CLK/Machines/PCCompatible/Memory.hpp

//
//  Memory.hpp
//  Clock Signal
//
//  Created by Thomas Harte on 01/12/2023.
//  Copyright © 2023 Thomas Harte. All rights reserved.
//

#pragma once

#include "Registers.hpp"
#include "Segments.hpp"

#include "../../InstructionSets/x86/AccessType.hpp"

#include <array>

namespace PCCompatible {

// TODO: send writes to the ROM area off to nowhere.
struct Memory {
	public:
		using AccessType = InstructionSet::x86::AccessType;

		// Constructor.
		Memory(Registers &registers, const Segments &segments) : registers_(registers), segments_(segments) {}

		//
		// Preauthorisation call-ins. Since only an 8088 is currently modelled, all accesses are implicitly authorised.
		//
		void preauthorise_stack_write([[maybe_unused]] uint32_t length) {}
		void preauthorise_stack_read([[maybe_unused]] uint32_t length) {}
		void preauthorise_read([[maybe_unused]] InstructionSet::x86::Source segment, [[maybe_unused]] uint16_t start, [[maybe_unused]] uint32_t length) {}
		void preauthorise_read([[maybe_unused]] uint32_t start, [[maybe_unused]] uint32_t length) {}

		//
		// Access call-ins.
		//

		// Accesses an address based on segment:offset.
		template <typename IntT, AccessType type>
		typename InstructionSet::x86::Accessor<IntT, type>::type access(InstructionSet::x86::Source segment, uint16_t offset) {
			const uint32_t physical_address = address(segment, offset);

			if constexpr (std::is_same_v<IntT, uint16_t>) {
				// If this is a 16-bit access that runs past the end of the segment, it'll wrap back
				// to the start. So the 16-bit value will need to be a local cache.
				if(offset == 0xffff) {
					return split_word<type>(physical_address, address(segment, 0));
				}
			}

			return access<IntT, type>(physical_address);
		}

		// Accesses an address based on physical location.
		template <typename IntT, AccessType type>
		typename InstructionSet::x86::Accessor<IntT, type>::type access(uint32_t address) {
			// Dispense with the single-byte case trivially.
			if constexpr (std::is_same_v<IntT, uint8_t>) {
				return memory[address];
			} else if(address != 0xf'ffff) {
				return *reinterpret_cast<IntT *>(&memory[address]);
			} else {
				return split_word<type>(address, 0);
			}
		}

		template <typename IntT>
		void write_back() {
			if constexpr (std::is_same_v<IntT, uint16_t>) {
				if(write_back_address_[0] != NoWriteBack) {
					memory[write_back_address_[0]] = write_back_value_ & 0xff;
					memory[write_back_address_[1]] = write_back_value_ >> 8;
					write_back_address_[0]  = 0;
				}
			}
		}

		//
		// Direct write.
		//
		template <typename IntT>
		void preauthorised_write(InstructionSet::x86::Source segment, uint16_t offset, IntT value) {
			// Bytes can be written without further ado.
			if constexpr (std::is_same_v<IntT, uint8_t>) {
				memory[address(segment, offset) & 0xf'ffff] = value;
				return;
			}

			// Words that straddle the segment end must be split in two.
			if(offset == 0xffff) {
				memory[address(segment, offset) & 0xf'ffff] = value & 0xff;
				memory[address(segment, 0x0000) & 0xf'ffff] = value >> 8;
				return;
			}

			const uint32_t target = address(segment, offset) & 0xf'ffff;

			// Words that straddle the end of physical RAM must also be split in two.
			if(target == 0xf'ffff) {
				memory[0xf'ffff] = value & 0xff;
				memory[0x0'0000] = value >> 8;
				return;
			}

			// It's safe just to write then.
			*reinterpret_cast<uint16_t *>(&memory[target]) = value;
		}

		//
		// Helper for instruction fetch.
		//
		std::pair<const uint8_t *, size_t> next_code() {
			const uint32_t start = segments_.cs_base_ + registers_.ip();
			return std::make_pair(&memory[start], 0x10'000 - start);
		}

		std::pair<const uint8_t *, size_t> all() {
			return std::make_pair(memory.data(), 0x10'000);
		}

		//
		// External access.
		//
		void install(size_t address, const uint8_t *data, size_t length) {
			std::copy(data, data + length, memory.begin() + std::vector<uint8_t>::difference_type(address));
		}

		uint8_t *at(uint32_t address) {
			return &memory[address];
		}

	private:
		std::array<uint8_t, 1024*1024> memory{0xff};
		Registers &registers_;
		const Segments &segments_;

		uint32_t segment_base(InstructionSet::x86::Source segment) {
			using Source = InstructionSet::x86::Source;
			switch(segment) {
				default:			return segments_.ds_base_;
				case Source::ES:	return segments_.es_base_;
				case Source::CS:	return segments_.cs_base_;
				case Source::SS:	return segments_.ss_base_;
			}
		}

		uint32_t address(InstructionSet::x86::Source segment, uint16_t offset) {
			return (segment_base(segment) + offset) & 0xf'ffff;
		}

		template <AccessType type>
		typename InstructionSet::x86::Accessor<uint16_t, type>::type
		split_word(uint32_t low_address, uint32_t high_address) {
			if constexpr (is_writeable(type)) {
				write_back_address_[0] = low_address;
				write_back_address_[1] = high_address;

				// Prepopulate only if this is a modify.
				if constexpr (type == AccessType::ReadModifyWrite) {
					write_back_value_ = uint16_t(memory[write_back_address_[0]] | (memory[write_back_address_[1]] << 8));
				}

				return write_back_value_;
			} else {
				return uint16_t(memory[low_address] | (memory[high_address] << 8));
			}
		}

		static constexpr uint32_t NoWriteBack = 0;	// A low byte address of 0 can't require write-back.
		uint32_t write_back_address_[2] = {NoWriteBack, NoWriteBack};
		uint16_t write_back_value_;
};

}