Attempts to add 5.25" drive support to the IIgs.

I want to try some classic software.

Components/DiskII/DiskIIDrive.cpp

@@ -0,0 +1,46 @@
+//
+//  DiskIIDrive.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 20/11/2020.
+//  Copyright © 2020 Thomas Harte. All rights reserved.
+//
+
+#include "DiskIIDrive.hpp"
+
+using namespace Apple::Disk;
+
+DiskIIDrive::DiskIIDrive(int input_clock_rate) :
+	IWMDrive(input_clock_rate, 1) {
+	Drive::set_rotation_speed(300.0f);
+}
+
+void DiskIIDrive::set_enabled(bool enabled) {
+	set_motor_on(enabled);
+}
+
+void DiskIIDrive::set_control_lines(int lines) {
+	// If the stepper magnet selections have changed, and any is on, see how
+	// that moves the head.
+	if(lines ^ stepper_mask_ && lines) {
+		// Convert from a representation of bits set to the centre of pull.
+		int direction = 0;
+		if(lines&1) direction += (((stepper_position_ - 0) + 4)&7) - 4;
+		if(lines&2) direction += (((stepper_position_ - 2) + 4)&7) - 4;
+		if(lines&4) direction += (((stepper_position_ - 4) + 4)&7) - 4;
+		if(lines&8) direction += (((stepper_position_ - 6) + 4)&7) - 4;
+		const int bits_set = (lines&1) + ((lines >> 1)&1) + ((lines >> 2)&1) + ((lines >> 3)&1);
+		direction /= bits_set;
+
+		// Compare to the stepper position to decide whether that pulls in the current cog notch,
+		// or grabs a later one.
+		step(Storage::Disk::HeadPosition(-direction, 4));
+		stepper_position_ = (stepper_position_ - direction + 8) & 7;
+		printf("Step %0.2f\n", float(-direction) / 4.0f);
+	}
+	stepper_mask_ = lines;
+}
+
+bool DiskIIDrive::read() {
+	return !!(stepper_mask_ & 2) || get_is_read_only();
+}