Takes further steps towards supporting interrupts.

Specifically:
* introduces the necessary bus signalling; and
* adds corresponding functional steps.

Still to figure out: getting into and out of an interrupt cycle.

Processors/68000/68000.hpp

@@ -65,21 +65,25 @@ struct Microcycle {
 	/// A Reset cycle is one in which the RESET output is asserted.
 	static const int Reset			= 3;
 
+	/// The interrupt acknowledge cycle is that during which the 68000 seeks to obtain the vector for
+	/// an interrupt it plans to observe. Noted on a real 68000 by all FCs being set to 1.
+	static const int InterruptAcknowledge = 4;
+
 	/// Indicates that the address and both data select strobes are active.
-	static const int SelectWord		= 1 << 2;
+	static const int SelectWord		= 1 << 3;
 
 	/// Indicates that the address strobe and exactly one of the data strobes are active; you can determine
 	/// which by inspecting the low bit of the provided address. The RW line indicates a read.
-	static const int SelectByte		= 1 << 3;
+	static const int SelectByte		= 1 << 4;
 
 	/// If set, indicates a read. Otherwise, a write.
-	static const int Read 			= 1 << 4;
+	static const int Read 			= 1 << 5;
 
 	/// Contains the value of line FC0.
-	static const int IsData 		= 1 << 5;
+	static const int IsData 		= 1 << 6;
 
 	/// Contains the value of line FC1.
-	static const int IsProgram 		= 1 << 6;
+	static const int IsProgram 		= 1 << 7;
 
 	int operation = 0;
 	HalfCycles length = HalfCycles(4);

Processors/68000/Implementation/68000Implementation.hpp

@@ -1785,6 +1785,28 @@ template <class T, bool dtack_is_implicit, bool signal_will_perform> void Proces
 							address_[7].full += 6;
 						break;
 
+						case int(MicroOp::Action::PrepareINT):
+							accepted_interrupt_level_ = bus_interrupt_level_;
+							populate_trap_steps(0, get_status());
+						break;
+
+						case int(MicroOp::Action::PrepareINTVector):
+							// Bus error => spurious interrupt.
+							if(bus_error_) {
+								effective_address_[0].full = 24 << 4;
+								break;
+							}
+
+							// Valid peripheral address => autovectored interrupt.
+							if(is_peripheral_address_) {
+								effective_address_[0].full = (24 + accepted_interrupt_level_) << 4;
+								break;
+							}
+
+							// Otherwise, the vector is whatever we were just told it is.
+							effective_address_[0].full = source_bus_data_[0].halves.low.halves.low << 4;
+						break;
+
 						case int(MicroOp::Action::CopyNextWord):
 							next_word_ = prefetch_queue_.halves.low.full;
 						break;

Processors/68000/Implementation/68000Storage.cpp

@@ -147,6 +147,7 @@ struct ProcessorStorageConstructor {
 		* nf: fetch the SSP's LSW;
 		* _: hold the reset line active for the usual period.
 		* tas: perform the final 6 cycles of a TAS: like an n nw but with the address strobe active for the entire period.
+		* int: the interrupt acknowledge cycle.
 
 		Quite a lot of that is duplicative, implying both something about internal
 		state and something about what's observable on the bus, but it's helpful to
@@ -337,6 +338,19 @@ struct ProcessorStorageConstructor {
 				continue;
 			}
 
+			// Interrupt acknowledge.
+			if(token == "int") {
+				step.microcycle.operation = Microcycle::InterruptAcknowledge | Microcycle::IsData | Microcycle::IsProgram | Microcycle::NewAddress;
+				step.microcycle.address = &storage_.effective_address_[0].full;		// The selected interrupt should be in bits 1–3; but 0 should be set.
+				step.microcycle.value = &storage_.source_bus_data_[0].halves.low;
+				steps.push_back(step);
+
+				step.microcycle.operation = Microcycle::InterruptAcknowledge | Microcycle::IsData | Microcycle::IsProgram | Microcycle::SameAddress | Microcycle::SelectByte;
+				steps.push_back(step);
+
+				continue;
+			}
+
 			std::cerr << "MC68000 program builder; Unknown access token " << token << std::endl;
 			assert(false);
 		}
@@ -3298,6 +3312,12 @@ struct ProcessorStorageConstructor {
 #undef dec
 		}
 
+		// Throw in the interrupt program.
+		const auto interrupt_pointer = storage_.all_micro_ops_.size();
+		op(Action::PrepareINT, seq("int"));	// Perform a cycle that will obtain an interrupt vector, or else dictate an autovector or a spurious interrupt.
+		op(Action::PrepareINTVector);		// The standard trap steps will be appended here, and PrepareINT will set them up according to the vector received.
+		op();
+
 #undef Dn
 #undef An
 #undef Ind
@@ -3321,21 +3341,31 @@ struct ProcessorStorageConstructor {
 #undef op
 #undef pseq
 
+		/*!
+			Iterates through the micro-sequence beginning at @c start, finalising bus_program
+			pointers that have been transiently stored as relative to @c arbitrary_base.
+		*/
+		const auto link_operations = [this](MicroOp *start, BusStep *arbitrary_base) {
+			while(!start->is_terminal()) {
+				const auto offset = size_t(start->bus_program - arbitrary_base);
+				assert(offset >= 0 &&  offset < storage_.all_bus_steps_.size());
+				start->bus_program = &storage_.all_bus_steps_[offset];
+				++start;
+			}
+		};
+
 		// Finalise micro-op and program pointers.
 		for(size_t instruction = 0; instruction < 65536; ++instruction) {
 			if(micro_op_pointers[instruction] != std::numeric_limits<size_t>::max()) {
 				storage_.instructions[instruction].micro_operations = &storage_.all_micro_ops_[micro_op_pointers[instruction]];
-
+				link_operations(storage_.instructions[instruction].micro_operations, &arbitrary_base);
-				auto operation = storage_.instructions[instruction].micro_operations;
-				while(!operation->is_terminal()) {
-					const auto offset = size_t(operation->bus_program - &arbitrary_base);
-					assert(offset >= 0 &&  offset < storage_.all_bus_steps_.size());
-					operation->bus_program = &storage_.all_bus_steps_[offset];
-					++operation;
-				}
 			}
 		}
 
+		// Link up the interrupt micro ops.
+		storage_.interrupt_micro_ops_ = &storage_.all_micro_ops_[interrupt_pointer];
+		link_operations(storage_.interrupt_micro_ops_, &arbitrary_base);
+
 		printf("%lu total steps\n", storage_.all_bus_steps_.size());
 	}
 
@@ -3478,6 +3508,9 @@ CPU::MC68000::ProcessorStorage::ProcessorStorage()  {
 	long_exception_micro_ops_ = &all_micro_ops_[long_exception_offset];
 	long_exception_micro_ops_->bus_program = bus_error_steps_;
 
+	// Apply the TRAP steps to the interrupt routine.
+	interrupt_micro_ops_[1].bus_program = trap_steps_;
+
 	// Set initial state.
 	active_step_ = reset_bus_steps_;
 	effective_address_[0] = 0;

Processors/68000/Implementation/68000Storage.hpp

@@ -59,6 +59,8 @@ class ProcessorStorage {
 		bool bus_acknowledge_ = false;
 		bool halt_ = false;
 
+		int accepted_interrupt_level_ = 0;
+
 		// Generic sources and targets for memory operations;
 		// by convention: [0] = source, [1] = destination.
 		RegisterPair32 effective_address_[2];
@@ -294,6 +296,15 @@ class ProcessorStorage {
 				// (i) fills in the proper stack addresses to the bus steps for this micro-op; and
 				// (ii) adjusts the stack pointer appropriately.
 				PrepareRTE_RTR,
+
+				// Performs the necessary status word substitution for the current interrupt level,
+				// and does the first part of initialising the trap steps.
+				PrepareINT,
+
+				// Observes the bus_error_, valid_peripheral_address_ and/or the value currently in
+				// source_bus_data_ to determine an interrupt vector, and fills in the final trap
+				// steps detail appropriately.
+				PrepareINTVector,
 			};
 			static const int SourceMask = 1 << 30;
 			static const int DestinationMask = 1 << 29;