diff --git a/Components/6821/6821.hpp b/Components/6821/6821.hpp
new file mode 100644
index 000000000..334b40031
--- /dev/null
+++ b/Components/6821/6821.hpp
@@ -0,0 +1,32 @@
+//
+//  6821.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 19/03/2026.
+//  Copyright © 2026 Thomas Harte. All rights reserved.
+//
+
+#ifndef _821_hpp
+#define _821_hpp
+
+#include "Numeric/SizedInt.hpp"
+
+namespace Motorola::MC6821 {
+
+template <typename BusHandlerT>
+class MC6821 {
+public:
+	void write(const Numeric::SizedInt<2> address, const uint8_t value) {
+		(void)address;
+		(void)value;
+	}
+
+	uint8_t read(const Numeric::SizedInt<2> address) {
+		(void)address;
+		return 0;
+	}
+};
+
+};
+
+#endif /* _821_hpp */
diff --git a/Machines/Thomson/MO5/MO5.cpp b/Machines/Thomson/MO5/MO5.cpp
index f4725f040..f06776380 100644
--- a/Machines/Thomson/MO5/MO5.cpp
+++ b/Machines/Thomson/MO5/MO5.cpp
@@ -10,9 +10,19 @@
 
 #include "Machines/MachineTypes.hpp"
 #include "Processors/6809/6809.hpp"
+#include "Components/6821/6821.hpp"
 
 using namespace Thomson::MO5;
 
+// Video timing, as far as auto-translate lets me figure it out:
+//
+//	64 cycles/line;
+//	56 lines post signalled vsync, then 200 of video, then 56 more, for 312 total.
+//
+// Start of vsync is connected to CPU IRQ.
+//
+// Within a line: ??? Who knows ???
+//
 namespace {
 
 struct ConcreteMachine:
@@ -33,6 +43,8 @@ struct ConcreteMachine:
 
 		const auto &rom = roms.find(ROM::Name::ThomasonMO5v11)->second;
 		std::copy_n(rom.begin(), rom.size(), rom_.begin());
+
+		page_lower(false);
 	}
 
 	void run_for(const Cycles cycles) final {
@@ -49,21 +61,38 @@ struct ConcreteMachine:
 		const AddressT address,
 		CPU::M6809::data_t<read_write> value
 	) {
-		if constexpr (read_write != CPU::M6809::ReadWrite::NoData) {
-			if constexpr (CPU::M6809::is_read(read_write)) {
-				if(address >= 0xc000) {
-					value = rom_[address - 0xc000];
-				} else {
-					value = ram_[address];
+		if constexpr (read_write == CPU::M6809::ReadWrite::NoData) {
+			return Cycles(0);
+		} else {
+			if(address >= 0xa7c0 && address < 0xa800) {
+				switch(address) {
+					case 0xa7c0:	case 0xa7c1:	case 0xa7c2:	case 0xa7c3:
+						if constexpr (CPU::M6809::is_read(read_write)) {
+							value = system_pia_.read(+address);
+						} else {
+							system_pia_.write(+address, value);
+						}
+					break;
 				}
 			} else {
-				ram_[address] = value;
-				printf("%04x: RAM <- 0x%02x [S: %04x]\n", +address, value, m6809_.registers().s);
+				if constexpr (CPU::M6809::is_read(read_write)) {
+					if(address < 0x2000) {
+						value = start_pointer_[address];
+					} else if(address >= 0xc000) {
+						value = rom_[address - 0xc000];
+					} else {
+						value = ram_[address];
+					}
+				} else {
+					if(address < 0x2000) {
+						start_pointer_[address] = value;
+					} else {
+						ram_[address] = value;
+					}
+				}
 			}
 		}
 
-		// TODO: the lowest 8kb of memory can actually be paged, so the linear representation above isn't accurate.
-
 		return Cycles(0);
 	}
 
@@ -75,8 +104,15 @@ private:
 	};
 	CPU::M6809::Processor<M6809Traits> m6809_;
 
-	std::array<uint8_t, 0xf000 + 0x2000> ram_;
+	std::array<uint8_t, 0x10000 + 0x2000> ram_;
 	std::array<uint8_t, 0x4000> rom_;
+	uint8_t *start_pointer_ = nullptr;
+
+	void page_lower(const bool attributes) {
+		start_pointer_ = &ram_[attributes ? 0 : 0xf000];
+	}
+
+	Motorola::MC6821::MC6821<int> system_pia_;
 
 	// MARK: - ScanProducer.
 
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index 53b1eb4ab..f9dbaef9e 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -2531,6 +2531,7 @@
 		4BDACBEA22FFA5D20045EF7E /* ncr5380.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = ncr5380.cpp; sourceTree = "<group>"; };
 		4BDACBEB22FFA5D20045EF7E /* ncr5380.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = ncr5380.hpp; sourceTree = "<group>"; };
 		4BDB3D8522833321002D3CEE /* Keyboard.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = Keyboard.hpp; sourceTree = "<group>"; };
+		4BDB734F2F6CA1A200964D69 /* 6821.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = 6821.hpp; sourceTree = "<group>"; };
 		4BDCC5F81FB27A5E001220C5 /* ROMMachine.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = ROMMachine.hpp; sourceTree = "<group>"; };
 		4BDDBA981EF3451200347E61 /* Z80MachineCycleTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Z80MachineCycleTests.swift; sourceTree = "<group>"; };
 		4BE0151C286A8C8E00EA42E9 /* MemorySwitches.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = MemorySwitches.hpp; sourceTree = "<group>"; };
@@ -5276,6 +5277,7 @@
 				4BC080DD26A481C100D03FD8 /* 6526 */,
 				4B1E85791D174DEC001EF87D /* 6532 */,
 				4BC9DF4C1D04691600F44158 /* 6560 */,
+				4BDB73512F6CA1A200964D69 /* 6821 */,
 				4BE845221F2FF7F400A5EA22 /* 6845 */,
 				4BB307B8235001C300457D33 /* 6850 */,
 				4BD9137C1F3115AC009BCF85 /* 8255 */,
@@ -5591,6 +5593,14 @@
 			path = 5380;
 			sourceTree = "<group>";
 		};
+		4BDB73512F6CA1A200964D69 /* 6821 */ = {
+			isa = PBXGroup;
+			children = (
+				4BDB734F2F6CA1A200964D69 /* 6821.hpp */,
+			);
+			path = 6821;
+			sourceTree = "<group>";
+		};
 		4BE211FB253FC80800435408 /* AppleIIgs */ = {
 			isa = PBXGroup;
 			children = (