Sketched out the generic interface for a disk, documenting it and the tape interface while I'm here.

OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj

@@ -42,6 +42,7 @@
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+		4BAB62AD1D3272D200DF5BA0 /* Disk.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BAB62AB1D3272D200DF5BA0 /* Disk.cpp */; };
 		4BB298EE1B587D8400A49093 /* 6502_functional_test.bin in Resources */ = {isa = PBXBuildFile; fileRef = 4BB297E01B587D8300A49093 /* 6502_functional_test.bin */; };
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@@ -415,6 +416,9 @@
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 		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>"; };
+		4BAB62AB1D3272D200DF5BA0 /* Disk.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = Disk.cpp; sourceTree = "<group>"; };
+		4BAB62AC1D3272D200DF5BA0 /* Disk.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = Disk.hpp; sourceTree = "<group>"; };
+		4BAB62AE1D32730D00DF5BA0 /* Storage.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = Storage.hpp; sourceTree = "<group>"; };
 		4BB297E01B587D8300A49093 /* 6502_functional_test.bin */ = {isa = PBXFileReference; lastKnownFileType = archive.macbinary; path = 6502_functional_test.bin; sourceTree = "<group>"; };
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 		4BB297E51B587D8300A49093 /*  start */ = {isa = PBXFileReference; lastKnownFileType = file; path = " start"; sourceTree = "<group>"; };
@@ -941,7 +945,9 @@
 		4B69FB391C4D908A00B5F0AA /* Storage */ = {
 			isa = PBXGroup;
 			children = (
+				4BAB62AA1D3272D200DF5BA0 /* Disk */,
 				4B69FB3A1C4D908A00B5F0AA /* Tape */,
+				4BAB62AE1D32730D00DF5BA0 /* Storage.hpp */,
 			);
 			name = Storage;
 			path = ../../Storage;
@@ -969,6 +975,15 @@
 			path = Formats;
 			sourceTree = "<group>";
 		};
+		4BAB62AA1D3272D200DF5BA0 /* Disk */ = {
+			isa = PBXGroup;
+			children = (
+				4BAB62AB1D3272D200DF5BA0 /* Disk.cpp */,
+				4BAB62AC1D3272D200DF5BA0 /* Disk.hpp */,
+			);
+			path = Disk;
+			sourceTree = "<group>";
+		};
 		4BB297E41B587D8300A49093 /* Wolfgang Lorenz 6502 test suite */ = {
 			isa = PBXGroup;
 			children = (
@@ -1818,6 +1833,7 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				4BAB62AD1D3272D200DF5BA0 /* Disk.cpp in Sources */,
 				4BC9DF4F1D04691600F44158 /* 6560.cpp in Sources */,
 				4BBF99151C8FBA6F0075DAFB /* CRTOpenGL.cpp in Sources */,
 				4B0CCC451C62D0B3001CAC5F /* CRT.cpp in Sources */,

Storage/Disk/Disk.cpp

@@ -0,0 +1,9 @@
+//
+//  Disk.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 10/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#include "Disk.hpp"

Storage/Disk/Disk.hpp

@@ -0,0 +1,66 @@
+//
+//  Disk.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 10/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#ifndef Disk_hpp
+#define Disk_hpp
+
+#include <memory>
+#include "../Storage.hpp"
+
+namespace Storage {
+
+/*!
+	Models a single track on a disk as a series of events, each event being of arbitrary length
+	and resulting in either a flux transition or the sensing of an index hole.
+	
+	Subclasses should implement @c get_next_event.
+*/
+class Track {
+	public:
+		struct Event {
+			enum {
+				IndexHole, FluxTransition
+			} type;
+			Time length;
+		};
+
+		virtual Event get_next_event() = 0;
+};
+
+/*!
+	Models a disk as a collection of tracks, providing a range of possible track positions and allowing
+	a point sampling of the track beneath any of those positions (if any).
+
+	The intention is not that tracks necessarily be evenly spaced; a head_position_count of 3 wih track
+	A appearing in positions 0 and 1, and track B appearing in position 2 is an appropriate use of this API
+	if it matches the media.
+
+	The track returned is point sampled only; if a particular disk drive has a sufficiently large head to
+	pick up multiple tracks at once then the drive responsible for asking for multiple tracks and for
+	merging the results.
+*/
+class Disk {
+	public:
+
+		/*!
+			Returns the number of discrete positions that this disk uses to model its complete surface area.
+
+			This is not necessarily a track count. There is no implicit guarantee that every position will
+			return a distinct track, or — if the media is holeless — will return any track at all.
+		*/
+		virtual unsigned int get_head_position_count() = 0;
+
+		/*!
+			Returns the @c Track at @c position if there are any detectable events there; returns @c nullptr otherwise.
+		*/
+		virtual std::shared_ptr<Track> get_track_at_position(unsigned int position) = 0;
+};
+
+}
+
+#endif /* Disk_hpp */

Storage/Storage.hpp

@@ -0,0 +1,20 @@
+//
+//  Storage.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 10/07/2016.
+//  Copyright © 2016 Thomas Harte. All rights reserved.
+//
+
+#ifndef Storage_hpp
+#define Storage_hpp
+
+namespace Storage {
+
+struct Time {
+	unsigned int length, clock_rate;
+};
+
+}
+
+#endif /* Storage_h */

Storage/Tape/Tape.cpp

@@ -10,7 +10,7 @@
 
 using namespace Storage;
 
-void Tape::seek(Tape::Time seek_time)
+void Tape::seek(Time seek_time)
 {
 	// TODO: as best we can
 }

Storage/Tape/Tape.hpp

@@ -11,15 +11,23 @@
 
 #include <memory>
 #include "../../SignalProcessing/Stepper.hpp"
+#include "../Storage.hpp"
 
 namespace Storage {
 
+/*!
+	Models a tape as a sequence of pulses, each pulse being of arbitrary length and described
+	by their relationship with zero:
+		- high pulses exit from zero upward before returning to it;
+		- low pulses exit from zero downward before returning to it;
+		- zero pulses run along zero.
+		
+	Subclasses should implement at least @c get_next_pulse and @c reset to provide a serial feeding
+	of pulses and the ability to return to the start of the feed. They may also implement @c seek if
+	a better implementation than a linear search from the @c reset time can be implemented.
+*/
 class Tape {
 	public:
-		struct Time {
-			unsigned int length, clock_rate;
-		};
-
 		struct Pulse {
 			enum {
 				High, Low, Zero
@@ -30,9 +38,16 @@ class Tape {
 		virtual Pulse get_next_pulse() = 0;
 		virtual void reset() = 0;
 
-		virtual void seek(Time seek_time);
+		virtual void seek(Time seek_time);	// TODO
 };
 
+/*!
+	Provides a helper for: (i) retaining a reference to a tape; and (ii) running the tape at a certain
+	input clock rate.
+
+	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 {
 	public:
 		TapePlayer(unsigned int input_clock_rate);