Attempts mostly to implement 6850 output.

Components/6850/6850.cpp

@@ -13,22 +13,39 @@
 
 using namespace Motorola::ACIA;
 
+ACIA::ACIA() {}
+
 uint8_t ACIA::read(int address) {
 	if(address&1) {
 		LOG("Read from receive register");
 	} else {
 		LOG("Read status");
-		return status_;
+		return
+			((next_transmission_ == NoTransmission) ? 0x02 : 0x00) |
+			(data_carrier_detect.read() ? 0x04 : 0x00) |
+			(clear_to_send.read() ? 0x08 : 0x00)
+		;
+
+		// b0: receive data full.
+		// b1: transmit data empty.
+		// b2: DCD.
+		// b3: CTS.
+		// b4: framing error (i.e. no first stop bit where expected).
+		// b5: receiver overran.
+		// b6: parity error.
+		// b7: IRQ state.
 	}
 	return 0x00;
 }
 
 void ACIA::write(int address, uint8_t value) {
 	if(address&1) {
-		LOG("Write to transmit register");
+		next_transmission_ = value;
+		consider_transmission();
 	} else {
 		if((value&3) == 3) {
-			LOG("Reset");
+			transmit.reset_writing();
+			request_to_send.reset_writing();
 		} else {
 			switch(value & 3) {
 				default:
@@ -38,30 +55,77 @@ void ACIA::write(int address, uint8_t value) {
 			}
 			switch((value >> 2) & 7) {
 				default:
-				case 0:	word_size_ = 7; stop_bits_ = 2; parity_ = Parity::Even;	break;
-				case 1:	word_size_ = 7; stop_bits_ = 2; parity_ = Parity::Odd;	break;
-				case 2:	word_size_ = 7; stop_bits_ = 1; parity_ = Parity::Even;	break;
-				case 3:	word_size_ = 7; stop_bits_ = 1; parity_ = Parity::Odd;	break;
-				case 4:	word_size_ = 8; stop_bits_ = 2; parity_ = Parity::None;	break;
-				case 5:	word_size_ = 8; stop_bits_ = 1; parity_ = Parity::None;	break;
-				case 6:	word_size_ = 8; stop_bits_ = 1; parity_ = Parity::Even;	break;
-				case 7:	word_size_ = 8; stop_bits_ = 1; parity_ = Parity::Odd;	break;
+				case 0:	data_bits_ = 7; stop_bits_ = 2; parity_ = Parity::Even;	break;
+				case 1:	data_bits_ = 7; stop_bits_ = 2; parity_ = Parity::Odd;	break;
+				case 2:	data_bits_ = 7; stop_bits_ = 1; parity_ = Parity::Even;	break;
+				case 3:	data_bits_ = 7; stop_bits_ = 1; parity_ = Parity::Odd;	break;
+				case 4:	data_bits_ = 8; stop_bits_ = 2; parity_ = Parity::None;	break;
+				case 5:	data_bits_ = 8; stop_bits_ = 1; parity_ = Parity::None;	break;
+				case 6:	data_bits_ = 8; stop_bits_ = 1; parity_ = Parity::Even;	break;
+				case 7:	data_bits_ = 8; stop_bits_ = 1; parity_ = Parity::Odd;	break;
 			}
 			switch((value >> 5) & 3) {
-				case 0:	set_ready_to_transmit(false); transmit_interrupt_enabled_ = false;	break;
-				case 1:	set_ready_to_transmit(false); transmit_interrupt_enabled_ = true;	break;
-				case 2:	set_ready_to_transmit(true); transmit_interrupt_enabled_ = false;	break;
-				case 3:	set_ready_to_transmit(false); transmit_interrupt_enabled_ = false;	break;	/* TODO: transmit a break level on the transmit output. */
+				case 0:	request_to_send.write(false); transmit_interrupt_enabled_ = false;	break;
+				case 1:	request_to_send.write(false); transmit_interrupt_enabled_ = true;	break;
+				case 2:	request_to_send.write(true); transmit_interrupt_enabled_ = false;	break;
+				case 3:
+					request_to_send.write(false);
+					transmit_interrupt_enabled_ = false;
+					transmit.reset_writing();
+					transmit.write(false);
+				break;
 			}
 			receive_interrupt_enabled_ = value & 0x80;
-			LOG("Write to control register");
 		}
 	}
 }
 
-void ACIA::run_for(HalfCycles) {
+void ACIA::run_for(HalfCycles length) {
+	// Transmission.
+	int transmit_advance = length.as_int();
+	if(next_transmission_ != NoTransmission) {
+		const auto write_data_time_remaining = transmit.write_data_time_remaining();
+		if(transmit_advance > write_data_time_remaining) {
+			transmit.flush_writing();
+			transmit_advance -= write_data_time_remaining;
+			consider_transmission();
+		}
+	}
+	transmit.advance_writer(transmit_advance);
+
+	// Reception.
 }
 
-void ACIA::set_ready_to_transmit(bool) {
+void ACIA::consider_transmission() {
+	if(next_transmission_ != NoTransmission && !transmit.write_data_time_remaining()) {
+		// Establish start bit and [7 or 8] data bits.
+		if(data_bits_ == 7) next_transmission_ &= 0x7f;
+		int transmission = next_transmission_ << 1;
 
+		// Add a parity bit, if any.
+		int mask = 0x2 << data_bits_;
+		if(parity_ != Parity::None) {
+			next_transmission_ ^= next_transmission_ >> 4;
+			next_transmission_ ^= next_transmission_ >> 2;
+			next_transmission_ ^= next_transmission_ >> 1;
+
+			if((next_transmission_&1) != (parity_ == Parity::Even)) {
+				transmission |= mask;
+			}
+			mask <<= 1;
+		}
+
+		// Add stop bits.
+		for(int c = 0; c < stop_bits_; ++c) {
+			transmission |= mask;
+			mask <<= 1;
+		}
+
+		// Output all that.
+		const int total_bits = 1 + data_bits_ + stop_bits_ + (parity_ != Parity::None);
+		transmit.write(divider_, total_bits, transmission);
+
+		// Mark the transmit register as empty again.
+		next_transmission_ = NoTransmission;
+	}
 }

Components/6850/6850.hpp

@@ -11,12 +11,15 @@
 
 #include <cstdint>
 #include "../../ClockReceiver/ClockReceiver.hpp"
+#include "../SerialPort/SerialPort.hpp"
 
 namespace Motorola {
 namespace ACIA {
 
 class ACIA {
 	public:
+		ACIA();
+
 		/*!
 			Reads from the ACIA.
 
@@ -37,17 +40,28 @@ class ACIA {
 
 		void run_for(HalfCycles);
 
+		// Input lines.
+		Serial::Line receive;
+		Serial::Line clear_to_send;
+		Serial::Line data_carrier_detect;
+
+		// Output lines.
+		Serial::Line transmit;
+		Serial::Line request_to_send;
+
 	private:
 		int divider_ = 1;
-		uint8_t status_ = 0x00;
 		enum class Parity {
 			Even, Odd, None
 		} parity_ = Parity::None;
-		int word_size_ = 7, stop_bits_ = 2;
+		int data_bits_ = 7, stop_bits_ = 2;
+
+		static const int NoTransmission = 0x100;
+		int next_transmission_ = NoTransmission;
+		void consider_transmission();
+
 		bool receive_interrupt_enabled_ = false;
 		bool transmit_interrupt_enabled_ = false;
-
-		void set_ready_to_transmit(bool);
 };
 
 }

Components/SerialPort/SerialPort.cpp

@@ -7,3 +7,68 @@
 //
 
 #include "SerialPort.hpp"
+
+using namespace Serial;
+
+void Line::advance_writer(int cycles) {
+	remaining_delays_ = std::max(remaining_delays_ - cycles, 0);
+	while(!events_.empty()) {
+		if(events_.front().delay < cycles) {
+			cycles -= events_.front().delay;
+			auto iterator = events_.begin() + 1;
+			while(iterator != events_.end() && iterator->type != Event::Delay) {
+				level_ = iterator->type == Event::SetHigh;
+				++iterator;
+			}
+			events_.erase(events_.begin(), iterator);
+		} else {
+			events_.front().delay -= cycles;
+			break;
+		}
+	}
+}
+
+void Line::write(bool level) {
+	if(!events_.empty()) {
+		events_.emplace_back();
+		events_.back().type = level ? Event::SetHigh : Event::SetLow;
+	} else {
+		level_ = level;
+	}
+}
+
+void Line::write(int cycles, int count, int levels) {
+	remaining_delays_ += count*cycles;
+
+	auto event = events_.size();
+	events_.resize(events_.size() + size_t(count)*2);
+	while(count--) {
+		events_[event].type = Event::Delay;
+		events_[event].delay = cycles;
+		events_[event+1].type = (levels&1) ? Event::SetHigh : Event::SetLow;
+		event += 2;
+	}
+}
+
+int Line::write_data_time_remaining() {
+	return remaining_delays_;
+}
+
+void Line::reset_writing() {
+	events_.clear();
+}
+
+void Line::flush_writing() {
+	for(const auto &event : events_) {
+		switch(event.type) {
+			default: break;
+			case Event::SetHigh:	level_ = true;	break;
+			case Event::SetLow:		level_ = false;	break;
+		}
+	}
+	events_.clear();
+}
+
+bool Line::read() {
+	return level_;
+}

Components/SerialPort/SerialPort.hpp

@@ -9,19 +9,9 @@
 #ifndef SerialPort_hpp
 #define SerialPort_hpp
 
-namespace Serial {
+#include <vector>
 
-/// Signal is an amalgamation of the RS-232-esque signals and those available on the Macintosh
-/// and therefore often associated with RS-422.
-enum class Signal {
-	Receive,
-	Transmit,
-	ClearToSend,
-	RequestToSend,
-	DataCarrierDetect,
-	OutputHandshake,
-	InputHandshake
-};
+namespace Serial {
 
 /*!
 	@c Line connects a single reader and a single writer, allowing timestamped events to be
@@ -34,29 +24,43 @@ enum class Signal {
 */
 class Line {
 	public:
-		void connect_reader(int clock_rate);
-		void disconnect_reader();
+		/// Advances the read position by @c cycles relative to the writer's
+		/// clock rate.
+		void advance_writer(int cycles);
 
-		void connect_writer(int clock_rate);
-		void disconnect_writer();
+		/// Sets the line to @c level.
+		void write(bool level);
 
-		/// Sets the line to @c level after @c cycles relative to the writer's
-		/// clock rate have elapsed from the final event currently posted.
-		void write(int cycles, bool level);
-
-		/// Enqueues @c count level changes, the first occurring @c cycles
+		/// Enqueues @c count level changes, the first occurring immediately
 		/// after the final event currently posted and each subsequent event
-		/// occurring @c cycles after the previous. The levels to output are
+		/// occurring @c cycles after the previous. An additional gap of @c cycles
+		/// is scheduled after the final output. The levels to output are
 		/// taken from @c levels which is read from lsb to msb. @c cycles is
 		/// relative to the writer's clock rate.
 		void write(int cycles, int count, int levels);
 
-		/// Advances the read position by @c cycles relative to the reader's
-		/// clock rate.
-		void advance_reader(int cycles);
+		/// @returns the number of cycles until currently enqueued write data is exhausted.
+		int write_data_time_remaining();
 
-		/// @returns The instantaneous level of this line at the current read position.
+		/// Eliminates all future write states, leaving the output at whatever it is now.
+		void reset_writing();
+
+		/// Applies all pending write changes instantly.
+		void flush_writing();
+
+		/// @returns The instantaneous level of this line.
 		bool read();
+
+	private:
+		struct Event {
+			enum Type {
+				Delay, SetHigh, SetLow
+			} type;
+			int delay;
+		};
+		std::vector<Event> events_;
+		int remaining_delays_ = 0;
+		bool level_ = false;
 };
 
 /*!

Machines/AtariST/AtariST.cpp

@@ -39,6 +39,8 @@ class ConcreteMachine:
 	public:
 		ConcreteMachine(const Target &target, const ROMMachine::ROMFetcher &rom_fetcher) :
 			mc68000_(*this),
+			keyboard_acia_(),
+			midi_acia_(),
 			ay_(audio_queue_),
 			speaker_(ay_) {
 			set_clock_rate(CLOCK_RATE);