CLK/Components/6526/Implementation/6526Storage.hpp

//
//  6526Storage.hpp
//  Clock Signal
//
//  Created by Thomas Harte on 18/07/2021.
//  Copyright © 2021 Thomas Harte. All rights reserved.
//

#pragma once

#include <array>

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

namespace MOS::MOS6526 {

class TODBase {
public:
	template <bool is_timer2> void set_control(const uint8_t value) {
		if constexpr (is_timer2) {
			write_alarm = value & 0x80;
		} else {
			is_50Hz = value & 0x80;
		}
	}

protected:
	bool write_alarm = false, is_50Hz = false;
};

template <bool is_8250> class TODStorage {};

template <> class TODStorage<false>: public TODBase {
private:
	bool increment_ = true, latched_ = false;
	int divider_ = 0;
	std::array<uint8_t, 4> value_;
	std::array<uint8_t, 4> latch_;
	std::array<uint8_t, 4> alarm_;

	static constexpr uint8_t masks[4] = {0xf, 0x3f, 0x3f, 0x1f};

	void bcd_increment(uint8_t &value) {
		++value;
		if((value&0x0f) > 0x09) value += 0x06;
	}

public:
	template <int byte> void write(const uint8_t v) {
		if(write_alarm) {
			alarm_[byte] = v & masks[byte];
		} else {
			value_[byte] = v & masks[byte];

			if constexpr (byte == 0) {
				increment_ = true;
			}
			if constexpr (byte == 3) {
				increment_ = false;
			}
		}
	}

	template <int byte> uint8_t read() {
		if(latched_) {
			const uint8_t result = latch_[byte];
			if constexpr (byte == 0) {
				latched_ = false;
			}
			return result;
		}

		if constexpr (byte == 3) {
			latched_ = true;
			latch_ = value_;
		}
		return value_[byte];
	}

	bool advance(int count) {
		if(!increment_) {
			return false;
		}

		while(count--) {
			// Increment the pre-10ths divider.
			++divider_;
			if(divider_ < 5) continue;
			if(divider_ < 6 && !is_50Hz) continue;
			divider_ = 0;

			// Increments 10ths of a second. One BCD digit.
			++value_[0];
			if(value_[0] < 10) {
				continue;
			}

			// Increment seconds. Actual BCD needed from here onwards.
			bcd_increment(value_[1]);
			if(value_[1] != 60) {
				continue;
			}
			value_[1] = 0;

			// Increment minutes.
			bcd_increment(value_[2]);
			if(value_[2] != 60) {
				continue;
			}
			value_[2] = 0;

			// TODO: increment hours, keeping AM/PM separate?
		}

		return false;	// TODO: test against alarm.
	}
};

template <> class TODStorage<true>: public TODBase {
private:
	uint32_t increment_mask_ = uint32_t(~0);
	uint32_t latch_ = 0;
	uint32_t value_ = 0;
	uint32_t alarm_ = 0xff'ffff;

public:
	template <int byte> void write(uint8_t v) {
		if constexpr (byte == 3) {
			return;
		}
		constexpr int shift = byte << 3;

		// Write to either the alarm or the current value as directed;
		// writing to any part of the current value other than the LSB
		// pauses incrementing until the LSB is written.
		const uint32_t mask = uint32_t(~(0xff << shift));
		if(write_alarm) {
			alarm_ = (alarm_ & mask) | uint32_t(v << shift);
		} else {
			value_ = (value_ & mask) | uint32_t(v << shift);
			increment_mask_ = (byte == 0) ? uint32_t(~0) : 0;
		}
	}

	template <int byte> uint8_t read() {
		if constexpr (byte == 3) {
			return 0xff;	// Assumed. Just a guess.
		}
		constexpr int shift = byte << 3;

		if constexpr (byte == 2) {
			latch_ = value_ | 0xff00'0000;
		}

		const uint32_t source = latch_ ? latch_ : value_;
		const uint8_t result = uint8_t((source >> shift) & 0xff);

		if constexpr (byte == 0) {
			latch_ = 0;
		}

		return result;
	}

	bool advance(int count) {
		// The 8250 uses a simple binary counter to replace the
		// 6526's time-of-day clock. So this is easy.
		const uint32_t distance_to_alarm = (alarm_ - value_) & 0xff'ffff;
		const auto increment = uint32_t(count) & increment_mask_;
		value_ = (value_ + increment) & 0xff'ffff;
		return distance_to_alarm <= increment;
	}
};

struct MOS6526Storage {
	bool cnt_state_ = false;	// Inactive by default.
	bool cnt_edge_ = false;
	bool flag_state_ = false;
	HalfCycles half_divider_;

	uint8_t output_[2] = {0, 0};
	uint8_t data_direction_[2] = {0, 0};

	uint8_t interrupt_control_ = 0;
	uint8_t interrupt_state_ = 0;

	uint8_t shift_data_ = 0;
	uint8_t shift_register_ = 0;
	int shift_bits_ = 0;
	bool shifter_is_output_ = false;

	struct Counter {
		uint16_t reload = 0;
		uint16_t value = 0;
		uint8_t control = 0;

		template <int shift, bool is_8250> void set_reload(uint8_t v) {
			reload = (reload & (0xff00 >> shift)) | uint16_t(v << shift);

			if constexpr (shift == 8) {
				// This seems to be a special 8250 feature per the Amiga
				// Hardware Reference Manual; cf. Appendix F.
				if(is_8250) {
					control |= 1;
					pending |= ReloadInOne;
				} else {
					if(!(control&1)) {
						pending |= ReloadInOne;
					}
				}
			}

			// If this write has hit during a reload cycle, reload.
			if(pending & ReloadNow) {
				value = reload;
			}
		}

		template <bool is_counter_2> void set_control(uint8_t v) {
			control = v;

			if(v&2) {
				Log::Logger<Log::Source::MOS6526> log;
				log.error().append("UNIMPLEMENTED: PB strobe");
			}
		}

		template <bool is_counter_2> bool advance(bool chained_input, bool cnt_state, bool cnt_edge) {
			// TODO: remove most of the conditionals here in favour of bit shuffling.

			pending = (pending & PendingClearMask) << 1;

			//
			// Apply feeder states inputs: anything that
			// will take effect in the future.
			//

			// Schedule a force reload if requested.
			if(control & 0x10) {
				pending |= ReloadInOne;
				control &= ~0x10;
			}

			// Keep a history of the one-shot bit.
			if(control & 0x08) {
				pending |= OneShotInOne;
			}

			// Determine whether an input clock is applicable.
			if constexpr(is_counter_2) {
				switch(control&0x60) {
					case 0x00:	// Count Phi2 pulses.
						pending |= TestInputNow;
					break;
					case 0x20:	// Count negative CNTs, with an extra cycle of delay.
						pending |= cnt_edge ? TestInputInOne : 0;
					break;
					case 0x40:	// Count timer A reloads.
						pending |= chained_input ? TestInputNow : 0;
					break;
					case 0x60:	// Count timer A transitions when CNT is low.
						pending |= chained_input && cnt_state ? TestInputNow : 0;
					break;
				}
			} else {
				if(!(control&0x20)) {
					pending |= TestInputNow;
				} else if (cnt_edge) {
					pending |= TestInputInOne;
				}
			}
			if(pending&TestInputNow && control&1) {
				pending |= ApplyClockInTwo;
			}


			//
			// Perform a timer tick and decide whether a reload is prompted.
			//
			if(pending & ApplyClockNow) {
				--value;
			}

			const bool should_reload = !value && (pending & ApplyClockInOne);

			// Schedule a reload if so ordered.
			if(should_reload) {
				pending |= ReloadNow;	// Combine this decision with a deferred
										// input from the force-reoad test above.

				// If this was one-shot, stop.
				if(pending&(OneShotInOne | OneShotNow)) {
					control &= ~1;
					pending &= ~(ApplyClockInOne|ApplyClockInTwo);	// Cancel scheduled ticks.
				}
			}

			// Reload if scheduled.
			if(pending & ReloadNow) {
				value = reload;
				pending &= ~ApplyClockInOne;	// Skip next decrement.
			}


			return should_reload;
		}

	private:
		int pending = 0;

		static constexpr int ReloadInOne = 1 << 0;
		static constexpr int ReloadNow = 1 << 1;

		static constexpr int OneShotInOne = 1 << 2;
		static constexpr int OneShotNow = 1 << 3;

		static constexpr int ApplyClockInTwo = 1 << 4;
		static constexpr int ApplyClockInOne = 1 << 5;
		static constexpr int ApplyClockNow = 1 << 6;

		static constexpr int TestInputInOne = 1 << 7;
		static constexpr int TestInputNow = 1 << 8;

		static constexpr int PendingClearMask = ~(ReloadNow | OneShotNow | ApplyClockNow);
	} counter_[2];

	static constexpr int InterruptInOne = 1 << 0;
	static constexpr int InterruptNow = 1 << 1;
	static constexpr int PendingClearMask = ~(InterruptNow);
	int pending_ = 0;
};

}