diff --git a/Machines/ZX8081/ZX8081.cpp b/Machines/ZX8081/ZX8081.cpp
index 6778c6187..8aa0c4e6c 100644
--- a/Machines/ZX8081/ZX8081.cpp
+++ b/Machines/ZX8081/ZX8081.cpp
@@ -99,8 +99,12 @@ HalfCycles Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &c
 		} break;
 
 		case CPU::Z80::PartialMachineCycle::Interrupt:
+			// resetting event is M1 and IOREQ both simultaneously having leading edges;
+			// that happens 2 cycles before the end of INTACK. So the timer was reset and
+			// now has advanced twice.
+			horizontal_counter_ = HalfCycles(2);
+
 			*cycle.value = 0xff;
-			horizontal_counter_ = 0;
 		break;
 
 		case CPU::Z80::PartialMachineCycle::Refresh:
@@ -109,7 +113,7 @@ HalfCycles Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &c
 			// final two cycles of an opcode fetch. Therefore communicate a transient signalling
 			// of the IRQ line if necessary.
 			if(!(address & 0x40)) {
-				set_interrupt_line(true, -2);
+				set_interrupt_line(true, Cycles(-2));
 				set_interrupt_line(false);
 			}
 			if(has_latched_video_byte_) {
@@ -126,8 +130,7 @@ HalfCycles Machine::perform_machine_cycle(const CPU::Z80::PartialMachineCycle &c
 			}
 		break;
 
-		case CPU::Z80::PartialMachineCycle::ReadOpcodeStart:
-		case CPU::Z80::PartialMachineCycle::ReadOpcodeWait:
+		case CPU::Z80::PartialMachineCycle::ReadOpcode:
 			// Check for use of the fast tape hack.
 			if(use_fast_tape_hack_ && address == tape_trap_address_ && tape_player_.has_tape()) {
 				uint64_t prior_offset = tape_player_.get_tape()->get_offset();
diff --git a/Processors/Z80/Z80.hpp b/Processors/Z80/Z80.hpp
index ce4165264..bf6eae7d9 100644
--- a/Processors/Z80/Z80.hpp
+++ b/Processors/Z80/Z80.hpp
@@ -104,7 +104,7 @@ struct PartialMachineCycle {
 		return operation <= Operation::BusAcknowledge;
 	}
 	inline bool is_wait() const {
-		return operation >= Operation::ReadWait && operation <= Operation::InterruptWait;
+		return operation >= Operation::ReadOpcodeWait && operation <= Operation::InterruptWait;
 	}
 };
 
@@ -1894,7 +1894,7 @@ template <class T> class Processor {
 			how many cycles before now the line changed state. The value may not be longer than the
 			current machine cycle. If called at any other time, this must be zero.
 		*/
-		void set_interrupt_line(bool value, int offset = 0) {
+		void set_interrupt_line(bool value, HalfCycles offset = 0) {
 			if(irq_line_ == value) return;
 
 			// IRQ requests are level triggered and masked.
@@ -1908,7 +1908,7 @@ template <class T> class Processor {
 			// If this change happened at least one cycle ago then: (i) we're promised that this is a machine
 			// cycle per the contract on supplying an offset; and (ii) that means it happened before the lines
 			// were sampled. So adjust the most recent sample.
-			if(offset < 0) {
+			if(offset <= HalfCycles(-2)) {
 				last_request_status_ = (last_request_status_ & ~Interrupt::IRQ) | (request_status_ & Interrupt::IRQ);
 			}
 		}