diff --git a/Machines/PCCompatible/CGA.hpp b/Machines/PCCompatible/CGA.hpp
index 897924656..ce47ee18e 100644
--- a/Machines/PCCompatible/CGA.hpp
+++ b/Machines/PCCompatible/CGA.hpp
@@ -15,6 +15,7 @@
 
 namespace PCCompatible {
 
+// TODO: border colour isn't currently honoured.
 class CGA {
 	public:
 		CGA() : crtc_(Motorola::CRTC::Personality::HD6845S, outputter_) {}
@@ -102,8 +103,7 @@ class CGA {
 				crt(910, 8, Outputs::Display::Type::NTSC60, Outputs::Display::InputDataType::Red2Green2Blue2)
 			{
 				crt.set_visible_area(Outputs::Display::Rect(0.097f, 0.095f, 0.82f, 0.82f));
-//				crt.set_display_type(Outputs::Display::DisplayType::CompositeColour);	// TODO: needs to be a user option.
-				crt.set_display_type(Outputs::Display::DisplayType::RGB);	// TODO: needs to be a user option.
+				crt.set_display_type(Outputs::Display::DisplayType::RGB);
 			}
 
 			void set_mode(uint8_t control) {
diff --git a/Machines/PCCompatible/PCCompatible.cpp b/Machines/PCCompatible/PCCompatible.cpp
index a0e57a8dd..55529d43b 100644
--- a/Machines/PCCompatible/PCCompatible.cpp
+++ b/Machines/PCCompatible/PCCompatible.cpp
@@ -15,6 +15,7 @@
 #include "Memory.hpp"
 #include "PIC.hpp"
 #include "PIT.hpp"
+#include "RTC.hpp"
 
 #include "../../InstructionSets/x86/Decoder.hpp"
 #include "../../InstructionSets/x86/Flags.hpp"
@@ -549,6 +550,25 @@ class i8255PortHandler : public Intel::i8255::PortHandler {
 				if(drive_count) low_switches_ |= 0xb0001;
 			}
 
+		/// Supplies a hint about the user's display choice. If the high switches haven't been read yet and this is a CGA device,
+		/// this hint will be used to select between 40- and 80-column default display.
+		void hint_is_composite(bool composite) {
+			if(high_switches_observed_) {
+				return;
+			}
+
+			switch(high_switches_ & 3) {
+				// Do nothing if a non-CGA card is in use.
+				case 0b00:	case 0b11:
+					break;
+
+				default:
+					high_switches_ &= ~0b11;
+					high_switches_ |= composite ? 0b01 : 0b10;
+				break;
+			}
+		}
+
 		void set_value(int port, uint8_t value) {
 			switch(port) {
 				case 1:
@@ -571,6 +591,7 @@ class i8255PortHandler : public Intel::i8255::PortHandler {
 		uint8_t get_value(int port) {
 			switch(port) {
 				case 0:
+					high_switches_observed_ = true;
 					return enable_keyboard_ ? keyboard_.read() : uint8_t((high_switches_ << 4) | low_switches_);
 						// Guesses that switches is high and low combined as below.
 
@@ -580,6 +601,7 @@ class i8255PortHandler : public Intel::i8255::PortHandler {
 					// b5: timer 2 output;	[TODO]
 					// b4: cassette data input; [TODO]
 					// b3...b0: whichever of the high and low switches is selected.
+					high_switches_observed_ |= use_high_switches_;
 					return
 						use_high_switches_ ? high_switches_ : low_switches_;
 			}
@@ -587,6 +609,7 @@ class i8255PortHandler : public Intel::i8255::PortHandler {
 		};
 
 	private:
+		bool high_switches_observed_ = false;
 		uint8_t high_switches_ = 0;
 		uint8_t low_switches_ = 0;
 
@@ -601,8 +624,8 @@ using PPI = Intel::i8255::i8255<i8255PortHandler>;
 template <VideoAdaptor video>
 class IO {
 	public:
-		IO(PIT &pit, DMA &dma, PPI &ppi, PIC &pic, typename Adaptor<video>::type &card, FloppyController &fdc) :
-			pit_(pit), dma_(dma), ppi_(ppi), pic_(pic), video_(card), fdc_(fdc) {}
+		IO(PIT &pit, DMA &dma, PPI &ppi, PIC &pic, typename Adaptor<video>::type &card, FloppyController &fdc, RTC &rtc) :
+			pit_(pit), dma_(dma), ppi_(ppi), pic_(pic), video_(card), fdc_(fdc), rtc_(rtc) {}
 
 		template <typename IntT> void out(uint16_t port, IntT value) {
 			static constexpr uint16_t crtc_base =
@@ -617,6 +640,9 @@ class IO {
 					}
 				break;
 
+				case 0x0070:	rtc_.write<0>(uint8_t(value));	break;
+				case 0x0071:	rtc_.write<1>(uint8_t(value));	break;
+
 				// On the XT the NMI can be masked by setting bit 7 on I/O port 0xA0.
 				case 0x00a0:
 					printf("TODO: NMIs %s\n", (value & 0x80) ? "masked" : "unmasked");
@@ -751,6 +777,8 @@ class IO {
 				case 0x006c:	case 0x006d:	case 0x006e:	case 0x006f:
 				return ppi_.read(port);
 
+				case 0x0071:	return rtc_.read();
+
 				case 0x0080:	return dma_.pages.page<0>();
 				case 0x0081:	return dma_.pages.page<1>();
 				case 0x0082:	return dma_.pages.page<2>();
@@ -804,6 +832,7 @@ class IO {
 		PIC &pic_;
 		typename Adaptor<video>::type &video_;
 		FloppyController &fdc_;
+		RTC &rtc_;
 };
 
 class FlowController {
@@ -880,7 +909,7 @@ class ConcreteMachine:
 			ppi_handler_(speaker_, keyboard_, video, DriveCount),
 			pit_(pit_observer_),
 			ppi_(ppi_handler_),
-			context(pit_, dma_, ppi_, pic_, video_, fdc_)
+			context(pit_, dma_, ppi_, pic_, video_, fdc_, rtc_)
 		{
 			// Set up DMA source/target.
 			dma_.set_memory(&context.memory);
@@ -892,9 +921,10 @@ class ConcreteMachine:
 
 			// Fetch the BIOS. [8088 only, for now]
 			const auto bios = ROM::Name::PCCompatibleGLaBIOS;
+			const auto tick = ROM::Name::PCCompatibleGLaTICK;
 			const auto font = Video::FontROM;
 
-			ROM::Request request = ROM::Request(bios) && ROM::Request(font);
+			ROM::Request request = ROM::Request(bios) && ROM::Request(tick) && ROM::Request(font);
 			auto roms = rom_fetcher(request);
 			if(!request.validate(roms)) {
 				throw ROMMachine::Error::MissingROMs;
@@ -903,6 +933,9 @@ class ConcreteMachine:
 			const auto &bios_contents = roms.find(bios)->second;
 			context.memory.install(0x10'0000 - bios_contents.size(), bios_contents.data(), bios_contents.size());
 
+			const auto &tick_contents = roms.find(tick)->second;
+			context.memory.install(0xd'0000, tick_contents.data(), tick_contents.size());
+
 			// Give the video card something to read from.
 			const auto &font_contents = roms.find(font)->second;
 			video_.set_source(context.memory.at(Video::BaseAddress), font_contents);
@@ -1076,6 +1109,10 @@ class ConcreteMachine:
 
 		void set_display_type(Outputs::Display::DisplayType display_type) override {
 			video_.set_display_type(display_type);
+			ppi_handler_.hint_is_composite(
+				(display_type == Outputs::Display::DisplayType::CompositeColour) ||
+				(display_type == Outputs::Display::DisplayType::CompositeMonochrome)
+			);
 		}
 
 		Outputs::Display::DisplayType get_display_type() const override {
@@ -1095,15 +1132,16 @@ class ConcreteMachine:
 
 		PIT pit_;
 		PPI ppi_;
+		RTC rtc_;
 
 		PCCompatible::KeyboardMapper keyboard_mapper_;
 
 		struct Context {
-			Context(PIT &pit, DMA &dma, PPI &ppi, PIC &pic, typename Adaptor<video>::type &card, FloppyController &fdc) :
+			Context(PIT &pit, DMA &dma, PPI &ppi, PIC &pic, typename Adaptor<video>::type &card, FloppyController &fdc, RTC &rtc) :
 				segments(registers),
 				memory(registers, segments),
 				flow_controller(registers, segments),
-				io(pit, dma, ppi, pic, card, fdc)
+				io(pit, dma, ppi, pic, card, fdc, rtc)
 			{
 				reset();
 			}
diff --git a/Machines/PCCompatible/RTC.hpp b/Machines/PCCompatible/RTC.hpp
new file mode 100644
index 000000000..819df78cb
--- /dev/null
+++ b/Machines/PCCompatible/RTC.hpp
@@ -0,0 +1,96 @@
+//
+//  RTC.hpp
+//  Clock Signal
+//
+//  Created by Thomas Harte on 06/12/2023.
+//  Copyright © 2023 Thomas Harte. All rights reserved.
+//
+
+#ifndef RTC_h
+#define RTC_h
+
+#include <ctime>
+
+namespace PCCompatible {
+
+class RTC {
+	public:
+		template <int address>
+		void write(uint8_t value) {
+			switch(address) {
+				default: break;
+				case 0:
+					selected_ = value & 0x7f;
+					// NMI not yet supported.
+				break;
+				case 1:
+					write_register(value);
+				break;
+			}
+		}
+
+		uint8_t read() {
+			std::time_t now = std::time(NULL);
+			std::tm *time_date = std::localtime(&now);
+
+			switch(selected_) {
+				default:
+				return 0xff;
+
+				case 0x00:	return bcd(time_date->tm_sec);			// Seconds [0-59]
+				case 0x01:	return 0;	// Seconds alarm
+				case 0x02:	return bcd(time_date->tm_min);			// Minutes [0-59]
+				case 0x03:	return 0;	// Minutes alarm
+				case 0x04:
+					// Hours [1-12 or 0-23]
+					if(statusB_ & 2) {
+						return bcd(time_date->tm_hour);
+					}
+					return
+						// TODO: determine why GLaTICK signals an RTC fault if I enable communication of PM as below.
+//						((time_date->tm_hour >= 12) ? 0x80 : 0x00) |
+						bcd(1 + (time_date->tm_hour + 11)%12);
+				break;
+				case 0x05:	return 0;	// Hours alarm
+				case 0x06:	return bcd(time_date->tm_wday + 1);		// Day of the week [Sunday = 1]
+				case 0x07:	return bcd(time_date->tm_mon);			// Date of the Month [1-31]
+				case 0x08:	return bcd(time_date->tm_mon + 1);		// Month [1-12]
+				case 0x09:	return bcd(time_date->tm_year % 100);	// Year [0-99]
+				case 0x32:	return bcd(19 + time_date->tm_year / 100);	// Century
+
+				case 0x0a:	return statusA_;
+				case 0x0b:	return statusB_;
+			}
+		}
+
+	private:
+		int selected_;
+
+		uint8_t statusA_ = 0x00;
+		uint8_t statusB_ = 0x00;
+
+		template <typename IntT>
+		uint8_t bcd(IntT input) {
+			// If calendar is in binary format, don't convert.
+			if(statusB_ & 4) {
+				return uint8_t(input);
+			}
+
+			// Convert a one or two digit number to BCD.
+			return uint8_t(
+				(input % 10) +
+				((input / 10) * 16)
+			);
+		}
+
+		void write_register(uint8_t value) {
+			switch(selected_) {
+				default: break;
+				case 0x0a:	statusA_ = value;	break;
+			}
+		}
+};
+
+}
+
+#endif /* RTC_h */
diff --git a/Machines/Utility/ROMCatalogue.cpp b/Machines/Utility/ROMCatalogue.cpp
index 6536cd5e9..9720ffa75 100644
--- a/Machines/Utility/ROMCatalogue.cpp
+++ b/Machines/Utility/ROMCatalogue.cpp
@@ -570,6 +570,9 @@ Description::Description(Name name) {
 		case Name::PCCompatibleGLaBIOS:
 			*this = Description(name, "PCCompatible", "8088 GLaBIOS 0.2.5", "GLABIOS_0.2.5_8T.ROM", 8 * 1024, 0x9576944cu);
 		break;
+		case Name::PCCompatibleGLaTICK:
+			*this = Description(name, "PCCompatible", "AT GLaTICK 0.8.5", "GLaTICK_0.8.5_AT.ROM", 2 * 1024, 0x371ea3f1u);
+		break;
 		case Name::PCCompatiblePhoenix80286BIOS:
 			*this = Description(name, "PCCompatible", "Phoenix 80286 BIOS 3.05", "Phoenix 80286 ROM BIOS Version 3.05.bin", 32 * 1024, 0x8d0d318au);
 		break;
diff --git a/Machines/Utility/ROMCatalogue.hpp b/Machines/Utility/ROMCatalogue.hpp
index b0603e830..3a15692ee 100644
--- a/Machines/Utility/ROMCatalogue.hpp
+++ b/Machines/Utility/ROMCatalogue.hpp
@@ -134,6 +134,7 @@ enum Name {
 
 	// PCCompatible.
 	PCCompatibleGLaBIOS,
+	PCCompatibleGLaTICK,
 	PCCompatiblePhoenix80286BIOS,
 	PCCompatibleMDAFont,
 	PCCompatibleCGAFont,
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index 173d32e10..812890027 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -1194,6 +1194,7 @@
 		42AD55302A0C4D5000ACE410 /* 68000Storage.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = 68000Storage.hpp; sourceTree = "<group>"; };
 		42AD55312A0C4D5000ACE410 /* 68000Implementation.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = 68000Implementation.hpp; sourceTree = "<group>"; };
 		42E5C3922AC46A7700DA093D /* Carbon.framework */ = {isa = PBXFileReference; lastKnownFileType = wrapper.framework; name = Carbon.framework; path = System/Library/Frameworks/Carbon.framework; sourceTree = SDKROOT; };
+		42EB81252B21788200429AF4 /* RTC.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = RTC.hpp; sourceTree = "<group>"; };
 		4B018B88211930DE002A3937 /* 65C02_extended_opcodes_test.bin */ = {isa = PBXFileReference; lastKnownFileType = archive.macbinary; name = 65C02_extended_opcodes_test.bin; path = "Klaus Dormann/65C02_extended_opcodes_test.bin"; sourceTree = "<group>"; };
 		4B01A6871F22F0DB001FD6E3 /* Z80MemptrTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Z80MemptrTests.swift; sourceTree = "<group>"; };
 		4B0333AD2094081A0050B93D /* AppleDSK.cpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.cpp; path = AppleDSK.cpp; sourceTree = "<group>"; };
@@ -2405,6 +2406,7 @@
 				4267A9C82B0D4EC2008A59BB /* PIC.hpp */,
 				4267A9C72B0C26FA008A59BB /* PIT.hpp */,
 				423820142B1A23C200964EFE /* Registers.hpp */,
+				42EB81252B21788200429AF4 /* RTC.hpp */,
 				423820152B1A23E100964EFE /* Segments.hpp */,
 			);
 			path = PCCompatible;
diff --git a/ROMImages/PCCompatible/GLaTICK_0.8.5_AT.ROM b/ROMImages/PCCompatible/GLaTICK_0.8.5_AT.ROM
new file mode 100644
index 000000000..5d767b240
Binary files /dev/null and b/ROMImages/PCCompatible/GLaTICK_0.8.5_AT.ROM differ
diff --git a/ROMImages/PCCompatible/readme.txt b/ROMImages/PCCompatible/readme.txt
index faf0be717..d15254705 100644
--- a/ROMImages/PCCompatible/readme.txt
+++ b/ROMImages/PCCompatible/readme.txt
@@ -1,11 +1,14 @@
 Expected files:
 
-GLABIOS_0.2.5_8T.ROM — the 8088 GlaBIOS ROM.
+GLABIOS_0.2.5_8T.ROM — the 8088 GLaBIOS ROM.
+GLaTICK_0.8.5_AT.ROM — the GLaBIOS AT RTC option ROM.
 Phoenix 80286 ROM BIOS Version 3.05.bin — Phoenix's 80286 AT-clone BIOS.
 EUMDA9.F14 — a dump of the MDA font.
 CGA.F08 — a dump of the CGA font.
 
 
-GlaBIOS is an open-source GPLv3 alternative BIOS for XT clones, available from https://glabios.org/
+GLaBIOS is an open-source GPLv3 alternative BIOS for XT clones, available from https://glabios.org/
+
+GLaTICK is a real-time clock option ROM, also available from available from https://glabios.org/
 
 The MDA and CGA fonts are in the form offered at https://github.com/viler-int10h/vga-text-mode-fonts i.e. it's 256 lots of 14 bytes, the first 14 being the content of character 0, the next 14 being the content of character 1, etc.
\ No newline at end of file