How can we implement a new opcode in the Ethereum Virtual Machine.

Question

How can we implement a new opcode in the Ethereum Virtual Machine. For eg : I want to implement an opcode logic such that the opcode number of this operation is 0x21. The EVM-implementation I am using is in go-lang.

Answer 1

I can only give you a general guide on how to do this:

1) Modify core/vm/instructions.go to add your instruction code, something like:

func opAdd(pc *uint64, evm *EVM, contract *Contract, memory *Memory, stack *Stack) ([]byte, error) {
    x, y := stack.pop(), stack.pop()
    stack.push(math.U256(x.Add(x, y)))

    evm.interpreter.intPool.put(y)

    return nil, nil
}

2) Modify core/vm/gas_table.go to add the gas cost for your instruction, something like this:

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

3) Modify core/vm/jump_table.go to add your new instruction opcode, something like this:

    ADD: {
        execute:       opAdd,
        gasCost:       constGasFunc(GasFastestStep),
        validateStack: makeStackFunc(2, 1),
        valid:         true,
    },

4) Modify core/vm/opcodes.go to add your new opcode.

That should be it. This is for geth 1.7.2, I didn't look at 1.8 yet so can't tell.

Answer 2

Well, it will be difficult to provide a copy&paste solution for this. However, for orientation:

You'll first have to at least understand how the code in this geth module works: https://github.com/ethereum/go-ethereum/tree/master/core/vm

opcodes.go / interpreter.go / evm.go are good entry points to start with.