diff --git a/Components/9918/Implementation/9918.cpp b/Components/9918/Implementation/9918.cpp
index 4ff388901..5adb22a24 100644
--- a/Components/9918/Implementation/9918.cpp
+++ b/Components/9918/Implementation/9918.cpp
@@ -806,41 +806,77 @@ void Base<personality>::commit_register(int reg, uint8_t value) {
 			break;
 
 			case 32:
+				Storage<personality>::command_context_.source_x =
+					(Storage<personality>::command_context_.source_x & ~0xff) |
+					value;
+			break;
 			case 33:
-				LOG("TODO: Yamaha command source x; " << PADHEX(2) << +value);
+				Storage<personality>::command_context_.source_x =
+					(Storage<personality>::command_context_.source_x & ~0x100) |
+					((value & 1) << 8);
 			break;
 
 			case 34:
+				Storage<personality>::command_context_.source_y =
+					(Storage<personality>::command_context_.source_y & ~0xff) |
+					value;
+			break;
 			case 35:
-				LOG("TODO: Yamaha command source y; " << PADHEX(2) << +value);
+				Storage<personality>::command_context_.source_y =
+					(Storage<personality>::command_context_.source_y & ~0x300) |
+					((value & 3) << 8);
 			break;
 
 			case 36:
+				Storage<personality>::command_context_.destination_x =
+					(Storage<personality>::command_context_.destination_x & ~0xff) |
+					value;
+			break;
 			case 37:
-				LOG("TODO: Yamaha command destination x; " << PADHEX(2) << +value);
+				Storage<personality>::command_context_.destination_x =
+					(Storage<personality>::command_context_.destination_x & ~0x100) |
+					((value & 1) << 8);
 			break;
 
 			case 38:
+				Storage<personality>::command_context_.destination_y =
+					(Storage<personality>::command_context_.destination_y & ~0xff) |
+					value;
+			break;
 			case 39:
-				LOG("TODO: Yamaha command destination y; " << PADHEX(2) << +value);
+				Storage<personality>::command_context_.destination_y =
+					(Storage<personality>::command_context_.destination_y & ~0x300) |
+					((value & 3) << 8);
 			break;
 
 			case 40:
+				Storage<personality>::command_context_.size_x =
+					(Storage<personality>::command_context_.size_x & ~0xff) |
+					value;
+			break;
 			case 41:
-				LOG("TODO: Yamaha command size x; " << PADHEX(2) << +value);
+				Storage<personality>::command_context_.size_x =
+					(Storage<personality>::command_context_.size_x & ~0x100) |
+					((value & 1) << 8);
 			break;
 
 			case 42:
+				Storage<personality>::command_context_.size_y =
+					(Storage<personality>::command_context_.size_y & ~0xff) |
+					value;
+			break;
 			case 43:
-				LOG("TODO: Yamaha command size y; " << PADHEX(2) << +value);
+				Storage<personality>::command_context_.size_y =
+					(Storage<personality>::command_context_.size_y & ~0x300) |
+					((value & 3) << 8);
 			break;
 
 			case 44:
-				LOG("TODO: Yamaha command colour; " << PADHEX(2) << +value);
+				Storage<personality>::command_context_.colour = value;
 			break;
 
 			case 45:
-				LOG("TODO: Yamaha VRAM bank selection addressing and command arguments; " << PADHEX(2) << +value);
+				Storage<personality>::command_context_.arguments = value;
 				// b6: 0 = video RAM; 1 = expansion RAM.
 				// b5: MXD (???)
 				// b4: MXS
diff --git a/Components/9918/Implementation/9918Base.hpp b/Components/9918/Implementation/9918Base.hpp
index 90ccb5ee5..b5b797501 100644
--- a/Components/9918/Implementation/9918Base.hpp
+++ b/Components/9918/Implementation/9918Base.hpp
@@ -15,7 +15,9 @@
 #include "../../../Numeric/BitReverse.hpp"
 #include "../../../Outputs/CRT/CRT.hpp"
 
+#include "AccessEnums.hpp"
 #include "PersonalityTraits.hpp"
+#include "YamahaCommands.hpp"
 
 #include <array>
 #include <cassert>
@@ -27,44 +29,6 @@
 namespace TI {
 namespace TMS {
 
-// The screen mode is a necessary predecessor to picking the line mode,
-// which is the thing latched per line.
-enum class ScreenMode {
-	// Original TMS modes.
-	Blank,
-	Text,
-	MultiColour,
-	ColouredText,
-	Graphics,
-
-	// 8-bit Sega modes.
-	SMSMode4,
-
-	// New Yamaha V9938 modes.
-	YamahaText80,
-	YamahaGraphics3,
-	YamahaGraphics4,
-	YamahaGraphics5,
-	YamahaGraphics6,
-	YamahaGraphics7,
-
-	// Rebranded Yamaha V9938 modes.
-	YamahaGraphics1 = ColouredText,
-	YamahaGraphics2 = Graphics,
-};
-
-enum class LineMode {
-	Text,
-	Character,
-	Refresh,
-	SMS,
-	Yamaha,
-};
-
-enum class MemoryAccess {
-	Read, Write, None
-};
-
 // Temporary buffers collect a representation of each line prior to pixel serialisation.
 //
 // TODO: either template on personality, to avoid having to be the union of all potential footprints,
@@ -217,6 +181,10 @@ template <Personality personality> struct Storage<personality, std::enable_if_t<
 		next_event_ = no_sprites_events;
 	}
 
+	// Command engine state.
+	CommandContext command_context_;
+	Command *command_ = nullptr;
+
 	Storage() noexcept {
 		// Perform sanity checks on the event lists.
 #ifndef NDEBUG
diff --git a/Components/9918/Implementation/AccessEnums.hpp b/Components/9918/Implementation/AccessEnums.hpp
new file mode 100644
index 000000000..e7f94b55a
--- /dev/null
+++ b/Components/9918/Implementation/AccessEnums.hpp
@@ -0,0 +1,57 @@
+//
+//  AccessEnums.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 26/01/2023.
+//  Copyright © 2023 Thomas Harte. All rights reserved.
+//
+
+#ifndef AccessEnums_hpp
+#define AccessEnums_hpp
+
+
+namespace TI {
+namespace TMS {
+
+// The screen mode is a necessary predecessor to picking the line mode,
+// which is the thing latched per line.
+enum class ScreenMode {
+	// Original TMS modes.
+	Blank,
+	Text,
+	MultiColour,
+	ColouredText,
+	Graphics,
+
+	// 8-bit Sega modes.
+	SMSMode4,
+
+	// New Yamaha V9938 modes.
+	YamahaText80,
+	YamahaGraphics3,
+	YamahaGraphics4,
+	YamahaGraphics5,
+	YamahaGraphics6,
+	YamahaGraphics7,
+
+	// Rebranded Yamaha V9938 modes.
+	YamahaGraphics1 = ColouredText,
+	YamahaGraphics2 = Graphics,
+};
+
+enum class LineMode {
+	Text,
+	Character,
+	Refresh,
+	SMS,
+	Yamaha,
+};
+
+enum class MemoryAccess {
+	Read, Write, None
+};
+
+}
+}
+
+#endif /* AccessEnums_hpp */
diff --git a/Components/9918/Implementation/YamahaCommands.hpp b/Components/9918/Implementation/YamahaCommands.hpp
new file mode 100644
index 000000000..692c411c0
--- /dev/null
+++ b/Components/9918/Implementation/YamahaCommands.hpp
@@ -0,0 +1,46 @@
+//
+//  YamahaCommands.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 26/01/2023.
+//  Copyright © 2023 Thomas Harte. All rights reserved.
+//
+
+#ifndef YamahaCommands_hpp
+#define YamahaCommands_hpp
+
+#include "AccessEnums.hpp"
+
+namespace TI {
+namespace TMS {
+
+struct CommandContext {
+	int source_x = 0, source_y = 0;
+	int destination_x = 0, destination_y = 0;
+	int size_x = 0, size_y = 0;
+	uint8_t colour = 0;
+	uint8_t arguments = 0;
+};
+
+struct Command {
+	// In net:
+	//
+	// This command is blocked until @c access has been performed, reading
+	// from or writing to @c value. It should not be performed until at least
+	// @c cycles have passed.
+	MemoryAccess access = MemoryAccess::None;
+	int cycles = 0;
+	uint8_t value = 0;
+
+	/// Current command parameters.
+	CommandContext &context;
+	Command(CommandContext &context) : context(context) {}
+
+	/// Request that the fields above are updated given the completed access.
+	virtual bool next() = 0;
+};
+
+}
+}
+
+#endif /* YamahaCommands_hpp */
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index b807c9c8b..261c7967f 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -2216,6 +2216,8 @@
 		4BF0BC67297108D100CCA2B5 /* MemorySlotHandler.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = MemorySlotHandler.cpp; sourceTree = "<group>"; };
 		4BF0BC6F2973318E00CCA2B5 /* RP5C01.cpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.cpp; path = RP5C01.cpp; sourceTree = "<group>"; };
 		4BF0BC702973318E00CCA2B5 /* RP5C01.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = RP5C01.hpp; sourceTree = "<group>"; };
+		4BF0BC732982E54700CCA2B5 /* YamahaCommands.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = YamahaCommands.hpp; sourceTree = "<group>"; };
+		4BF0BC742982E6D300CCA2B5 /* AccessEnums.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = AccessEnums.hpp; sourceTree = "<group>"; };
 		4BF40A5525424C770033EA39 /* LanguageCardSwitches.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = LanguageCardSwitches.hpp; sourceTree = "<group>"; };
 		4BF40A5A254263140033EA39 /* AuxiliaryMemorySwitches.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = AuxiliaryMemorySwitches.hpp; sourceTree = "<group>"; };
 		4BF437EC209D0F7E008CBD6B /* SegmentParser.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = SegmentParser.cpp; sourceTree = "<group>"; };
@@ -4751,10 +4753,12 @@
 			children = (
 				4B43983829620FB1006B0BFC /* 9918.cpp */,
 				4BD388411FE34E010042B588 /* 9918Base.hpp */,
+				4BF0BC742982E6D300CCA2B5 /* AccessEnums.hpp */,
 				4B43983C29621024006B0BFC /* ClockConverter.hpp */,
 				4B43983F2967459B006B0BFC /* Draw.hpp */,
 				4B43983E29628538006B0BFC /* Fetch.hpp */,
 				4B262BFF29691F55002EC0F7 /* PersonalityTraits.hpp */,
+				4BF0BC732982E54700CCA2B5 /* YamahaCommands.hpp */,
 			);
 			path = Implementation;
 			sourceTree = "<group>";