diff --git a/Machines/ZX8081/ZX8081.cpp b/Machines/ZX8081/ZX8081.cpp
index 3f772c133..3d955c268 100644
--- a/Machines/ZX8081/ZX8081.cpp
+++ b/Machines/ZX8081/ZX8081.cpp
@@ -35,6 +35,7 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 	if(previous_counter < vsync_start_cycle_ && horizontal_counter_ >= vsync_start_cycle_) {
 		video_->run_for_cycles(vsync_start_cycle_ - previous_counter);
 		set_hsync(true);
+		line_counter_ = (line_counter_ + 1) & 7;
 		if(nmi_is_enabled_) {
 			set_non_maskable_interrupt_line(true);
 		}
@@ -66,17 +67,20 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 	bool is_opcode_read = false;
 	switch(cycle.operation) {
 		case CPU::Z80::PartialMachineCycle::Output:
-			set_vsync(false);
-			line_counter_ = 0;
-
 			if(!(address & 2)) nmi_is_enabled_ = false;
 			if(!(address & 1)) nmi_is_enabled_ = is_zx81_;
+			if((address&3) == 3) {
+				if(!nmi_is_enabled_) {
+					set_vsync(false);
+					line_counter_ = 0;
+				}
+			}
 		break;
 
 		case CPU::Z80::PartialMachineCycle::Input: {
 			uint8_t value = 0xff;
 			if(!(address&1)) {
-				set_vsync(true);
+				if(!nmi_is_enabled_) set_vsync(true);
 
 				uint16_t mask = 0x100;
 				for(int c = 0; c < 8; c++) {
@@ -90,7 +94,6 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 		} break;
 
 		case CPU::Z80::PartialMachineCycle::Interrupt:
-			line_counter_ = (line_counter_ + 1) & 7;
 			*cycle.value = 0xff;
 			horizontal_counter_ = 0;
 		break;
@@ -106,9 +109,11 @@ int Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &cycle) {
 			}
 			if(latched_video_byte_) {
 				size_t char_address = (size_t)((address & 0xff00) | ((latched_video_byte_ & 0x3f) << 3) | line_counter_);
+				uint8_t mask = (latched_video_byte_ & 0x80) ? 0x00 : 0xff;
 				if(char_address < ram_base_) {
-					uint8_t mask = (latched_video_byte_ & 0x80) ? 0x00 : 0xff;
 					latched_video_byte_ = rom_[char_address & rom_mask_] ^ mask;
+				} else {
+					latched_video_byte_ = ram_[address & ram_mask_] ^ mask;
 				}
 
 				video_->output_byte(latched_video_byte_);
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 2cfe830df..0bbefe859 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme	
@@ -68,13 +68,14 @@
       </AdditionalOptions>
    </TestAction>
    <LaunchAction
-      buildConfiguration = "Release"
+      buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
       launchStyle = "0"
       useCustomWorkingDirectory = "NO"
       ignoresPersistentStateOnLaunch = "NO"
       debugDocumentVersioning = "YES"
+      enableAddressSanitizer = "YES"
       debugServiceExtension = "internal"
       allowLocationSimulation = "NO">
       <BuildableProductRunnable