diff --git a/Machines/Atari/ST/Video.hpp b/Machines/Atari/ST/Video.hpp
index 371d0a4d7..ef61a5609 100644
--- a/Machines/Atari/ST/Video.hpp
+++ b/Machines/Atari/ST/Video.hpp
@@ -15,6 +15,9 @@
 
 #include <vector>
 
+// Testing hook; not for any other user.
+class VideoTester;
+
 namespace Atari {
 namespace ST {
 
@@ -278,6 +281,8 @@ class Video {
 			}
 			pending_events_.emplace(insertion_point, type, delay);
 		}
+
+		friend class ::VideoTester;
 };
 
 }
diff --git a/OSBindings/Mac/Clock SignalTests/AtariSTVideoTests.mm b/OSBindings/Mac/Clock SignalTests/AtariSTVideoTests.mm
index 62a632cb5..f2419e096 100644
--- a/OSBindings/Mac/Clock SignalTests/AtariSTVideoTests.mm	
+++ b/OSBindings/Mac/Clock SignalTests/AtariSTVideoTests.mm	
@@ -12,6 +12,14 @@
 
 #include "../../../Machines/Atari/ST/Video.hpp"
 
+// Implement Atari::ST::Video's friend class, to expose some
+// internal state.
+struct VideoTester {
+	static bool vsync(Atari::ST::Video &video) {
+		return video.vertical_.sync;
+	}
+};
+
 @interface AtariSTVideoTests : XCTestCase
 @end
 
@@ -272,13 +280,14 @@ struct RunLength {
 
 // MARK: - Address Reload Timing tests
 
-/// Tests that the current video address is reloaded constantly throughout hsync
-- (void)testHsyncReload {
+/// Tests that the current video address is reloaded constantly throughout vsync (subject to caveat: observed .
+- (void)testVsyncReload {
+
 	// Set an initial video address of 0.
 	[self setVideoBaseAddress:0];
 
 	// Find next area of non-vsync.
-	while(_video->vsync()) {
+	while(VideoTester::vsync(*_video)) {
 		_video->run_for(Cycles(1));
 	}
 
@@ -287,7 +296,7 @@ struct RunLength {
 
 	// Find next area of vsync, checking that the address isn't
 	// reloaded before then.
-	while(!_video->vsync()) {
+	while(!VideoTester::vsync(*_video)) {
 		XCTAssertNotEqual([self currentVideoAddress], 0x800000);
 		_video->run_for(Cycles(1));
 	}
@@ -305,7 +314,7 @@ struct RunLength {
 
 	// Find end of vertical sync, set a different base address,
 	// check that it doesn't become current.
-	while(_video->vsync()) {
+	while(VideoTester::vsync(*_video)) {
 		_video->run_for(Cycles(1));
 	}
 	[self setVideoBaseAddress:0];