diff --git a/Machines/Atari/ST/AtariST.cpp b/Machines/Atari/ST/AtariST.cpp
index 413b94acb..e3683e4d1 100644
--- a/Machines/Atari/ST/AtariST.cpp
+++ b/Machines/Atari/ST/AtariST.cpp
@@ -71,7 +71,8 @@ class ConcreteMachine:
 			Memory::Fuzz(ram_);
 
 			std::vector<ROMMachine::ROM> rom_descriptions = {
-				{"AtariST", "the TOS ROM", "tos100.img", 192*1024, 0x1a586c64}
+//				{"AtariST", "the UK TOS 1.00 ROM", "tos100.img", 192*1024, 0x1a586c64}
+				{"AtariST", "the UK TOS 1.04 ROM", "tos104.img", 192*1024, 0xa50d1d43}
 			};
 			const auto roms = rom_fetcher(rom_descriptions);
 			if(!roms[0]) {
diff --git a/Machines/Atari/ST/Video.cpp b/Machines/Atari/ST/Video.cpp
index d98f690ef..4a975c622 100644
--- a/Machines/Atari/ST/Video.cpp
+++ b/Machines/Atari/ST/Video.cpp
@@ -11,6 +11,7 @@
 #include "../../../Outputs/Log.hpp"
 
 #include <algorithm>
+#include <cstring>
 
 using namespace Atari::ST;
 
@@ -377,9 +378,13 @@ void Video::Shifter::flush_output(OutputMode next_mode) {
 		case OutputMode::Sync:	crt_.output_sync(duration_);	break;
 		case OutputMode::Blank:	crt_.output_blank(duration_);	break;
 		case OutputMode::Border: {
-			uint16_t *const colour_pointer = reinterpret_cast<uint16_t *>(crt_.begin_data(1));
-			if(colour_pointer) *colour_pointer = border_colour_;
-			crt_.output_level(duration_);
+//			if(!border_colour_) {
+//				crt_.output_blank(duration_);
+//			} else {
+				uint16_t *const colour_pointer = reinterpret_cast<uint16_t *>(crt_.begin_data(1));
+				if(colour_pointer) *colour_pointer = border_colour_;
+				crt_.output_level(duration_);
+//			}
 		} break;
 		case OutputMode::Pixels: {
 			crt_.output_data(duration_, pixel_pointer_);
@@ -428,9 +433,34 @@ void Video::Shifter::output_border(int duration, OutputBpp bpp) {
 }
 
 void Video::Shifter::output_pixels(int duration, OutputBpp bpp) {
-	// If the shifter is empty, redirect this to an output_level call.
+	// If the shifter is empty and there's no pixel buffer at present,
+	// redirect this to an output_level call. Otherwise, do a quick
+	// memset-type fill, since the special case has been detected anyway.
 	if(!output_shifter_) {
-		output_border(duration, bpp);
+		if(!pixel_buffer_) {
+			output_border(duration, bpp);
+		} else {
+			duration_ += duration;
+
+			switch(bpp_) {
+				case OutputBpp::One: {
+					const size_t pixels = size_t(duration << 1);
+					memset(&pixel_buffer_[pixel_pointer_], 0, pixels * sizeof(uint16_t));
+					pixel_pointer_ += pixels;
+				} break;
+
+				default:
+				case OutputBpp::Four:
+					assert(!(duration & 1));
+					duration >>= 1;
+				case OutputBpp::Two: {
+					while(duration--) {
+						pixel_buffer_[pixel_pointer_] = palette_[0];
+						++pixel_pointer_;
+					}
+				} break;
+			}
+		}
 		return;
 	}
 
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 47f9c7286..1465a4f62 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/xcshareddata/xcschemes/Clock Signal.xcscheme	
@@ -67,7 +67,7 @@
       </Testables>
    </TestAction>
    <LaunchAction
-      buildConfiguration = "Debug"
+      buildConfiguration = "Release"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
       enableASanStackUseAfterReturn = "YES"