diff --git a/Components/9918/9918.cpp b/Components/9918/9918.cpp
index 2e417fa5e..7343aebf9 100644
--- a/Components/9918/9918.cpp
+++ b/Components/9918/9918.cpp
@@ -560,10 +560,14 @@ uint8_t TMS9918::get_register(int address) {
 	}
 
 	// Figure out the number of internal cycles until the next line interrupt.
-	const int local_cycles_until_line_interrupt = ((mode_timing_.line_interrupt_position - column_ + 342) % 342) + (next_line_interrupt_row - row_) * 342;
+	int local_cycles_until_line_interrupt = ((mode_timing_.line_interrupt_position - column_ + 342) % 342) + (next_line_interrupt_row - row_) * 342;
+	local_cycles_until_line_interrupt <<= 2;
+	local_cycles_until_line_interrupt -= cycles_error_;
 
-	// Map that to input cycles by multiplying by 2/3 (and incrementing on any carry).
-	auto time_until_line_interrupt = HalfCycles( (local_cycles_until_line_interrupt * 6 + 7) / 4);
+	// Map that to input cycles by multiplying by 2/3 (and incrementing on any carry, TODO: allowing for current error).
+	auto time_until_line_interrupt = HalfCycles(local_cycles_until_line_interrupt / 3);
+
+	if(!generate_interrupts_) return time_until_line_interrupt;
 
 	// Return whichever interrupt is closer.
 	return std::min(time_until_frame_interrupt, time_until_line_interrupt);
diff --git a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj
index 6f56f10e9..95080054f 100644
--- a/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
+++ b/OSBindings/Mac/Clock Signal.xcodeproj/project.pbxproj	
@@ -315,6 +315,7 @@
 		4B9BE401203A0C0600FFAE60 /* MultiSpeaker.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B9BE3FE203A0C0600FFAE60 /* MultiSpeaker.cpp */; };
 		4BA0F68E1EEA0E8400E9489E /* ZX8081.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4BA0F68C1EEA0E8400E9489E /* ZX8081.cpp */; };
 		4BA61EB01D91515900B3C876 /* NSData+StdVector.mm in Sources */ = {isa = PBXBuildFile; fileRef = 4BA61EAF1D91515900B3C876 /* NSData+StdVector.mm */; };
+		4BA91E1D216D85BA00F79557 /* MasterSystemVDPTests.mm in Sources */ = {isa = PBXBuildFile; fileRef = 4BA91E1C216D85BA00F79557 /* MasterSystemVDPTests.mm */; };
 		4BAD13441FF709C700FD114A /* MSX.cpp in Sources */ = {isa = PBXBuildFile; fileRef = 4B0E61051FF34737002A9DBD /* MSX.cpp */; };
 		4BAE49582032881E004BE78E /* CSZX8081.mm in Sources */ = {isa = PBXBuildFile; fileRef = 4B14978E1EE4B4D200CE2596 /* CSZX8081.mm */; };
 		4BAE495920328897004BE78E /* ZX8081OptionsPanel.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4B95FA9C1F11893B0008E395 /* ZX8081OptionsPanel.swift */; };
@@ -1041,6 +1042,7 @@
 		4BA141C12073100800A31EC9 /* Target.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = Target.hpp; sourceTree = "<group>"; };
 		4BA61EAE1D91515900B3C876 /* NSData+StdVector.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; path = "NSData+StdVector.h"; sourceTree = "<group>"; };
 		4BA61EAF1D91515900B3C876 /* NSData+StdVector.mm */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.objcpp; path = "NSData+StdVector.mm"; sourceTree = "<group>"; };
+		4BA91E1C216D85BA00F79557 /* MasterSystemVDPTests.mm */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.objcpp; path = MasterSystemVDPTests.mm; sourceTree = "<group>"; };
 		4BA9C3CF1D8164A9002DDB61 /* MediaTarget.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = MediaTarget.hpp; sourceTree = "<group>"; };
 		4BAA167B21582B1D008A3276 /* Target.hpp */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.cpp.h; path = Target.hpp; sourceTree = "<group>"; };
 		4BAB62AC1D3272D200DF5BA0 /* Disk.hpp */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.cpp.h; path = Disk.hpp; sourceTree = "<group>"; };
@@ -2818,10 +2820,11 @@
 		4BB73EB51B587A5100552FC2 /* Clock SignalTests */ = {
 			isa = PBXGroup;
 			children = (
-				4B98A0601FFADCDE00ADF63B /* MSXStaticAnalyserTests.mm */,
 				4B5073091DDFCFDF00C48FBD /* ArrayBuilderTests.mm */,
 				4B924E981E74D22700B76AF1 /* AtariStaticAnalyserTests.mm */,
 				4BB2A9AE1E13367E001A5C23 /* CRCTests.mm */,
+				4BA91E1C216D85BA00F79557 /* MasterSystemVDPTests.mm */,
+				4B98A0601FFADCDE00ADF63B /* MSXStaticAnalyserTests.mm */,
 				4B121F9A1E06293F00BFDA12 /* PCMSegmentEventSourceTests.mm */,
 				4BD4A8CF1E077FD20020D856 /* PCMTrackTests.mm */,
 				4B2AF8681E513FC20027EE29 /* TIATests.mm */,
@@ -4000,6 +4003,7 @@
 				4BB73EB71B587A5100552FC2 /* AllSuiteATests.swift in Sources */,
 				4B01A6881F22F0DB001FD6E3 /* Z80MemptrTests.swift in Sources */,
 				4B121F9B1E06293F00BFDA12 /* PCMSegmentEventSourceTests.mm in Sources */,
+				4BA91E1D216D85BA00F79557 /* MasterSystemVDPTests.mm in Sources */,
 				4B98A0611FFADCDE00ADF63B /* MSXStaticAnalyserTests.mm in Sources */,
 				4BEF6AAC1D35D1C400E73575 /* DPLLTests.swift in Sources */,
 				4B3BA0CF1D318B44005DD7A7 /* MOS6522Bridge.mm in Sources */,
diff --git a/OSBindings/Mac/Clock SignalTests/MasterSystemVDPTests.mm b/OSBindings/Mac/Clock SignalTests/MasterSystemVDPTests.mm
new file mode 100644
index 000000000..0047ac20b
--- /dev/null
+++ b/OSBindings/Mac/Clock SignalTests/MasterSystemVDPTests.mm	
@@ -0,0 +1,69 @@
+//
+//  MasterSystemVDPTests.m
+//  Clock SignalTests
+//
+//  Created by Thomas Harte on 09/10/2018.
+//  Copyright © 2018 Thomas Harte. All rights reserved.
+//
+
+#import <XCTest/XCTest.h>
+#import <OpenGL/OpenGL.h>
+
+#include "9918.hpp"
+
+@interface MasterSystemVDPTests : XCTestCase
+@end
+
+@implementation MasterSystemVDPTests {
+	NSOpenGLContext *_openGLContext;
+}
+
+- (void)setUp {
+    [super setUp];
+
+	// Create a valid OpenGL context, so that a VDP can be constructed.
+	NSOpenGLPixelFormatAttribute attributes[] =
+	{
+		NSOpenGLPFAOpenGLProfile,	NSOpenGLProfileVersion3_2Core,
+		0
+	};
+
+	NSOpenGLPixelFormat *pixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:attributes];
+	_openGLContext = [[NSOpenGLContext alloc] initWithFormat:pixelFormat shareContext:nil];
+	[_openGLContext makeCurrentContext];
+}
+
+- (void)tearDown {
+    // Put teardown code here. This method is called after the invocation of each test method in the class.
+    _openGLContext = nil;
+
+    [super tearDown];
+}
+
+- (void)testLineInterrupt {
+	TI::TMS::TMS9918 vdp(TI::TMS::Personality::SMSVDP);
+
+	// Disable end-of-frame interrupts, enable line interrupts.
+	vdp.set_register(1, 0x00);
+	vdp.set_register(1, 0x81);
+
+	vdp.set_register(1, 0x10);
+	vdp.set_register(1, 0x80);
+
+	// Set a line interrupt to occur in five lines.
+	vdp.set_register(1, 5);
+	vdp.set_register(1, 0x8a);
+
+	// Get time until interrupt.
+	int time_until_interrupt = vdp.get_time_until_interrupt().as_int() - 1;
+
+	// Check interrupt flag isn't set prior to the reported time.
+	vdp.run_for(HalfCycles(time_until_interrupt));
+	NSAssert(!vdp.get_interrupt_line(), @"Interrupt line went active early");
+
+	// Check interrupt flag is set at the reported time.
+	vdp.run_for(HalfCycles(1));
+	NSAssert(vdp.get_interrupt_line(), @"Interrupt line wasn't set when promised");
+}
+
+@end