Factor out spin lock, get a bit stricter on PointerSets.

Concurrency/SpinLock.cpp

@@ -0,0 +1,9 @@
+//
+//  SpinLock.cpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/02/2026.
+//  Copyright © 2026 Thomas Harte. All rights reserved.
+//
+
+#include "SpinLock.hpp"

Concurrency/SpinLock.hpp

@@ -0,0 +1,45 @@
+//
+//  SpinLock.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 14/02/2026.
+//  Copyright © 2026 Thomas Harte. All rights reserved.
+//
+
+#pragma once
+
+namespace Concurrency {
+
+enum class Barrier {
+	Relaxed,
+	AcquireRelease,
+};
+
+/*!
+	A basic spin lock. Applies a memory barrier as per the template.
+
+	Standard warnings apply: having revealed nothing to the scheduler, a holder of this lock might sleep
+	and block other eligble work.
+*/
+template <Barrier type>
+class SpinLock {
+public:
+	void lock() {
+		while(flag_.test_and_set(LockMemoryOrder));
+	}
+
+	void unlock() {
+		flag_.clear(UnlockMemoryOrder);
+	}
+
+private:
+	static constexpr auto LockMemoryOrder =
+		type == Barrier::Relaxed ? std::memory_order_relaxed : std::memory_order_acquire;
+	static constexpr auto UnlockMemoryOrder =
+		type == Barrier::Relaxed ? std::memory_order_relaxed : std::memory_order_release;
+
+	// Note to self: this is guaranteed to construct in a clear state since C++20.
+	std::atomic_flag flag_;
+};
+
+}

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

@@ -1671,6 +1671,7 @@
 		4B55CE5E1C3B7D960093A61B /* MachineDocument.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = MachineDocument.swift; sourceTree = "<group>"; };
 		4B55DD8020DF06680043F2E5 /* MachinePicker.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = MachinePicker.swift; sourceTree = "<group>"; };
 		4B55DD8220DF06680043F2E5 /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.xib; name = Base; path = Base.lproj/MachinePicker.xib; sourceTree = "<group>"; };
+		4B56172C2F40D446003CB7FE /* SpinLock.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = SpinLock.hpp; sourceTree = "<group>"; };
 		4B58601C1F806AB200AEE2E3 /* MFMSectorDump.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = MFMSectorDump.cpp; sourceTree = "<group>"; };
 		4B58601D1F806AB200AEE2E3 /* MFMSectorDump.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = MFMSectorDump.hpp; sourceTree = "<group>"; };
 		4B59199A1DAC6C46005BB85C /* OricTAP.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = OricTAP.cpp; sourceTree = "<group>"; };
@@ -3245,6 +3246,7 @@
 			isa = PBXGroup;
 			children = (
 				4B3940E61DA83C8300427841 /* AsyncTaskQueue.hpp */,
+				4B56172C2F40D446003CB7FE /* SpinLock.hpp */,
 			);
 			name = Concurrency;
 			path = ../../Concurrency;

Outputs/ScanTargets/BufferingScanTarget.cpp

@@ -32,9 +32,6 @@ BufferingScanTarget::BufferingScanTarget() {
 	// Ensure proper initialisation of the two atomic pointer sets.
 	read_pointers_.store(write_pointers_, std::memory_order_relaxed);
 	submit_pointers_.store(write_pointers_, std::memory_order_relaxed);
-
-	// Establish initial state for is_updating_.
-	is_updating_.clear(std::memory_order_relaxed);
 }
 
 // MARK: - Producer; pixel data.
@@ -223,6 +220,8 @@ void BufferingScanTarget::announce(
 		is_first_in_frame_ = true;
 		previous_frame_was_complete_ = frame_is_complete_;
 		frame_is_complete_ = true;
+
+//		frames_[frame_pointer_]
 	}
 
 	// Proceed from here only if a change in visibility has occurred.

Outputs/ScanTargets/BufferingScanTarget.hpp

@@ -10,6 +10,7 @@
 
 #include "Outputs/ScanTarget.hpp"
 #include "Outputs/DisplayMetrics.hpp"
+#include "Concurrency/SpinLock.hpp"
 
 #include <array>
 #include <atomic>
@@ -88,12 +89,6 @@ public:
 		size_t first_scan;
 	};
 
-	struct Frame {
-		size_t first_scan;
-		size_t first_line;
-		bool previous_frame_was_complete;
-	};
-
 	/// Sets the area of memory to use as a scan buffer.
 	void set_scan_buffer(Scan *buffer, size_t size);
 
@@ -157,9 +152,8 @@ public:
 	/// change to modals, occurs simultaneously.
 	template <typename FuncT>
 	void perform(FuncT &&function) {
-		while(is_updating_.test_and_set(std::memory_order_acquire));
+		std::lock_guard guard(is_updating_);
 		function();
-		is_updating_.clear(std::memory_order_release);
 	}
 
 	/// @returns new Modals if any have been set since the last call to get_new_modals().
@@ -289,8 +283,8 @@ private:
 		// an ambiguity in the C++ standard; cf. https://stackoverflow.com/questions/17430377
 		PointerSet() noexcept = default;
 
-		// Squeezing this struct into 64 bits makes the std::atomics more likely
-		// to be lock free; they are under LLVM x86-64.
+		// Squeezing this struct into 128 bits makes the std::atomics more likely
+		// to be lock free; cf. `lock cmpxchg16b`.
 
 		// Points to the vended area in the write area texture.
 		// The vended area is always preceded by a guard pixel, so a
@@ -302,20 +296,23 @@ private:
 
 		// Points into the line buffer.
 		uint16_t line = 0;
+
+		// Points into the frame list.
+		uint8_t frame = 0;
 	};
+	static_assert(std::atomic<PointerSet>::is_always_lock_free);
 
 	/// A pointer to the final thing currently cleared for submission.
-	std::atomic<PointerSet> submit_pointers_;
+	alignas(64) std::atomic<PointerSet> submit_pointers_;
 
 	/// A pointer to the first thing not yet submitted for display; this is
 	/// atomic since it also acts as the buffer into which the write_pointers_
 	/// may run and is therefore used by both producer and consumer.
-	std::atomic<PointerSet> read_pointers_;
+	alignas(64) std::atomic<PointerSet> read_pointers_;
 
-	std::atomic<PointerSet> read_ahead_pointers_;
+	alignas(64) std::atomic<PointerSet> read_ahead_pointers_;
 
-	/// This is used as a spinlock to guard `perform` calls.
-	std::atomic_flag is_updating_;
+	Concurrency::SpinLock<Concurrency::Barrier::AcquireRelease> is_updating_;
 
 	/// A mutex for gettng access to anything the producer modifies — i.e. the write_pointers_,
 	/// data_type_size_ and write_area_texture_, and all other state to do with capturing
@@ -353,6 +350,14 @@ private:
 	size_t output_area_counter_ = 0;
 	size_t output_area_next_returned_ = 0;
 #endif
+
+	struct Frame {
+		size_t first_scan;
+		size_t first_line;
+		bool previous_frame_was_complete;
+	};
+	std::array<Frame, 5> frames_;
+	size_t frame_pointer_ = 0;
 };
 
 }