Formalised clock-rate multiplication within disk drives, discovered that the stepper didn't have ideal behaviour for my timed event loop and hence nailed down the semantics a ilttle more.

(obiter: the 1540 now appears to discern the correct sequence of bits. Framing is off in my test printfs but that's neither here nor there).

Machines/Commodore/1540/C1540.cpp

@@ -14,7 +14,7 @@ using namespace Commodore::C1540;
 
 Machine::Machine() :
 	_shift_register(0),
-	Storage::DiskDrive(1000000, 300)
+	Storage::DiskDrive(1000000, 16, 300)
 {
 	// create a serial port and a VIA to run it
 	_serialPortVIA.reset(new SerialPortVIA);
@@ -115,7 +115,7 @@ void Machine::mos6522_did_change_interrupt_status(void *mos6522)
 
 void Machine::process_input_bit(int value, unsigned int cycles_since_index_hole)
 {
-	_shift_register = (_shift_register >> 1) | (value << 10);
+	_shift_register = (_shift_register << 1) | value;
 
 	static int bitCount = 0;
 	bitCount++;

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

SignalProcessing/Stepper.hpp

@@ -19,6 +19,9 @@ namespace SignalProcessing {
 	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
 {
@@ -33,7 +36,7 @@ class Stepper
 			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),

Storage/Disk/DiskDrive.cpp

@@ -10,17 +10,19 @@
 
 using namespace Storage;
 
-DiskDrive::DiskDrive(unsigned int clock_rate, unsigned int revolutions_per_minute) :
-	_clock_rate(clock_rate * 16),
+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 * 16)
+	TimedEventLoop(clock_rate * clock_rate_multiplier)
 {}
 
 void DiskDrive::set_expected_bit_length(Time bit_length)
 {
 	_bit_length = bit_length;
+	printf("expected bit length: %0.4f\n", bit_length.get_float() * 1000.0);
 
 	// 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
@@ -54,6 +56,28 @@ void DiskDrive::step(int direction)
 void DiskDrive::set_track()
 {
 	_track = _disk->get_track_at_position((unsigned int)_head_position);
+
+//	printf("***");
+//	int calibration = -1;
+//	int bit = 0, shift = 0;
+//	while(1)
+//	{
+//		Track::Event event = _track->get_next_event();
+//		if(event.type == Track::Event::IndexHole) break;
+//		if(calibration < 0) calibration = event.length.length;
+//
+//		int number_of_bits = event.length.length / calibration;
+//		while(number_of_bits > 1)
+//		{
+//			shift = (shift << 1)&0xff;
+//			number_of_bits--;
+//			bit++; if(bit == 8) { printf("%02x.", shift); bit = 0; }
+//		}
+//		shift = ((shift << 1) | 0x01)&0xff;
+//		bit++; if(bit == 8) { printf("%02x.", shift); bit = 0; }
+//	}
+//	printf("***\n");
+
 	reset_timer();
 	get_next_event();
 }
@@ -62,11 +86,13 @@ void DiskDrive::run_for_cycles(int number_of_cycles)
 {
 	if(has_disk())
 	{
-		_cycles_since_index_hole += (unsigned int)number_of_cycles;
-
-		number_of_cycles *= 16;
-		_pll->run_for_cycles(number_of_cycles);
-		TimedEventLoop::run_for_cycles(number_of_cycles);
+		number_of_cycles *= _clock_rate_multiplier;
+		while(number_of_cycles--)
+		{
+			_cycles_since_index_hole ++;
+			_pll->run_for_cycles(1);
+			TimedEventLoop::run_for_cycles(1);
+		}
 	}
 }
 
@@ -90,6 +116,7 @@ void DiskDrive::get_next_event()
 	event_interval.clock_rate *= _revolutions_per_minute;
 	event_interval.simplify();
 	set_next_event_time_interval(event_interval);
+//	printf("bit length: %0.4f\n", event_interval.get_float() * 1000.0);
 }
 
 void DiskDrive::process_next_event()

Storage/Disk/DiskDrive.hpp

@@ -17,7 +17,7 @@ namespace Storage {
 
 class DiskDrive: public DigitalPhaseLockedLoop::Delegate, public TimedEventLoop {
 	public:
-		DiskDrive(unsigned int clock_rate, unsigned int revolutions_per_minute);
+		DiskDrive(unsigned int clock_rate, unsigned int clock_rate_multiplier, unsigned int revolutions_per_minute);
 
 		void set_expected_bit_length(Time bit_length);
 
@@ -43,6 +43,7 @@ class DiskDrive: public DigitalPhaseLockedLoop::Delegate, public TimedEventLoop
 	private:
 		Time _bit_length;
 		unsigned int _clock_rate;
+		unsigned int _clock_rate_multiplier;
 		unsigned int _revolutions_per_minute;
 
 		std::shared_ptr<DigitalPhaseLockedLoop> _pll;

Storage/Disk/PCMTrack.cpp

@@ -32,6 +32,8 @@ PCMTrack::Event PCMTrack::get_next_event()
 	while(_segment_pointer < _segments.size())
 	{
 		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].number_of_bits)
 		{
@@ -39,7 +41,7 @@ PCMTrack::Event PCMTrack::get_next_event()
 			// 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 * _segments[_segment_pointer].length_of_a_bit.length;
+			_next_event.length.length += bit_length;
 
 			if(bit) return _next_event;
 		}
@@ -74,5 +76,12 @@ void PCMTrack::fix_length()
 		_next_event.length.clock_rate += _segments[c].length_of_a_bit.length * _segments[c].number_of_bits * multiplier;
 	}
 
+	uint8_t *data = _segments[0].data.get();
+	for(int c = 0; c < _segments[0].number_of_bits >> 3; c++)
+	{
+		printf("%02x.", data[c]);
+	}
+	printf("===\n");
+
 	_segment_pointer = _bit_pointer = 0;
 }

Storage/Storage.hpp

@@ -22,6 +22,11 @@ struct Time {
 		length /= common_divisor;
 		clock_rate /= common_divisor;
 	}
+
+	inline float get_float()
+	{
+		return (float)length / (float)clock_rate;
+	}
 };
 
 }