diff --git a/InstructionSets/CachingExecutor.hpp b/InstructionSets/CachingExecutor.hpp
index c5fc80753..ec1d58722 100644
--- a/InstructionSets/CachingExecutor.hpp
+++ b/InstructionSets/CachingExecutor.hpp
@@ -94,7 +94,7 @@ template <
 		// work for executors to fill this array, but subsequently performers can be
 		// indexed by array position, which is a lot more compact than a generic pointer.
 		std::array<Performer, max_performer_count> performers_;
-
+		ProgramCounterType program_counter_;
 
 		/*!
 			Moves the current point of execution to @c address, updating necessary performer caches
@@ -104,6 +104,7 @@ template <
 			// Set flag to terminate any inner loop currently running through
 			// previously-parsed content.
 			has_branched_ = true;
+			program_counter_ = address;
 
 			// Temporary implementation: just interpret.
 			program_.clear();
@@ -137,11 +138,36 @@ template <
 			}
 		}
 
-	private:
-		bool has_branched_ = false;
+		/*!
+			Runs for @c duration; the intention is that subclasses provide a method
+			that is clear about units, and call this to count down in whatever units they
+			count down in.
+		*/
+		void run_for(int duration) {
+			remaining_duration_ += duration;
+
+			while(remaining_duration_ > 0) {
+				has_branched_ = false;
+				while(remaining_duration_ > 0 && !has_branched_) {
+					(static_cast<Executor *>(this)->*performers_[program_index_])();
+					++program_index_;
+				}
+			}
+		}
+
+		/*!
+			Should be called by a specific executor to subtract from the remaining
+			running duration.
+		*/
+		inline void subtract_duration(int duration) {
+			remaining_duration_ -= duration;
+		}
 
 	private:
+		bool has_branched_ = false;
+		int remaining_duration_ = 0;
 		std::vector<PerformerIndex> program_;
+		size_t program_index_ = 0;
 
 		/* TODO: almost below here can be shoved off into an LRUCache object, or similar. */
 
diff --git a/InstructionSets/M50740/Executor.cpp b/InstructionSets/M50740/Executor.cpp
index def6927e6..6469e76e0 100644
--- a/InstructionSets/M50740/Executor.cpp
+++ b/InstructionSets/M50740/Executor.cpp
@@ -37,9 +37,11 @@ void Executor::set_rom(const std::vector<uint8_t> &rom) {
 	reset();
 
 	// TEMPORARY: just to test initial wiring.
-	for(int c = 0; c < 1300; c++) {
-		run_to_branch();
-	}
+	run_for(Cycles(13000));
+}
+
+void Executor::run_for(Cycles cycles) {
+	CachingExecutor::run_for(cycles.as<int>());
 }
 
 void Executor::reset() {
@@ -126,6 +128,12 @@ template<bool is_brk> inline void Executor::perform_interrupt() {
 }
 
 template <Operation operation, AddressingMode addressing_mode> void Executor::perform() {
+	printf("%04x\t%02x\t%d %d\t[x:%02x s:%02x]\n", program_counter_ & 0x1fff, memory_[program_counter_ & 0x1fff], int(operation), int(addressing_mode), x_, s_);
+
+	// Post cycle cost; this emulation _does not provide accurate timing_.
+	// TODO: post actual cycle counts. For now count instructions only.
+	subtract_duration(1);
+
 	// Deal with all modes that don't access memory up here;
 	// those that access memory will go through a slightly longer
 	// sequence below that wraps the address and checks whether
@@ -135,8 +143,6 @@ template <Operation operation, AddressingMode addressing_mode> void Executor::pe
 #define next8()		memory_[(program_counter_ + 1) & 0x1fff]
 #define next16()	(memory_[(program_counter_ + 1) & 0x1fff] | (memory_[(program_counter_ + 2) & 0x1fff] << 8))
 
-	printf("%04x\t%02x\t%d %d\t[x:%02x s:%02x]\n", program_counter_ & 0x1fff, memory_[program_counter_ & 0x1fff], int(operation), int(addressing_mode), x_, s_);
-
 	// Underlying assumption below: the instruction stream will never
 	// overlap with IO ports.
 	switch(addressing_mode) {
@@ -204,7 +210,7 @@ template <Operation operation, AddressingMode addressing_mode> void Executor::pe
 			case AddressingMode::AbsoluteY:		address = next16() + y_;			break;
 			case AddressingMode::ZeroPage:		address = next8();					break;
 			case AddressingMode::ZeroPageX:		address = (next8() + x_) & 0xff;	break;
-			case AddressingMode::ZeroPageY:		address = (next8() + x_) & 0xff;	break;
+			case AddressingMode::ZeroPageY:		address = (next8() + y_) & 0xff;	break;
 
 			case AddressingMode::ZeroPageIndirect:
 				address = next8();
@@ -398,6 +404,10 @@ template <Operation operation> void Executor::perform(uint8_t *operand [[maybe_u
 			overflow_result_ = uint8_t(*operand << 1);
 		break;
 
+		case Operation::TST:
+			set_nz(*operand);
+		break;
+
 	/*
 		Operations affected by the index mode flag: ADC, AND, CMP, EOR, LDA, ORA, and SBC.
 	*/
@@ -514,9 +524,3 @@ template <Operation operation> void Executor::perform(uint8_t *operand [[maybe_u
 	}
 #undef set_nz
 }
-
-void Executor::set_program_counter(uint16_t address) {
-	printf("--- %04x ---\n", (address & 0x1fff) - 0x1000);
-	program_counter_ = address;
-	CachingExecutor::set_program_counter(address);
-}
diff --git a/InstructionSets/M50740/Executor.hpp b/InstructionSets/M50740/Executor.hpp
index 744c4da43..7e2617c5c 100644
--- a/InstructionSets/M50740/Executor.hpp
+++ b/InstructionSets/M50740/Executor.hpp
@@ -12,6 +12,7 @@
 #include "Instruction.hpp"
 #include "Parser.hpp"
 #include "../CachingExecutor.hpp"
+#include "../../ClockReceiver/ClockReceiver.hpp"
 
 #include <cstdint>
 #include <vector>
@@ -34,6 +35,11 @@ class Executor: public CachingExecutor {
 		void set_rom(const std::vector<uint8_t> &rom);
 		void reset();
 
+		/*!
+			Runs, in discrete steps, the minimum number of instructions as it takes to complete at least @c cycles.
+		*/
+		void run_for(Cycles cycles);
+
 	private:
 		// MARK: - CachingExecutor-facing interface.
 
@@ -106,8 +112,6 @@ class Executor: public CachingExecutor {
 		*/
 		template <Operation operation, AddressingMode addressing_mode> void perform();
 
-		void set_program_counter(uint16_t address);
-
 	private:
 		// MARK: - Instruction set state.
 
@@ -115,7 +119,6 @@ class Executor: public CachingExecutor {
 		uint8_t memory_[0x2000];
 
 		// Registers.
-		uint16_t program_counter_;
 		uint8_t a_, x_, y_, s_;
 
 		uint8_t negative_result_ = 0;