CLK/Components/6850/6850.cpp

//
//  6850.cpp
//  Clock Signal
//
//  Created by Thomas Harte on 10/10/2019.
//  Copyright © 2019 Thomas Harte. All rights reserved.
//

#include "6850.hpp"

#include <cassert>

using namespace Motorola::ACIA;

const HalfCycles ACIA::SameAsTransmit;

ACIA::ACIA(HalfCycles transmit_clock_rate, HalfCycles receive_clock_rate) :
	transmit_clock_rate_(transmit_clock_rate),
	receive_clock_rate_((receive_clock_rate != SameAsTransmit) ? receive_clock_rate : transmit_clock_rate) {
	transmit.set_writer_clock_rate(transmit_clock_rate);
	request_to_send.set_writer_clock_rate(transmit_clock_rate);
}

uint8_t ACIA::read(int address) {
	if(address&1) {
		overran_ = false;
		received_data_ |= NoValueMask;
		update_interrupt_line();
		return uint8_t(received_data_);
	} else {
		return get_status();
	}
}

void ACIA::reset() {
	transmit.reset_writing();
	transmit.write(true);
	request_to_send.reset_writing();

	bits_received_ = bits_incoming_ = 0;
	receive_interrupt_enabled_ = transmit_interrupt_enabled_ = false;
	overran_ = false;
	next_transmission_ = received_data_ = NoValueMask;

	update_interrupt_line();
	assert(!interrupt_line_);
}

void ACIA::write(int address, uint8_t value) {
	if(address&1) {
		next_transmission_ = value;
		consider_transmission();
		update_interrupt_line();
	} else {
		if((value&3) == 3) {
			reset();
		} else {
			switch(value & 3) {
				default:
				case 0: divider_ = 1;	break;
				case 1: divider_ = 16;	break;
				case 2: divider_ = 64;	break;
			}
			switch((value >> 2) & 7) {
				default:
				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:	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.set_read_delegate(this, Storage::Time(divider_ * 2, int(receive_clock_rate_.as_integral())));
			receive_interrupt_enabled_ = value & 0x80;

			update_interrupt_line();
		}
	}
	update_clocking_observer();
}

void ACIA::consider_transmission() {
	if(next_transmission_ != NoValueMask && !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) {
			transmission |= parity(uint8_t(next_transmission_)) ? mask : 0;
			mask <<= 1;
		}

		// Add stop bits.
		for(int c = 0; c < stop_bits_; ++c) {
			transmission |= mask;
			mask <<= 1;
		}

		// Output all that.
		const int total_bits = expected_bits();
		transmit.write(divider_ * 2, total_bits, transmission);

		// Mark the transmit register as empty again.
		next_transmission_ = NoValueMask;
	}
}

ClockingHint::Preference ACIA::preferred_clocking() const {
	// Real-time clocking is required if a transmission is ongoing; this is a courtesy for whomever
	// is on the receiving end.
	if(transmit.transmission_data_time_remaining() > 0) return ClockingHint::Preference::RealTime;

	// If a bit reception is ongoing that might lead to an interrupt, ask for real-time clocking
	// because it's unclear when the interrupt might come.
	if(bits_incoming_ && receive_interrupt_enabled_) return ClockingHint::Preference::RealTime;

	// No clocking required then.
	return ClockingHint::Preference::None;
}

bool ACIA::get_interrupt_line() const {
	return interrupt_line_;
}

int ACIA::expected_bits() {
	return 1 + data_bits_ + stop_bits_ + (parity_ != Parity::None);
}

uint8_t ACIA::parity(uint8_t value) {
	value ^= value >> 4;
	value ^= value >> 2;
	value ^= value >> 1;
	return value ^ (parity_ == Parity::Even);
}

bool ACIA::serial_line_did_produce_bit(Serial::Line *, int bit) {
	// Shift this bit into the 11-bit input register; this is big enough to hold
	// the largest transmission symbol.
	++bits_received_;
	bits_incoming_ = (bits_incoming_ >> 1) | (bit << 10);

	// If that's the now-expected number of bits, update.
	const int bit_target = expected_bits();
	if(bits_received_ >= bit_target) {
		bits_received_ = 0;
		overran_ |= get_status() & 1;
		received_data_ = uint8_t(bits_incoming_ >> (12 - bit_target));
		update_interrupt_line();
		update_clocking_observer();
		return false;
	}

	// TODO: overrun, and parity.

	// Keep receiving, and consider a potential clocking change.
	if(bits_received_ == 1) update_clocking_observer();
	return true;
}

void ACIA::set_interrupt_delegate(InterruptDelegate *delegate) {
	interrupt_delegate_ = delegate;
}

void ACIA::update_interrupt_line() {
	const bool old_line = interrupt_line_;

	/*
		"Bit 7 of the control register is the rie bit. When the rie bit is high, the rdrf, ndcd,
		and ovr bits will assert the nirq output. When the rie bit is low, nirq generation is disabled."

		rie = read interrupt enable
		rdrf = receive data register full (status word bit 0)
		ndcd = data carrier detect (status word bit 2)
		over = receiver overrun (status word bit 5)

		"Bit 1 of the status register is the tdre bit. When high, the tdre bit indicates that data has been
		transferred from the transmitter data register to the output shift register. At this point, the a6850
		is ready to accept a new transmit data byte. However, if the ncts signal is high, the tdre bit remains
		low regardless of the status of the transmitter data register. Also, if transmit interrupt is enabled,
		the nirq output is asserted."

		tdre = transmitter data register empty
		ncts = clear to send
	*/
	const auto status = get_status();
	interrupt_line_ =
		(receive_interrupt_enabled_ && (status & 0x25)) ||
		(transmit_interrupt_enabled_ && (status & 0x02));

	if(interrupt_delegate_ && old_line != interrupt_line_) {
		interrupt_delegate_->acia6850_did_change_interrupt_status(this);
	}
}

uint8_t ACIA::get_status() {
	return
		((received_data_ & NoValueMask) ? 0x00 : 0x01) |
		((next_transmission_ == NoValueMask) ? 0x02 : 0x00) |
//		(data_carrier_detect.read() ? 0x04 : 0x00) |
//		(clear_to_send.read() ? 0x08 : 0x00) |
		(overran_ ? 0x20 : 0x00) |
		(interrupt_line_ ? 0x80 : 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.
}