diff --git a/OSBindings/Mac/Clock SignalTests/Z80InterruptTests.swift b/OSBindings/Mac/Clock SignalTests/Z80InterruptTests.swift
index 5c25b9028..43ddc4a3b 100644
--- a/OSBindings/Mac/Clock SignalTests/Z80InterruptTests.swift	
+++ b/OSBindings/Mac/Clock SignalTests/Z80InterruptTests.swift	
@@ -20,6 +20,11 @@ class Z80InterruptTests: XCTestCase {
 	}
 
 	func testNMI() {
+		// Per http://www.z80.info/interrup.htm NMI should take 11 cycles to get to 0x66:
+		// M1 cycle: 5 T states to do an opcode read and decrement SP
+		// M2 cycle: 3 T states write high byte of PC to the stack and decrement SP
+		// M3 cycle: 3 T states write the low byte of PC and jump to #0066.
+
 		let machine = CSTestMachineZ80()
 
 		// start the PC at 0x0100 and install two NOPs for it
@@ -153,7 +158,8 @@ class Z80InterruptTests: XCTestCase {
 
 	func testIRQMode1() {
 		// In interrupt mode 1, receipt of an IRQ means that the interrupt flag is disabled,
-		// the PC is pushed to the stack and execution resumes at 0x38.
+		// the PC is pushed to the stack and execution resumes at 0x38. This should take
+		// 13 cycles total, per http://www.z80.info/interrup.htm
 
 		let machine = CSTestMachineZ80()
 
diff --git a/Processors/Z80/Z80.hpp b/Processors/Z80/Z80.hpp
index a6db04055..e353ec3fe 100644
--- a/Processors/Z80/Z80.hpp
+++ b/Processors/Z80/Z80.hpp
@@ -847,11 +847,11 @@ template <class T, bool uses_bus_request> class Processor {
 			};
 			MicroOp irq_mode1_program[] = {
 				{ MicroOp::BeginIRQ },
-				BusOp(IntAckStart(7, operation_)),
-				BusOp(IntWait(operation_)),
-				BusOp(IntAckEnd(operation_)),
-				BusOp(Refresh(4)),
-				Push(pc_),
+				BusOp(IntAckStart(7, operation_)),	// 7 half cycles (including  +
+				BusOp(IntWait(operation_)),			// [potentially 2 half cycles] +
+				BusOp(IntAckEnd(operation_)),		// Implicitly 3 half cycles +
+				BusOp(Refresh(4)),					// 4 half cycles +
+				Push(pc_),							// 12 half cycles = 26 half cycles = 13 cycles
 				{ MicroOp::Move16, &temp16_.full, &pc_.full },
 				{ MicroOp::MoveToNextProgram }
 			};