diff --git a/InstructionSets/M50740/Executor.cpp b/InstructionSets/M50740/Executor.cpp
index 449fab886..c0b1c554b 100644
--- a/InstructionSets/M50740/Executor.cpp
+++ b/InstructionSets/M50740/Executor.cpp
@@ -38,7 +38,9 @@ Executor::Executor(PortHandler &port_handler) : port_handler_(port_handler) {
 		}
 	}
 
+	// Fuzz RAM; then set anything that may be replaced by ROM to FF.
 	Memory::Fuzz(memory_);
+	memset(&memory_[0x100], 0xff, memory_.size() - 0x100);
 }
 
 void Executor::set_rom(const std::vector<uint8_t> &rom) {
@@ -63,8 +65,8 @@ void Executor::reset() {
 uint8_t Executor::read(uint16_t address) {
 	address &= 0x1fff;
 
-	// Deal with a RAM access quickly.
-	if(address < 0x60) return memory_[address];
+	// Deal with RAM and ROM accesses quickly.
+	if(address < 0x60 || address >= 0x100) return memory_[address];
 
 	port_handler_.run_ports_for(cycles_since_port_handler_.flush<Cycles>());
 	switch(address) {
@@ -118,6 +120,12 @@ void Executor::write(uint16_t address, uint8_t value) {
 		return;
 	}
 
+	// ROM 'writes' are almost as easy (albeit unexpected).
+	if(address >= 0x100) {
+		LOG("Attempted ROM write of " << PADHEX(2) << value << " to " << PADHEX(4) << address);
+		return;
+	}
+
 	// Push time to the port handler.
 	port_handler_.run_ports_for(cycles_since_port_handler_.flush<Cycles>());
 
diff --git a/Machines/Apple/ADB/Mouse.cpp b/Machines/Apple/ADB/Mouse.cpp
index 49c8e2e89..af3bec90d 100644
--- a/Machines/Apple/ADB/Mouse.cpp
+++ b/Machines/Apple/ADB/Mouse.cpp
@@ -8,27 +8,9 @@
 
 #include "Mouse.hpp"
 
-#define LOG_PREFIX "[Mouse] "
-#include "../../../Outputs/Log.hpp"
-
 using namespace Apple::ADB;
 
-Mouse::Mouse(Bus &bus) : ReactiveDevice(bus) {}
+Mouse::Mouse(Bus &bus) : ReactiveDevice(bus, 3) {}
 
-void Mouse::adb_bus_did_observe_event(Bus::Event event, uint8_t value) {
-	if(!next_is_command_ && event != Bus::Event::Attention) {
-		return;
-	}
-
-	if(next_is_command_ && event == Bus::Event::Byte) {
-		next_is_command_ = false;
-
-		const auto command = decode_command(value);
-		LOG(command);
-		if(command.device != 3) {
-			return;
-		}
-	} else if(event == Bus::Event::Attention) {
-		next_is_command_ = true;
-	}
+void Mouse::perform_command(const Command &) {
 }
diff --git a/Machines/Apple/ADB/Mouse.hpp b/Machines/Apple/ADB/Mouse.hpp
index 5749d1cb4..ebbd0cbb9 100644
--- a/Machines/Apple/ADB/Mouse.hpp
+++ b/Machines/Apple/ADB/Mouse.hpp
@@ -18,10 +18,7 @@ class Mouse: public ReactiveDevice {
 	public:
 		Mouse(Bus &);
 
-		void adb_bus_did_observe_event(Bus::Event event, uint8_t value) override;
-
-	private:
-		bool next_is_command_ = false;
+		void perform_command(const Command &command) override;
 };
 
 }
diff --git a/Machines/Apple/ADB/ReactiveDevice.cpp b/Machines/Apple/ADB/ReactiveDevice.cpp
index b21fe45d4..2c402bfb3 100644
--- a/Machines/Apple/ADB/ReactiveDevice.cpp
+++ b/Machines/Apple/ADB/ReactiveDevice.cpp
@@ -8,9 +8,15 @@
 
 #include "ReactiveDevice.hpp"
 
+#define LOG_PREFIX "[ADB device] "
+#include "../../../Outputs/Log.hpp"
+
 using namespace Apple::ADB;
 
-ReactiveDevice::ReactiveDevice(Apple::ADB::Bus &bus) : bus_(bus), device_id_(bus.add_device(this)) {}
+ReactiveDevice::ReactiveDevice(Apple::ADB::Bus &bus, uint8_t adb_device_id) :
+	bus_(bus),
+	device_id_(bus.add_device(this)),
+	adb_device_id_(adb_device_id) {}
 
 void ReactiveDevice::post_response(const std::vector<uint8_t> &&response) {
 	response_ = std::move(response);
@@ -53,3 +59,22 @@ void ReactiveDevice::advance_state(double microseconds) {
 	constexpr double low_periods[] = {66, 33};
 	bus_.set_device_output(device_id_, microseconds_at_bit_ > low_periods[bit]);
 }
+
+void ReactiveDevice::adb_bus_did_observe_event(Bus::Event event, uint8_t value) {
+	if(!next_is_command_ && event != Bus::Event::Attention) {
+		return;
+	}
+
+	if(next_is_command_ && event == Bus::Event::Byte) {
+		next_is_command_ = false;
+
+		const auto command = decode_command(value);
+		LOG(command);
+		if(command.device == adb_device_id_) {
+			// TODO: handle fixed commands here (like register 3?)
+			perform_command(command);
+		}
+	} else if(event == Bus::Event::Attention) {
+		next_is_command_ = true;
+	}
+}
diff --git a/Machines/Apple/ADB/ReactiveDevice.hpp b/Machines/Apple/ADB/ReactiveDevice.hpp
index ab7ade2c6..67e96ddf0 100644
--- a/Machines/Apple/ADB/ReactiveDevice.hpp
+++ b/Machines/Apple/ADB/ReactiveDevice.hpp
@@ -19,10 +19,14 @@ namespace ADB {
 
 class ReactiveDevice: public Bus::Device {
 	protected:
-		ReactiveDevice(Bus &bus);
+		ReactiveDevice(Bus &bus, uint8_t adb_device_id);
 
 		void post_response(const std::vector<uint8_t> &&response);
+		virtual void perform_command(const Command &command) = 0;
+
+	private:
 		void advance_state(double microseconds) override;
+		void adb_bus_did_observe_event(Bus::Event event, uint8_t value) override;
 
 	private:
 		Bus &bus_;
@@ -31,6 +35,9 @@ class ReactiveDevice: public Bus::Device {
 		std::vector<uint8_t> response_;
 		int bit_offset_ = 0;
 		double microseconds_at_bit_ = 0;
+
+		bool next_is_command_ = false;
+		uint8_t adb_device_id_;
 };
 
 }