diff --git a/Components/6522/6522.hpp b/Components/6522/6522.hpp
index ecb118d91..4ec1fde0a 100644
--- a/Components/6522/6522.hpp
+++ b/Components/6522/6522.hpp
@@ -364,7 +364,7 @@ class MOS6522IRQDelegate {
 				virtual void mos6522_did_change_interrupt_status(void *mos6522) = 0;
 		};
 
-		void set_delegate(Delegate *delegate)
+		void set_interrupt_delegate(Delegate *delegate)
 		{
 			_delegate = delegate;
 		}
diff --git a/Machines/Commodore/1540/C1540.cpp b/Machines/Commodore/1540/C1540.cpp
index 8724a2f2b..0e0e3455b 100644
--- a/Machines/Commodore/1540/C1540.cpp
+++ b/Machines/Commodore/1540/C1540.cpp
@@ -7,11 +7,14 @@
 //
 
 #include "C1540.hpp"
-#include <string.h>
+#include <string>
+#include "../../../Storage/Disk/Encodings/CommodoreGCR.hpp"
 
 using namespace Commodore::C1540;
 
-Machine::Machine()
+Machine::Machine() :
+	_shift_register(0),
+	Storage::DiskDrive(1000000, 16, 300)
 {
 	// create a serial port and a VIA to run it
 	_serialPortVIA.reset(new SerialPortVIA);
@@ -22,8 +25,12 @@ Machine::Machine()
 	_serialPortVIA->set_serial_port(_serialPort);
 
 	// set this instance as the delegate to receive interrupt requests from both VIAs
-	_serialPortVIA->set_delegate(this);
+	_serialPortVIA->set_interrupt_delegate(this);
+	_driveVIA.set_interrupt_delegate(this);
 	_driveVIA.set_delegate(this);
+
+	// set a bit rate
+	set_expected_bit_length(Storage::Encodings::CommodoreGCR::length_of_a_bit_in_time_zone(3));
 }
 
 void Machine::set_serial_bus(std::shared_ptr<::Commodore::Serial::Bus> serial_bus)
@@ -33,7 +40,11 @@ void Machine::set_serial_bus(std::shared_ptr<::Commodore::Serial::Bus> serial_bu
 
 unsigned int Machine::perform_bus_operation(CPU6502::BusOperation operation, uint16_t address, uint8_t *value)
 {
-//	if(operation == CPU6502::BusOperation::ReadOpcode && (address >= 0xF556 && address <= 0xF56D)) printf("%04x\n", address);
+//	static bool log = false;
+//	if(operation == CPU6502::BusOperation::ReadOpcode && (address == 0xF3C0)) log = true;
+//	if(operation == CPU6502::BusOperation::ReadOpcode && log) printf("%04x\n", address);
+//	if(operation == CPU6502::BusOperation::ReadOpcode) printf("%04x\n", address);
+//	if(operation == CPU6502::BusOperation::ReadOpcode && (address >= 0xF510 && address <= 0xF553)) printf("%04x\n", address);
 //	if(operation == CPU6502::BusOperation::ReadOpcode && (address == 0xE887)) printf("A: %02x\n", get_value_of_register(CPU6502::Register::A));
 
 /*	static bool log = false;
@@ -91,9 +102,11 @@ void Machine::set_rom(const uint8_t *rom)
 	memcpy(_rom, rom, sizeof(_rom));
 }
 
-void Machine::set_disk(std::shared_ptr<Storage::Disk> disk)
+void Machine::run_for_cycles(int number_of_cycles)
 {
-	_disk = disk;
+	CPU6502::Processor<Machine>::run_for_cycles(number_of_cycles);
+	if(_driveVIA.get_motor_enabled()) // TODO: motor speed up/down
+		Storage::DiskDrive::run_for_cycles(number_of_cycles);
 }
 
 #pragma mark - 6522 delegate
@@ -103,3 +116,46 @@ void Machine::mos6522_did_change_interrupt_status(void *mos6522)
 	// both VIAs are connected to the IRQ line
 	set_irq_line(_serialPortVIA->get_interrupt_line() || _driveVIA.get_interrupt_line());
 }
+
+#pragma mark - Disk drive
+
+void Machine::process_input_bit(int value, unsigned int cycles_since_index_hole)
+{
+	_shift_register = (_shift_register << 1) | value;
+	if((_shift_register & 0x3ff) == 0x3ff)
+	{
+		_driveVIA.set_sync_detected(true);
+		_bit_window_offset = -1;
+	}
+	else
+	{
+		_driveVIA.set_sync_detected(false);
+	}
+	_bit_window_offset++;
+	if(_bit_window_offset == 8)
+	{
+		_driveVIA.set_data_input((uint8_t)_shift_register);
+		_bit_window_offset = 0;
+		if(_driveVIA.get_should_set_overflow())
+		{
+			set_overflow_line(true);
+		}
+	}
+	else
+		set_overflow_line(false);
+}
+
+// the 1540 does not recognise index holes
+void Machine::process_index_hole()	{}
+
+#pragma mak - Drive VIA delegate
+
+void Machine::drive_via_did_step_head(void *driveVIA, int direction)
+{
+	step(direction);
+}
+
+void Machine::drive_via_did_set_data_density(void *driveVIA, int density)
+{
+	set_expected_bit_length(Storage::Encodings::CommodoreGCR::length_of_a_bit_in_time_zone((unsigned int)density));
+}
diff --git a/Machines/Commodore/1540/C1540.hpp b/Machines/Commodore/1540/C1540.hpp
index 1935b1e5d..f052ea7bc 100644
--- a/Machines/Commodore/1540/C1540.hpp
+++ b/Machines/Commodore/1540/C1540.hpp
@@ -15,6 +15,7 @@
 #include "../SerialBus.hpp"
 
 #include "../../../Storage/Disk/Disk.hpp"
+#include "../../../Storage/Disk/DiskDrive.hpp"
 
 namespace Commodore {
 namespace C1540 {
@@ -100,12 +101,12 @@ class SerialPortVIA: public MOS::MOS6522<SerialPortVIA>, public MOS::MOS6522IRQD
 	An implementation of the drive VIA in a Commodore 1540 — the VIA that is used to interface with the disk.
 
 	It is wired up such that Port B contains:
-		Bits 0/1:	head step direction (TODO)
-		Bit 2:		motor control (TODO)
+		Bits 0/1:	head step direction
+		Bit 2:		motor control
 		Bit 3:		LED control (TODO)
 		Bit 4:		write protect photocell status (TODO)
-		Bits 5/6:	write density (TODO)
-		Bit 7:		0 if sync marks are currently being detected, 1 otherwise;
+		Bits 5/6:	read/write density
+		Bit 7:		0 if sync marks are currently being detected, 1 otherwise.
 
 	... and Port A contains the byte most recently read from the disk or the byte next to write to the disk, depending on data direction.
 
@@ -114,28 +115,80 @@ class SerialPortVIA: public MOS::MOS6522<SerialPortVIA>, public MOS::MOS6522IRQD
 */
 class DriveVIA: public MOS::MOS6522<DriveVIA>, public MOS::MOS6522IRQDelegate {
 	public:
+		class Delegate {
+			public:
+				virtual void drive_via_did_step_head(void *driveVIA, int direction) = 0;
+				virtual void drive_via_did_set_data_density(void *driveVIA, int density) = 0;
+		};
+		void set_delegate(Delegate *delegate)
+		{
+			_delegate = delegate;
+		}
+
 		using MOS6522IRQDelegate::set_interrupt_status;
 
+		// write protect tab uncovered
+		DriveVIA() : _port_b(0xff), _port_a(0xff), _delegate(nullptr) {}
+
 		uint8_t get_port_input(Port port) {
-			if(port)
-			{
-				return 0xff;	// imply not sync, write protect tab uncovered
+			return port ? _port_b : _port_a;
+		}
+
+		void set_sync_detected(bool sync_detected) {
+			_port_b = (_port_b & 0x7f) | (sync_detected ? 0x00 : 0x80);
+		}
+
+		void set_data_input(uint8_t value) {
+			_port_a = value;
+		}
+
+		bool get_should_set_overflow() {
+			return _should_set_overflow;
+		}
+
+		bool get_motor_enabled() {
+			return _drive_motor;
+		}
+
+		void set_control_line_output(Port port, Line line, bool value) {
+			if(port == Port::A && line == Line::Two) {
+				_should_set_overflow = value;
 			}
-			return 0xff;
 		}
 
 		void set_port_output(Port port, uint8_t value, uint8_t direction_mask) {
 			if(port)
 			{
-//				if(value&4)
-//				{
-//					printf("Head step: %d\n", value&3);
-//					printf("Motor: %s\n", value&4 ? "On" : "Off");
+				// record drive motor state
+				_drive_motor = !!(value&4);
+
+				// check for a head step
+				int step_difference = ((value&3) - (_previous_port_b_output&3))&3;
+				if(step_difference)
+				{
+					if(_delegate) _delegate->drive_via_did_step_head(this, (step_difference == 1) ? 1 : -1);
+				}
+
+				// check for a change in density
+				int density_difference = (_previous_port_b_output^value) & (3 << 5);
+				if(density_difference && _delegate)
+				{
+					_delegate->drive_via_did_set_data_density(this, (value >> 5)&3);
+				}
+
+				// TODO: something with the drive LED
 //					printf("LED: %s\n", value&8 ? "On" : "Off");
-//					printf("Density: %d\n", (value >> 5)&3);
-//				}
+
+				_previous_port_b_output = value;
 			}
 		}
+
+	private:
+		uint8_t _port_b, _port_a;
+		bool _should_set_overflow;
+		bool _drive_motor;
+		uint8_t _previous_port_b_output;
+		Delegate *_delegate;
 };
 
 /*!
@@ -161,7 +214,9 @@ class SerialPort : public ::Commodore::Serial::Port {
 */
 class Machine:
 	public CPU6502::Processor<Machine>,
-	public MOS::MOS6522IRQDelegate::Delegate {
+	public MOS::MOS6522IRQDelegate::Delegate,
+	public DriveVIA::Delegate,
+	public Storage::DiskDrive {
 
 	public:
 		Machine();
@@ -176,10 +231,7 @@ class Machine:
 		*/
 		void set_serial_bus(std::shared_ptr<::Commodore::Serial::Bus> serial_bus);
 
-		/*!
-			Sets the disk from which this 1540 is reading data.
-		*/
-		void set_disk(std::shared_ptr<Storage::Disk> disk);
+		void run_for_cycles(int number_of_cycles);
 
 		// to satisfy CPU6502::Processor
 		unsigned int perform_bus_operation(CPU6502::BusOperation operation, uint16_t address, uint8_t *value);
@@ -187,6 +239,10 @@ class Machine:
 		// to satisfy MOS::MOS6522::Delegate
 		virtual void mos6522_did_change_interrupt_status(void *mos6522);
 
+		// to satisfy DriveVIA::Delegate
+		void drive_via_did_step_head(void *driveVIA, int direction);
+		void drive_via_did_set_data_density(void *driveVIA, int density);
+
 	private:
 		uint8_t _ram[0x800];
 		uint8_t _rom[0x4000];
@@ -196,6 +252,10 @@ class Machine:
 		DriveVIA _driveVIA;
 
 		std::shared_ptr<Storage::Disk> _disk;
+
+		int _shift_register, _bit_window_offset;
+		virtual void process_input_bit(int value, unsigned int cycles_since_index_hole);
+		virtual void process_index_hole();
 };
 
 }
diff --git a/Machines/Commodore/Vic-20/Vic20.cpp b/Machines/Commodore/Vic-20/Vic20.cpp
index 2b4f3d3ba..03b24f7ff 100644
--- a/Machines/Commodore/Vic-20/Vic20.cpp
+++ b/Machines/Commodore/Vic-20/Vic20.cpp
@@ -30,8 +30,8 @@ Machine::Machine() :
 	_serialPort->set_user_port_via(_userPortVIA);
 
 	// wire up the 6522s, tape and machine
-	_userPortVIA->set_delegate(this);
-	_keyboardVIA->set_delegate(this);
+	_userPortVIA->set_interrupt_delegate(this);
+	_keyboardVIA->set_interrupt_delegate(this);
 	_tape.set_delegate(this);
 
 	// establish the memory maps
@@ -169,6 +169,11 @@ void Machine::set_rom(ROMSlot slot, size_t length, const uint8_t *data)
 				_c1540->set_rom(data);
 				_c1540->run_for_cycles(2000000);	// pretend it booted a couple of seconds ago
 			}
+			else
+			{
+				_driveROM.reset(new uint8_t[length]);
+				memcpy(_driveROM.get(), data, length);
+			}
 		return;
 	}
 
@@ -221,6 +226,13 @@ void Machine::set_disk(std::shared_ptr<Storage::Disk> disk)
 
 	// hand it the disk
 	_c1540->set_disk(disk);
+
+	// install the ROM if it was previously set
+	if(_driveROM)
+	{
+		_c1540->set_rom(_driveROM.get());
+		_driveROM.reset();
+	}
 }
 
 #pragma mark - Typer
diff --git a/Machines/Commodore/Vic-20/Vic20.hpp b/Machines/Commodore/Vic-20/Vic20.hpp
index 279fa5249..fed2b4f11 100644
--- a/Machines/Commodore/Vic-20/Vic20.hpp
+++ b/Machines/Commodore/Vic-20/Vic20.hpp
@@ -287,6 +287,7 @@ class Machine:
 		uint8_t _screenMemory[0x1000];
 		uint8_t _colorMemory[0x0400];
 		uint8_t _junkMemory[0x0400];
+		std::unique_ptr<uint8_t> _driveROM;
 
 		uint8_t *_videoMemoryMap[16];
 		uint8_t *_processorReadMemoryMap[64];
diff --git a/Machines/Electron/Electron.cpp b/Machines/Electron/Electron.cpp
index c6a0e2688..cc4c02be3 100644
--- a/Machines/Electron/Electron.cpp
+++ b/Machines/Electron/Electron.cpp
@@ -1008,7 +1008,7 @@ inline void Tape::run_for_cycles(unsigned int number_of_cycles)
 		{
 			if(_is_running)
 			{
-				TapePlayer::run_for_cycles(number_of_cycles);
+				TapePlayer::run_for_cycles((int)number_of_cycles);
 			}
 		}
 		else
diff --git a/NumberTheory/Factors.cpp b/NumberTheory/Factors.cpp
new file mode 100644
index 000000000..067615653
--- /dev/null
+++ b/NumberTheory/Factors.cpp
@@ -0,0 +1,34 @@
+//
+//  Factors.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 29/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#include "Factors.hpp"
+
+unsigned int NumberTheory::greatest_common_divisor(unsigned int a, unsigned int b)
+{
+	if(a < b)
+	{
+		unsigned int swap = b;
+		b = a;
+		a = swap;
+	}
+
+	while(1) {
+		if(!a) return b;
+		if(!b) return a;
+
+		unsigned int remainder = a%b;
+		a = b;
+		b = remainder;
+	}
+}
+
+unsigned int NumberTheory::least_common_multiple(unsigned int a, unsigned int b)
+{
+	unsigned int gcd = greatest_common_divisor(a, b);
+	return (a*b) / gcd;
+}
diff --git a/NumberTheory/Factors.hpp b/NumberTheory/Factors.hpp
new file mode 100644
index 000000000..bdc92c011
--- /dev/null
+++ b/NumberTheory/Factors.hpp
@@ -0,0 +1,17 @@
+//
+//  Factors.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 29/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#ifndef Factors_hpp
+#define Factors_hpp
+
+namespace NumberTheory {
+	unsigned int greatest_common_divisor(unsigned int a, unsigned int b);
+	unsigned int least_common_multiple(unsigned int a, unsigned int b);
+}
+
+#endif /* Factors_hpp */
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index 06204d82f..124da7db1 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -7,6 +7,7 @@
 	objects = {
 
 /* Begin PBXBuildFile section */
+		4B0BE4281D3481E700D5256B /* DigitalPhaseLockedLoop.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B0BE4261D3481E700D5256B /* DigitalPhaseLockedLoop.cpp */; };
 		4B0CCC451C62D0B3001CAC5F /* CRT.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B0CCC421C62D0B3001CAC5F /* CRT.cpp */; };
 		4B14145B1B58879D00E04248 /* CPU6502.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B1414571B58879D00E04248 /* CPU6502.cpp */; };
 		4B14145D1B5887A600E04248 /* CPU6502.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B1414571B58879D00E04248 /* CPU6502.cpp */; };
@@ -39,6 +40,7 @@
 		4B69FB3D1C4D908A00B5F0AA /* Tape.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B69FB3B1C4D908A00B5F0AA /* Tape.cpp */; };
 		4B69FB441C4D941400B5F0AA /* TapeUEF.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B69FB421C4D941400B5F0AA /* TapeUEF.cpp */; };
 		4B69FB461C4D950F00B5F0AA /* libz.tbd in Frameworks */ = {isa = PBXBuildFile; fileRef = 4B69FB451C4D950F00B5F0AA /* libz.tbd */; };
+		4B6C73BD1D387AE500AFCFCA /* DiskDrive.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B6C73BB1D387AE500AFCFCA /* DiskDrive.cpp */; };
 		4B73C71A1D036BD90074D992 /* Vic20Document.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4B73C7191D036BD90074D992 /* Vic20Document.swift */; };
 		4B73C71D1D036C030074D992 /* Vic20Document.xib in Resources */ = {isa = PBXBuildFile; fileRef = 4B73C71B1D036C030074D992 /* Vic20Document.xib */; };
 		4B92EACA1B7C112B00246143 /* 6502TimingTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4B92EAC91B7C112B00246143 /* 6502TimingTests.swift */; };
@@ -312,6 +314,9 @@
 		4BB299F71B587D8400A49093 /* txan in Resources */ = {isa = PBXBuildFile; fileRef = 4BB298EB1B587D8400A49093 /* txan */; };
 		4BB299F81B587D8400A49093 /* txsn in Resources */ = {isa = PBXBuildFile; fileRef = 4BB298EC1B587D8400A49093 /* txsn */; };
 		4BB299F91B587D8400A49093 /* tyan in Resources */ = {isa = PBXBuildFile; fileRef = 4BB298ED1B587D8400A49093 /* tyan */; };
+		4BB697C71D4B558F00248BDF /* Factors.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BB697C51D4B558F00248BDF /* Factors.cpp */; };
+		4BB697CB1D4B6D3E00248BDF /* TimedEventLoop.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BB697C91D4B6D3E00248BDF /* TimedEventLoop.cpp */; };
+		4BB697CE1D4BA44400248BDF /* CommodoreGCR.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BB697CC1D4BA44400248BDF /* CommodoreGCR.cpp */; };
 		4BB73EA21B587A5100552FC2 /* AppDelegate.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4BB73EA11B587A5100552FC2 /* AppDelegate.swift */; };
 		4BB73EA71B587A5100552FC2 /* Atari2600Document.xib in Resources */ = {isa = PBXBuildFile; fileRef = 4BB73EA51B587A5100552FC2 /* Atari2600Document.xib */; };
 		4BB73EA91B587A5100552FC2 /* Assets.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = 4BB73EA81B587A5100552FC2 /* Assets.xcassets */; };
@@ -332,6 +337,8 @@
 		4BC9DF4F1D04691600F44158 /* 6560.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BC9DF4D1D04691600F44158 /* 6560.cpp */; };
 		4BC9E1EE1D23449A003FCEE4 /* 6502InterruptTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4BC9E1ED1D23449A003FCEE4 /* 6502InterruptTests.swift */; };
 		4BD5F1951D13528900631CD1 /* CSBestEffortUpdater.m in Sources */ = {isa = PBXBuildFile; fileRef = 4BD5F1941D13528900631CD1 /* CSBestEffortUpdater.m */; };
+		4BEF6AAA1D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.mm in Sources */ = {isa = PBXBuildFile; fileRef = 4BEF6AA91D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.mm */; };
+		4BEF6AAC1D35D1C400E73575 /* DPLLTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4BEF6AAB1D35D1C400E73575 /* DPLLTests.swift */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -354,6 +361,8 @@
 /* Begin PBXFileReference section */
 		4B046DC31CFE651500E9E45E /* CRTMachine.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = CRTMachine.hpp; sourceTree = "<group>"; };
 		4B0B6E121C9DBD5D00FFB60D /* CRTConstants.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = CRTConstants.hpp; sourceTree = "<group>"; };
+		4B0BE4261D3481E700D5256B /* DigitalPhaseLockedLoop.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = DigitalPhaseLockedLoop.cpp; sourceTree = "<group>"; };
+		4B0BE4271D3481E700D5256B /* DigitalPhaseLockedLoop.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = DigitalPhaseLockedLoop.hpp; sourceTree = "<group>"; };
 		4B0CCC421C62D0B3001CAC5F /* CRT.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = CRT.cpp; sourceTree = "<group>"; };
 		4B0CCC431C62D0B3001CAC5F /* CRT.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = CRT.hpp; sourceTree = "<group>"; };
 		4B1414501B58848C00E04248 /* ClockSignal-Bridging-Header.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = "ClockSignal-Bridging-Header.h"; sourceTree = "<group>"; };
@@ -415,6 +424,8 @@
 		4B69FB421C4D941400B5F0AA /* TapeUEF.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = TapeUEF.cpp; sourceTree = "<group>"; };
 		4B69FB431C4D941400B5F0AA /* TapeUEF.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = TapeUEF.hpp; sourceTree = "<group>"; };
 		4B69FB451C4D950F00B5F0AA /* libz.tbd */ = {isa = PBXFileReference; lastKnownFileType = "sourcecode.text-based-dylib-definition"; name = libz.tbd; path = usr/lib/libz.tbd; sourceTree = SDKROOT; };
+		4B6C73BB1D387AE500AFCFCA /* DiskDrive.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = DiskDrive.cpp; sourceTree = "<group>"; };
+		4B6C73BC1D387AE500AFCFCA /* DiskDrive.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = DiskDrive.hpp; sourceTree = "<group>"; };
 		4B73C7191D036BD90074D992 /* Vic20Document.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Vic20Document.swift; sourceTree = "<group>"; };
 		4B73C71C1D036C030074D992 /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.xib; name = Base; path = "Clock Signal/Base.lproj/Vic20Document.xib"; sourceTree = SOURCE_ROOT; };
 		4B92EAC91B7C112B00246143 /* 6502TimingTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = 6502TimingTests.swift; sourceTree = "<group>"; };
@@ -692,6 +703,12 @@
 		4BB298EB1B587D8400A49093 /* txan */ = {isa = PBXFileReference; lastKnownFileType = file; path = txan; sourceTree = "<group>"; };
 		4BB298EC1B587D8400A49093 /* txsn */ = {isa = PBXFileReference; lastKnownFileType = file; path = txsn; sourceTree = "<group>"; };
 		4BB298ED1B587D8400A49093 /* tyan */ = {isa = PBXFileReference; lastKnownFileType = file; path = tyan; sourceTree = "<group>"; };
+		4BB697C51D4B558F00248BDF /* Factors.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; name = Factors.cpp; path = ../../NumberTheory/Factors.cpp; sourceTree = "<group>"; };
+		4BB697C61D4B558F00248BDF /* Factors.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; name = Factors.hpp; path = ../../NumberTheory/Factors.hpp; sourceTree = "<group>"; };
+		4BB697C91D4B6D3E00248BDF /* TimedEventLoop.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = TimedEventLoop.cpp; sourceTree = "<group>"; };
+		4BB697CA1D4B6D3E00248BDF /* TimedEventLoop.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = TimedEventLoop.hpp; sourceTree = "<group>"; };
+		4BB697CC1D4BA44400248BDF /* CommodoreGCR.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; name = CommodoreGCR.cpp; path = Encodings/CommodoreGCR.cpp; sourceTree = "<group>"; };
+		4BB697CD1D4BA44400248BDF /* CommodoreGCR.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; name = CommodoreGCR.hpp; path = Encodings/CommodoreGCR.hpp; sourceTree = "<group>"; };
 		4BB73E9E1B587A5100552FC2 /* Clock Signal.app */ = {isa = PBXFileReference; explicitFileType = wrapper.application; includeInIndex = 0; path = "Clock Signal.app"; sourceTree = BUILT_PRODUCTS_DIR; };
 		4BB73EA11B587A5100552FC2 /* AppDelegate.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AppDelegate.swift; sourceTree = "<group>"; };
 		4BB73EA61B587A5100552FC2 /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.xib; name = Base; path = Base.lproj/Atari2600Document.xib; sourceTree = "<group>"; };
@@ -734,6 +751,9 @@
 		4BCA98C21D065CA20062F44C /* 6522.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = 6522.hpp; sourceTree = "<group>"; };
 		4BD5F1931D13528900631CD1 /* CSBestEffortUpdater.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = CSBestEffortUpdater.h; path = Updater/CSBestEffortUpdater.h; sourceTree = "<group>"; };
 		4BD5F1941D13528900631CD1 /* CSBestEffortUpdater.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; name = CSBestEffortUpdater.m; path = Updater/CSBestEffortUpdater.m; sourceTree = "<group>"; };
+		4BEF6AA81D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = DigitalPhaseLockedLoopBridge.h; sourceTree = "<group>"; };
+		4BEF6AA91D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = DigitalPhaseLockedLoopBridge.mm; sourceTree = "<group>"; };
+		4BEF6AAB1D35D1C400E73575 /* DPLLTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = DPLLTests.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -884,13 +904,15 @@
 			isa = PBXGroup;
 			children = (
 				4B3BA0C51D318B44005DD7A7 /* C1540Bridge.h */,
-				4B3BA0C61D318B44005DD7A7 /* C1540Bridge.mm */,
 				4B3BA0C71D318B44005DD7A7 /* Clock SignalTests-Bridging-Header.h */,
+				4BEF6AA81D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.h */,
 				4B3BA0C81D318B44005DD7A7 /* MOS6522Bridge.h */,
-				4B3BA0C91D318B44005DD7A7 /* MOS6522Bridge.mm */,
 				4B3BA0CA1D318B44005DD7A7 /* MOS6532Bridge.h */,
-				4B3BA0CB1D318B44005DD7A7 /* MOS6532Bridge.mm */,
 				4B3BA0CC1D318B44005DD7A7 /* TestMachine.h */,
+				4B3BA0C61D318B44005DD7A7 /* C1540Bridge.mm */,
+				4BEF6AA91D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.mm */,
+				4B3BA0C91D318B44005DD7A7 /* MOS6522Bridge.mm */,
+				4B3BA0CB1D318B44005DD7A7 /* MOS6532Bridge.mm */,
 				4B3BA0CD1D318B44005DD7A7 /* TestMachine.mm */,
 			);
 			path = Bridges;
@@ -951,9 +973,11 @@
 		4B69FB391C4D908A00B5F0AA /* Storage */ = {
 			isa = PBXGroup;
 			children = (
+				4BB697C91D4B6D3E00248BDF /* TimedEventLoop.cpp */,
+				4BAB62AE1D32730D00DF5BA0 /* Storage.hpp */,
+				4BB697CA1D4B6D3E00248BDF /* TimedEventLoop.hpp */,
 				4BAB62AA1D3272D200DF5BA0 /* Disk */,
 				4B69FB3A1C4D908A00B5F0AA /* Tape */,
-				4BAB62AE1D32730D00DF5BA0 /* Storage.hpp */,
 			);
 			name = Storage;
 			path = ../../Storage;
@@ -984,11 +1008,16 @@
 		4BAB62AA1D3272D200DF5BA0 /* Disk */ = {
 			isa = PBXGroup;
 			children = (
-				4BAB62B21D327F7E00DF5BA0 /* Formats */,
+				4B0BE4261D3481E700D5256B /* DigitalPhaseLockedLoop.cpp */,
 				4BAB62AB1D3272D200DF5BA0 /* Disk.cpp */,
-				4BAB62AC1D3272D200DF5BA0 /* Disk.hpp */,
+				4B6C73BB1D387AE500AFCFCA /* DiskDrive.cpp */,
 				4BAB62B61D3302CA00DF5BA0 /* PCMTrack.cpp */,
+				4B0BE4271D3481E700D5256B /* DigitalPhaseLockedLoop.hpp */,
+				4BAB62AC1D3272D200DF5BA0 /* Disk.hpp */,
+				4B6C73BC1D387AE500AFCFCA /* DiskDrive.hpp */,
 				4BAB62B71D3302CA00DF5BA0 /* PCMTrack.hpp */,
+				4BB697CF1D4BA44900248BDF /* Encodings */,
+				4BAB62B21D327F7E00DF5BA0 /* Formats */,
 			);
 			path = Disk;
 			sourceTree = "<group>";
@@ -1274,6 +1303,24 @@
 			path = "Wolfgang Lorenz 6502 test suite";
 			sourceTree = "<group>";
 		};
+		4BB697C81D4B559300248BDF /* NumberTheory */ = {
+			isa = PBXGroup;
+			children = (
+				4BB697C51D4B558F00248BDF /* Factors.cpp */,
+				4BB697C61D4B558F00248BDF /* Factors.hpp */,
+			);
+			name = NumberTheory;
+			sourceTree = "<group>";
+		};
+		4BB697CF1D4BA44900248BDF /* Encodings */ = {
+			isa = PBXGroup;
+			children = (
+				4BB697CC1D4BA44400248BDF /* CommodoreGCR.cpp */,
+				4BB697CD1D4BA44400248BDF /* CommodoreGCR.hpp */,
+			);
+			name = Encodings;
+			sourceTree = "<group>";
+		};
 		4BB73E951B587A5100552FC2 = {
 			isa = PBXGroup;
 			children = (
@@ -1283,13 +1330,17 @@
 				4BB73EC01B587A5100552FC2 /* Clock SignalUITests */,
 				4BC9DF4A1D04691600F44158 /* Components */,
 				4BB73EDC1B587CA500552FC2 /* Machines */,
+				4BB697C81D4B559300248BDF /* NumberTheory */,
 				4B366DFD1B5C165F0026627B /* Outputs */,
 				4BB73EDD1B587CA500552FC2 /* Processors */,
 				4BB73E9F1B587A5100552FC2 /* Products */,
 				4B2409591C45DF85004DA684 /* SignalProcessing */,
 				4B69FB391C4D908A00B5F0AA /* Storage */,
 			);
+			indentWidth = 4;
 			sourceTree = "<group>";
+			tabWidth = 4;
+			usesTabs = 1;
 		};
 		4BB73E9F1B587A5100552FC2 /* Products */ = {
 			isa = PBXGroup;
@@ -1330,6 +1381,7 @@
 				4B1E85801D176468001EF87D /* 6532Tests.swift */,
 				4BB73EB61B587A5100552FC2 /* AllSuiteATests.swift */,
 				4B3BA0C21D318AEB005DD7A7 /* C1540Tests.swift */,
+				4BEF6AAB1D35D1C400E73575 /* DPLLTests.swift */,
 				4B1414611B58888700E04248 /* KlausDormannTests.swift */,
 				4B14145F1B58885000E04248 /* WolfgangLorenzTests.swift */,
 				4B3BA0C41D318B44005DD7A7 /* Bridges */,
@@ -1853,14 +1905,17 @@
 			files = (
 				4BAB62AD1D3272D200DF5BA0 /* Disk.cpp in Sources */,
 				4BC9DF4F1D04691600F44158 /* 6560.cpp in Sources */,
+				4BB697CE1D4BA44400248BDF /* CommodoreGCR.cpp in Sources */,
 				4BBF99151C8FBA6F0075DAFB /* CRTOpenGL.cpp in Sources */,
 				4B0CCC451C62D0B3001CAC5F /* CRT.cpp in Sources */,
+				4BB697C71D4B558F00248BDF /* Factors.cpp in Sources */,
 				4B55CE591C3B7D360093A61B /* ElectronDocument.swift in Sources */,
 				4B4DC82B1D2C27A4003C5BF8 /* SerialBus.cpp in Sources */,
 				4BC3B74F1CD194CC00F86E85 /* Shader.cpp in Sources */,
 				4B55CE581C3B7D360093A61B /* Atari2600Document.swift in Sources */,
 				4BBB14311CD2CECE00BDB55C /* IntermediateShader.cpp in Sources */,
 				4BD5F1951D13528900631CD1 /* CSBestEffortUpdater.m in Sources */,
+				4B0BE4281D3481E700D5256B /* DigitalPhaseLockedLoop.cpp in Sources */,
 				4B73C71A1D036BD90074D992 /* Vic20Document.swift in Sources */,
 				4BBF99181C8FBA6F0075DAFB /* TextureTarget.cpp in Sources */,
 				4BC76E691C98E31700E6EF73 /* FIRFilter.cpp in Sources */,
@@ -1871,12 +1926,14 @@
 				4BAB62B51D327F7E00DF5BA0 /* G64.cpp in Sources */,
 				4BBF99141C8FBA6F0075DAFB /* CRTInputBufferBuilder.cpp in Sources */,
 				4B2409551C45AB05004DA684 /* Speaker.cpp in Sources */,
+				4B6C73BD1D387AE500AFCFCA /* DiskDrive.cpp in Sources */,
 				4B4DC8281D2C2470003C5BF8 /* C1540.cpp in Sources */,
 				4B1E85751D170228001EF87D /* Typer.cpp in Sources */,
 				4B2E2D9D1C3A070400138695 /* Electron.cpp in Sources */,
 				4BAB62B81D3302CA00DF5BA0 /* PCMTrack.cpp in Sources */,
 				4B69FB3D1C4D908A00B5F0AA /* Tape.cpp in Sources */,
 				4B55CE5D1C3B7D6F0093A61B /* CSOpenGLView.m in Sources */,
+				4BB697CB1D4B6D3E00248BDF /* TimedEventLoop.cpp in Sources */,
 				4B2A53A31D117D36003C6002 /* CSVic20.mm in Sources */,
 				4B2A53A21D117D36003C6002 /* CSElectron.mm in Sources */,
 				4B2E2D9A1C3A06EC00138695 /* Atari2600.cpp in Sources */,
@@ -1897,10 +1954,12 @@
 				4B14145D1B5887A600E04248 /* CPU6502.cpp in Sources */,
 				4B1E85811D176468001EF87D /* 6532Tests.swift in Sources */,
 				4BC9E1EE1D23449A003FCEE4 /* 6502InterruptTests.swift in Sources */,
+				4BEF6AAA1D35CE9E00E73575 /* DigitalPhaseLockedLoopBridge.mm in Sources */,
 				4B3BA0CE1D318B44005DD7A7 /* C1540Bridge.mm in Sources */,
 				4B3BA0D11D318B44005DD7A7 /* TestMachine.mm in Sources */,
 				4B92EACA1B7C112B00246143 /* 6502TimingTests.swift in Sources */,
 				4BB73EB71B587A5100552FC2 /* AllSuiteATests.swift in Sources */,
+				4BEF6AAC1D35D1C400E73575 /* DPLLTests.swift in Sources */,
 				4B3BA0CF1D318B44005DD7A7 /* MOS6522Bridge.mm in Sources */,
 				4BC751B21D157E61006C31D9 /* 6522Tests.swift in Sources */,
 				4B3BA0D01D318B44005DD7A7 /* MOS6532Bridge.mm in Sources */,
diff --git a/OSBindings/Mac/Clock Signal/Documents/Vic20Document.swift b/OSBindings/Mac/Clock Signal/Documents/Vic20Document.swift
index 42e9b97ed..0c446427c 100644
--- a/OSBindings/Mac/Clock Signal/Documents/Vic20Document.swift	
+++ b/OSBindings/Mac/Clock Signal/Documents/Vic20Document.swift	
@@ -32,7 +32,7 @@ class Vic20Document: MachineDocument {
 			vic20.setCharactersROM(characters)
 		}
 
-		if let drive = dataForResource("1541", ofType: "bin", inDirectory: "ROMImages/Commodore1540") {
+		if let drive = dataForResource("1540", ofType: "bin", inDirectory: "ROMImages/Commodore1540") {
 			vic20.setDriveROM(drive)
 		}
 	}
diff --git a/OSBindings/Mac/Clock SignalTests/Bridges/Clock SignalTests-Bridging-Header.h b/OSBindings/Mac/Clock SignalTests/Bridges/Clock SignalTests-Bridging-Header.h
index 8333797fa..23c391390 100644
--- a/OSBindings/Mac/Clock SignalTests/Bridges/Clock SignalTests-Bridging-Header.h	
+++ b/OSBindings/Mac/Clock SignalTests/Bridges/Clock SignalTests-Bridging-Header.h	
@@ -6,3 +6,4 @@
 #import "MOS6522Bridge.h"
 #import "MOS6532Bridge.h"
 #import "C1540Bridge.h"
+#import "DigitalPhaseLockedLoopBridge.h"
diff --git a/OSBindings/Mac/Clock SignalTests/Bridges/DigitalPhaseLockedLoopBridge.h b/OSBindings/Mac/Clock SignalTests/Bridges/DigitalPhaseLockedLoopBridge.h
new file mode 100644
index 000000000..9f39f0f7d
--- /dev/null
+++ b/OSBindings/Mac/Clock SignalTests/Bridges/DigitalPhaseLockedLoopBridge.h	
@@ -0,0 +1,20 @@
+//
+//  DigitalPhaseLockedLoopBridge.h
+//  Clock Signal
+//
+//  Created by Thomas Harte on 12/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#import <Foundation/Foundation.h>
+
+@interface DigitalPhaseLockedLoopBridge : NSObject
+
+- (instancetype)initWithClocksPerBit:(NSUInteger)clocksPerBit tolerance:(NSUInteger)tolerance historyLength:(NSUInteger)historyLength;
+
+- (void)runForCycles:(NSUInteger)cycles;
+- (void)addPulse;
+
+@property(nonatomic) NSUInteger stream;
+
+@end
diff --git a/OSBindings/Mac/Clock SignalTests/Bridges/DigitalPhaseLockedLoopBridge.mm b/OSBindings/Mac/Clock SignalTests/Bridges/DigitalPhaseLockedLoopBridge.mm
new file mode 100644
index 000000000..7a4068f10
--- /dev/null
+++ b/OSBindings/Mac/Clock SignalTests/Bridges/DigitalPhaseLockedLoopBridge.mm	
@@ -0,0 +1,61 @@
+//
+//  DigitalPhaseLockedLoopBridge.m
+//  Clock Signal
+//
+//  Created by Thomas Harte on 12/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#import "DigitalPhaseLockedLoopBridge.h"
+
+#include "DigitalPhaseLockedLoop.hpp"
+#include <memory>
+
+@interface DigitalPhaseLockedLoopBridge(BitPushing)
+- (void)pushBit:(int)value;
+@end
+
+class DigitalPhaseLockedLoopDelegate: public Storage::DigitalPhaseLockedLoop::Delegate {
+	public:
+		__weak DigitalPhaseLockedLoopBridge *bridge;
+
+		void digital_phase_locked_loop_output_bit(int value)
+		{
+			[bridge pushBit:value ? 1 : 0];
+		}
+};
+
+@implementation DigitalPhaseLockedLoopBridge
+{
+	std::unique_ptr<Storage::DigitalPhaseLockedLoop> _digitalPhaseLockedLoop;
+	DigitalPhaseLockedLoopDelegate _delegate;
+}
+
+- (instancetype)initWithClocksPerBit:(NSUInteger)clocksPerBit tolerance:(NSUInteger)tolerance historyLength:(NSUInteger)historyLength
+{
+	self = [super init];
+	if(self)
+	{
+		_digitalPhaseLockedLoop.reset(new Storage::DigitalPhaseLockedLoop((unsigned int)clocksPerBit, (unsigned int)tolerance, (unsigned int)historyLength));
+		_delegate.bridge = self;
+		_digitalPhaseLockedLoop->set_delegate(&_delegate);
+	}
+	return self;
+}
+
+- (void)runForCycles:(NSUInteger)cycles
+{
+	_digitalPhaseLockedLoop->run_for_cycles((unsigned int)cycles);
+}
+
+- (void)addPulse
+{
+	_digitalPhaseLockedLoop->add_pulse();
+}
+
+- (void)pushBit:(int)value
+{
+	_stream = (_stream << 1) | value;
+}
+
+@end
diff --git a/OSBindings/Mac/Clock SignalTests/DPLLTests.swift b/OSBindings/Mac/Clock SignalTests/DPLLTests.swift
new file mode 100644
index 000000000..01d99c054
--- /dev/null
+++ b/OSBindings/Mac/Clock SignalTests/DPLLTests.swift	
@@ -0,0 +1,61 @@
+//
+//  DPLLTests.swift
+//  Clock Signal
+//
+//  Created by Thomas Harte on 12/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+import XCTest
+
+class DPLLTests: XCTestCase {
+
+	func testRegularNibblesOnPLL(pll: DigitalPhaseLockedLoopBridge, bitLength: UInt) {
+		// clock in two 1s, a 0, and a 1, 200 times over
+		for _ in 0 ..< 200 {
+			pll.runForCycles(bitLength/2)
+			pll.addPulse()
+			pll.runForCycles(bitLength)
+			pll.addPulse()
+			pll.runForCycles(bitLength*2)
+			pll.addPulse()
+			pll.runForCycles(bitLength/2)
+		}
+
+		XCTAssert((pll.stream&0xffffffff) == 0xdddddddd, "PLL should have synchronised and clocked repeating 0xd nibbles; got \(String(pll.stream, radix: 16, uppercase: false))")
+	}
+
+	func testPerfectInput() {
+		let pll = DigitalPhaseLockedLoopBridge(clocksPerBit: 100, tolerance: 20, historyLength: 3)
+		testRegularNibblesOnPLL(pll, bitLength: 100)
+	}
+
+	func testFastButRegular() {
+		let pll = DigitalPhaseLockedLoopBridge(clocksPerBit: 100, tolerance: 20, historyLength: 3)
+		testRegularNibblesOnPLL(pll, bitLength: 90)
+	}
+
+	func testSlowButRegular() {
+		let pll = DigitalPhaseLockedLoopBridge(clocksPerBit: 100, tolerance: 20, historyLength: 3)
+		testRegularNibblesOnPLL(pll, bitLength: 110)
+	}
+
+	func testTwentyPercentSinePattern() {
+		let pll = DigitalPhaseLockedLoopBridge(clocksPerBit: 100, tolerance: 20, historyLength: 3)
+		var angle = 0.0
+
+		// clock in two 1s, a 0, and a 1, 200 times over
+		for _ in 0 ..< 200 {
+			let bitLength: UInt = UInt(100 + 20 * sin(angle))
+
+			pll.runForCycles(bitLength/2)
+			pll.addPulse()
+			pll.runForCycles((bitLength*3)/2)
+
+			angle = angle + 0.1
+		}
+
+		let endOfStream = pll.stream&0xffffffff;
+		XCTAssert(endOfStream == 0xaaaaaaaa || endOfStream == 0x55555555, "PLL should have synchronised and clocked repeating 0xa or 0x5 nibbles; got \(String(pll.stream, radix: 16, uppercase: false))")
+	}
+}
diff --git a/Processors/6502/CPU6502.hpp b/Processors/6502/CPU6502.hpp
index 4b0434eb8..3134a8965 100644
--- a/Processors/6502/CPU6502.hpp
+++ b/Processors/6502/CPU6502.hpp
@@ -472,6 +472,7 @@ template <class T> class Processor {
 		bool _ready_line_is_enabled;
 
 		bool _irq_line_is_enabled, _irq_request_history;
+		bool _nmi_line_is_enabled, _set_overflow_line_is_enabled;
 
 		/*!
 			Gets the program representing an RST response.
@@ -549,7 +550,10 @@ template <class T> class Processor {
 			_interruptFlag(Flag::Interrupt),
 			_s(0),
 			_nextBusOperation(BusOperation::None),
-			_interrupt_requests(InterruptRequestFlags::PowerOn)
+			_interrupt_requests(InterruptRequestFlags::PowerOn),
+			_irq_line_is_enabled(false),
+			_nmi_line_is_enabled(false),
+			_set_overflow_line_is_enabled(false)
 		{
 			// only the interrupt flag is defined upon reset but get_flags isn't going to
 			// mask the other flags so we need to do that, at least
@@ -1237,6 +1241,19 @@ template <class T> class Processor {
 			_irq_line_is_enabled = active;
 		}
 
+		/*!
+			Sets the current level of the set overflow line.
+
+			@param active @c true if the line is logically active; @c false otherwise.
+		*/
+		inline void set_overflow_line(bool active)
+		{
+			// a leading edge will set the overflow flag
+			if(active && !_set_overflow_line_is_enabled)
+				_overflowFlag = Flag::Overflow;
+			_set_overflow_line_is_enabled = active;
+		}
+
 		/*!
 			Sets the current level of the NMI line.
 
@@ -1245,7 +1262,9 @@ template <class T> class Processor {
 		inline void set_nmi_line(bool active)
 		{
 			// NMI is edge triggered, not level
-			_interrupt_requests |= (active ? InterruptRequestFlags::NMI : 0);
+			if(active && !_nmi_line_is_enabled)
+				_interrupt_requests |= InterruptRequestFlags::NMI;
+			_nmi_line_is_enabled = active;
 		}
 
 		/*!
diff --git a/SignalProcessing/Stepper.hpp b/SignalProcessing/Stepper.hpp
index f91beb2a0..a874d8a51 100644
--- a/SignalProcessing/Stepper.hpp
+++ b/SignalProcessing/Stepper.hpp
@@ -13,19 +13,41 @@
 
 namespace SignalProcessing {
 
+/*!
+	Allows a repeating action running at an input rate to determine how many times it should
+	trigger an action that runs at an unrelated output rate; therefore it allows something with one
+	clock to sample something with another.
+
+	Uses a Bresenham-like error term internally for full-integral storage with no drift.
+
+	Pegs the beginning of both clocks to the time at which the stepper is created. So e.g. a stepper
+	that converts from an input clock of 1200 to an output clock of 2 will first fire on cycle 600.
+*/
 class Stepper
 {
 	public:
+		/*!
+			Establishes a stepper with a one-to-one conversion rate.
+		*/
 		Stepper() : Stepper(1,1) {}
 
+		/*!
+			Establishes a stepper that will receive steps at the @c input_rate and dictate the number
+			of steps that should be taken at the @c output_rate.
+		*/
 		Stepper(uint64_t output_rate, uint64_t input_rate) :
-			accumulated_error_(0),
+			accumulated_error_(-((int64_t)input_rate << 1)),
 			input_rate_(input_rate),
 			output_rate_(output_rate),
 			whole_step_(output_rate / input_rate),
 			adjustment_up_((int64_t)(output_rate % input_rate) << 1),
 			adjustment_down_((int64_t)input_rate << 1) {}
 
+		/*!
+			Advances one step at the input rate.
+
+			@returns the number of output steps.
+		*/
 		inline uint64_t step()
 		{
 			uint64_t update = whole_step_;
@@ -38,11 +60,34 @@ class Stepper
 			return update;
 		}
 
+		/*!
+			Advances by @c number_of_steps steps at the input rate.
+
+			@returns the number of output steps.
+		*/
+		inline uint64_t step(uint64_t number_of_steps)
+		{
+			uint64_t update = whole_step_ * number_of_steps;
+			accumulated_error_ += adjustment_up_ * (int64_t)number_of_steps;
+			if(accumulated_error_ > 0)
+			{
+				update += 1 + (uint64_t)(accumulated_error_ / adjustment_down_);
+				accumulated_error_ = (accumulated_error_ % adjustment_down_) - adjustment_down_;
+			}
+			return update;
+		}
+
+		/*!
+			@returns the output rate.
+		*/
 		inline uint64_t get_output_rate()
 		{
 			return output_rate_;
 		}
 
+		/*!
+			@returns the input rate.
+		*/
 		inline uint64_t get_input_rate()
 		{
 			return input_rate_;
diff --git a/Storage/Disk/DigitalPhaseLockedLoop.cpp b/Storage/Disk/DigitalPhaseLockedLoop.cpp
new file mode 100644
index 000000000..999e17b2d
--- /dev/null
+++ b/Storage/Disk/DigitalPhaseLockedLoop.cpp
@@ -0,0 +1,76 @@
+//
+//  DigitalPhaseLockedLoop.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 11/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#include "DigitalPhaseLockedLoop.hpp"
+#include <algorithm>
+#include <cstdlib>
+
+using namespace Storage;
+
+DigitalPhaseLockedLoop::DigitalPhaseLockedLoop(int clocks_per_bit, int tolerance, size_t length_of_history) :
+	_clocks_per_bit(clocks_per_bit),
+	_tolerance(tolerance),
+
+	_phase(0),
+	_window_length(clocks_per_bit),
+
+	_phase_error_pointer(0)
+{
+	_phase_error_history.reset(new std::vector<int>(length_of_history, 0));
+}
+
+void DigitalPhaseLockedLoop::run_for_cycles(int number_of_cycles)
+{
+	_phase += number_of_cycles;
+	int windows_crossed = _phase / _window_length;
+	if(windows_crossed)
+	{
+		// check whether this triggers any 0s, if anybody cares
+		if(_delegate)
+		{
+			if(_window_was_filled) windows_crossed--;
+			for(int c = 0; c < windows_crossed; c++)
+				_delegate->digital_phase_locked_loop_output_bit(0);
+		}
+
+		_window_was_filled = false;
+		_phase %= _window_length;
+	}
+}
+
+void DigitalPhaseLockedLoop::add_pulse()
+{
+	if(!_window_was_filled)
+	{
+		if(_delegate) _delegate->digital_phase_locked_loop_output_bit(1);
+		_window_was_filled = true;
+		post_phase_error(_phase - (_window_length >> 1));
+	}
+}
+
+void DigitalPhaseLockedLoop::post_phase_error(int error)
+{
+	// use a simple spring mechanism as a lowpass filter for phase
+	_phase -= (error + 1) >> 1;
+
+	// use the average of the last few errors to affect frequency
+	std::vector<int> *phase_error_history = _phase_error_history.get();
+	size_t phase_error_history_size = phase_error_history->size();
+
+	(*phase_error_history)[_phase_error_pointer] = error;
+	_phase_error_pointer = (_phase_error_pointer + 1)%phase_error_history_size;
+
+	int total_error = 0;
+	for(size_t c = 0; c < phase_error_history_size; c++)
+	{
+		total_error += (*phase_error_history)[c];
+	}
+	int denominator = (int)(phase_error_history_size * 4);
+	_window_length += (total_error + (denominator >> 1)) / denominator;
+	_window_length = std::max(std::min(_window_length, _clocks_per_bit + _tolerance), _clocks_per_bit - _tolerance);
+}
diff --git a/Storage/Disk/DigitalPhaseLockedLoop.hpp b/Storage/Disk/DigitalPhaseLockedLoop.hpp
new file mode 100644
index 000000000..da257b445
--- /dev/null
+++ b/Storage/Disk/DigitalPhaseLockedLoop.hpp
@@ -0,0 +1,69 @@
+//
+//  DigitalPhaseLockedLoop.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 11/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#ifndef DigitalPhaseLockedLoop_hpp
+#define DigitalPhaseLockedLoop_hpp
+
+#include <memory>
+#include <vector>
+
+namespace Storage {
+
+class DigitalPhaseLockedLoop {
+	public:
+		/*!
+			Instantiates a @c DigitalPhaseLockedLoop.
+
+			@param clocks_per_bit The expected number of cycles between each bit of input.
+			@param tolerance The maximum tolerance for bit windows — extremes will be clocks_per_bit ± tolerance.
+			@param length_of_history The number of historic pulses to consider in locking to phase.
+		*/
+		DigitalPhaseLockedLoop(int clocks_per_bit, int tolerance, size_t length_of_history);
+
+		/*!
+			Runs the loop, impliedly posting no pulses during that period.
+
+			@c number_of_cycles The time to run the loop for.
+		*/
+		void run_for_cycles(int number_of_cycles);
+
+		/*!
+			Announces a pulse at the current time.
+		*/
+		void add_pulse();
+
+		/*!
+			A receiver for PCM output data; called upon every recognised bit.
+		*/
+		class Delegate {
+			public:
+				virtual void digital_phase_locked_loop_output_bit(int value) = 0;
+		};
+		void set_delegate(Delegate *delegate)
+		{
+			_delegate = delegate;
+		}
+
+	private:
+		Delegate *_delegate;
+
+		void post_phase_error(int error);
+		std::unique_ptr<std::vector<int>> _phase_error_history;
+		size_t _phase_error_pointer;
+
+		int _phase;
+		int _window_length;
+		bool _window_was_filled;
+
+		int _clocks_per_bit;
+		int _tolerance;
+};
+
+}
+
+#endif /* DigitalPhaseLockedLoop_hpp */
diff --git a/Storage/Disk/DiskDrive.cpp b/Storage/Disk/DiskDrive.cpp
new file mode 100644
index 000000000..90a11a977
--- /dev/null
+++ b/Storage/Disk/DiskDrive.cpp
@@ -0,0 +1,118 @@
+//
+//  DiskDrive.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#include "DiskDrive.hpp"
+
+using namespace Storage;
+
+DiskDrive::DiskDrive(unsigned int clock_rate, unsigned int clock_rate_multiplier, unsigned int revolutions_per_minute) :
+	_clock_rate(clock_rate * clock_rate_multiplier),
+	_clock_rate_multiplier(clock_rate_multiplier),
+	_revolutions_per_minute(revolutions_per_minute),
+	_head_position(0),
+
+	TimedEventLoop(clock_rate * clock_rate_multiplier)
+{}
+
+void DiskDrive::set_expected_bit_length(Time bit_length)
+{
+	_bit_length = bit_length;
+
+	// this conversion doesn't need to be exact because there's a lot of variation to be taken
+	// account of in rotation speed, air turbulence, etc, so a direct conversion will do
+	int clocks_per_bit = (int)((bit_length.length * _clock_rate) / bit_length.clock_rate);
+	_pll.reset(new DigitalPhaseLockedLoop(clocks_per_bit, clocks_per_bit / 5, 3));
+	_pll->set_delegate(this);
+}
+
+void DiskDrive::set_disk(std::shared_ptr<Disk> disk)
+{
+	_disk = disk;
+	set_track();
+}
+
+bool DiskDrive::has_disk()
+{
+	return (bool)_disk;
+}
+
+bool DiskDrive::get_is_track_zero()
+{
+	return _head_position == 0;
+}
+
+void DiskDrive::step(int direction)
+{
+	_head_position = std::max(_head_position + direction, 0);
+	set_track();
+}
+
+void DiskDrive::set_track()
+{
+	_track = _disk->get_track_at_position((unsigned int)_head_position);
+	reset_timer();
+	get_next_event();
+}
+
+void DiskDrive::run_for_cycles(int number_of_cycles)
+{
+	if(has_disk())
+	{
+		number_of_cycles *= _clock_rate_multiplier;
+		while(number_of_cycles--)
+		{
+			_cycles_since_index_hole ++;
+			_pll->run_for_cycles(1);
+			TimedEventLoop::run_for_cycles(1);
+		}
+	}
+}
+
+#pragma mark - Track timed event loop
+
+void DiskDrive::get_next_event()
+{
+	if(_track)
+		_current_event = _track->get_next_event();
+	else
+	{
+		_current_event.length.length = 1;
+		_current_event.length.clock_rate = 1;
+		_current_event.type = Track::Event::IndexHole;
+	}
+
+	// divide interval, which is in terms of a rotation of the disk, by rotation speed, and
+	// convert it into revolutions per second
+	Time event_interval = _current_event.length;
+	event_interval.length *= 60;
+	event_interval.clock_rate *= _revolutions_per_minute;
+	event_interval.simplify();
+	set_next_event_time_interval(event_interval);
+}
+
+void DiskDrive::process_next_event()
+{
+	switch(_current_event.type)
+	{
+		case Track::Event::FluxTransition:
+			_pll->add_pulse();
+		break;
+		case Track::Event::IndexHole:
+			_cycles_since_index_hole = 0;
+			process_index_hole();
+		break;
+	}
+	get_next_event();
+}
+
+#pragma mark - PLL delegate
+
+void DiskDrive::digital_phase_locked_loop_output_bit(int value)
+{
+	process_input_bit(value, _cycles_since_index_hole);
+}
diff --git a/Storage/Disk/DiskDrive.hpp b/Storage/Disk/DiskDrive.hpp
new file mode 100644
index 000000000..7d3cb560a
--- /dev/null
+++ b/Storage/Disk/DiskDrive.hpp
@@ -0,0 +1,62 @@
+//
+//  DiskDrive.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#ifndef DiskDrive_hpp
+#define DiskDrive_hpp
+
+#include "Disk.hpp"
+#include "DigitalPhaseLockedLoop.hpp"
+#include "../TimedEventLoop.hpp"
+
+namespace Storage {
+
+class DiskDrive: public DigitalPhaseLockedLoop::Delegate, public TimedEventLoop {
+	public:
+		DiskDrive(unsigned int clock_rate, unsigned int clock_rate_multiplier, unsigned int revolutions_per_minute);
+
+		void set_expected_bit_length(Time bit_length);
+
+		void set_disk(std::shared_ptr<Disk> disk);
+		bool has_disk();
+
+		void run_for_cycles(int number_of_cycles);
+
+		bool get_is_track_zero();
+		void step(int direction);
+		void set_motor_on(bool motor_on);
+
+		// to satisfy DigitalPhaseLockedLoop::Delegate
+		void digital_phase_locked_loop_output_bit(int value);
+
+	protected:
+		virtual void process_input_bit(int value, unsigned int cycles_since_index_hole) = 0;
+		virtual void process_index_hole() = 0;
+
+		// for TimedEventLoop
+		virtual void process_next_event();
+
+	private:
+		Time _bit_length;
+		unsigned int _clock_rate;
+		unsigned int _clock_rate_multiplier;
+		unsigned int _revolutions_per_minute;
+
+		std::shared_ptr<DigitalPhaseLockedLoop> _pll;
+		std::shared_ptr<Disk> _disk;
+		std::shared_ptr<Track> _track;
+		int _head_position;
+		unsigned int _cycles_since_index_hole;
+		void set_track();
+
+		inline void get_next_event();
+		Track::Event _current_event;
+};
+
+}
+
+#endif /* DiskDrive_hpp */
diff --git a/Storage/Disk/Encodings/CommodoreGCR.cpp b/Storage/Disk/Encodings/CommodoreGCR.cpp
new file mode 100644
index 000000000..36667141d
--- /dev/null
+++ b/Storage/Disk/Encodings/CommodoreGCR.cpp
@@ -0,0 +1,20 @@
+//
+//  CommodoreGCR.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 29/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#include "CommodoreGCR.hpp"
+
+using namespace Storage;
+
+Time Storage::Encodings::CommodoreGCR::length_of_a_bit_in_time_zone(unsigned int time_zone)
+{
+	Time duration;
+	// the speed zone divides a 4Mhz clock by 13, 14, 15 or 16, with higher-numbered zones being faster (i.e. each bit taking less time)
+	duration.length = 16 - time_zone;
+	duration.clock_rate = 4000000;
+	return duration;
+}
diff --git a/Storage/Disk/Encodings/CommodoreGCR.hpp b/Storage/Disk/Encodings/CommodoreGCR.hpp
new file mode 100644
index 000000000..0617af963
--- /dev/null
+++ b/Storage/Disk/Encodings/CommodoreGCR.hpp
@@ -0,0 +1,22 @@
+//
+//  CommodoreGCR.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 29/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#ifndef CommodoreGCR_hpp
+#define CommodoreGCR_hpp
+
+#include "../../Storage.hpp"
+
+namespace Storage {
+namespace Encodings {
+namespace CommodoreGCR {
+	Time length_of_a_bit_in_time_zone(unsigned int time_zone);
+}
+}
+}
+
+#endif /* CommodoreGCR_hpp */
diff --git a/Storage/Disk/Formats/G64.cpp b/Storage/Disk/Formats/G64.cpp
index 67b243137..d68d8c299 100644
--- a/Storage/Disk/Formats/G64.cpp
+++ b/Storage/Disk/Formats/G64.cpp
@@ -10,6 +10,7 @@
 
 #include <vector>
 #include "../PCMTrack.hpp"
+#include "../Encodings/CommodoreGCR.hpp"
 
 using namespace Storage;
 
@@ -40,7 +41,8 @@ G64::G64(const char *file_name)
 	_maximum_track_size = (uint16_t)fgetc(_file);
 	_maximum_track_size |= (uint16_t)fgetc(_file) << 8;
 
-	get_track_at_position(0);
+//	for(size_t c = 0; c < _number_of_tracks; c++)
+//		get_track_at_position(c);
 }
 
 G64::~G64()
@@ -116,14 +118,14 @@ std::shared_ptr<Track> G64::get_track_at_position(unsigned int position)
 		unsigned int start_byte_in_current_speed = 0;
 		for(unsigned int byte = 0; byte < track_length; byte ++)
 		{
-			unsigned int byte_speed = speed_zone_contents[byte >> 2] >> (6 -  (byte&3)*2);
+			unsigned int byte_speed = speed_zone_contents[byte >> 2] >> (6 - (byte&3)*2);
 			if(byte_speed != current_speed || byte == (track_length-1))
 			{
 				unsigned int number_of_bytes = byte - start_byte_in_current_speed;
 
 				PCMSegment segment;
-				segment.duration.length = number_of_bytes * 8;
-				segment.duration.clock_rate = 4000000 / (13 + current_speed);	// the speed zone divides a 4Mhz clock by 13, 14, 15 or 16; TODO: is this the right way around? Is zone 3 the fastest or the slowest?
+				segment.number_of_bits = number_of_bytes * 8;
+				segment.length_of_a_bit = Encodings::CommodoreGCR::length_of_a_bit_in_time_zone(current_speed);
 				segment.data.reset(new uint8_t[number_of_bytes]);
 				memcpy(segment.data.get(), &track_contents.get()[start_byte_in_current_speed], number_of_bytes);
 				segments.push_back(std::move(segment));
@@ -138,8 +140,8 @@ std::shared_ptr<Track> G64::get_track_at_position(unsigned int position)
 	else
 	{
 		PCMSegment segment;
-		segment.duration.length = track_length * 8;
-		segment.duration.clock_rate = 1; // this is arbitrary; if supplying only one PCMSegment then it'll naturally fill the track
+		segment.number_of_bits = track_length * 8;
+		segment.length_of_a_bit = Encodings::CommodoreGCR::length_of_a_bit_in_time_zone((unsigned int)speed_zone_offset);
 		segment.data = std::move(track_contents);
 
 		resulting_track.reset(new PCMTrack(std::move(segment)));
diff --git a/Storage/Disk/PCMTrack.cpp b/Storage/Disk/PCMTrack.cpp
index a25d31527..32dc79a9d 100644
--- a/Storage/Disk/PCMTrack.cpp
+++ b/Storage/Disk/PCMTrack.cpp
@@ -7,34 +7,10 @@
 //
 
 #include "PCMTrack.hpp"
+#include "../../NumberTheory/Factors.hpp"
 
 using namespace Storage;
 
-unsigned int greatest_common_divisor(unsigned int a, unsigned int b)
-{
-	if(a < b)
-	{
-		unsigned int swap = b;
-		b = a;
-		a = swap;
-	}
-
-	while(1) {
-		if(!a) return b;
-		if(!b) return a;
-
-		unsigned int remainder = a%b;
-		a = b;
-		b = remainder;
-	}
-}
-
-unsigned int least_common_multiple(unsigned int a, unsigned int b)
-{
-	unsigned int gcd = greatest_common_divisor(a, b);
-	return (a*b) / gcd;
-}
-
 PCMTrack::PCMTrack(std::vector<PCMSegment> segments)
 {
 	_segments = std::move(segments);
@@ -55,15 +31,17 @@ PCMTrack::Event PCMTrack::get_next_event()
 	_next_event.length.length = 0;
 	while(_segment_pointer < _segments.size())
 	{
-		unsigned int clock_multiplier = _track_clock_rate / _segments[_segment_pointer].duration.clock_rate;
+		unsigned int clock_multiplier = _track_clock_rate / _segments[_segment_pointer].length_of_a_bit.clock_rate;
+		unsigned int bit_length = clock_multiplier * _segments[_segment_pointer].length_of_a_bit.length;
+
 		const uint8_t *segment_data = _segments[_segment_pointer].data.get();
-		while(_bit_pointer < _segments[_segment_pointer].duration.length)
+		while(_bit_pointer < _segments[_segment_pointer].number_of_bits)
 		{
 			// for timing simplicity, bits are modelled as happening at the end of their window
 			// TODO: should I account for the converse bit ordering? Or can I assume MSB first?
 			int bit = segment_data[_bit_pointer >> 3] & (0x80 >> (_bit_pointer&7));
 			_bit_pointer++;
-			_next_event.length.length += clock_multiplier;
+			_next_event.length.length += bit_length;
 
 			if(bit) return _next_event;
 		}
@@ -84,18 +62,18 @@ PCMTrack::Event PCMTrack::get_next_event()
 void PCMTrack::fix_length()
 {
 	// find the least common multiple of all segment clock rates
-	_track_clock_rate = _segments[0].duration.clock_rate;
+	_track_clock_rate = _segments[0].length_of_a_bit.clock_rate;
 	for(size_t c = 1; c < _segments.size(); c++)
 	{
-		_track_clock_rate = least_common_multiple(_track_clock_rate, _segments[c].duration.clock_rate);
+		_track_clock_rate = NumberTheory::least_common_multiple(_track_clock_rate, _segments[c].length_of_a_bit.clock_rate);
 	}
 
-	// therby determine the total length, storing it to next_event as the divisor
+	// thereby determine the total length, storing it to next_event as the track-total divisor
 	_next_event.length.clock_rate = 0;
 	for(size_t c = 0; c < _segments.size(); c++)
 	{
-		unsigned int multiplier = _track_clock_rate / _segments[c].duration.clock_rate;
-		_next_event.length.clock_rate += _segments[c].duration.length * multiplier;
+		unsigned int multiplier = _track_clock_rate / _segments[c].length_of_a_bit.clock_rate;
+		_next_event.length.clock_rate += _segments[c].length_of_a_bit.length * _segments[c].number_of_bits * multiplier;
 	}
 
 	_segment_pointer = _bit_pointer = 0;
diff --git a/Storage/Disk/PCMTrack.hpp b/Storage/Disk/PCMTrack.hpp
index 0cfc0f451..758846926 100644
--- a/Storage/Disk/PCMTrack.hpp
+++ b/Storage/Disk/PCMTrack.hpp
@@ -15,22 +15,13 @@
 namespace Storage {
 
 /*!
-	A segment of PCM-sampled data. The clock rate in the duration is taken to be relative to all other
-	segments that comprise a track rather than absolute, and the length is taken to be the number of
-	bits from @c data that are actually present.
+	A segment of PCM-sampled data.
 
 	Bits from each byte are taken MSB to LSB.
-
-	Actual segment lengths will be calculated such that all segments that comprise a track exactly fill the track.
-
-	So the segment for a track with only a single segment may supply any clock rate other than 0. It will exactly
-	fill the track, so if it has 7 samples then there will be at most a flux transition every 1/7th of a rotation.
-
-	If a track consists of two segments, one with clock rate 1 and one with clock rate 2, the second will be
-	clocked twice as fast as the first.
 */
 struct PCMSegment {
-	Time duration;
+	Time length_of_a_bit;
+	unsigned int number_of_bits;
 	std::unique_ptr<uint8_t> data;
 };
 
diff --git a/Storage/Storage.hpp b/Storage/Storage.hpp
index ac65da093..91abe794e 100644
--- a/Storage/Storage.hpp
+++ b/Storage/Storage.hpp
@@ -9,10 +9,24 @@
 #ifndef Storage_hpp
 #define Storage_hpp
 
+#include "../NumberTheory/Factors.hpp"
+
 namespace Storage {
 
 struct Time {
 	unsigned int length, clock_rate;
+
+	inline void simplify()
+	{
+		unsigned int common_divisor = NumberTheory::greatest_common_divisor(length, clock_rate);
+		length /= common_divisor;
+		clock_rate /= common_divisor;
+	}
+
+	inline float get_float()
+	{
+		return (float)length / (float)clock_rate;
+	}
 };
 
 }
diff --git a/Storage/Tape/Tape.cpp b/Storage/Tape/Tape.cpp
index 822093b1c..8717a5ce4 100644
--- a/Storage/Tape/Tape.cpp
+++ b/Storage/Tape/Tape.cpp
@@ -7,6 +7,7 @@
 //
 
 #include "Tape.hpp"
+#include "../../NumberTheory/Factors.hpp"
 
 using namespace Storage;
 
@@ -16,12 +17,13 @@ void Tape::seek(Time seek_time)
 }
 
 TapePlayer::TapePlayer(unsigned int input_clock_rate) :
-	_input_clock_rate(input_clock_rate)
+	TimedEventLoop(input_clock_rate)
 {}
 
 void TapePlayer::set_tape(std::shared_ptr<Storage::Tape> tape)
 {
 	_tape = tape;
+	reset_timer();
 	get_next_pulse();
 }
 
@@ -32,38 +34,34 @@ bool TapePlayer::has_tape()
 
 void TapePlayer::get_next_pulse()
 {
-	_input.time_into_pulse = 0;
+	// get the new pulse
 	if(_tape)
-		_input.current_pulse = _tape->get_next_pulse();
+		_current_pulse = _tape->get_next_pulse();
 	else
 	{
-		_input.current_pulse.length.length = 1;
-		_input.current_pulse.length.clock_rate = 1;
-		_input.current_pulse.type = Storage::Tape::Pulse::Zero;
-	}
-	if(_input.pulse_stepper == nullptr || _input.current_pulse.length.clock_rate != _input.pulse_stepper->get_output_rate())
-	{
-		_input.pulse_stepper.reset(new SignalProcessing::Stepper(_input.current_pulse.length.clock_rate, _input_clock_rate));
+		_current_pulse.length.length = 1;
+		_current_pulse.length.clock_rate = 1;
+		_current_pulse.type = Storage::Tape::Pulse::Zero;
 	}
+
+	set_next_event_time_interval(_current_pulse.length);
 }
 
-void TapePlayer::run_for_cycles(unsigned int number_of_cycles)
+void TapePlayer::run_for_cycles(int number_of_cycles)
 {
 	if(has_tape())
 	{
-		while(number_of_cycles--)
-		{
-			_input.time_into_pulse += (unsigned int)_input.pulse_stepper->step();
-			while(_input.time_into_pulse >= _input.current_pulse.length.length)
-			{
-				run_for_input_pulse();
-			}
-		}
+		::TimedEventLoop::run_for_cycles(number_of_cycles);
 	}
 }
 
 void TapePlayer::run_for_input_pulse()
 {
-	process_input_pulse(_input.current_pulse);
+	jump_to_next_event();
+}
+
+void TapePlayer::process_next_event()
+{
+	process_input_pulse(_current_pulse);
 	get_next_pulse();
 }
diff --git a/Storage/Tape/Tape.hpp b/Storage/Tape/Tape.hpp
index 47babe011..47ebea129 100644
--- a/Storage/Tape/Tape.hpp
+++ b/Storage/Tape/Tape.hpp
@@ -10,8 +10,7 @@
 #define Tape_hpp
 
 #include <memory>
-#include "../../SignalProcessing/Stepper.hpp"
-#include "../Storage.hpp"
+#include "../TimedEventLoop.hpp"
 
 namespace Storage {
 
@@ -48,29 +47,25 @@ class Tape {
 	Will call @c process_input_pulse instantaneously upon reaching *the end* of a pulse. Therefore a subclass
 	can decode pulses into data within process_input_pulse, using the supplied pulse's @c length and @c type.
 */
-class TapePlayer {
+class TapePlayer: public TimedEventLoop {
 	public:
 		TapePlayer(unsigned int input_clock_rate);
 
 		void set_tape(std::shared_ptr<Storage::Tape> tape);
 		bool has_tape();
 
-		void run_for_cycles(unsigned int number_of_cycles);
+		void run_for_cycles(int number_of_cycles);
 		void run_for_input_pulse();
 
 	protected:
+		virtual void process_next_event();
 		virtual void process_input_pulse(Tape::Pulse pulse) = 0;
 
 	private:
 		inline void get_next_pulse();
 
-		unsigned int _input_clock_rate;
 		std::shared_ptr<Storage::Tape> _tape;
-		struct {
-			Tape::Pulse current_pulse;
-			std::unique_ptr<SignalProcessing::Stepper> pulse_stepper;
-			uint32_t time_into_pulse;
-		} _input;
+		Tape::Pulse _current_pulse;
 };
 
 }
diff --git a/Storage/TimedEventLoop.cpp b/Storage/TimedEventLoop.cpp
new file mode 100644
index 000000000..b0851b7f8
--- /dev/null
+++ b/Storage/TimedEventLoop.cpp
@@ -0,0 +1,72 @@
+//
+//  TimedEventLoop.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 29/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#include "TimedEventLoop.hpp"
+#include "../NumberTheory/Factors.hpp"
+
+using namespace Storage;
+
+TimedEventLoop::TimedEventLoop(unsigned int input_clock_rate) :
+	_input_clock_rate(input_clock_rate) {}
+
+void TimedEventLoop::run_for_cycles(int number_of_cycles)
+{
+	_time_into_interval += (unsigned int)_stepper->step((uint64_t)number_of_cycles);
+	while(_time_into_interval >= _event_interval.length)
+	{
+		process_next_event();
+	}
+}
+
+void TimedEventLoop::reset_timer()
+{
+	_time_into_interval = 0;
+	_stepper.reset();
+}
+
+void TimedEventLoop::jump_to_next_event()
+{
+	reset_timer();
+	process_next_event();
+}
+
+void TimedEventLoop::set_next_event_time_interval(Time interval)
+{
+	// figure out how much time has been run since the last bit ended
+	if(_stepper)
+	{
+		_time_into_interval -= _event_interval.length;
+		if(_time_into_interval)
+		{
+			// simplify the quotient
+			unsigned int common_divisor = NumberTheory::greatest_common_divisor(_time_into_interval, _event_interval.clock_rate);
+			_time_into_interval /= common_divisor;
+			_event_interval.clock_rate /= common_divisor;
+
+			// build a quotient that is the sum of the time overrun plus the incoming time and adjust the time overrun
+			// to be in terms of the new quotient
+			unsigned int denominator = NumberTheory::least_common_multiple(_event_interval.clock_rate, interval.clock_rate);
+			interval.length *= denominator / interval.clock_rate;
+			interval.clock_rate = denominator;
+			_time_into_interval *= denominator / _event_interval.clock_rate;
+		}
+	}
+	else
+	{
+		_time_into_interval = 0;
+	}
+
+	// store new interval
+	_event_interval = interval;
+
+	// adjust stepper if required
+	if(!_stepper || _event_interval.clock_rate != _stepper->get_output_rate())
+	{
+		_stepper.reset(new SignalProcessing::Stepper(_event_interval.clock_rate, _input_clock_rate));
+	}
+}
diff --git a/Storage/TimedEventLoop.hpp b/Storage/TimedEventLoop.hpp
new file mode 100644
index 000000000..232ff8c6d
--- /dev/null
+++ b/Storage/TimedEventLoop.hpp
@@ -0,0 +1,40 @@
+//
+//  TimedEventLoop.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 29/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#ifndef TimedEventLoop_hpp
+#define TimedEventLoop_hpp
+
+#include "Storage.hpp"
+
+#include <memory>
+#include "../SignalProcessing/Stepper.hpp"
+
+namespace Storage {
+
+	class TimedEventLoop {
+		public:
+			TimedEventLoop(unsigned int input_clock_rate);
+			void run_for_cycles(int number_of_cycles);
+
+		protected:
+			void reset_timer();
+			void set_next_event_time_interval(Time interval);
+			void jump_to_next_event();
+
+			virtual void process_next_event() = 0;
+
+		private:
+			unsigned int _input_clock_rate;
+			Time _event_interval;
+			std::unique_ptr<SignalProcessing::Stepper> _stepper;
+			uint32_t _time_into_interval;
+	};
+
+}
+
+#endif /* TimedEventLoop_hpp */