CLK/Components/5380/ncr5380.cpp

//
//  ncr5380.cpp
//  Clock Signal
//
//  Created by Thomas Harte on 10/08/2019.
//  Copyright © 2019 Thomas Harte. All rights reserved.
//

#include "ncr5380.hpp"

#include "../../Outputs/Log.hpp"

using namespace NCR::NCR5380;
using SCSI::Line;

NCR5380::NCR5380(SCSI::Bus &bus, int clock_rate) :
	bus_(bus),
	clock_rate_(clock_rate) {
	device_id_ = bus_.add_device();
}

void NCR5380::write(int address, uint8_t value) {
	switch(address & 7) {
		case 0:
			LOG("[SCSI 0] Set current SCSI bus state to " << PADHEX(2) << int(value));
			data_bus_ = value;
		break;

		case 1: {
			LOG("[SCSI 1] Initiator command register set: " << PADHEX(2) << int(value));
			initiator_command_ = value;

			bus_output_ &= ~(Line::Reset | Line::Acknowledge | Line::Busy | Line::SelectTarget | Line::Attention);
			if(value & 0x80) bus_output_ |= Line::Reset;
			if(value & 0x08) bus_output_ |= Line::Busy;
			if(value & 0x04) bus_output_ |= Line::SelectTarget;

			/* bit 5 = differential enable if this were a 5381 */

			test_mode_ = value & 0x40;
			assert_data_bus_ = value & 0x01;
			update_control_output();
		} break;

		case 2:
			LOG("[SCSI 2] Set mode: " << PADHEX(2) << int(value));
			mode_ = value;

			// bit 7: 1 = use block mode DMA mode (if DMA mode is also enabled)
			// bit 6: 1 = be a SCSI target; 0 = be an initiator
			// bit 5: 1 = check parity
			// bit 4: 1 = generate an interrupt if parity checking is enabled and an error is found
			// bit 3: 1 = generate an interrupt when an EOP is received from the DMA controller
			// bit 2: 1 = generate an interrupt and reset low 6 bits of register 1 if an unexpected loss of Line::Busy occurs
			// bit 1: 1 = use DMA mode
			// bit 0: 1 = begin arbitration mode (device ID should be in register 0)
			arbitration_in_progress_ = false;
			switch(mode_ & 0x3) {
				case 0x0:
					bus_output_ &= ~SCSI::Line::Busy;
					dma_request_ = false;
					set_execution_state(ExecutionState::None);
				break;

				case 0x1:
					arbitration_in_progress_ = true;
					set_execution_state(ExecutionState::WatchingBusy);
					lost_arbitration_ = false;
				break;

				default:
					assert_data_bus_ = false; // TODO: proper logic for this.
					set_execution_state(ExecutionState::PerformingDMA);
				break;
			}
			update_control_output();
		break;

		case 3: {
			LOG("[SCSI 3] Set target command: " << PADHEX(2) << int(value));
			target_command_ = value;
			update_control_output();
		} break;

		case 4:
			LOG("[SCSI 4] Set select enabled: " << PADHEX(2) << int(value));
		break;

		case 5:
			LOG("[SCSI 5] Start DMA send: " << PADHEX(2) << int(value));
		break;

		case 6:
			LOG("[SCSI 6] Start DMA target receive: " << PADHEX(2) << int(value));
		break;

		case 7:
			LOG("[SCSI 7] Start DMA initiator receive: " << PADHEX(2) << int(value));
		break;
	}

	// Data is output only if the data bus is asserted.
	if(assert_data_bus_) {
		bus_output_ |= data_bus_;
	} else {
		bus_output_ &= ~SCSI::Line::Data;
	}

	// In test mode, still nothing is output. Otherwise throw out
	// the current value of bus_output_.
	if(test_mode_) {
		bus_.set_device_output(device_id_, SCSI::DefaultBusState);
	} else {
		bus_.set_device_output(device_id_, bus_output_);
	}
}

uint8_t NCR5380::read(int address) {
	switch(address & 7) {
		case 0:
			LOG("[SCSI 0] Get current SCSI bus state: " << PADHEX(2) << (bus_.get_state() & 0xff));

			if(dma_request_ && state_ == ExecutionState::PerformingDMA) {
				dma_acknowledge_ = true;
				dma_request_ = false;
				update_control_output();
				bus_.set_device_output(device_id_, bus_output_);
			}
		return uint8_t(bus_.get_state());

		case 1:
			LOG("[SCSI 1] Initiator command register get: " << (arbitration_in_progress_ ? 'p' : '-') <<  (lost_arbitration_ ? 'l' : '-'));
		return
			// Bits repeated as they were set.
			(initiator_command_ & ~0x60) |

			// Arbitration in progress.
			(arbitration_in_progress_ ? 0x40 : 0x00) |

			// Lost arbitration.
			(lost_arbitration_ ? 0x20 : 0x00);

		case 2:
			LOG("[SCSI 2] Get mode");
		return mode_;

		case 3:
			LOG("[SCSI 3] Get target command");
		return target_command_;

		case 4: {
			const auto bus_state = bus_.get_state();
			const uint8_t result =
				((bus_state & Line::Reset)			? 0x80 : 0x00) |
				((bus_state & Line::Busy)			? 0x40 : 0x00) |
				((bus_state & Line::Request)		? 0x20 : 0x00) |
				((bus_state & Line::Message)		? 0x10 : 0x00) |
				((bus_state & Line::Control)		? 0x08 : 0x00) |
				((bus_state & Line::Input)			? 0x04 : 0x00) |
				((bus_state & Line::SelectTarget)	? 0x02 : 0x00) |
				((bus_state & Line::Parity)			? 0x01 : 0x00);
			LOG("[SCSI 4] Get current bus state: " << PADHEX(2) << int(result));
			return result;
		}

		case 5: {
			const auto bus_state = bus_.get_state();
			const bool phase_matches =
				(target_output() & (Line::Message | Line::Control | Line::Input)) ==
				(bus_state & (Line::Message | Line::Control | Line::Input));

			const uint8_t result =
				/* b7 = end of DMA */
				((dma_request_ && state_ == ExecutionState::PerformingDMA) ? 0x40 : 0x00)	|
				/* b5 = parity error */
				/* b4 = IRQ active */
				(phase_matches ? 0x08 : 0x00)	|
				/* b2 = busy error */
				((bus_state & Line::Attention) ? 0x02 : 0x00) |
				((bus_state & Line::Acknowledge) ? 0x01 : 0x00);
			LOG("[SCSI 5] Get bus and status: " << PADHEX(2) << int(result));
			return result;
		}

		case 6:
			LOG("[SCSI 6] Get input data");
		return 0xff;

		case 7:
			LOG("[SCSI 7] Reset parity/interrupt");
		return 0xff;
	}
	return 0;
}

SCSI::BusState NCR5380::target_output() {
	SCSI::BusState output = SCSI::DefaultBusState;
	if(target_command_ & 0x08) output |= Line::Request;
	if(target_command_ & 0x04) output |= Line::Message;
	if(target_command_ & 0x02) output |= Line::Control;
	if(target_command_ & 0x01) output |= Line::Input;
	return output;
}

void NCR5380::update_control_output() {
	bus_output_ &= ~(Line::Request | Line::Message | Line::Control | Line::Input | Line::Acknowledge | Line::Attention);
	if(mode_ & 0x40) {
		// This is a target; C/D, I/O, /MSG and /REQ are signalled on the bus.
		bus_output_ |= target_output();
	} else {
		// This is an initiator; /ATN and /ACK are signalled on the bus.
		if(
			(initiator_command_ & 0x10) ||
			(state_ == ExecutionState::PerformingDMA && dma_acknowledge_)
		) bus_output_ |= Line::Acknowledge;
		if(initiator_command_ & 0x02) bus_output_ |= Line::Attention;
	}
}

void NCR5380::run_for(Cycles cycles) {
	if(state_ == ExecutionState::None) return;

	const auto bus_state = bus_.get_state();
	++time_in_state_;
	switch(state_) {
		default: break;

		/*
			Official documentation:

				Arbitration is accomplished using a bus-free filter to continuously monitor BSY.
				If BSY remains inactive for at least 400 nsec then the SCSI bus is considered free
				and arbitration may begin. Arbitration will begin if the bus is free, SEL is inactive
				and the ARBITRATION bit (port 2, bit 0) is active. Once arbitration has begun
				(BSY asserted), an arbitration delay of 2.2 /Lsec must elapse before the data bus
				can be examined to deter- mine if arbitration has been won. This delay must be
				implemented in the controlling software driver.

			Personal notes:

				I'm discounting that "arbitratation is accomplished" opening, and assuming that what needs
				to happen is:

					(i) wait for BSY to be inactive;
					(ii) count 400 nsec;
					(iii) check that BSY and SEL are inactive.
		*/

		case ExecutionState::WatchingBusy:
			if(bus_state & SCSI::Line::Busy) {
				// Arbitration is lost only if a non-busy state had previously been observed.
				if(time_in_state_ > 1) {
					lost_arbitration_ = true;
					set_execution_state(ExecutionState::None);
				} else {
					time_in_state_ = 0;
				}
			} /* else {
				arbitration_in_progress_ = true;
			}*/

			// Check for having hit 400ns (more or less) since BSY was inactive.
			if(!lost_arbitration_ && time_in_state_ == int(int64_t(400) * int64_t(clock_rate_) / int64_t(1000000000))) {
//				arbitration_in_progress_ = false;
				if(bus_.get_state() & SCSI::Line::SelectTarget) {
					lost_arbitration_ = true;
					set_execution_state(ExecutionState::None);
				} else {
					bus_output_ &= ~SCSI::Line::Busy;
					set_execution_state(ExecutionState::None);
				}
			}

			/* TODO: there's a bug here, given that the dropping of Busy isn't communicated onward. */
		break;

		case ExecutionState::PerformingDMA:
			// Signal a DMA request if the request line is active, i.e. meaningful data is
			// on the bus, and this device hasn't yet acknowledged it.
			switch(bus_state & (SCSI::Line::Request | SCSI::Line::Acknowledge)) {
				case 0:
					dma_request_ = false;
				break;
				case SCSI::Line::Request:
					dma_request_ = true;
				break;
				case SCSI::Line::Request | SCSI::Line::Acknowledge:
					dma_request_ = false;
				break;
				case SCSI::Line::Acknowledge:
					dma_acknowledge_ = false;
					dma_request_ = false;
					update_control_output();
					bus_.set_device_output(device_id_, bus_output_);
				break;
			}
		break;
	}
}

void NCR5380::set_execution_state(ExecutionState state) {
	time_in_state_ = 0;
	state_ = state;
	update_clocking_observer();
}

ClockingHint::Preference NCR5380::preferred_clocking() {
	// Request real-time clocking if any sort of timed bus watching is ongoing,
	// given that there's no knowledge in here as to what clocking other devices
	// on the SCSI bus might be enjoying.
	return (state_ < ExecutionState::WatchingBusy) ? ClockingHint::Preference::None : ClockingHint::Preference::RealTime;
}