Tweaked timing of mode 0, per contradictory information. Wrote a failing test of mode 2.

OSBindings/Mac/Clock SignalTests/Z80InterruptTests.swift

@@ -73,7 +73,46 @@ class Z80InterruptTests: XCTestCase {
 		XCTAssertEqual(machine.value(for: .programCounter), 0x0105)
 	}
 
+	func testIRQMode0() {
+		// In interrupt mode 0, receipt of an IRQ causes an instruction to be read from the bus and
+		// executed, with a two-cycle penalty. The test machine posts 0x21 during an interrupt acknowledge
+		// cycle so the instruction that is executed will be LD HL, nnnn
+
+		let machine = CSTestMachineZ80()
+
+		// start the PC at 0x0100 and install two NOPs for it and then one copy of 0x83, then
+		// something that isn't 0x83, and ensuring interrupts are enabled and in mode 1
+		machine.setValue(0x0100, for: .programCounter)
+		machine.setValue(1, for: .IFF1)
+		machine.setValue(0, for: .IM)
+		machine.setValue(0x1010, for: .HL)
+		machine.setValue(0x00, atAddress: 0x0100)
+		machine.setValue(0x00, atAddress: 0x0101)
+		machine.setValue(0x83, atAddress: 0x0102)
+		machine.setValue(0x9b, atAddress: 0x0103)
+
+		// run for four cycles, and signal an IRQ
+		machine.runForNumber(ofCycles: 4)
+		machine.irqLine = true
+
+		// run for four more cycles to get to where the IRQ should be recognised
+		machine.runForNumber(ofCycles: 4)
+		XCTAssertEqual(machine.value(for: .programCounter), 0x0102)
+
+		// run for twelve more cycles, to complete a LD HL, nnnn with the additional two cycles
+		// of cost for it being an IRQ program
+		machine.runForNumber(ofCycles: 12)
+
+		// confirm that the PC is still where it was, but HL is now 0x8383, and interrupts are disabled
+		XCTAssertEqual(machine.value(for: .programCounter), 0x0102)
+		XCTAssertEqual(machine.value(for: .HL), 0x8383)
+		XCTAssertEqual(machine.value(for: .IFF1), 0)
+	}
+
 	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.
+
 		let machine = CSTestMachineZ80()
 
 		// start the PC at 0x0100 and install three NOPs for it, ensuring interrupts are enabled
@@ -106,4 +145,47 @@ class Z80InterruptTests: XCTestCase {
 		XCTAssertEqual(machine.value(atAddress: 0xfffe), 0x02)
 		XCTAssertEqual(machine.value(for: .IFF1), 0)
 	}
+
+	func testIRQMode2() {
+		// In interrupt mode 2, the current bus value is combined with the I register to look
+		// up a vector from memory; the PC calls that vector. Total cost: 19 cycles.
+
+		let machine = CSTestMachineZ80()
+
+		// start the PC at 0x0100 and install two NOPs for it and then one copy of 0x83, then
+		// something that isn't 0x83, and ensuring interrupts are enabled and in mode 1
+		machine.setValue(0x0100, for: .programCounter)
+		machine.setValue(1, for: .IFF1)
+		machine.setValue(2, for: .IM)
+		machine.setValue(0x00, atAddress: 0x0100)
+		machine.setValue(0x00, atAddress: 0x0101)
+
+		// set I to 0x0200 to establish the location of our vector table, and because the test
+		// machine will post a 0x21 in response to the interrupt cycle, at 0x0221 put the
+		// arbitrarily-chosen address 0x8049
+		machine.setValue(0x02, for: .I)
+		machine.setValue(0x49, atAddress: 0x0221)
+		machine.setValue(0x80, atAddress: 0x0222)
+
+		// put the stack at the top of memory
+		machine.setValue(0, for: .stackPointer)
+
+		// run for four cycles, and signal an IRQ
+		machine.runForNumber(ofCycles: 4)
+		machine.irqLine = true
+
+		// run for four more cycles to get to where the IRQ should be recognised
+		machine.runForNumber(ofCycles: 4)
+		XCTAssertEqual(machine.value(for: .programCounter), 0x0102)
+
+		// run for nineteen more cycles, to complete the interrupt beginning
+		machine.runForNumber(ofCycles: 19)
+
+		// confirm that the PC is now at 0x8049, the old is on the stack, and interrupts
+		// are disabled
+		XCTAssertEqual(machine.value(for: .programCounter), 0x8049)
+		XCTAssertEqual(machine.value(atAddress: 0xffff), 0x01)
+		XCTAssertEqual(machine.value(atAddress: 0xfffe), 0x02)
+		XCTAssertEqual(machine.value(for: .IFF1), 0)
+	}
 }

Processors/Z80/Z80.hpp

@@ -707,7 +707,7 @@ template <class T> class Processor: public MicroOpScheduler<MicroOp> {
 			};
 			MicroOp irq_mode0_program[] = {
 				{ MicroOp::BeginIRQMode0 },
-				{ MicroOp::BusOperation, nullptr, nullptr, {BusOperation::Interrupt, 7, nullptr, &operation_}},
+				{ MicroOp::BusOperation, nullptr, nullptr, {BusOperation::Interrupt, 6, nullptr, &operation_}},
 				{ MicroOp::DecodeOperationNoRChange }
 			};
 			MicroOp irq_mode1_program[] = {