CLK/Machines/Apple/AppleII/LanguageCardSwitches.hpp

//
//  LanguageCardSwitches.hpp
//  Clock Signal
//
//  Created by Thomas Harte on 22/10/2020.
//  Copyright © 2020 Thomas Harte. All rights reserved.
//

#pragma once

#include "MemorySwitches.hpp"

namespace Apple::II {

/*!
	Models the language card soft switches, present on any Apple II with a language card and provided built-in from the IIe onwards.

	Relevant memory accesses should be fed to this class; it'll call:
		* machine.set_language_card_paging() if the proper mapped state changes.
*/
template <typename Machine> class LanguageCardSwitches {
public:
	struct State {
		/// When RAM is visible in the range $D000–$FFFF:
		/// @c true indicates that bank 2 should be used between $D000 and $DFFF;
		/// @c false indicates bank 1.
		bool bank2 = true;

		/// @c true indicates that RAM should be readable in the range $D000–$FFFF;
		/// @c false indicates ROM should be readable.
		bool read = false;

		/// @c true indicates that ROM is selected for 'writing' in the range $D000–$FFFF (i.e. writes are a no-op);
		/// @c false indicates that RAM is selected for writing.
		bool write = false;

		bool operator != (const State &rhs) const {
			return
				bank2 != rhs.bank2 ||
				read != rhs.read ||
				write != rhs.write;
		}
	};

	LanguageCardSwitches(Machine &machine) : machine_(machine) {}

	/// Used by an owner to forward any access to $c08x.
	void access(uint16_t address, bool is_read) {
		const auto previous_state = state_;

		// Quotes below taken from Understanding the Apple II, p. 5-28 and 5-29.

		// "A3 controls the 4K bank selection"; 0 = bank 2, 1 = bank 1.
		state_.bank2 = !(address & 8);

		// "Access to $C080, $C083, $C084, $0087, $C088, $C08B, $C08C, or $C08F sets the READ ENABLE flip-flop"
		// (other accesses reset it)
		state_.read = !(((address&2) >> 1) ^ (address&1));

		// "The WRITE ENABLE' flip-flop is reset by an odd read access to the $C08X range when the PRE-WRITE flip-flop is set."
		if(pre_write_ && is_read && (address&1)) state_.write = false;

		// "[The WRITE ENABLE' flip-flop] is set by an even access in the $C08X range."
		if(!(address&1)) state_.write = true;

		// ("Any other type of access causes the WRITE ENABLE' flip-flop to hold its current state.")

		// "The PRE-WRITE flip-flop is set by an odd read access in the $C08X range. It is reset by an even access or a write access."
		pre_write_ = is_read ? (address&1) : false;

		// Apply whatever the net effect of all that is to the memory map.
		if(previous_state != state_) {
			machine_.template set_paging<PagingType::LanguageCard>();
		}
	}

	/// Provides read-only access to the current language card switch state.
	const State &state() const {
		return state_;
	}

	/// Provides relevant parts of the IIgs interface.
	void set_state(uint8_t value) {
		const auto previous_state = state_;

		// Bit 3: 1 => enable ROM, 0 => enable RAM.
		state_.read = !(value & 0x08);
		// Bit 2: 1 => select bank 2, 0 => select bank 1. [per errata to the Hardware Reference
		// correcting the original, which lists them the other way around]
		state_.bank2 = value & 0x04;

		if(previous_state != state_) {
			machine_.template set_paging<PagingType::LanguageCard>();
		}
	}

	uint8_t get_state() const {
		return
			(state_.read ? 0x00 : 0x08) |
			(state_.bank2 ? 0x04 : 0x00);
	}

private:
	Machine &machine_;
	State state_;

	// This is an additional flip flop contained on the language card, but
	// it is one step removed from current banking state, so I've excluded it
	// from the State struct.
	bool pre_write_ = false;
};

}