diff --git a/Machines/Apple/AppleIIgs/MemoryMap.hpp b/Machines/Apple/AppleIIgs/MemoryMap.hpp
index 2abeeaa0c..c20ac389f 100644
--- a/Machines/Apple/AppleIIgs/MemoryMap.hpp
+++ b/Machines/Apple/AppleIIgs/MemoryMap.hpp
@@ -228,12 +228,14 @@ class MemoryMap {
 };
 
 #define MemoryMapRegion(map, address) map.regions[map.region_map[address >> 8]]
-#define MemoryMapRead(region, address, value) *value = region.read ? region.read[address] : 0xff;
+#define MemoryMapRead(region, address, value) *value = region.read ? region.read[address] : 0xff
 #define MemoryMapWrite(map, region, address, value) \
-	region.write[address] = *value;	\
-	if(region.flags & (MemoryMap::Region::IsShadowed)) {	\
-		const uint32_t shadowed_address = (address & 0xffff) + (uint32_t(0xe1 - (region.flags & MemoryMap::Region::IsShadowedE0)) << 16);	\
-		map.regions[map.region_map[shadowed_address >> 8]].write[shadowed_address] = *value;	\
+	if(region.write) {	\
+		region.write[address] = *value;	\
+		if(region.flags & (MemoryMap::Region::IsShadowed)) {	\
+			const uint32_t shadowed_address = (address & 0xffff) + (uint32_t(0xe1 - (region.flags & MemoryMap::Region::IsShadowedE0)) << 16);	\
+			map.regions[map.region_map[shadowed_address >> 8]].write[shadowed_address] = *value;	\
+		}	\
 	}
 
 }
diff --git a/OSBindings/Mac/Clock SignalTests/IIgsMemoryMapTests.mm b/OSBindings/Mac/Clock SignalTests/IIgsMemoryMapTests.mm
index e75d1aeb4..d97bf677d 100644
--- a/OSBindings/Mac/Clock SignalTests/IIgsMemoryMapTests.mm	
+++ b/OSBindings/Mac/Clock SignalTests/IIgsMemoryMapTests.mm	
@@ -10,11 +10,15 @@
 
 #include "../../../Machines/Apple/AppleIIgs/MemoryMap.hpp"
 
+namespace {
+	using MemoryMap = Apple::IIgs::MemoryMap;
+}
+
 @interface IIgsMemoryMapTests : XCTestCase
 @end
 
 @implementation IIgsMemoryMapTests {
-	Apple::IIgs::MemoryMap _memoryMap;
+	MemoryMap _memoryMap;
 	std::vector<uint8_t> _ram;
 	std::vector<uint8_t> _rom;
 }
@@ -25,10 +29,37 @@
 	_memoryMap.set_storage(_ram, _rom);
 }
 
-- (void)testNothing {
+- (void)testHigherRAM {
+	// Fill memory via the map.
+	for(int address = 0x020000; address < 0x800000; ++address) {
+		const auto &region = MemoryMapRegion(_memoryMap, address);
+		const uint8_t value = uint8_t(address ^ (address >> 8));
+		MemoryMapWrite(_memoryMap, region, address, &value);
+	}
+
+	// Test by direct access.
+	for(int address = 0x020000; address < 0x800000; ++address) {
+		const uint8_t value = uint8_t(address ^ (address >> 8));
+		XCTAssertEqual(_ram[address], value);
+	}
 }
 
-- (void)testSomeOtherNothing {
+- (void)testROMReadonly {
+	_rom[0] = 0xc0;
+
+	// Test that ROM can be read in the correct location.
+	const uint32_t address = 0xfc0000;
+	const auto &region = MemoryMapRegion(_memoryMap, address);
+	uint8_t value;
+	MemoryMapRead(region, address, &value);
+
+	XCTAssertEqual(value, 0xc0);
+
+	// Try writing to it, and check that nothing happened.
+	value = 0xfc;
+	MemoryMapWrite(_memoryMap, region, address, &value);
+
+	XCTAssertEqual(_rom[0], 0xc0);
 }
 
 @end