++++
 cmake/CLK_SOURCES.cmake                       |  1 +
 5 files changed, 119 insertions(+), 7 deletions(-)
 create mode 100644 Machines/Thomson/MO5/CD90-640.cpp
 create mode 100644 Machines/Thomson/MO5/CD90-640.hpp

diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
new file mode 100644
index 000000000..97b3c6601
--- /dev/null
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -0,0 +1,30 @@
+//
+//  CD90-640.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 04/04/2026.
+//  Copyright © 2026 Thomas Harte. All rights reserved.
+//
+
+#include "CD90-640.hpp"
+
+using namespace Thomson;
+
+CD90_640::CD90_640() : WD::WD1770(P1770) {
+	emplace_drives(2, 8000000, 300, 2);
+}
+
+uint8_t CD90_640::control() {
+	return control_;
+}
+
+void CD90_640::set_control(const uint8_t value) {
+	control_ = value;
+	// TODO: What do these bits mean?
+}
+
+void CD90_640::set_activity_observer(Activity::Observer *const observer) {
+	for_all_drives([observer] (Storage::Disk::Drive &drive, size_t index) {
+		drive.set_activity_observer(observer, "Drive " + std::to_string(index+1), true);
+	});
+}
diff --git a/Machines/Thomson/MO5/CD90-640.hpp b/Machines/Thomson/MO5/CD90-640.hpp
new file mode 100644
index 000000000..da2339c7f
--- /dev/null
+++ b/Machines/Thomson/MO5/CD90-640.hpp
@@ -0,0 +1,49 @@
+//
+//  CD90-640.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 04/04/2026.
+//  Copyright © 2026 Thomas Harte. All rights reserved.
+//
+
+#pragma once
+
+#include "Components/1770/1770.hpp"
+
+#include <string>
+
+namespace Thomson {
+
+class CD90_640 final : public WD::WD1770 {
+public:
+	CD90_640();
+
+	void set_disk(std::shared_ptr<Storage::Disk::Disk>, size_t drive);
+	const Storage::Disk::Disk *disk(const std::string &);
+	void set_activity_observer(Activity::Observer *);
+
+	template <int address>
+	uint8_t read() {
+		if(address & 8) {
+			return control();
+		} else {
+			return WD::WD1770::read(address);
+		}
+	}
+
+	template <int address>
+	void write(const uint8_t value) {
+		if(address & 8) {
+			set_control(value);
+		} else {
+			WD::WD1770::write(address, value);
+		}
+	}
+
+private:
+	uint8_t control();
+	void set_control(uint8_t);
+	uint8_t control_;
+};
+
+}
diff --git a/Machines/Thomson/MO5/MO5.cpp b/Machines/Thomson/MO5/MO5.cpp
index 1ac68caa8..3d9175b69 100644
--- a/Machines/Thomson/MO5/MO5.cpp
+++ b/Machines/Thomson/MO5/MO5.cpp
@@ -8,6 +8,7 @@
 
 #include "MO5.hpp"
 
+#include "CD90-640.hpp"
 #include "Video.hpp"
 
 #include "Activity/Source.hpp"
@@ -129,9 +130,21 @@ struct ConcreteMachine:
 		if(typer_) {
 			typer_->run_for(duration);
 		}
+		if(has_floppy) {
+			fdc_.run_for(duration * 8);	// My WD1770 has a nominal clock of 8Mhz.
+		}
 
 		if constexpr (read_write != CPU::M6809::ReadWrite::NoData) {
 			if(address >= 0xa7c0 && address < 0xa800) {
+				const auto unmapped = [&] {
+					if constexpr (CPU::M6809::is_read(read_write)) {
+						value = 0xff;
+						Log::info().append("Unhandled read at %04x", +address);
+					} else {
+						Log::info().append("Unhandled write: %02x -> %04x", +value, +address);
+					}
+				};
+
 				switch(address) {
 					case 0xa7c0:	access<0xa7c0, read_write>(system_pia_, value);				break;
 					case 0xa7c1:	access<0xa7c1, read_write>(system_pia_, value);				break;
@@ -149,13 +162,14 @@ struct ConcreteMachine:
 						}
 					break;
 
+					case 0xa7d0:	if(has_floppy) access<0xa7d0, read_write>(fdc_, value); else unmapped();	break;
+					case 0xa7d1:	if(has_floppy) access<0xa7d1, read_write>(fdc_, value);	else unmapped();	break;
+					case 0xa7d2:	if(has_floppy) access<0xa7d2, read_write>(fdc_, value);	else unmapped();	break;
+					case 0xa7d3:	if(has_floppy) access<0xa7d3, read_write>(fdc_, value);	else unmapped();	break;
+					case 0xa7d8:	if(has_floppy) access<0xa7d8, read_write>(fdc_, value);	else unmapped();	break;
+
 					default:
-						if constexpr (CPU::M6809::is_read(read_write)) {
-							value = 0xff;
-							Log::info().append("Unhandled read at %04x", +address);
-						} else {
-							Log::info().append("Unhandled write: %02x -> %04x", +value, +address);
-						}
+						unmapped();
 					break;
 				}
 			} else {
@@ -480,9 +494,14 @@ private:
 	Motorola::MC6821::MC6821<SoundAndGamePIAPortHandler, 2, 1> sound_and_game_pia_;
 
 	JustInTimeActor<Video, Cycles> video_;
+
+	// MARK: - Tape and disk.
+
 	Storage::Tape::BinaryTapePlayer tape_player_;
 	bool allow_fast_tape_hack_ = true;
 
+	Thomson::CD90_640 fdc_;
+
 	// MARK: - AudioProducer.
 
 	Concurrency::AsyncTaskQueue<false> audio_queue_;
@@ -584,8 +603,11 @@ private:
 
 	// MARK: - Activity Source.
 
-	void set_activity_observer(Activity::Observer *observer) override {
+	void set_activity_observer(Activity::Observer *const observer) override {
 		tape_player_.set_activity_observer(observer);
+		if(has_floppy) {
+			fdc_.set_activity_observer(observer);
+		}
 	}
 
 	// MARK: - Configuration options.
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index f385ced30..642255bc5 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -133,6 +133,9 @@
 		4B055AEE1FAE9BBF0060FFFF /* Keyboard.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B86E2591F8C628F006FAA45 /* Keyboard.cpp */; };
 		4B055AEF1FAE9BF00060FFFF /* Typer.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B2B3A471F9B8FA70062DABF /* Typer.cpp */; };
 		4B055AF21FAE9C1C0060FFFF /* OpenGL.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = 4B055AF01FAE9C080060FFFF /* OpenGL.framework */; };
+		4B062D1A2F81427D00472717 /* CD90-640.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D192F81427D00472717 /* CD90-640.cpp */; };
+		4B062D1B2F81427D00472717 /* CD90-640.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D192F81427D00472717 /* CD90-640.cpp */; };
+		4B062D1C2F81427D00472717 /* CD90-640.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D192F81427D00472717 /* CD90-640.cpp */; };
 		4B06AACB2C645ED00034D014 /* AY38910.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B4A762E1DB1A3FA007AAE2E /* AY38910.cpp */; };
 		4B06AACC2C645EDF0034D014 /* Archimedes.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BB505842B9634F30031C43C /* Archimedes.cpp */; };
 		4B06AACD2C645EE50034D014 /* Enterprise.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B051CA12676F52200CA44E8 /* Enterprise.cpp */; };
@@ -1405,6 +1408,8 @@
 		4B055ABE1FAE98000060FFFF /* MachineForTarget.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = MachineForTarget.cpp; sourceTree = "<group>"; };
 		4B055ABF1FAE98000060FFFF /* MachineForTarget.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = MachineForTarget.hpp; sourceTree = "<group>"; };
 		4B055AF01FAE9C080060FFFF /* OpenGL.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = OpenGL.framework; path = System/Library/Frameworks/OpenGL.framework; sourceTree = SDKROOT; };
+		4B062D182F81427D00472717 /* CD90-640.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = "CD90-640.hpp"; sourceTree = "<group>"; };
+		4B062D192F81427D00472717 /* CD90-640.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = "CD90-640.cpp"; sourceTree = "<group>"; };
 		4B08A2741EE35D56008B7065 /* Z80InterruptTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Z80InterruptTests.swift; sourceTree = "<group>"; };
 		4B08A2761EE39306008B7065 /* TestMachine.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = TestMachine.h; sourceTree = "<group>"; };
 		4B08A2771EE39306008B7065 /* TestMachine.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = TestMachine.mm; sourceTree = "<group>"; };
@@ -5350,8 +5355,10 @@
 		4BCA6FAA2F589C0C000B9537 /* MO5 */ = {
 			isa = PBXGroup;
 			children = (
+				4B062D192F81427D00472717 /* CD90-640.cpp */,
 				4BCA6FA92F589C0C000B9537 /* MO5.cpp */,
 				4BBC7DC02F6D94E500E71794 /* Video.cpp */,
+				4B062D182F81427D00472717 /* CD90-640.hpp */,
 				4BBC7DC52F70BAFB00E71794 /* Keyboard.hpp */,
 				4BCA6FA82F589C0C000B9537 /* MO5.hpp */,
 				4BBC7DBF2F6D94E500E71794 /* Video.hpp */,
@@ -6538,6 +6545,7 @@
 				4B0ACC03237756F6008902D0 /* Line.cpp in Sources */,
 				4B055AB11FAE86070060FFFF /* Tape.cpp in Sources */,
 				4B96F7CF263E33B10092AEE1 /* DSK.cpp in Sources */,
+				4B062D1C2F81427D00472717 /* CD90-640.cpp in Sources */,
 				4B2B946626377C0200E7097C /* SZX.cpp in Sources */,
 				4BEDA43225B3C700000C2DBD /* Executor.cpp in Sources */,
 				4BC1317B2346DF2B00E4FF3D /* MSA.cpp in Sources */,
@@ -6865,6 +6873,7 @@
 				4B7C681627517A59001671EC /* Sprites.cpp in Sources */,
 				4B0ACC02237756ED008902D0 /* Line.cpp in Sources */,
 				4B5617402F44D533003CB7FE /* ROMCatalogue.cpp in Sources */,
+				4B062D1B2F81427D00472717 /* CD90-640.cpp in Sources */,
 				4B89453A201967B4007DE474 /* StaticAnalyser.cpp in Sources */,
 				4BB697CB1D4B6D3E00248BDF /* TimedEventLoop.cpp in Sources */,
 				4BDACBEC22FFA5D20045EF7E /* ncr5380.cpp in Sources */,
@@ -7018,6 +7027,7 @@
 				4B778F0E23A5EC4F0000D260 /* Tape.cpp in Sources */,
 				4B778F2D23A5EF190000D260 /* MFMDiskController.cpp in Sources */,
 				4B06AAD92C645F5D0034D014 /* SN76489.cpp in Sources */,
+				4B062D1A2F81427D00472717 /* CD90-640.cpp in Sources */,
 				423BDC4A2AB24699008E37B6 /* 8088Tests.mm in Sources */,
 				4B778F2723A5EEF60000D260 /* BinaryDump.cpp in Sources */,
 				4BFCA1241ECBDCB400AC40C1 /* AllRAMProcessor.cpp in Sources */,
diff --git a/cmake/CLK_SOURCES.cmake b/cmake/CLK_SOURCES.cmake
index 3d066c7d0..655b44af7 100644
--- a/cmake/CLK_SOURCES.cmake
+++ b/cmake/CLK_SOURCES.cmake
@@ -151,6 +151,7 @@ set(CLK_SOURCES
 	Machines/Sinclair/ZX8081/Video.cpp
 	Machines/Sinclair/ZX8081/ZX8081.cpp
 	Machines/Sinclair/ZXSpectrum/ZXSpectrum.cpp
+	Machines/Thomson/MO5/CD90-640.cpp
 	Machines/Thomson/MO5/MO5.cpp
 	Machines/Thomson/MO5/Video.cpp
 	Machines/Utility/MachineForTarget.cpp

From 3be3772abfe30fafdf74506052f4489612689e1f Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 09:58:00 -0400
Subject: [PATCH 07/23] Increase logging, guess that this is a 1793?

---
 Machines/Thomson/MO5/CD90-640.cpp | 4 +++-
 Machines/Thomson/MO5/CD90-640.hpp | 5 +++++
 Outputs/Log.hpp                   | 4 ++--
 3 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
index 97b3c6601..0eff1dc54 100644
--- a/Machines/Thomson/MO5/CD90-640.cpp
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -10,15 +10,17 @@
 
 using namespace Thomson;
 
-CD90_640::CD90_640() : WD::WD1770(P1770) {
+CD90_640::CD90_640() : WD::WD1770(P1793) {
 	emplace_drives(2, 8000000, 300, 2);
 }
 
 uint8_t CD90_640::control() {
+	Logger::info().append("Read control: %02x", control_);
 	return control_;
 }
 
 void CD90_640::set_control(const uint8_t value) {
+	Logger::info().append("Set control to %02x", value);
 	control_ = value;
 	// TODO: What do these bits mean?
 }
diff --git a/Machines/Thomson/MO5/CD90-640.hpp b/Machines/Thomson/MO5/CD90-640.hpp
index da2339c7f..b9701ae96 100644
--- a/Machines/Thomson/MO5/CD90-640.hpp
+++ b/Machines/Thomson/MO5/CD90-640.hpp
@@ -9,6 +9,7 @@
 #pragma once
 
 #include "Components/1770/1770.hpp"
+#include "Outputs/Log.hpp"
 
 #include <string>
 
@@ -24,6 +25,7 @@ public:
 
 	template <int address>
 	uint8_t read() {
+		Logger::info().append("Read %04x", address);
 		if(address & 8) {
 			return control();
 		} else {
@@ -33,6 +35,7 @@ public:
 
 	template <int address>
 	void write(const uint8_t value) {
+		Logger::info().append("Write %02x -> %04x", value, address);
 		if(address & 8) {
 			set_control(value);
 		} else {
@@ -44,6 +47,8 @@ private:
 	uint8_t control();
 	void set_control(uint8_t);
 	uint8_t control_;
+
+	using Logger = Log::Logger<Log::Source::Floppy>;
 };
 
 }
diff --git a/Outputs/Log.hpp b/Outputs/Log.hpp
index 293202210..f303f18d3 100644
--- a/Outputs/Log.hpp
+++ b/Outputs/Log.hpp
@@ -122,9 +122,9 @@ constexpr EnabledLevel enabled_level(const Source source) {
 //		case Source::PCPOST:
 			return EnabledLevel::None;
 
-		case Source::Floppy:
+//		case Source::Floppy:
 //		case Source::Keyboard:
-			return EnabledLevel::Errors;
+//			return EnabledLevel::Errors;
 	}
 }
 

From ec915504d123e3faa8f3223012762bd8ebafbff9 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 10:49:33 -0400
Subject: [PATCH 08/23] Attempt FD file support.

---
 Analyser/Static/StaticAnalyser.cpp            |  3 ++
 Analyser/Static/Thomson/StaticAnalyser.cpp    |  6 +++
 Analyser/Static/Thomson/Target.hpp            |  6 ++-
 Machines/Thomson/MO5/CD90-640.cpp             | 17 +++++++
 Machines/Thomson/MO5/MO5.cpp                  | 17 ++++++-
 .../Clock Signal.xcodeproj/project.pbxproj    | 14 +++++-
 OSBindings/Mac/Clock Signal/Info.plist        | 22 ++++++++
 Storage/Disk/DiskImage/Formats/FD.cpp         | 50 +++++++++++++++++++
 Storage/Disk/DiskImage/Formats/FD.hpp         | 36 +++++++++++++
 cmake/CLK_SOURCES.cmake                       |  1 +
 10 files changed, 167 insertions(+), 5 deletions(-)
 create mode 100644 Storage/Disk/DiskImage/Formats/FD.cpp
 create mode 100644 Storage/Disk/DiskImage/Formats/FD.hpp

diff --git a/Analyser/Static/StaticAnalyser.cpp b/Analyser/Static/StaticAnalyser.cpp
index 0c026a450..cf9c3233e 100644
--- a/Analyser/Static/StaticAnalyser.cpp
+++ b/Analyser/Static/StaticAnalyser.cpp
@@ -51,6 +51,7 @@
 #include "Storage/Disk/DiskImage/Formats/G64.hpp"
 #include "Storage/Disk/DiskImage/Formats/DMK.hpp"
 #include "Storage/Disk/DiskImage/Formats/FAT12.hpp"
+#include "Storage/Disk/DiskImage/Formats/FD.hpp"
 #include "Storage/Disk/DiskImage/Formats/HFE.hpp"
 #include "Storage/Disk/DiskImage/Formats/IPF.hpp"
 #include "Storage/Disk/DiskImage/Formats/IMD.hpp"
@@ -240,6 +241,8 @@ static Media GetMediaAndPlatforms(const std::string &file_name, TargetPlatform::
 	accumulator.try_standard<Disk::DiskImageHolder<Disk::FAT12>>(TargetPlatform::MSX, "dsk");
 	accumulator.try_standard<Disk::DiskImageHolder<Disk::OricMFMDSK>>(TargetPlatform::Oric, "dsk");
 
+	accumulator.try_standard<Disk::DiskImageHolder<Disk::FD>>(TargetPlatform::ThomsonMO, "fd");
+
 	accumulator.try_standard<Disk::DiskImageHolder<Disk::G64>>(TargetPlatform::Commodore8bit, "g64");
 
 	accumulator.try_standard<MassStorage::HDV>(TargetPlatform::AppleII, "hdv");
diff --git a/Analyser/Static/Thomson/StaticAnalyser.cpp b/Analyser/Static/Thomson/StaticAnalyser.cpp
index 844464a6e..a14ca0d46 100644
--- a/Analyser/Static/Thomson/StaticAnalyser.cpp
+++ b/Analyser/Static/Thomson/StaticAnalyser.cpp
@@ -45,6 +45,12 @@ Analyser::Static::TargetList Analyser::Static::Thomson::GetTargets(
 		}
 	}
 
+	// Currently: accept all floppy disks.
+	if(!media.disks.empty()) {
+		target->floppy = MOTarget::Floppy::CD90_640;
+		target->media.disks = media.disks;
+	}
+
 	if(!target->media.empty()) {
 		destination.push_back(std::move(target));
 	}
diff --git a/Analyser/Static/Thomson/Target.hpp b/Analyser/Static/Thomson/Target.hpp
index be20f6754..6cf60d7d2 100644
--- a/Analyser/Static/Thomson/Target.hpp
+++ b/Analyser/Static/Thomson/Target.hpp
@@ -18,12 +18,16 @@ namespace Analyser::Static::Thomson {
 struct MOTarget: public Analyser::Static::Target, public Reflection::StructImpl<MOTarget> {
 	std::string loading_command;
 
+	ReflectableEnum(Floppy, None, CD90_640);
+	Floppy floppy = Floppy::None;
+
 	MOTarget() : Analyser::Static::Target(Machine::ThomsonMO) {}
 
 private:
 	friend Reflection::StructImpl<MOTarget>;
 	void declare_fields() {
-		// None yet.
+		AnnounceEnum(Floppy);
+		DeclareField(floppy);
 	}
 };
 
diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
index 0eff1dc54..094389034 100644
--- a/Machines/Thomson/MO5/CD90-640.cpp
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -30,3 +30,20 @@ void CD90_640::set_activity_observer(Activity::Observer *const observer) {
 		drive.set_activity_observer(observer, "Drive " + std::to_string(index+1), true);
 	});
 }
+
+// TODO: the code below is fairly boilerplate; can it be factored out?
+
+void CD90_640::set_disk(std::shared_ptr<Storage::Disk::Disk> disk, const size_t drive) {
+	get_drive(drive).set_disk(disk);
+}
+
+const Storage::Disk::Disk *CD90_640::disk(const std::string &name) {
+	const Storage::Disk::Disk *result = nullptr;
+	for_all_drives( [&](Storage::Disk::Drive &drive, size_t) {
+		const auto disk = drive.disk();
+		if(disk && disk->represents(name)) {
+			result = disk;
+		}
+	});
+	return result;
+}
diff --git a/Machines/Thomson/MO5/MO5.cpp b/Machines/Thomson/MO5/MO5.cpp
index 3d9175b69..b249907f3 100644
--- a/Machines/Thomson/MO5/MO5.cpp
+++ b/Machines/Thomson/MO5/MO5.cpp
@@ -594,7 +594,16 @@ private:
 			tape_player_.set_tape(media.tapes.front(), TargetPlatform::ThomsonMO);
 		}
 
-		return !media.tapes.empty();
+		if(!has_floppy) {
+			return !media.tapes.empty();
+		}
+
+		size_t index = 0;
+		for(auto &disk: media.disks) {
+			fdc_.set_disk(disk, index);
+			++ index;
+		}
+		return !media.tapes.empty() || !media.disks.empty();
 	}
 
 	ChangeEffect effect_for_file_did_change(const std::string &) const override {
@@ -644,5 +653,9 @@ std::unique_ptr<Machine> Machine::ThomsonMO(
 ) {
 	using Target = Analyser::Static::Thomson::MOTarget;
 	const Target *const thomson_target = dynamic_cast<const Target *>(target);
-	return std::make_unique<ConcreteMachine<true>>(*thomson_target, rom_fetcher);
+
+	switch(thomson_target->floppy) {
+		case Target::Floppy::None:		return std::make_unique<ConcreteMachine<false>>(*thomson_target, rom_fetcher);
+		case Target::Floppy::CD90_640:	return std::make_unique<ConcreteMachine<true>>(*thomson_target, rom_fetcher);
+	}
 }
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index 642255bc5..4da662ed5 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -136,6 +136,9 @@
 		4B062D1A2F81427D00472717 /* CD90-640.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D192F81427D00472717 /* CD90-640.cpp */; };
 		4B062D1B2F81427D00472717 /* CD90-640.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D192F81427D00472717 /* CD90-640.cpp */; };
 		4B062D1C2F81427D00472717 /* CD90-640.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D192F81427D00472717 /* CD90-640.cpp */; };
+		4B062D1F2F81589C00472717 /* FD.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D1E2F81589C00472717 /* FD.cpp */; };
+		4B062D202F81589C00472717 /* FD.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D1E2F81589C00472717 /* FD.cpp */; };
+		4B062D212F81589C00472717 /* FD.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B062D1E2F81589C00472717 /* FD.cpp */; };
 		4B06AACB2C645ED00034D014 /* AY38910.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B4A762E1DB1A3FA007AAE2E /* AY38910.cpp */; };
 		4B06AACC2C645EDF0034D014 /* Archimedes.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BB505842B9634F30031C43C /* Archimedes.cpp */; };
 		4B06AACD2C645EE50034D014 /* Enterprise.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B051CA12676F52200CA44E8 /* Enterprise.cpp */; };
@@ -1410,6 +1413,8 @@
 		4B055AF01FAE9C080060FFFF /* OpenGL.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = OpenGL.framework; path = System/Library/Frameworks/OpenGL.framework; sourceTree = SDKROOT; };
 		4B062D182F81427D00472717 /* CD90-640.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = "CD90-640.hpp"; sourceTree = "<group>"; };
 		4B062D192F81427D00472717 /* CD90-640.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = "CD90-640.cpp"; sourceTree = "<group>"; };
+		4B062D1D2F81589C00472717 /* FD.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = FD.hpp; sourceTree = "<group>"; };
+		4B062D1E2F81589C00472717 /* FD.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = FD.cpp; sourceTree = "<group>"; };
 		4B08A2741EE35D56008B7065 /* Z80InterruptTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Z80InterruptTests.swift; sourceTree = "<group>"; };
 		4B08A2761EE39306008B7065 /* TestMachine.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = TestMachine.h; sourceTree = "<group>"; };
 		4B08A2771EE39306008B7065 /* TestMachine.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = TestMachine.mm; sourceTree = "<group>"; };
@@ -3418,6 +3423,7 @@
 				4B4518911F75FD1B00926311 /* D64.cpp */,
 				4BAF2B4C2004580C00480230 /* DMK.cpp */,
 				4BEBFB4B2002C4BF000708CC /* FAT12.cpp */,
+				4B062D1E2F81589C00472717 /* FD.cpp */,
 				4B4518931F75FD1B00926311 /* G64.cpp */,
 				4B4518951F75FD1B00926311 /* HFE.cpp */,
 				42EB81262B23AAC300429AF4 /* IMD.cpp */,
@@ -3425,13 +3431,13 @@
 				4B7773512DE894E400933F03 /* JFD.cpp */,
 				4BB4BFAE22A42F290069048D /* MacintoshIMG.cpp */,
 				4B58601C1F806AB200AEE2E3 /* MFMSectorDump.cpp */,
+				4B5617322F42ADB8003CB7FE /* MOOF.cpp */,
 				4BC131782346DF2B00E4FF3D /* MSA.cpp */,
 				4B0F94FC208C1A1600FE41D9 /* NIB.cpp */,
 				4B4518971F75FD1B00926311 /* OricMFMDSK.cpp */,
 				423820422B1A90BE00964EFE /* PCBooter.cpp */,
 				4B4518991F75FD1B00926311 /* SSD.cpp */,
 				4B7BA03323C58B1E00B98D9E /* STX.cpp */,
-				4B5617322F42ADB8003CB7FE /* MOOF.cpp */,
 				4B6ED2EE208E2F8A0047B343 /* WOZ.cpp */,
 				4B80CD75256CA15E00176FCC /* 2MG.hpp */,
 				4B45188E1F75FD1B00926311 /* AcornADF.hpp */,
@@ -3441,6 +3447,7 @@
 				4B4518921F75FD1B00926311 /* D64.hpp */,
 				4BAF2B4D2004580C00480230 /* DMK.hpp */,
 				4BEBFB4C2002C4BF000708CC /* FAT12.hpp */,
+				4B062D1D2F81589C00472717 /* FD.hpp */,
 				4B4518941F75FD1B00926311 /* G64.hpp */,
 				4B4518961F75FD1B00926311 /* HFE.hpp */,
 				42EB81272B23AAC300429AF4 /* IMD.hpp */,
@@ -3448,13 +3455,13 @@
 				4B7773502DE894E400933F03 /* JFD.hpp */,
 				4BB4BFAF22A42F290069048D /* MacintoshIMG.hpp */,
 				4B58601D1F806AB200AEE2E3 /* MFMSectorDump.hpp */,
+				4B5617312F42ADB8003CB7FE /* MOOF.hpp */,
 				4BC131792346DF2B00E4FF3D /* MSA.hpp */,
 				4B0F94FD208C1A1600FE41D9 /* NIB.hpp */,
 				4B4518981F75FD1B00926311 /* OricMFMDSK.hpp */,
 				423820432B1A90BE00964EFE /* PCBooter.hpp */,
 				4B45189A1F75FD1B00926311 /* SSD.hpp */,
 				4B7BA03223C58B1E00B98D9E /* STX.hpp */,
-				4B5617312F42ADB8003CB7FE /* MOOF.hpp */,
 				4B6ED2EF208E2F8A0047B343 /* WOZ.hpp */,
 				4BFDD7891F7F2DB4008579B9 /* Utility */,
 			);
@@ -6596,6 +6603,7 @@
 				4B055A911FAE85B50060FFFF /* Cartridge.cpp in Sources */,
 				4B8DD39826360DDF00B3C866 /* Z80.cpp in Sources */,
 				4B894525201967B4007DE474 /* Tape.cpp in Sources */,
+				4B062D1F2F81589C00472717 /* FD.cpp in Sources */,
 				4B055AB21FAE860F0060FFFF /* CommodoreTAP.cpp in Sources */,
 				4BC6236F26F426B400F83DFE /* FAT.cpp in Sources */,
 				4B0F1BFD260300D900B85C66 /* ZXSpectrum.cpp in Sources */,
@@ -6868,6 +6876,7 @@
 				4B894528201967B4007DE474 /* Disk.cpp in Sources */,
 				4B7773542DE894E400933F03 /* JFD.cpp in Sources */,
 				4B2E86CF25D8D8C70024F1E9 /* Keyboard.cpp in Sources */,
+				4B062D202F81589C00472717 /* FD.cpp in Sources */,
 				4B4C81CA28B56CF800F84AE9 /* MacintoshVolume.cpp in Sources */,
 				4BBB70A4202011C2002FE009 /* MultiMediaTarget.cpp in Sources */,
 				4B7C681627517A59001671EC /* Sprites.cpp in Sources */,
@@ -7023,6 +7032,7 @@
 				4B778F5923A5F2D00000D260 /* Z80.cpp in Sources */,
 				4BD971392BFC3D9D00C907AA /* ArchimedesStaticAnalyserTests.mm in Sources */,
 				4BBC7DC22F6D94E500E71794 /* Video.cpp in Sources */,
+				4B062D212F81589C00472717 /* FD.cpp in Sources */,
 				4B08A2751EE35D56008B7065 /* Z80InterruptTests.swift in Sources */,
 				4B778F0E23A5EC4F0000D260 /* Tape.cpp in Sources */,
 				4B778F2D23A5EF190000D260 /* MFMDiskController.cpp in Sources */,
diff --git a/OSBindings/Mac/Clock Signal/Info.plist b/OSBindings/Mac/Clock Signal/Info.plist
index 791091967..8a06b7185 100644
--- a/OSBindings/Mac/Clock Signal/Info.plist	
+++ b/OSBindings/Mac/Clock Signal/Info.plist	
@@ -875,6 +875,28 @@
 			<key>NSDocumentClass</key>
 			<string>$(PRODUCT_MODULE_NAME).MachineDocument</string>
 		</dict>
+		<dict>
+			<key>CFBundleTypeExtensions</key>
+			<array>
+				<string>fd</string>
+			</array>
+			<key>CFBundleTypeIconFile</key>
+			<string>floppy525</string>
+			<key>CFBundleTypeName</key>
+			<string>Thomson Disk Image</string>
+			<key>CFBundleTypeOSTypes</key>
+			<array>
+				<string>????</string>
+			</array>
+			<key>CFBundleTypeRole</key>
+			<string>Editor</string>
+			<key>LSHandlerRank</key>
+			<string>Owner</string>
+			<key>LSTypeIsPackage</key>
+			<false/>
+			<key>NSDocumentClass</key>
+			<string>$(PRODUCT_MODULE_NAME).MachineDocument</string>
+		</dict>
 	</array>
 	<key>CFBundleExecutable</key>
 	<string>$(EXECUTABLE_NAME)</string>
diff --git a/Storage/Disk/DiskImage/Formats/FD.cpp b/Storage/Disk/DiskImage/Formats/FD.cpp
new file mode 100644
index 000000000..85b8573a8
--- /dev/null
+++ b/Storage/Disk/DiskImage/Formats/FD.cpp
@@ -0,0 +1,50 @@
+//
+//  FD.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 04/04/2026.
+//  Copyright © 2026 Thomas Harte. All rights reserved.
+//
+
+#include "FD.hpp"
+
+namespace {
+// Per https://en.wikipedia.org/wiki/List_of_floppy_disk_formats the Thomson
+// uses 16 256-byte sectors per track.
+constexpr int tracks_per_side = 40;
+constexpr int sectors_per_track = 16;
+constexpr int sector_size = 1;
+
+constexpr int bytes_per_track = sectors_per_track * (128 << sector_size);
+constexpr int bytes_per_side = tracks_per_side * bytes_per_track;
+}
+
+using namespace Storage::Disk;
+
+FD::FD(const std::string &file_name) : MFMSectorDump(file_name) {
+	// Disk has two heads if the suffix is .dsd or if it's too large to be an SSD.
+	const bool is_double_sided = file_.stats().st_size > bytes_per_side;
+
+	if(
+		file_.stats().st_size != bytes_per_side &&
+		file_.stats().st_size != bytes_per_side * 2
+	) throw Error::InvalidFormat;
+
+	head_count_ = is_double_sided ? 2 : 1;
+
+	set_geometry(sectors_per_track, sector_size, 0, Encodings::MFM::Density::Double);
+}
+
+HeadPosition FD::maximum_head_position() const {
+	return HeadPosition(tracks_per_side);
+}
+
+int FD::head_count() const {
+	return head_count_;
+}
+
+long FD::get_file_offset_for_position(const Track::Address address) const {
+	// TODO: RESOLVE GUESS HERE.
+	// Guess is that data is interleaved.
+	return (address.position.as_int() * head_count_ + address.head) * bytes_per_track;
+}
diff --git a/Storage/Disk/DiskImage/Formats/FD.hpp b/Storage/Disk/DiskImage/Formats/FD.hpp
new file mode 100644
index 000000000..a491dc033
--- /dev/null
+++ b/Storage/Disk/DiskImage/Formats/FD.hpp
@@ -0,0 +1,36 @@
+//
+//  FD.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 04/04/2026.
+//  Copyright © 2026 Thomas Harte. All rights reserved.
+//
+
+#pragma once
+
+#include "MFMSectorDump.hpp"
+
+namespace Storage::Disk {
+
+/*!
+	Provides a @c Disk containing an FD disk image: a decoded sector dump of a Thomson disk.
+*/
+class FD: public MFMSectorDump {
+public:
+	/*!
+		Construct an @c FD containing content from the file with name @c file_name.
+
+		@throws Storage::FileHolder::Error::CantOpen if this file can't be opened.
+		@throws Error::InvalidFormat if the file doesn't appear to contain a .FD format image.
+	*/
+	FD(const std::string &file_name);
+
+	HeadPosition maximum_head_position() const final;
+	int head_count() const final;
+
+private:
+	long get_file_offset_for_position(Track::Address) const final;
+	int head_count_;
+};
+
+}
diff --git a/cmake/CLK_SOURCES.cmake b/cmake/CLK_SOURCES.cmake
index 655b44af7..6cdb58b2c 100644
--- a/cmake/CLK_SOURCES.cmake
+++ b/cmake/CLK_SOURCES.cmake
@@ -207,6 +207,7 @@ set(CLK_SOURCES
 	Storage/Disk/DiskImage/Formats/D64.cpp
 	Storage/Disk/DiskImage/Formats/DMK.cpp
 	Storage/Disk/DiskImage/Formats/FAT12.cpp
+	Storage/Disk/DiskImage/Formats/FD.cpp
 	Storage/Disk/DiskImage/Formats/G64.cpp
 	Storage/Disk/DiskImage/Formats/HFE.cpp
 	Storage/Disk/DiskImage/Formats/IMD.cpp

From 9e7d1d79e95412a2b97179d60cf4ec93e5dc6644 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 10:55:14 -0400
Subject: [PATCH 09/23] Force motor on, as initial step.

---
 Machines/Thomson/MO5/CD90-640.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
index 094389034..c24e21fbf 100644
--- a/Machines/Thomson/MO5/CD90-640.cpp
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -35,6 +35,9 @@ void CD90_640::set_activity_observer(Activity::Observer *const observer) {
 
 void CD90_640::set_disk(std::shared_ptr<Storage::Disk::Disk> disk, const size_t drive) {
 	get_drive(drive).set_disk(disk);
+
+	// TODO: what is really in charge of motor control?
+	get_drive(drive).set_motor_on(true);
 }
 
 const Storage::Disk::Disk *CD90_640::disk(const std::string &name) {

From 9ab3d52eb2188df5186dc0ffe33b0d8636ba9f2a Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 11:05:01 -0400
Subject: [PATCH 10/23] Force drive selection; verify that the index hole is
 going by.

---
 Machines/Thomson/MO5/CD90-640.cpp |  3 ++-
 Machines/Thomson/MO5/CD90-640.hpp | 15 +++++++++------
 2 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
index c24e21fbf..bc8fcdf98 100644
--- a/Machines/Thomson/MO5/CD90-640.cpp
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -36,8 +36,9 @@ void CD90_640::set_activity_observer(Activity::Observer *const observer) {
 void CD90_640::set_disk(std::shared_ptr<Storage::Disk::Disk> disk, const size_t drive) {
 	get_drive(drive).set_disk(disk);
 
-	// TODO: what is really in charge of motor control?
+	// TODO: what is really in charge of motor control and drive selection?
 	get_drive(drive).set_motor_on(true);
+	set_drive(1 << drive);
 }
 
 const Storage::Disk::Disk *CD90_640::disk(const std::string &name) {
diff --git a/Machines/Thomson/MO5/CD90-640.hpp b/Machines/Thomson/MO5/CD90-640.hpp
index b9701ae96..4af1f4b5e 100644
--- a/Machines/Thomson/MO5/CD90-640.hpp
+++ b/Machines/Thomson/MO5/CD90-640.hpp
@@ -25,12 +25,15 @@ public:
 
 	template <int address>
 	uint8_t read() {
-		Logger::info().append("Read %04x", address);
-		if(address & 8) {
-			return control();
-		} else {
-			return WD::WD1770::read(address);
-		}
+		const uint8_t result = [&] {
+			if(address & 8) {
+				return control();
+			} else {
+				return WD::WD1770::read(address);
+			}
+		} ();
+		Logger::info().append("Read %02x <- %04x", result, address);
+		return result;
 	}
 
 	template <int address>

From 04c59b3b42beb3f545403f28fd857117bb30ddf7 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 17:18:21 -0400
Subject: [PATCH 11/23] Experiment with head load for motor.

---
 Machines/Thomson/MO5/CD90-640.cpp | 9 +++++++++
 Machines/Thomson/MO5/CD90-640.hpp | 2 ++
 2 files changed, 11 insertions(+)

diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
index bc8fcdf98..39b80e873 100644
--- a/Machines/Thomson/MO5/CD90-640.cpp
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -51,3 +51,12 @@ const Storage::Disk::Disk *CD90_640::disk(const std::string &name) {
 	});
 	return result;
 }
+
+//void CD90_640::set_head_load_request(const bool head_load) {
+//	// Turn all motors on or off; if off then unload the head instantly.
+//	for_all_drives([head_load] (Storage::Disk::Drive &drive, size_t) {
+//		drive.set_motor_on(head_load);
+//	});
+//	Logger::info().append("Head load: %d", head_load);
+//	set_head_loaded(head_load);
+//}
diff --git a/Machines/Thomson/MO5/CD90-640.hpp b/Machines/Thomson/MO5/CD90-640.hpp
index 4af1f4b5e..9cf6228eb 100644
--- a/Machines/Thomson/MO5/CD90-640.hpp
+++ b/Machines/Thomson/MO5/CD90-640.hpp
@@ -52,6 +52,8 @@ private:
 	uint8_t control_;
 
 	using Logger = Log::Logger<Log::Source::Floppy>;
+
+//	void set_head_load_request(bool) override;
 };
 
 }

From 899898508231c563735914f0b466429c67dd7226 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 18:07:22 -0400
Subject: [PATCH 12/23] Add `const`s.

---
 Storage/Disk/DiskImage/Formats/AcornADF.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Storage/Disk/DiskImage/Formats/AcornADF.cpp b/Storage/Disk/DiskImage/Formats/AcornADF.cpp
index 4a3d4f4fb..65904cd6c 100644
--- a/Storage/Disk/DiskImage/Formats/AcornADF.cpp
+++ b/Storage/Disk/DiskImage/Formats/AcornADF.cpp
@@ -51,7 +51,7 @@ AcornADF::AcornADF(const std::string &file_name) : MFMSectorDump(file_name) {
 	//
 	// So then .ADF files might be track-interleaved and might not be.
 
-	const auto has_identifier = [&](long location, bool permit_hugo, bool permit_nick) -> bool {
+	const auto has_identifier = [&](const long location, const bool permit_hugo, const bool permit_nick) -> bool {
 		file_.seek(location, Whence::SET);
 
 		uint8_t bytes[4];
@@ -119,6 +119,6 @@ int AcornADF::head_count() const {
 	return head_count_;
 }
 
-long AcornADF::get_file_offset_for_position(Track::Address address) const {
+long AcornADF::get_file_offset_for_position(const Track::Address address) const {
 	return (address.position.as_int() * head_count_ + address.head) * (128 << sector_size_) * sectors_per_track_;
 }

From 43bb7e8a2807e91be44750af739d2fe2a878611f Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 18:08:50 -0400
Subject: [PATCH 13/23] Add `const`s, enforce line length.

---
 Storage/Disk/DiskImage/Formats/MFMSectorDump.cpp | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/Storage/Disk/DiskImage/Formats/MFMSectorDump.cpp b/Storage/Disk/DiskImage/Formats/MFMSectorDump.cpp
index ff31fa1a3..a653c03c0 100644
--- a/Storage/Disk/DiskImage/Formats/MFMSectorDump.cpp
+++ b/Storage/Disk/DiskImage/Formats/MFMSectorDump.cpp
@@ -14,14 +14,19 @@ using namespace Storage::Disk;
 
 MFMSectorDump::MFMSectorDump(const std::string &file_name) : file_(file_name) {}
 
-void MFMSectorDump::set_geometry(int sectors_per_track, uint8_t sector_size, uint8_t first_sector, Encodings::MFM::Density density) {
+void MFMSectorDump::set_geometry(
+	const int sectors_per_track,
+	const uint8_t sector_size,
+	const uint8_t first_sector,
+	const Encodings::MFM::Density density
+) {
 	sectors_per_track_ = sectors_per_track;
 	sector_size_ = sector_size;
 	density_ = density;
 	first_sector_ = first_sector;
 }
 
-std::unique_ptr<Track> MFMSectorDump::track_at_position(Track::Address address) const {
+std::unique_ptr<Track> MFMSectorDump::track_at_position(const Track::Address address) const {
 	if(address.head >= head_count()) return nullptr;
 	if(address.position.as_largest() >= maximum_head_position().as_largest()) return nullptr;
 

From 02c17a5b53b779a2aaf81fe410b321c4d0876a17 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 22:38:03 -0400
Subject: [PATCH 14/23] Apply what a schematic seems to imply.

---
 Components/1770/1770.cpp          |  1 +
 Machines/Thomson/MO5/CD90-640.cpp | 47 ++++++++++++++++++++-----------
 Machines/Thomson/MO5/CD90-640.hpp |  2 +-
 3 files changed, 32 insertions(+), 18 deletions(-)

diff --git a/Components/1770/1770.cpp b/Components/1770/1770.cpp
index e1be73aba..fda3469f3 100644
--- a/Components/1770/1770.cpp
+++ b/Components/1770/1770.cpp
@@ -207,6 +207,7 @@ void WD1770::posit_event(const int new_event_type) {
 			status.data_request = false;
 			status.spin_up = false;
 		});
+		// TODO: potentially should disable motors?
 	} else {
 		if(!(interesting_event_mask_ & int(new_event_type))) return;
 		interesting_event_mask_ &= ~new_event_type;
diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
index 39b80e873..34c81c8a3 100644
--- a/Machines/Thomson/MO5/CD90-640.cpp
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -10,19 +10,45 @@
 
 using namespace Thomson;
 
-CD90_640::CD90_640() : WD::WD1770(P1793) {
+CD90_640::CD90_640() : WD::WD1770(P1770) {
 	emplace_drives(2, 8000000, 300, 2);
 }
 
 uint8_t CD90_640::control() {
-	Logger::info().append("Read control: %02x", control_);
+//	Logger::info().append("Read control: %02x [?]", control_);
 	return control_;
 }
 
 void CD90_640::set_control(const uint8_t value) {
-	Logger::info().append("Set control to %02x", value);
+//	Logger::info().append("Write control: %02x", value);
 	control_ = value;
-	// TODO: What do these bits mean?
+
+	// Following along from the schematic in https://github.com/OlivierP-To8/CD90-640/ :
+	//
+	//	The 74LS123 on the second page is fed with:
+	//
+	//		D0 = external data line 7
+	//		D1 = line 0
+	//		D2 = line 1
+	//		D3 = line 2
+	//
+	//	It then routes:
+	//
+	//		Q0 = DDEN select
+	//		Q1 = side sleect
+	//		Q2, Q3 = drive selects
+	//
+	set_is_double_density(!(value & 0x80));
+	for_all_drives( [&](Storage::Disk::Drive &drive, size_t) {
+		drive.set_head(value & 1);
+	});
+	set_drive((value >> 1) & 3);
+}
+
+void CD90_640::set_motor_on(const bool motor) {
+	for_all_drives( [&](Storage::Disk::Drive &drive, size_t) {
+		drive.set_motor_on(motor);
+	});
 }
 
 void CD90_640::set_activity_observer(Activity::Observer *const observer) {
@@ -35,10 +61,6 @@ void CD90_640::set_activity_observer(Activity::Observer *const observer) {
 
 void CD90_640::set_disk(std::shared_ptr<Storage::Disk::Disk> disk, const size_t drive) {
 	get_drive(drive).set_disk(disk);
-
-	// TODO: what is really in charge of motor control and drive selection?
-	get_drive(drive).set_motor_on(true);
-	set_drive(1 << drive);
 }
 
 const Storage::Disk::Disk *CD90_640::disk(const std::string &name) {
@@ -51,12 +73,3 @@ const Storage::Disk::Disk *CD90_640::disk(const std::string &name) {
 	});
 	return result;
 }
-
-//void CD90_640::set_head_load_request(const bool head_load) {
-//	// Turn all motors on or off; if off then unload the head instantly.
-//	for_all_drives([head_load] (Storage::Disk::Drive &drive, size_t) {
-//		drive.set_motor_on(head_load);
-//	});
-//	Logger::info().append("Head load: %d", head_load);
-//	set_head_loaded(head_load);
-//}
diff --git a/Machines/Thomson/MO5/CD90-640.hpp b/Machines/Thomson/MO5/CD90-640.hpp
index 9cf6228eb..9fff33b52 100644
--- a/Machines/Thomson/MO5/CD90-640.hpp
+++ b/Machines/Thomson/MO5/CD90-640.hpp
@@ -53,7 +53,7 @@ private:
 
 	using Logger = Log::Logger<Log::Source::Floppy>;
 
-//	void set_head_load_request(bool) override;
+	void set_motor_on(bool) override;
 };
 
 }

From 7b3799b8b652f82eec5a2d89b542971b346a8145 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 23:04:33 -0400
Subject: [PATCH 15/23] Shorten index pulse time.

---
 Storage/Disk/Drive.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Storage/Disk/Drive.cpp b/Storage/Disk/Drive.cpp
index 0e8dd3d03..42ad569c9 100644
--- a/Storage/Disk/Drive.cpp
+++ b/Storage/Disk/Drive.cpp
@@ -352,8 +352,8 @@ void Drive::process_next_event() {
 		}
 		cycles_since_index_hole_ = 0;
 
-		// Begin a 2ms period of holding the index line pulse active.
-		index_pulse_remaining_ = Cycles((get_input_clock_rate() * 2) / 1000);
+		// Begin a 1ms period of holding the index line pulse active.
+		index_pulse_remaining_ = Cycles((get_input_clock_rate() * 1) / 1000);
 	}
 	if(
 		event_delegate_ &&

From 377e75f4d42a3193e9fe54bee59e377d4b34b347 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 23:04:43 -0400
Subject: [PATCH 16/23] Log more unexpected writes.

---
 Machines/Thomson/MO5/MO5.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Machines/Thomson/MO5/MO5.cpp b/Machines/Thomson/MO5/MO5.cpp
index b249907f3..b9280e2f3 100644
--- a/Machines/Thomson/MO5/MO5.cpp
+++ b/Machines/Thomson/MO5/MO5.cpp
@@ -236,7 +236,7 @@ struct ConcreteMachine:
 						start_pointer_[address] = value;
 					} else {
 						if constexpr (!CPU::M6809::is_read(read_write)) {
-							if(address >= 0xc000) {
+							if(address >= 0xa000) {
 								Log::info().append("ROM write at %04x", +address);
 							}
 						}

From 011ec055d5f91780187dfbcdedb506f049a63a41 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 23:14:24 -0400
Subject: [PATCH 17/23] Increase RPM; these are 5.25" drives.

---
 Machines/Thomson/MO5/CD90-640.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
index 34c81c8a3..3ce073f8d 100644
--- a/Machines/Thomson/MO5/CD90-640.cpp
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -11,7 +11,7 @@
 using namespace Thomson;
 
 CD90_640::CD90_640() : WD::WD1770(P1770) {
-	emplace_drives(2, 8000000, 300, 2);
+	emplace_drives(2, 8000000, 360, 2);
 }
 
 uint8_t CD90_640::control() {

From e37c9d759d2b47a8df691ead928e16e093fa4ea5 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 23:27:33 -0400
Subject: [PATCH 18/23] Empirically it seems like sectors are 1-indexed.

---
 Storage/Disk/DiskImage/Formats/FD.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Storage/Disk/DiskImage/Formats/FD.cpp b/Storage/Disk/DiskImage/Formats/FD.cpp
index 85b8573a8..96192ea37 100644
--- a/Storage/Disk/DiskImage/Formats/FD.cpp
+++ b/Storage/Disk/DiskImage/Formats/FD.cpp
@@ -32,7 +32,7 @@ FD::FD(const std::string &file_name) : MFMSectorDump(file_name) {
 
 	head_count_ = is_double_sided ? 2 : 1;
 
-	set_geometry(sectors_per_track, sector_size, 0, Encodings::MFM::Density::Double);
+	set_geometry(sectors_per_track, sector_size, 1, Encodings::MFM::Density::Double);
 }
 
 HeadPosition FD::maximum_head_position() const {

From b8b81530544aa5d9b0fc847b4d6fef7e9b56bf2e Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 23:27:55 -0400
Subject: [PATCH 19/23] Slow drives slightly.

---
 Machines/Thomson/MO5/CD90-640.cpp | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/Machines/Thomson/MO5/CD90-640.cpp b/Machines/Thomson/MO5/CD90-640.cpp
index 3ce073f8d..f3e5cc3a8 100644
--- a/Machines/Thomson/MO5/CD90-640.cpp
+++ b/Machines/Thomson/MO5/CD90-640.cpp
@@ -11,16 +11,17 @@
 using namespace Thomson;
 
 CD90_640::CD90_640() : WD::WD1770(P1770) {
-	emplace_drives(2, 8000000, 360, 2);
+	// 325 is a peculiar RPM, but seems to match a spin-up test in the disk ROM that polls the WD1770's status register
+	// index hole bit and counts time. Furthermore there are other machines with unusual RPMs. Could definitely
+	// still just imply an issue elsewhere in the emulator though.
+	emplace_drives(2, 8000000, 325, 2);
 }
 
 uint8_t CD90_640::control() {
-//	Logger::info().append("Read control: %02x [?]", control_);
-	return control_;
+	return control_;	// Possibly only b7 is loaded?
 }
 
 void CD90_640::set_control(const uint8_t value) {
-//	Logger::info().append("Write control: %02x", value);
 	control_ = value;
 
 	// Following along from the schematic in https://github.com/OlivierP-To8/CD90-640/ :

From 701af5758ce8b21a7cad04fcb22de44de9801c35 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 23:29:45 -0400
Subject: [PATCH 20/23] Updated guess: not interleaved.

---
 Storage/Disk/DiskImage/Formats/FD.cpp | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/Storage/Disk/DiskImage/Formats/FD.cpp b/Storage/Disk/DiskImage/Formats/FD.cpp
index 96192ea37..b8fd1bb31 100644
--- a/Storage/Disk/DiskImage/Formats/FD.cpp
+++ b/Storage/Disk/DiskImage/Formats/FD.cpp
@@ -44,7 +44,6 @@ int FD::head_count() const {
 }
 
 long FD::get_file_offset_for_position(const Track::Address address) const {
-	// TODO: RESOLVE GUESS HERE.
-	// Guess is that data is interleaved.
-	return (address.position.as_int() * head_count_ + address.head) * bytes_per_track;
+	// Determined empirically: disk images are not interleaved.
+	return  address.head * bytes_per_side + address.position.as_int() * bytes_per_track;
 }

From b7e10b3c3ac849d2f45d423d3e6d42af89c66d37 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 23:35:30 -0400
Subject: [PATCH 21/23] Remove space, add brackets for clarity.

---
 Storage/Disk/DiskImage/Formats/FD.cpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Storage/Disk/DiskImage/Formats/FD.cpp b/Storage/Disk/DiskImage/Formats/FD.cpp
index b8fd1bb31..93862e035 100644
--- a/Storage/Disk/DiskImage/Formats/FD.cpp
+++ b/Storage/Disk/DiskImage/Formats/FD.cpp
@@ -45,5 +45,5 @@ int FD::head_count() const {
 
 long FD::get_file_offset_for_position(const Track::Address address) const {
 	// Determined empirically: disk images are not interleaved.
-	return  address.head * bytes_per_side + address.position.as_int() * bytes_per_track;
+	return (address.head * bytes_per_side) + (address.position.as_int() * bytes_per_track);
 }

From 8ec21f058bd2eebe3157e97754107067f0bc2f96 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sat, 4 Apr 2026 23:41:20 -0400
Subject: [PATCH 22/23] Reduce logging.

---
 Outputs/Log.hpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Outputs/Log.hpp b/Outputs/Log.hpp
index f303f18d3..293202210 100644
--- a/Outputs/Log.hpp
+++ b/Outputs/Log.hpp
@@ -122,9 +122,9 @@ constexpr EnabledLevel enabled_level(const Source source) {
 //		case Source::PCPOST:
 			return EnabledLevel::None;
 
-//		case Source::Floppy:
+		case Source::Floppy:
 //		case Source::Keyboard:
-//			return EnabledLevel::Errors;
+			return EnabledLevel::Errors;
 	}
 }
 

From b3300e487fde1ea48163d32d80cc5f5cc6d99eb0 Mon Sep 17 00:00:00 2001
From: Thomas Harte <thomas.harte@gmail.com>
Date: Sun, 5 Apr 2026 10:04:01 -0400
Subject: [PATCH 23/23] Revise heads/tracks guesswork.

---
 Storage/Disk/DiskImage/Formats/FD.cpp | 30 +++++++++++++++++----------
 Storage/Disk/DiskImage/Formats/FD.hpp |  1 +
 2 files changed, 20 insertions(+), 11 deletions(-)

diff --git a/Storage/Disk/DiskImage/Formats/FD.cpp b/Storage/Disk/DiskImage/Formats/FD.cpp
index 93862e035..0bf8dfe94 100644
--- a/Storage/Disk/DiskImage/Formats/FD.cpp
+++ b/Storage/Disk/DiskImage/Formats/FD.cpp
@@ -11,32 +11,40 @@
 namespace {
 // Per https://en.wikipedia.org/wiki/List_of_floppy_disk_formats the Thomson
 // uses 16 256-byte sectors per track.
-constexpr int tracks_per_side = 40;
 constexpr int sectors_per_track = 16;
 constexpr int sector_size = 1;
 
 constexpr int bytes_per_track = sectors_per_track * (128 << sector_size);
-constexpr int bytes_per_side = tracks_per_side * bytes_per_track;
 }
 
 using namespace Storage::Disk;
 
 FD::FD(const std::string &file_name) : MFMSectorDump(file_name) {
-	// Disk has two heads if the suffix is .dsd or if it's too large to be an SSD.
-	const bool is_double_sided = file_.stats().st_size > bytes_per_side;
+	// Guess disk geometry from file size.
+	switch(file_.stats().st_size) {
+		default: throw Error::InvalidFormat;
 
-	if(
-		file_.stats().st_size != bytes_per_side &&
-		file_.stats().st_size != bytes_per_side * 2
-	) throw Error::InvalidFormat;
+		case bytes_per_track * 40:
+			head_count_ = 1;
+			track_count_ = 40;
+		break;
 
-	head_count_ = is_double_sided ? 2 : 1;
+		case bytes_per_track * 80:
+			head_count_ = 1;
+			track_count_ = 80;
+		break;
+
+		case bytes_per_track * 160:
+			head_count_ = 2;
+			track_count_ = 80;
+		break;
+	}
 
 	set_geometry(sectors_per_track, sector_size, 1, Encodings::MFM::Density::Double);
 }
 
 HeadPosition FD::maximum_head_position() const {
-	return HeadPosition(tracks_per_side);
+	return HeadPosition(track_count_);
 }
 
 int FD::head_count() const {
@@ -45,5 +53,5 @@ int FD::head_count() const {
 
 long FD::get_file_offset_for_position(const Track::Address address) const {
 	// Determined empirically: disk images are not interleaved.
-	return (address.head * bytes_per_side) + (address.position.as_int() * bytes_per_track);
+	return ((address.head * track_count_) + address.position.as_int()) * bytes_per_track;
 }
diff --git a/Storage/Disk/DiskImage/Formats/FD.hpp b/Storage/Disk/DiskImage/Formats/FD.hpp
index a491dc033..81130e55b 100644
--- a/Storage/Disk/DiskImage/Formats/FD.hpp
+++ b/Storage/Disk/DiskImage/Formats/FD.hpp
@@ -31,6 +31,7 @@ public:
 private:
 	long get_file_offset_for_position(Track::Address) const final;
 	int head_count_;
+	int track_count_;
 };
 
 }