diff --git a/Storage/Disk/Drive.hpp b/Storage/Disk/Drive.hpp
index 0b45a43e8..234aa87f6 100644
--- a/Storage/Disk/Drive.hpp
+++ b/Storage/Disk/Drive.hpp
@@ -9,11 +9,14 @@
 #ifndef Drive_hpp
 #define Drive_hpp
 
-#include <memory>
-
 #include "Disk.hpp"
-#include "../../ClockReceiver/Sleeper.hpp"
+#include "PCMSegment.hpp"
+#include "PCMPatchedTrack.hpp"
+
 #include "../TimedEventLoop.hpp"
+#include "../../ClockReceiver/Sleeper.hpp"
+
+#include <memory>
 
 namespace Storage {
 namespace Disk {
@@ -78,16 +81,50 @@ class Drive: public Sleeper, public TimedEventLoop {
 		*/
 		void set_motor_on(bool);
 
+		/*!
+			@returns @c true if the motor is on; @c false otherwise.
+		*/
+		bool get_motor_on();
+
+		/*!
+			Begins write mode, initiating a PCM sampled region of data. Bits should be written via
+			@c write_bit. They will be written with the length set via @c set_expected_bit_length.
+			It is acceptable to supply a backlog of bits. Flux transition events will not be reported
+			while writing.
+
+			@param clamp_to_index_hole If @c true then writing will automatically be truncated by
+			the index hole. Writing will continue over the index hole otherwise.
+		*/
+		void begin_writing(bool clamp_to_index_hole);
+
+		/*!
+			Writes the bit @c value as the next in the PCM stream initiated by @c begin_writing.
+		*/
+		void write_bit(bool value);
+
+		/*!
+			Ends write mode, switching back to read mode. The drive will stop overwriting events.
+		*/
+		void end_writing();
+
 		/*!
 			Advances the drive by @c number_of_cycles cycles.
 		*/
 		void run_for(const Cycles cycles);
 
 		/*!
-			Provides a mechanism to receive track events as they occur.
+			Provides a mechanism to receive track events as they occur, including the synthetic
+			event of "you told me to output the following data, and I've done that now".
 		*/
 		struct EventDelegate {
+			/// Informs the delegate that @c event has been reached.
 			virtual void process_event(const Track::Event &event) = 0;
+
+			/*!
+				If the drive is in write mode, announces that all queued bits have now been written.
+				If the controller provides further bits now then there will be no gap in written data.
+			*/
+			virtual void process_write_completed() {}
 		};
 
 		/// Sets the current event delegate.
@@ -97,22 +134,53 @@ class Drive: public Sleeper, public TimedEventLoop {
 		bool is_sleeping();
 
 	private:
-		std::shared_ptr<Track> track_;
+		// Drives [usually] contain an entire disk; from that a certain track
+		// will be currently under the head.
 		std::shared_ptr<Disk> disk_;
-		int cycles_since_index_hole_ = 0;
-		Time rotational_multiplier_;
-
+		std::shared_ptr<Track> track_;
 		bool has_disk_ = false;
 
+		// Contains the multiplier that converts between track-relative lengths
+		// to real-time lengths — so it's the reciprocal of rotation speed.
+		Time rotational_multiplier_;
+
+		// A count of time since the index hole was last seen. Which is used to
+		// determine how far the drive is into a full rotation when switching to
+		// a new track.
+		int cycles_since_index_hole_ = 0;
+
+		// A record of head position and active head.
 		int head_position_ = 0;
 		unsigned int head_ = 0;
 
+		// Motor control state.
+		bool motor_is_on_ = false;
+
+		// If the drive is not currently reading then it is writing. While writing
+		// it can optionally be told to clamp to the index hole.
+		bool is_reading_;
+		bool clamp_writing_to_index_hole_;
+
+		// If writing is occurring then the drive will be accumulating a write segment,
+		// for addition to a patched track.
+		std::shared_ptr<PCMPatchedTrack> patched_track_;
+		PCMSegment write_segment_;
+		Time write_start_time_;
+
+		// Maintains appropriate counting to know when to indicate that writing
+		// is complete.
+		Time cycles_until_bits_written_;
+		Time cycles_per_bit_;
+
+		// TimedEventLoop call-ins and state.
 		void process_next_event();
 		void get_next_event(const Time &duration_already_passed);
 		Track::Event current_event_;
-		bool motor_is_on_ = false;
 
+		// Helper for track changes.
 		Time get_time_into_track();
+
+		// The target (if any) for track events.
 		EventDelegate *event_delegate_ = nullptr;
 };