diff --git a/InstructionSets/M50740/Executor.cpp b/InstructionSets/M50740/Executor.cpp
index e554635b9..3be5a4935 100644
--- a/InstructionSets/M50740/Executor.cpp
+++ b/InstructionSets/M50740/Executor.cpp
@@ -8,6 +8,7 @@
 
 #include "Executor.hpp"
 
+#include <algorithm>
 #include <cassert>
 
 using namespace InstructionSet::M50740;
@@ -18,13 +19,31 @@ Executor::Executor() {
 	Decoder decoder;
 	for(size_t c = 0; c < 256; c++) {
 		const auto instruction = decoder.instrucion_for_opcode(uint8_t(c));
-		performers_[c] = performer_lookup_.performer(instruction.operation, instruction.addressing_mode);
+
+		// Treat invalid as NOP, because I've got to do _something_.
+		if(instruction.operation == Operation::Invalid) {
+			performers_[c] = performer_lookup_.performer(Operation::NOP, instruction.addressing_mode);
+		} else {
+			performers_[c] = performer_lookup_.performer(instruction.operation, instruction.addressing_mode);
+		}
 	}
 
 	// TODO: read reset vector, etc. This is just the start of ROM.
 	set_program_counter(0x1400);
 }
 
+void Executor::set_rom(const std::vector<uint8_t> &rom) {
+	// Copy into place, and reset.
+	const auto length = std::min(size_t(0x1000), rom.size());
+	memcpy(&memory_[0x2000 - length], rom.data(), length);
+	reset();
+}
+
+void Executor::reset() {
+	// Just jump to the reset vector.
+	set_program_counter(uint16_t(memory_[0x1ffe] | (memory_[0x1fff] << 8)));
+}
+
 template <Operation operation, AddressingMode addressing_mode> void Executor::perform() {
 	// Deal with all modes that don't access memory up here;
 	// those that access memory will go through a slightly longer
diff --git a/InstructionSets/M50740/Executor.hpp b/InstructionSets/M50740/Executor.hpp
index b70a12f38..64b8133d9 100644
--- a/InstructionSets/M50740/Executor.hpp
+++ b/InstructionSets/M50740/Executor.hpp
@@ -13,6 +13,9 @@
 #include "Parser.hpp"
 #include "../CachingExecutor.hpp"
 
+#include <cstdint>
+#include <vector>
+
 namespace InstructionSet {
 namespace M50740 {
 
@@ -22,6 +25,8 @@ using CachingExecutor = CachingExecutor<Executor, 0x1fff, 256, Instruction, fals
 class Executor: public CachingExecutor {
 	public:
 		Executor();
+		void set_rom(const std::vector<uint8_t> &rom);
+		void reset();
 
 	private:
 		// MARK: - CachingExecutor-facing interface.
diff --git a/Machines/Apple/AppleIIgs/ADB.cpp b/Machines/Apple/AppleIIgs/ADB.cpp
index eed717c02..2bf02ac2a 100644
--- a/Machines/Apple/AppleIIgs/ADB.cpp
+++ b/Machines/Apple/AppleIIgs/ADB.cpp
@@ -211,3 +211,7 @@ uint8_t GLU::read_microcontroller_address(uint16_t address) {
 		return 0x40;
 	return 0;
 }
+
+void GLU::set_microcontroller_rom(const std::vector<uint8_t> &rom) {
+	executor_.set_rom(rom);
+}
diff --git a/Machines/Apple/AppleIIgs/ADB.hpp b/Machines/Apple/AppleIIgs/ADB.hpp
index 20efe25b9..2d5745f28 100644
--- a/Machines/Apple/AppleIIgs/ADB.hpp
+++ b/Machines/Apple/AppleIIgs/ADB.hpp
@@ -11,6 +11,7 @@
 
 #include <cstdint>
 #include <vector>
+#include "../../../InstructionSets/M50740/Executor.hpp"
 
 namespace Apple {
 namespace IIgs {
@@ -34,6 +35,8 @@ class GLU {
 		void set_status(uint8_t);
 		void clear_key_strobe();
 
+		void set_microcontroller_rom(const std::vector<uint8_t> &rom);
+
 	private:
 		bool is_rom03_ = false;
 		std::vector<uint8_t> next_command_;
@@ -52,6 +55,8 @@ class GLU {
 		void set_configuration_bytes(uint8_t *);
 
 		uint8_t read_microcontroller_address(uint16_t);
+
+		InstructionSet::M50740::Executor executor_;
 };
 
 }
diff --git a/Machines/Apple/AppleIIgs/AppleIIgs.cpp b/Machines/Apple/AppleIIgs/AppleIIgs.cpp
index 92f72ef59..1894a4163 100644
--- a/Machines/Apple/AppleIIgs/AppleIIgs.cpp
+++ b/Machines/Apple/AppleIIgs/AppleIIgs.cpp
@@ -88,13 +88,15 @@ class ConcreteMachine:
 				break;
 			}
 			rom_descriptions.push_back(video_->rom_description(Video::Video::CharacterROM::EnhancedIIe));
+			rom_descriptions.emplace_back(machine_name, "the Apple IIgs ADB microcontroller ROM", "341s0632-2", 4*1024, 0xe1c11fb0);
 
 			const auto roms = rom_fetcher(rom_descriptions);
-			if(!roms[0] || !roms[1]) {
+			if(!roms[0] || !roms[1] || !roms[2]) {
 				throw ROMMachine::Error::MissingROMs;
 			}
 			rom_ = *roms[0];
 			video_->set_character_rom(*roms[1]);
+			adb_glu_.set_microcontroller_rom(*roms[2]);
 
 			// Run only the currently-interesting self test.
 			rom_[0x36402] = 2;
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme b/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme
index 7516c5447..672c62faf 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme	
@@ -27,6 +27,8 @@
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
       shouldUseLaunchSchemeArgsEnv = "YES"
+      enableAddressSanitizer = "YES"
+      enableUBSanitizer = "YES"
       disableMainThreadChecker = "YES"
       codeCoverageEnabled = "YES">
       <MacroExpansion>
diff --git a/ROMImages/AppleIIGS/readme.txt b/ROMImages/AppleIIGS/readme.txt
index 4362bfc75..02d568fda 100644
--- a/ROMImages/AppleIIGS/readme.txt
+++ b/ROMImages/AppleIIGS/readme.txt
@@ -2,5 +2,6 @@ ROM files would ordinarily go here; they are copyright Apple so are not included
 
 Expected files (as usual, using the most conventional pre-existing names):
 
+341s0632-2.bin	— the ADB microcontroller ROM;
 APPLE2GS.ROM	- ROM version 1; and
 APPLE2GS.ROM2	- ROM version 3.