diff --git a/ClockReceiver/ClockReceiver.hpp b/ClockReceiver/ClockReceiver.hpp
index 6fe49cf1d..a621cbf1b 100644
--- a/ClockReceiver/ClockReceiver.hpp
+++ b/ClockReceiver/ClockReceiver.hpp
@@ -139,10 +139,10 @@ template <class T> class WrappedInt {
 			Severs from @c this the effect of dividing by @c divisor; @c this will end up with
 			the value of @c this modulo @c divisor and @c divided by @c divisor is returned.
 		*/
-		forceinline T divide(const T &divisor) {
-			T result(length_ / divisor.length_);
-			length_ %= divisor.length_;
-			return result;
+		template <typename Result = T> forceinline Result divide(const T &divisor) {
+			Result r;
+			static_cast<T *>(this)->fill(r, divisor);
+			return r;
 		}
 
 		/*!
@@ -177,6 +177,11 @@ class Cycles: public WrappedInt<Cycles> {
 			result.length_ = length_;
 			length_ = 0;
 		}
+
+		void fill(Cycles &result, const Cycles &divisor) {
+			result.length_ = length_ / divisor.length_;
+			length_ %= divisor.length_;
+		}
 };
 
 /// Describes an integer number of half cycles: single clock signal transitions.
@@ -215,6 +220,16 @@ class HalfCycles: public WrappedInt<HalfCycles> {
 			result.length_ = length_;
 			length_ = 0;
 		}
+
+		void fill(Cycles &result, const HalfCycles &divisor) {
+			result = Cycles(length_ / (divisor.length_ << 1));
+			length_ %= (divisor.length_ << 1);
+		}
+
+		void fill(HalfCycles &result, const HalfCycles &divisor) {
+			result.length_ = length_ / divisor.length_;
+			length_ %= divisor.length_;
+		}
 };
 
 // Create a specialisation of WrappedInt::flush for converting HalfCycles to Cycles
diff --git a/ClockReceiver/JustInTime.hpp b/ClockReceiver/JustInTime.hpp
index 4f7bafe05..778ba93e2 100644
--- a/ClockReceiver/JustInTime.hpp
+++ b/ClockReceiver/JustInTime.hpp
@@ -21,7 +21,7 @@
 	Machines that accumulate HalfCycle time but supply to a Cycle-counted device may supply a
 	separate @c TargetTimeScale at template declaration.
 */
-template <class T, class LocalTimeScale = HalfCycles, class TargetTimeScale = LocalTimeScale> class JustInTimeActor {
+template <class T, int divider = 1, class LocalTimeScale = HalfCycles, class TargetTimeScale = LocalTimeScale> class JustInTimeActor {
 	public:
 		/// Constructs a new JustInTimeActor using the same construction arguments as the included object.
 		template<typename... Args> JustInTimeActor(Args&&... args) : object_(std::forward<Args>(args)...) {}
@@ -44,8 +44,14 @@ template <class T, class LocalTimeScale = HalfCycles, class TargetTimeScale = Lo
 		}
 
 		/// Flushes all accumulated time.
-		inline void flush() {
-			if(!is_flushed_) object_.run_for(time_since_update_.template flush<TargetTimeScale>());
+		void flush() {
+			if(!is_flushed_) {
+				if(divider == 1) {
+					object_.run_for(time_since_update_.template flush<TargetTimeScale>());
+				} else {
+					object_.run_for(time_since_update_.template divide<TargetTimeScale>(LocalTimeScale(divider)));
+				}
+			}
 			is_flushed_ = true;
 		}
 
diff --git a/Components/6850/6850.cpp b/Components/6850/6850.cpp
index e935590ff..5b20be2cc 100644
--- a/Components/6850/6850.cpp
+++ b/Components/6850/6850.cpp
@@ -89,18 +89,20 @@ void ACIA::run_for(HalfCycles length) {
 	const int transmit_advance = length.as_int();
 	const auto write_data_time_remaining = transmit.write_data_time_remaining();
 
-	if(transmit_advance > write_data_time_remaining) {
-		if(next_transmission_ != NoTransmission) {
-			transmit.flush_writing();
-			consider_transmission();
-			transmit.advance_writer(transmit_advance - write_data_time_remaining);
+	if(write_data_time_remaining) {
+		if(transmit_advance > write_data_time_remaining) {
+			if(next_transmission_ != NoTransmission) {
+				transmit.flush_writing();
+				consider_transmission();
+				transmit.advance_writer(transmit_advance - write_data_time_remaining);
+			} else {
+				transmit.advance_writer(transmit_advance);
+				update_clocking_observer();
+				interrupt_request_ |= transmit_interrupt_enabled_;
+			}
 		} else {
 			transmit.advance_writer(transmit_advance);
-			update_clocking_observer();
-			interrupt_request_ |= transmit_interrupt_enabled_;
 		}
-	} else {
-		transmit.advance_writer(transmit_advance);
 	}
 
 	// Reception.
diff --git a/Machines/Apple/Macintosh/Macintosh.cpp b/Machines/Apple/Macintosh/Macintosh.cpp
index 09d91a47d..231b5f242 100644
--- a/Machines/Apple/Macintosh/Macintosh.cpp
+++ b/Machines/Apple/Macintosh/Macintosh.cpp
@@ -656,9 +656,11 @@ template <Analyser::Static::Macintosh::Target::Model model> class ConcreteMachin
 			return mouse_;
 		}
 
+		using IWMActor = JustInTimeActor<IWM, 1, HalfCycles, Cycles>;
+
 		class VIAPortHandler: public MOS::MOS6522::PortHandler {
 			public:
-				VIAPortHandler(ConcreteMachine &machine, RealTimeClock &clock, Keyboard &keyboard, DeferredAudio &audio, JustInTimeActor<IWM, HalfCycles, Cycles> &iwm, Inputs::QuadratureMouse &mouse) :
+				VIAPortHandler(ConcreteMachine &machine, RealTimeClock &clock, Keyboard &keyboard, DeferredAudio &audio, IWMActor &iwm, Inputs::QuadratureMouse &mouse) :
 					machine_(machine), clock_(clock), keyboard_(keyboard), audio_(audio), iwm_(iwm), mouse_(mouse) {}
 
 				using Port = MOS::MOS6522::Port;
@@ -764,14 +766,14 @@ template <Analyser::Static::Macintosh::Target::Model model> class ConcreteMachin
 				RealTimeClock &clock_;
 				Keyboard &keyboard_;
 				DeferredAudio &audio_;
-				JustInTimeActor<IWM, HalfCycles, Cycles> &iwm_;
+				IWMActor &iwm_;
 				Inputs::QuadratureMouse &mouse_;
 		};
 
 		CPU::MC68000::Processor<ConcreteMachine, true> mc68000_;
 
 		DriveSpeedAccumulator drive_speed_accumulator_;
-		JustInTimeActor<IWM, HalfCycles, Cycles> iwm_;
+		IWMActor iwm_;
 
 		DeferredAudio audio_;
 		Video video_;
diff --git a/Machines/AtariST/AtariST.cpp b/Machines/AtariST/AtariST.cpp
index 1a4d829a1..f161cfb8a 100644
--- a/Machines/AtariST/AtariST.cpp
+++ b/Machines/AtariST/AtariST.cpp
@@ -347,6 +347,8 @@ class ConcreteMachine:
 		forceinline void advance_time(HalfCycles length) {
 			cycles_since_audio_update_ += length;
 			mfp_ += length;
+			keyboard_acia_ += length;
+			midi_acia_ += length;
 
 			while(length >= cycles_until_video_event_) {
 				length -= cycles_until_video_event_;
@@ -366,12 +368,12 @@ class ConcreteMachine:
 		}
 
 		CPU::MC68000::Processor<ConcreteMachine, true> mc68000_;
-		JustInTimeActor<Video, HalfCycles> video_;
+		JustInTimeActor<Video> video_;
 		HalfCycles cycles_until_video_event_;
 
-		JustInTimeActor<Motorola::MFP68901::MFP68901, HalfCycles> mfp_;
-		JustInTimeActor<Motorola::ACIA::ACIA, HalfCycles> keyboard_acia_;
-		JustInTimeActor<Motorola::ACIA::ACIA, HalfCycles> midi_acia_;
+		JustInTimeActor<Motorola::MFP68901::MFP68901> mfp_;
+		JustInTimeActor<Motorola::ACIA::ACIA, 10> keyboard_acia_;
+		JustInTimeActor<Motorola::ACIA::ACIA, 10> midi_acia_;
 
 		Concurrency::DeferringAsyncTaskQueue audio_queue_;
 		GI::AY38910::AY38910 ay_;
diff --git a/Machines/ColecoVision/ColecoVision.cpp b/Machines/ColecoVision/ColecoVision.cpp
index ce8b85666..c2f347ec1 100644
--- a/Machines/ColecoVision/ColecoVision.cpp
+++ b/Machines/ColecoVision/ColecoVision.cpp
@@ -397,7 +397,7 @@ class ConcreteMachine:
 		}
 
 		CPU::Z80::Processor<ConcreteMachine, false, false> z80_;
-		JustInTimeActor<TI::TMS::TMS9918, HalfCycles> vdp_;
+		JustInTimeActor<TI::TMS::TMS9918, 1, HalfCycles> vdp_;
 
 		Concurrency::DeferringAsyncTaskQueue audio_queue_;
 		TI::SN76489 sn76489_;
diff --git a/Machines/MSX/MSX.cpp b/Machines/MSX/MSX.cpp
index df139190e..c048169f8 100644
--- a/Machines/MSX/MSX.cpp
+++ b/Machines/MSX/MSX.cpp
@@ -752,7 +752,7 @@ class ConcreteMachine:
 		};
 
 		CPU::Z80::Processor<ConcreteMachine, false, false> z80_;
-		JustInTimeActor<TI::TMS::TMS9918, HalfCycles> vdp_;
+		JustInTimeActor<TI::TMS::TMS9918> vdp_;
 		Intel::i8255::i8255<i8255PortHandler> i8255_;
 
 		Concurrency::DeferringAsyncTaskQueue audio_queue_;
diff --git a/Machines/MasterSystem/MasterSystem.cpp b/Machines/MasterSystem/MasterSystem.cpp
index 152ddb018..a005b4e3d 100644
--- a/Machines/MasterSystem/MasterSystem.cpp
+++ b/Machines/MasterSystem/MasterSystem.cpp
@@ -414,7 +414,7 @@ class ConcreteMachine:
 		Target::Region region_;
 		Target::PagingScheme paging_scheme_;
 		CPU::Z80::Processor<ConcreteMachine, false, false> z80_;
-		JustInTimeActor<TI::TMS::TMS9918, HalfCycles> vdp_;
+		JustInTimeActor<TI::TMS::TMS9918> vdp_;
 
 		Concurrency::DeferringAsyncTaskQueue audio_queue_;
 		TI::SN76489 sn76489_;