CLK/InstructionSets/x86/Decoder.hpp

//
//  Decoder.hpp
//  Clock Signal
//
//  Created by Thomas Harte on 1/1/21.
//  Copyright © 2021 Thomas Harte. All rights reserved.
//

#ifndef InstructionSets_x86_Decoder_hpp
#define InstructionSets_x86_Decoder_hpp

#include "Instruction.hpp"

#include <utility>

namespace CPU {
namespace Decoder {
namespace x86 {

enum class Model {
	i8086,
};

/*!
	Implements Intel x86 instruction decoding.

	This is an experimental implementation; it has not yet undergone significant testing.
*/
struct Decoder {
	public:
		Decoder(Model model);

		/*!
			@returns an @c Instruction plus a size; a positive size to indicate successful decoding; a
				negative size specifies the [negatived] number of further bytes the caller should ideally
				collect before calling again. The caller is free to call with fewer, but may not get a decoded
				instruction in response, and the decoder may still not be able to complete decoding
				even if given that number of bytes.
		*/
		std::pair<int, Instruction> decode(const uint8_t *source, size_t length);

	private:
		enum class Phase {
			/// Captures all prefixes and continues until an instruction byte is encountered.
			Instruction,
			/// Receives a ModRegRM byte and either populates the source_ and dest_ fields appropriately
			/// or completes decoding of the instruction, as per the instruction format.
			ModRegRM,
			/// Waits for sufficiently many bytes to pass for the required displacement and operand to be captured.
			/// Cf. displacement_size_ and operand_size_.
			AwaitingDisplacementOrOperand,
			/// Forms and returns an Instruction, and resets parsing state.
			ReadyToPost
		} phase_ = Phase::Instruction;

		/// During the ModRegRM phase, format dictates interpretation of the ModRegRM byte.
		///
		/// During the ReadyToPost phase, format determines how transiently-recorded fields
		/// are packaged into an Instruction.
		enum class ModRegRMFormat: uint8_t {
			// Parse the ModRegRM for mode, register and register/memory fields
			// and populate the source_ and destination_ fields appropriate.
			MemReg_Reg,
			Reg_MemReg,

			// Parse for mode and register/memory fields, populating both
			// source_ and destination_ fields with the result. Use the 'register'
			// field to pick an operation from the TEST/NOT/NEG/MUL/IMUL/DIV/IDIV group.
			MemRegTEST_to_IDIV,

			// Parse for mode and register/memory fields, populating both
			// source_ and destination_ fields with the result. Use the 'register'
			// field to check for the POP operation.
			MemRegPOP,

			// Parse for mode and register/memory fields, populating both
			// the destination_ field with the result and setting source_ to Immediate.
			// Use the 'register' field to check for the MOV operation.
			MemRegMOV,

			// Parse for mode and register/memory fields, populating the
			// destination_ field with the result. Use the 'register' field
			// to pick an operation from the ROL/ROR/RCL/RCR/SAL/SHR/SAR group.
			MemRegROL_to_SAR,

			// Parse for mode and register/memory fields, populating the
			// destination_ field with the result. Use the 'register' field
			// to pick an operation from the ADD/OR/ADC/SBB/AND/SUB/XOR/CMP group and
			// waits for an operand equal to the operation size.
			MemRegADD_to_CMP,

			// Parse for mode and register/memory fields, populating the
			// source_ field with the result. Fills destination_ with a segment
			// register based on the reg field.
			SegReg,

			// Parse for mode and register/memory fields, populating the
			// source_ and destination_ fields with the result. Uses the
			// 'register' field to pick INC or DEC.
			MemRegINC_DEC,

			// Parse for mode and register/memory fields, populating the
			// source_ and destination_ fields with the result. Uses the
			// 'register' field to pick from INC/DEC/CALL/JMP/PUSH, altering
			// the source to ::Immediate and setting an operand size if necessary.
			MemRegINC_to_PUSH,

			// Parse for mode and register/memory fields, populating the
			// source_ and destination_ fields with the result. Uses the
			// 'register' field to pick from ADD/ADC/SBB/SUB/CMP, altering
			// the source to ::Immediate and setting an appropriate operand size.
			MemRegADC_to_CMP,
		} modregrm_format_ = ModRegRMFormat::MemReg_Reg;

		// Ephemeral decoding state.
		Operation operation_ = Operation::Invalid;
		uint8_t instr_ = 0x00;	// TODO: is this desired, versus loading more context into ModRegRMFormat?
		int consumed_ = 0, operand_bytes_ = 0;

		// Source and destination locations.
		Source source_ = Source::None;
		Source destination_ = Source::None;

		// Immediate fields.
		int16_t displacement_ = 0;
		uint16_t operand_ = 0;
		uint64_t inward_data_ = 0;

		// Facts about the instruction.
		int displacement_size_ = 0;		// i.e. size of in-stream displacement, if any.
		int operand_size_ = 0;			// i.e. size of in-stream operand, if any.
		int operation_size_ = 0;		// i.e. size of data manipulated by the operation.

		// Prefix capture fields.
		Repetition repetition_ = Repetition::None;
		bool lock_ = false;
		Source segment_override_ = Source::None;

		/// Resets size capture and all fields with default values.
		void reset_parsing() {
			consumed_ = operand_bytes_ = 0;
			displacement_size_ = operand_size_ = 0;
			displacement_ = operand_ = 0;
			lock_ = false;
			segment_override_ = Source::None;
			repetition_ = Repetition::None;
			phase_ = Phase::Instruction;
			source_ = destination_ = Source::None;
		}
};

}
}
}

#endif /* InstructionSets_x86_Decoder_hpp */