type operation struct {
	// op is the operation function  执行函数
	execute executionFunc
	// gasCost is the gas function and returns the gas required for execution gas消耗函数
	gasCost gasFunc
	// validateStack validates the stack (size) for the operation 堆栈大小验证函数
	validateStack stackValidationFunc
	// memorySize returns the memory size required for the operation 需要的内存大小
	memorySize memorySizeFunc

	halts   bool // indicates whether the operation shoult halt further execution 表示操作是否停止进一步执行
	jumps   bool // indicates whether the program counter should not increment 指示程序计数器是否不增加
	writes  bool // determines whether this a state modifying operation 确定这是否是一个状态修改操作
	valid   bool // indication whether the retrieved operation is valid and known 指示检索到的操作是否有效并且已知
	reverts bool // determines whether the operation reverts state (implicitly halts)确定操作是否恢复状态（隐式停止）
	returns bool // determines whether the opertions sets the return data content 确定操作是否设置了返回数据内容
}

// NewByzantiumInstructionSet returns the frontier, homestead and
// byzantium instructions.
func NewByzantiumInstructionSet() [256]operation {
	// instructions that can be executed during the homestead phase.
	instructionSet := NewHomesteadInstructionSet()
	instructionSet[STATICCALL] = operation{
		execute:       opStaticCall,
		gasCost:       gasStaticCall,
		validateStack: makeStackFunc(6, 1),
		memorySize:    memoryStaticCall,
		valid:         true,
		returns:       true,
	}
	instructionSet[RETURNDATASIZE] = operation{
		execute:       opReturnDataSize,
		gasCost:       constGasFunc(GasQuickStep),
		validateStack: makeStackFunc(0, 1),
		valid:         true,
	}
	instructionSet[RETURNDATACOPY] = operation{
		execute:       opReturnDataCopy,
		gasCost:       gasReturnDataCopy,
		validateStack: makeStackFunc(3, 0),
		memorySize:    memoryReturnDataCopy,
		valid:         true,
	}
	instructionSet[REVERT] = operation{
		execute:       opRevert,
		gasCost:       gasRevert,
		validateStack: makeStackFunc(2, 0),
		memorySize:    memoryRevert,
		valid:         true,
		reverts:       true,
		returns:       true,
	}
	return instructionSet
}

// NewHomesteadInstructionSet returns the frontier and homestead
// instructions that can be executed during the homestead phase.
func NewHomesteadInstructionSet() [256]operation {
	instructionSet := NewFrontierInstructionSet()
	instructionSet[DELEGATECALL] = operation{
		execute:       opDelegateCall,
		gasCost:       gasDelegateCall,
		validateStack: makeStackFunc(6, 1),
		memorySize:    memoryDelegateCall,
		valid:         true,
		returns:       true,
	}
	return instructionSet
}

func opPc(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
	stack.push(evm.interpreter.intPool.get().SetUint64(*pc))
	return nil, nil
}

func opMsize(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
	stack.push(evm.interpreter.intPool.get().SetInt64(int64(memory.Len())))
	return nil, nil
}

func gasBalance(gt params.GasTable, evm *EVM, contract *Contract, stack *Stack, mem *Memory, memorySize uint64) (uint64, error) {
	return gt.Balance, nil
}

func gasExtCodeSize(gt params.GasTable, evm *EVM, contract *Contract, stack *Stack, mem *Memory, memorySize uint64) (uint64, error) {
	return gt.ExtcodeSize, nil
}

func gasSLoad(gt params.GasTable, evm *EVM, contract *Contract, stack *Stack, mem *Memory, memorySize uint64) (uint64, error) {
	return gt.SLoad, nil
}

func gasExp(gt params.GasTable, evm *EVM, contract *Contract, stack *Stack, mem *Memory, memorySize uint64) (uint64, error) {
	expByteLen := uint64((stack.data[stack.len()-2].BitLen() + 7) / 8)

	var (
		gas      = expByteLen * gt.ExpByte // no overflow check required. Max is 256 * ExpByte gas
		overflow bool
	)
	if gas, overflow = math.SafeAdd(gas, GasSlowStep); overflow {
		return 0, errGasUintOverflow
	}
	return gas, nil
}

// Config are the configuration options for the Interpreter
type Config struct {
	// Debug enabled debugging Interpreter options
	Debug bool
	// EnableJit enabled the JIT VM
	EnableJit bool
	// ForceJit forces the JIT VM
	ForceJit bool
	// Tracer is the op code logger
	Tracer Tracer
	// NoRecursion disabled Interpreter call, callcode,
	// delegate call and create.
	NoRecursion bool
	// Disable gas metering
	DisableGasMetering bool
	// Enable recording of SHA3/keccak preimages
	EnablePreimageRecording bool
	// JumpTable contains the EVM instruction table. This
	// may be left uninitialised and will be set to the default
	// table.
	JumpTable [256]operation
}

// Interpreter is used to run Ethereum based contracts and will utilise the
// passed evmironment to query external sources for state information.
// The Interpreter will run the byte code VM or JIT VM based on the passed
// configuration.
type Interpreter struct {
	evm      *EVM
	cfg      Config
	gasTable params.GasTable   // 标识了很多操作的Gas价格
	intPool  *intPool

	readOnly   bool   // Whether to throw on stateful modifications
	returnData []byte // Last CALL's return data for subsequent reuse 最后一个函数的返回值
}

// NewInterpreter returns a new instance of the Interpreter.
func NewInterpreter(evm *EVM, cfg Config) *Interpreter {
	// We use the STOP instruction whether to see
	// the jump table was initialised. If it was not
	// we'll set the default jump table.
	// 用一个STOP指令测试JumpTable是否已经被初始化了, 如果没有被初始化,那么设置为默认值
	if !cfg.JumpTable[STOP].valid { 
		switch {
		case evm.ChainConfig().IsByzantium(evm.BlockNumber):
			cfg.JumpTable = byzantiumInstructionSet
		case evm.ChainConfig().IsHomestead(evm.BlockNumber):
			cfg.JumpTable = homesteadInstructionSet
		default:
			cfg.JumpTable = frontierInstructionSet
		}
	}

	return &Interpreter{
		evm:      evm,
		cfg:      cfg,
		gasTable: evm.ChainConfig().GasTable(evm.BlockNumber),
		intPool:  newIntPool(),
	}
}

func (in *Interpreter) enforceRestrictions(op OpCode, operation operation, stack *Stack) error {
	if in.evm.chainRules.IsByzantium {
		if in.readOnly {
			// If the interpreter is operating in readonly mode, make sure no
			// state-modifying operation is performed. The 3rd stack item
			// for a call operation is the value. Transferring value from one
			// account to the others means the state is modified and should also
			// return with an error.
			if operation.writes || (op == CALL && stack.Back(2).BitLen() > 0) {
				return errWriteProtection
			}
		}
	}
	return nil
}

// Run loops and evaluates the contract's code with the given input data and returns
// the return byte-slice and an error if one occurred.
// 用给定的入参循环执行合约的代码，并返回返回的字节片段，如果发生错误则返回错误。
// It's important to note that any errors returned by the interpreter should be
// considered a revert-and-consume-all-gas operation. No error specific checks
// should be handled to reduce complexity and errors further down the in.
// 重要的是要注意，解释器返回的任何错误都会消耗全部gas。 为了减少复杂性,没有特别的错误处理流程。
func (in *Interpreter) Run(snapshot int, contract *Contract, input []byte) (ret []byte, err error) {
	// Increment the call depth which is restricted to 1024
	in.evm.depth++
	defer func() { in.evm.depth-- }()

	// Reset the previous call's return data. It's unimportant to preserve the old buffer
	// as every returning call will return new data anyway.
	in.returnData = nil

	// Don't bother with the execution if there's no code.
	if len(contract.Code) == 0 {
		return nil, nil
	}

	codehash := contract.CodeHash // codehash is used when doing jump dest caching
	if codehash == (common.Hash{}) {
		codehash = crypto.Keccak256Hash(contract.Code)
	}

	var (
		op    OpCode        // current opcode
		mem   = NewMemory() // bound memory
		stack = newstack()  // local stack
		// For optimisation reason we're using uint64 as the program counter.
		// It's theoretically possible to go above 2^64. The YP defines the PC
		// to be uint256. Practically much less so feasible.
		pc   = uint64(0) // program counter
		cost uint64
		// copies used by tracer
		stackCopy = newstack() // stackCopy needed for Tracer since stack is mutated by 63/64 gas rule 
		pcCopy uint64 // needed for the deferred Tracer
		gasCopy uint64 // for Tracer to log gas remaining before execution
		logged bool // deferred Tracer should ignore already logged steps
	)
	contract.Input = input

	defer func() {
		if err != nil && !logged && in.cfg.Debug {
			in.cfg.Tracer.CaptureState(in.evm, pcCopy, op, gasCopy, cost, mem, stackCopy, contract, in.evm.depth, err)
		}
	}()

	// The Interpreter main run loop (contextual). This loop runs until either an
	// explicit STOP, RETURN or SELFDESTRUCT is executed, an error occurred during
	// the execution of one of the operations or until the done flag is set by the
	// parent context.
	// 解释器的主要循环， 直到遇到STOP，RETURN，SELFDESTRUCT指令被执行，或者是遇到任意错误，或者说done 标志被父context设置。
	for atomic.LoadInt32(&in.evm.abort) == 0 {
		// Get the memory location of pc
		// 难道下一个需要执行的指令
		op = contract.GetOp(pc)

		if in.cfg.Debug {
			logged = false
			pcCopy = uint64(pc)
			gasCopy = uint64(contract.Gas)
			stackCopy = newstack()
			for _, val := range stack.data {
				stackCopy.push(val)
			}
		}

		// get the operation from the jump table matching the opcode
		// 通过JumpTable拿到对应的operation
		operation := in.cfg.JumpTable[op]
		// 这里检查了只读模式下面不能执行writes指令
		// staticCall的情况下会设置为readonly模式
		if err := in.enforceRestrictions(op, operation, stack); err != nil {
			return nil, err
		}

		// if the op is invalid abort the process and return an error
		if !operation.valid { //检查指令是否非法
			return nil, fmt.Errorf("invalid opcode 0x%x", int(op))
		}

		// validate the stack and make sure there enough stack items available
		// to perform the operation
		// 检查是否有足够的堆栈空间。 包括入栈和出栈
		if err := operation.validateStack(stack); err != nil {
			return nil, err
		}

		var memorySize uint64
		// calculate the new memory size and expand the memory to fit
		// the operation
		if operation.memorySize != nil { // 计算内存使用量，需要收费
			memSize, overflow := bigUint64(operation.memorySize(stack))
			if overflow {
				return nil, errGasUintOverflow
			}
			// memory is expanded in words of 32 bytes. Gas
			// is also calculated in words.
			if memorySize, overflow = math.SafeMul(toWordSize(memSize), 32); overflow {
				return nil, errGasUintOverflow
			}
		}

		if !in.cfg.DisableGasMetering { //这个参数在本地模拟执行的时候比较有用，可以不消耗或者检查GAS执行交易并得到返回结果
			// consume the gas and return an error if not enough gas is available.
			// cost is explicitly set so that the capture state defer method cas get the proper cost
			// 计算gas的Cost 并使用，如果不够，就返回OutOfGas错误。
			cost, err = operation.gasCost(in.gasTable, in.evm, contract, stack, mem, memorySize)
			if err != nil || !contract.UseGas(cost) {
				return nil, ErrOutOfGas
			}
		}
		if memorySize > 0 { //扩大内存范围
			mem.Resize(memorySize)
		}

		if in.cfg.Debug {
			in.cfg.Tracer.CaptureState(in.evm, pc, op, gasCopy, cost, mem, stackCopy, contract, in.evm.depth, err)
			logged = true
		}

		// execute the operation
		// 执行命令
		res, err := operation.execute(&pc, in.evm, contract, mem, stack)
		// verifyPool is a build flag. Pool verification makes sure the integrity
		// of the integer pool by comparing values to a default value.
		if verifyPool {
			verifyIntegerPool(in.intPool)
		}
		// if the operation clears the return data (e.g. it has returning data)
		// set the last return to the result of the operation.
		if operation.returns { //如果有返回值，那么就设置返回值。 注意只有最后一个返回有效果。
			in.returnData = res
		}

		switch {
		case err != nil:
			return nil, err
		case operation.reverts:
			return res, errExecutionReverted
		case operation.halts:
			return res, nil
		case !operation.jumps:
			pc++
		}
	}
	return nil, nil
}