CLK/InstructionSets/x86/Implementation/PerformImplementation.hpp

//
//  PerformImplementation.hpp
//  Clock Signal
//
//  Created by Thomas Harte on 05/10/2023.
//  Copyright © 2023 Thomas Harte. All rights reserved.
//

#ifndef PerformImplementation_h
#define PerformImplementation_h

#include "../../../Numeric/Carry.hpp"
#include "../../../Numeric/RegisterSizes.hpp"
#include "../Interrupts.hpp"

namespace InstructionSet::x86 {

template <Model model, typename IntT, typename InstructionT, typename RegistersT, typename MemoryT>
IntT *resolve(
	InstructionT &instruction,
	Source source,
	DataPointer pointer,
	RegistersT &registers,
	MemoryT &memory,
	IntT *none = nullptr,
	IntT *immediate = nullptr
);

template <Model model, Source source, typename IntT, typename InstructionT, typename RegistersT, typename MemoryT>
uint32_t address(
	InstructionT &instruction,
	DataPointer pointer,
	RegistersT &registers,
	MemoryT &memory
) {
	// TODO: non-word indexes and bases.
	if constexpr (source == Source::DirectAddress) {
		return instruction.offset();
	}

	uint32_t address;
	uint16_t zero = 0;
	address = *resolve<model, uint16_t>(instruction, pointer.index(), pointer, registers, memory, &zero);
	if constexpr (is_32bit(model)) {
		address <<= pointer.scale();
	}
	address += instruction.offset();

	if constexpr (source == Source::IndirectNoBase) {
		return address;
	}
	return address + *resolve<model, uint16_t>(instruction, pointer.base(), pointer, registers, memory);
}

template <Model model, typename IntT, typename InstructionT, typename RegistersT, typename MemoryT>
IntT *resolve(
	InstructionT &instruction,
	Source source,
	DataPointer pointer,
	RegistersT &registers,
	MemoryT &memory,
	IntT *none,
	IntT *immediate
) {
	// Rules:
	//
	// * if this is a memory access, set target_address and break;
	// * otherwise return the appropriate value.
	uint32_t target_address;
	switch(source) {
		case Source::eAX:
			// Slightly contorted if chain here and below:
			//
			//	(i) does the `constexpr` version of a `switch`; and
			//	(i) ensures .eax() etc aren't called on @c registers for 16-bit processors, so they need not implement 32-bit storage.
			if constexpr (is_32bit(model) && std::is_same_v<IntT, uint32_t>) 	{	return &registers.eax();	}
			else if constexpr (std::is_same_v<IntT, uint16_t>)					{	return &registers.ax();		}
			else if constexpr (std::is_same_v<IntT, uint8_t>)					{	return &registers.al();		}
			else 																{	return nullptr;				}
		case Source::eCX:
			if constexpr (is_32bit(model) && std::is_same_v<IntT, uint32_t>) 	{	return &registers.ecx();	}
			else if constexpr (std::is_same_v<IntT, uint16_t>)					{	return &registers.cx();		}
			else if constexpr (std::is_same_v<IntT, uint8_t>)					{	return &registers.cl();		}
			else 																{	return nullptr;				}
		case Source::eDX:
			if constexpr (is_32bit(model) && std::is_same_v<IntT, uint32_t>) 	{	return &registers.edx();	}
			else if constexpr (std::is_same_v<IntT, uint16_t>)					{	return &registers.dx();		}
			else if constexpr (std::is_same_v<IntT, uint8_t>)					{	return &registers.dl();		}
			else if constexpr (std::is_same_v<IntT, uint32_t>)					{	return nullptr;				}
		case Source::eBX:
			if constexpr (is_32bit(model) && std::is_same_v<IntT, uint32_t>) 	{	return &registers.ebx();	}
			else if constexpr (std::is_same_v<IntT, uint16_t>)					{	return &registers.bx();		}
			else if constexpr (std::is_same_v<IntT, uint8_t>)					{	return &registers.bl();		}
			else if constexpr (std::is_same_v<IntT, uint32_t>)					{	return nullptr;				}
		case Source::eSPorAH:
			if constexpr (is_32bit(model) && std::is_same_v<IntT, uint32_t>) 	{	return &registers.esp();	}
			else if constexpr (std::is_same_v<IntT, uint16_t>)					{	return &registers.sp();		}
			else if constexpr (std::is_same_v<IntT, uint8_t>)					{	return &registers.ah();		}
			else																{	return nullptr;				}
		case Source::eBPorCH:
			if constexpr (is_32bit(model) && std::is_same_v<IntT, uint32_t>) 	{	return &registers.ebp();	}
			else if constexpr (std::is_same_v<IntT, uint16_t>)					{	return &registers.bp();		}
			else if constexpr (std::is_same_v<IntT, uint8_t>)					{	return &registers.ch();		}
			else 																{	return nullptr;				}
		case Source::eSIorDH:
			if constexpr (is_32bit(model) && std::is_same_v<IntT, uint32_t>) 	{	return &registers.esi();	}
			else if constexpr (std::is_same_v<IntT, uint16_t>)					{	return &registers.si();		}
			else if constexpr (std::is_same_v<IntT, uint8_t>)					{	return &registers.dh();		}
			else 																{	return nullptr;				}
		case Source::eDIorBH:
			if constexpr (is_32bit(model) && std::is_same_v<IntT, uint32_t>) 	{	return &registers.edi();	}
			else if constexpr (std::is_same_v<IntT, uint16_t>)					{	return &registers.di();		}
			else if constexpr (std::is_same_v<IntT, uint8_t>)					{	return &registers.bh();		}
			else																{	return nullptr;				}

		case Source::ES:	if constexpr (std::is_same_v<IntT, uint16_t>) return &registers.es(); else return nullptr;
		case Source::CS:	if constexpr (std::is_same_v<IntT, uint16_t>) return &registers.cs(); else return nullptr;
		case Source::SS:	if constexpr (std::is_same_v<IntT, uint16_t>) return &registers.ss(); else return nullptr;
		case Source::DS:	if constexpr (std::is_same_v<IntT, uint16_t>) return &registers.ds(); else return nullptr;

		// 16-bit models don't have FS and GS.
		case Source::FS:	if constexpr (is_32bit(model) && std::is_same_v<IntT, uint16_t>) return &registers.fs(); else return nullptr;
		case Source::GS:	if constexpr (is_32bit(model) && std::is_same_v<IntT, uint16_t>) return &registers.gs(); else return nullptr;

		case Source::Immediate:
			*immediate = instruction.operand();
		return immediate;

		case Source::None:		return none;

		case Source::Indirect:
			target_address = address<model, Source::Indirect, IntT>(instruction, pointer, registers, memory);
		break;
		case Source::IndirectNoBase:
			target_address = address<model, Source::IndirectNoBase, IntT>(instruction, pointer, registers, memory);
		break;
		case Source::DirectAddress:
			target_address = address<model, Source::DirectAddress, IntT>(instruction, pointer, registers, memory);
		break;
	}

	// If execution has reached here then a memory fetch is required.
	// Do it and exit.
	const Source segment = pointer.segment(instruction.segment_override());
	return &memory.template access<IntT>(segment, target_address);
};

namespace Primitive {

//
// BEGIN TEMPORARY COPY AND PASTE SECTION.
//
// The following are largely excised from the M68k PerformImplementation.hpp; if there proves to be no
// reason further to specialise them, there'll be a factoring out. In some cases I've tightened the documentation.
//

/// @returns An int of type @c IntT with only the most-significant bit set.
template <typename IntT> constexpr IntT top_bit() {
	static_assert(!std::numeric_limits<IntT>::is_signed);
	constexpr IntT max = std::numeric_limits<IntT>::max();
	return max - (max >> 1);
}

/// @returns The number of bits in @c IntT.
template <typename IntT> constexpr int bit_size() {
	return sizeof(IntT) * 8;
}

/// @returns An int with the top bit indicating whether overflow occurred during the calculation of
///		• @c lhs + @c rhs (if @c is_add is true); or
///		• @c lhs - @c rhs (if @c is_add is false)
/// and the result was @c result. All other bits will be clear.
template <bool is_add, typename IntT>
IntT overflow(IntT lhs, IntT rhs, IntT result) {
	const IntT output_changed = result ^ lhs;
	const IntT input_differed = lhs ^ rhs;

	if constexpr (is_add) {
		return top_bit<IntT>() & output_changed & ~input_differed;
	} else {
		return top_bit<IntT>() & output_changed & input_differed;
	}
}
// NOTE TO FUTURE SELF: the original 68k `overflow` treats lhs and rhs the other way
// around, affecting subtractive overflow. Be careful.

//
// END COPY AND PASTE SECTION.
//

//
// Comments below on intended functioning of each operation come from the 1997 edition of the
// Intel Architecture Software Developer’s Manual; that year all such definitions still fitted within a
// single volume, Volume 2.
//
// Order Number 243191; e.g. https://www.ardent-tool.com/CPU/docs/Intel/IA/243191-002.pdf
//

inline void aaa(CPU::RegisterPair16 &ax, Status &status) {	// P. 313
	/*
		IF ((AL AND 0FH) > 9) OR (AF = 1)
			THEN
				AL ← (AL + 6);
				AH ← AH + 1;
				AF ← 1;
				CF ← 1;
			ELSE
				AF ← 0;
				CF ← 0;
			FI;
		AL ← AL AND 0FH;
	*/
	/*
		The AF and CF flags are set to 1 if the adjustment results in a decimal carry;
		otherwise they are cleared to 0. The OF, SF, ZF, and PF flags are undefined.
	*/
	if((ax.halves.low & 0x0f) > 9 || status.auxiliary_carry) {
		ax.halves.low += 6;
		++ax.halves.high;
		status.auxiliary_carry = status.carry = 1;
	} else {
		status.auxiliary_carry = status.carry = 0;
	}
	ax.halves.low &= 0x0f;
}

inline void aad(CPU::RegisterPair16 &ax, uint8_t imm, Status &status) {
	/*
		tempAL ← AL;
		tempAH ← AH;
		AL ← (tempAL + (tempAH * imm8)) AND FFH; (* imm8 is set to 0AH for the AAD mnemonic *)
		AH ← 0
	*/
	/*
		The SF, ZF, and PF flags are set according to the result;
		the OF, AF, and CF flags are undefined.
	*/
	ax.halves.low = ax.halves.low + (ax.halves.high * imm);
	ax.halves.high = 0;
	status.sign = ax.halves.low & 0x80;
	status.parity = status.zero = ax.halves.low;
}

template <typename FlowControllerT>
inline void aam(CPU::RegisterPair16 &ax, uint8_t imm, Status &status, FlowControllerT &flow_controller) {
	/*
		tempAL ← AL;
		AH ← tempAL / imm8; (* imm8 is set to 0AH for the AAD mnemonic *)
		AL ← tempAL MOD imm8;
	*/
	/*
		The SF, ZF, and PF flags are set according to the result.
		The OF, AF, and CF flags are undefined.
	*/
	/*
		If ... an immediate value of 0 is used, it will cause a #DE (divide error) exception.
	*/
	if(!imm) {
		flow_controller.interrupt(Interrupt::DivideByZero);
		return;
	}

	ax.halves.high = ax.halves.low / imm;
	ax.halves.low = ax.halves.low % imm;
	status.sign = ax.halves.low & 0x80;
	status.parity = status.zero = ax.halves.low;
}

inline void aas(CPU::RegisterPair16 &ax, Status &status) {
	/*
		IF ((AL AND 0FH) > 9) OR (AF = 1)
		THEN
			AL ← AL – 6;
			AH ← AH – 1;
			AF ← 1;
			CF ← 1;
		ELSE
			CF ← 0;
			AF ← 0;
		FI;
		AL ← AL AND 0FH;
	*/
	/*
		The AF and CF flags are set to 1 if there is a decimal borrow;
		otherwise, they are cleared to 0. The OF, SF, ZF, and PF flags are undefined.
	*/
	if((ax.halves.low & 0x0f) > 9 || status.auxiliary_carry) {
		ax.halves.low -= 6;
		--ax.halves.high;
		status.auxiliary_carry = status.carry = 1;
	} else {
		status.auxiliary_carry = status.carry = 0;
	}
	ax.halves.low &= 0x0f;
}

inline void daa(uint8_t &al, Status &status) {
	/*
		(as modified by https://www.felixcloutier.com/x86/daa ...)

        old_AL ← AL;
        old_CF ← CF;
        CF ← 0;

		IF (((AL AND 0FH) > 9) or AF = 1)
			THEN
				AL ← AL + 6;
				CF ← old_CF OR CarryFromLastAddition; (* CF OR carry from AL ← AL + 6 *)
				AF ← 1;
			ELSE
				AF ← 0;
		FI;
		IF ((old_AL > 99H) or old_CF = 1)
			THEN
				AL ← AL + 60H;
				CF ← 1;
			ELSE
				CF ← 0;
		FI;
	*/
	/*
		The CF and AF flags are set if the adjustment of the value results in a
		decimal carry in either digit of the result (see the “Operation” section above).
		The SF, ZF, and PF flags are set according to the result. The OF flag is undefined.
	*/
	const uint8_t old_al = al;
	const auto old_carry = status.carry;
	status.carry = 0;

	if((al & 0x0f) > 0x09 || status.auxiliary_carry) {
		status.carry = old_carry | (al > 0xf9);
		al += 0x06;
		status.auxiliary_carry = 1;
	} else {
		status.auxiliary_carry = 0;
	}

	if(old_al > 0x99 || old_carry) {
		al += 0x60;
		status.carry = 1;
	} else {
		status.carry = 0;
	}

	status.sign = al & 0x80;
	status.zero = status.parity = al;
}

inline void das(uint8_t &al, Status &status) {
	/*
		(as modified by https://www.felixcloutier.com/x86/daa ...)

        old_AL ← AL;
        old_CF ← CF;
        CF ← 0;

		IF (((AL AND 0FH) > 9) or AF = 1)
			THEN
				AL ← AL - 6;
				CF ← old_CF OR CarryFromLastAddition; (* CF OR borrow from AL ← AL - 6 *)
				AF ← 1;
			ELSE
				AF ← 0;
		FI;
		IF ((old_AL > 99H) or old_CF = 1)
			THEN
				AL ← AL - 60H;
				CF ← 1;
			ELSE
				CF ← 0;
		FI;
	*/
	/*
		The CF and AF flags are set if the adjustment of the value results in a
		decimal carry in either digit of the result (see the “Operation” section above).
		The SF, ZF, and PF flags are set according to the result. The OF flag is undefined.
	*/
	const uint8_t old_al = al;
	const auto old_carry = status.carry;
	status.carry = 0;

	if((al & 0x0f) > 0x09 || status.auxiliary_carry) {
		status.carry = old_carry | (al < 0x06);
		al -= 0x06;
		status.auxiliary_carry = 1;
	} else {
		status.auxiliary_carry = 0;
	}

	if(old_al > 0x99 || old_carry) {
		al -= 0x60;
		status.carry = 1;
	} else {
		status.carry = 0;
	}

	status.sign = al & 0x80;
	status.zero = status.parity = al;
}

template <typename IntT>
void adc(IntT &destination, IntT source, Status &status) {
	/*
		DEST ← DEST + SRC + CF;
	*/
	/*
		The OF, SF, ZF, AF, CF, and PF flags are set according to the result.
	*/
	const IntT result = destination + source + status.carry_bit<IntT>();

	status.carry = Numeric::carried_out<true, bit_size<IntT>() - 1>(destination, source, result);
	status.auxiliary_carry = Numeric::carried_in<4>(destination, source, result);
	status.sign = result & top_bit<IntT>();
	status.zero = status.parity = result;
	status.overflow = overflow<true, IntT>(destination, source, result);

	destination = result;
}

template <typename IntT>
void add(IntT &destination, IntT source, Status &status) {
	/*
		DEST ← DEST + SRC;
	*/
	/*
		The OF, SF, ZF, AF, CF, and PF flags are set according to the result.
	*/
	const IntT result = destination + source;

	status.carry = Numeric::carried_out<true, bit_size<IntT>() - 1>(destination, source, result);
	status.auxiliary_carry = Numeric::carried_in<4>(destination, source, result);
	status.sign = result & top_bit<IntT>();
	status.zero = status.parity = result;
	status.overflow = overflow<true, IntT>(destination, source, result);

	destination = result;
}

template <typename IntT>
void sbb(IntT &destination, IntT source, Status &status) {
	/*
		DEST ← DEST - (SRC + CF);
	*/
	/*
		The OF, SF, ZF, AF, CF, and PF flags are set according to the result.
	*/
	const IntT result = destination - source - status.carry_bit<IntT>();

	status.carry = Numeric::carried_out<false, bit_size<IntT>() - 1>(destination, source, result);
	status.auxiliary_carry = Numeric::carried_in<4>(destination, source, result);
	status.sign = result & top_bit<IntT>();
	status.zero = status.parity = result;
	status.overflow = overflow<false, IntT>(destination, source, result);

	destination = result;
}

template <typename IntT>
void sub(IntT &destination, IntT source, Status &status) {
	/*
		DEST ← DEST - SRC;
	*/
	/*
		The OF, SF, ZF, AF, CF, and PF flags are set according to the result.
	*/
	const IntT result = destination - source;

	status.carry = Numeric::carried_out<false, bit_size<IntT>() - 1>(destination, source, result);
	status.auxiliary_carry = Numeric::carried_in<4>(destination, source, result);
	status.sign = result & top_bit<IntT>();
	status.zero = status.parity = result;
	status.overflow = overflow<false, IntT>(destination, source, result);

	destination = result;
}

template <typename IntT>
void mul(IntT &destination_high, IntT &destination_low, IntT source, Status &status) {
	/*
		IF byte operation
			THEN
				AX ← AL * SRC
			ELSE (* word or doubleword operation *)
				IF OperandSize = 16 THEN
					DX:AX ← AX * SRC
				ELSE (* OperandSize = 32 *)
					EDX:EAX ← EAX * SRC
		FI;
	*/
	/*
		The OF and CF flags are cleared to 0 if the upper half of the result is 0;
		otherwise, they are set to 1. The SF, ZF, AF, and PF flags are undefined.
	*/
	destination_high = (destination_low * source) >> (8 * sizeof(IntT));
	destination_low *= source;
	status.overflow = status.carry = destination_high;
}

template <typename IntT>
void and_(IntT &destination, IntT source, Status &status) {
	/*
		DEST ← DEST AND SRC;
	*/
	/*
		The OF and CF flags are cleared; the SF, ZF, and PF flags are set according to the result.
		The state of the AF flag is undefined.
	*/
	destination &= source;

	status.overflow = 0;
	status.carry = 0;
	status.sign = destination & top_bit<IntT>();
	status.zero = status.parity = destination;
}

template <typename IntT, typename RegistersT, typename FlowControllerT>
inline void call_relative(IntT offset, RegistersT &registers, FlowControllerT &flow_controller) {
	flow_controller.call(registers.ip() + offset);
}

template <typename IntT, typename FlowControllerT>
inline void call_absolute(IntT target, FlowControllerT &flow_controller) {
	flow_controller.call(target);
}

template <Model model, typename InstructionT, typename FlowControllerT, typename RegistersT, typename MemoryT>
void call_far(InstructionT &instruction,
	FlowControllerT &flow_controller,
	RegistersT &registers,
	MemoryT &memory) {

	// TODO: eliminate 16-bit assumption below.
	uint16_t source_address = 0;
	auto pointer = instruction.destination();
	switch(pointer.template source<false>()) {
		default:
		case Source::Immediate:	flow_controller.call(instruction.segment(), instruction.offset());	return;

		case Source::Indirect:
			source_address = address<model, Source::Indirect, uint16_t>(instruction, pointer, registers, memory);
		break;
		case Source::IndirectNoBase:
			source_address = address<model, Source::IndirectNoBase, uint16_t>(instruction, pointer, registers, memory);
		break;
		case Source::DirectAddress:
			source_address = address<model, Source::DirectAddress, uint16_t>(instruction, pointer, registers, memory);
		break;
	}

	const Source source_segment = pointer.segment(instruction.segment_override());

	const uint16_t offset = memory.template access<uint16_t>(source_segment, source_address);
	source_address += 2;
	const uint16_t segment = memory.template access<uint16_t>(source_segment, source_address);
	flow_controller.call(segment, offset);
}

template <typename IntT>
void cbw(IntT &ax) {
	constexpr IntT test_bit = 1 << (sizeof(IntT) * 4 - 1);
	constexpr IntT low_half = (1 << (sizeof(IntT) * 4)) - 1;

	if(ax & test_bit) {
		ax |= ~low_half;
	} else {
		ax &= low_half;
	}
}

template <typename IntT>
void cwd(IntT &dx, IntT ax) {
	dx = ax & top_bit<IntT>() ? IntT(~0) : IntT(0);
}

inline void clc(Status &status) {	status.carry = 0;				}
inline void cld(Status &status) {	status.direction = 0;			}
inline void cli(Status &status) {	status.interrupt = 0;			}	// TODO: quite a bit more in protected mode.
inline void cmc(Status &status) {	status.carry = !status.carry;	}

}

template <
	Model model,
	DataSize data_size,
	typename InstructionT,
	typename FlowControllerT,
	typename RegistersT,
	typename MemoryT,
	typename IOT
> void perform(
	const InstructionT &instruction,
	Status &status,
	FlowControllerT &flow_controller,
	RegistersT &registers,
	MemoryT &memory,
	[[maybe_unused]] IOT &io
) {
	using IntT = typename DataSizeType<data_size>::type;
	using AddressT = typename AddressT<is_32bit(model)>::type;

	// Establish source() and destination() shorthand to fetch data if necessary.
	IntT immediate;
	auto source = [&]() -> IntT& {
		return *resolve<model, IntT>(
			instruction,
			instruction.source().template source<false>(),
			instruction.source(),
			registers,
			memory,
			nullptr,
			&immediate);
	};
	auto destination = [&]() -> IntT& {
		return *resolve<model, IntT>(
			instruction,
			instruction.destination().template source<false>(),
			instruction.destination(),
			registers,
			memory,
			nullptr,
			&immediate);
	};

	// Guide to the below:
	//
	//	* use hard-coded register names where appropriate;
	//	* return directly if there is definitely no possible write back to RAM;
	//	* otherwise use the source() and destination() lambdas, and break in order to allow a writeback if necessary.
	switch(instruction.operation) {
		default:
			assert(false);

		case Operation::AAA:	Primitive::aaa(registers.axp(), status);											return;
		case Operation::AAD:	Primitive::aad(registers.axp(), instruction.operand(), status);						return;
		case Operation::AAM:	Primitive::aam(registers.axp(), instruction.operand(), status, flow_controller);	return;
		case Operation::AAS:	Primitive::aas(registers.axp(), status);											return;
		case Operation::DAA:	Primitive::daa(registers.al(), status);												return;
		case Operation::DAS:	Primitive::das(registers.al(), status);												return;

		case Operation::CBW:
			if constexpr (data_size == DataSize::Word) {
				Primitive::cbw(registers.ax());
			} else if constexpr (is_32bit(model) && data_size == DataSize::DWord) {
				Primitive::cbw(registers.eax());
			}
		return;
		case Operation::CWD:
			if constexpr (data_size == DataSize::Word) {
				Primitive::cwd(registers.dx(), registers.ax());
			} else if constexpr (data_size == DataSize::DWord) {
				Primitive::cwd(registers.edx(), registers.eax());
			}
		return;

		case Operation::ESC:
		case Operation::NOP:	return;

		case Operation::HLT:	flow_controller.halt();		return;
		case Operation::WAIT:	flow_controller.wait();		return;

		case Operation::ADC:	Primitive::adc(destination(), source(), status);		break;
		case Operation::ADD:	Primitive::add(destination(), source(), status);		break;
		case Operation::SBB:	Primitive::sbb(destination(), source(), status);		break;
		case Operation::SUB:	Primitive::sub(destination(), source(), status);		break;
		case Operation::MUL:
			if constexpr (data_size == DataSize::Byte) {
				Primitive::mul(registers.ah(), registers.al(), source(), status);
			} else if constexpr (data_size == DataSize::Word) {
				Primitive::mul(registers.dx(), registers.ax(), source(), status);
			} else if constexpr (data_size == DataSize::DWord) {
				Primitive::mul(registers.edx(), registers.eax(), source(), status);
			}
		return;

		case Operation::AND:	Primitive::and_(destination(), source(), status);		break;

		case Operation::CALLrel:
			Primitive::call_relative(instruction.displacement(), registers, flow_controller);
		return;
		case Operation::CALLabs:
			Primitive::call_absolute(destination(), flow_controller);
		return;
		case Operation::CALLfar:
			Primitive::call_far<model>(instruction, flow_controller, registers, memory);
		return;

		case Operation::CLC:	Primitive::clc(status);				return;
		case Operation::CLD:	Primitive::cld(status);				return;
		case Operation::CLI:	Primitive::cli(status);				return;
		case Operation::CMC:	Primitive::cmc(status);				return;
	}

	// Write to memory if required to complete this operation.
	memory.template write_back<IntT>();
}

template <
	Model model,
	typename InstructionT,
	typename FlowControllerT,
	typename RegistersT,
	typename MemoryT,
	typename IOT
> void perform(
	const InstructionT &instruction,
	Status &status,
	FlowControllerT &flow_controller,
	RegistersT &registers,
	MemoryT &memory,
	IOT &io
) {
	// Dispatch to a function just like this that is specialised on data size.
	// Fetching will occur in that specialised function, per the overlapping
	// meaning of register names.
	switch(instruction.operation_size()) {
		case DataSize::Byte:
			perform<model, DataSize::Byte>(instruction, status, flow_controller, registers, memory, io);
		break;
		case DataSize::Word:
			perform<model, DataSize::Word>(instruction, status, flow_controller, registers, memory, io);
		break;
		case DataSize::DWord:
			if constexpr (is_32bit(model)) {
				perform<model, DataSize::DWord>(instruction, status, flow_controller, registers, memory, io);
			}
		break;
		case DataSize::None:
			perform<model, DataSize::None>(instruction, status, flow_controller, registers, memory, io);
		break;
	}
}

}

#endif /* PerformImplementation_h */