This is a workshop example to explain how to implement a new precompiled contract. A precompiled contract is an implemented class within an ethereum node, that is pinned to a fixed contract address.

Implementing a custom feature as a precompiled contract allows new functionality to be added to mining nodes without making changes to the EVM opcode, making it a much cleaner way to implement complex new feature. Some possible use of this is for adding new cryptographic functions, complex zero knowledge proof verifier, external call to oracles or another chain, etc.

Typically on the base chain, such changes will need to be implemented across all ethereum node implementations and also rquires a hardfork by the miners for adoption. Thus, they need to undergo the lengthy EIP process before mass adoption.

However, when used in building private networks of ethereum nodes, or as a layer-2 plasma chain, it can be very useful to easily add custom functionality to the private chain.

• Define a new class for the feature, that implements the interface PrecompiledContract. Make the necessary changes in core/vm/contracts.go
• Add the address of the class to the precompiled contract mapping, also in core/vm/contracts.go
• Create a corresponding function in the smart contract which calls the precompiled contract using the an assembly call() method, with the predefined address.

call(g, a, v, in, insize, out, outsize)

call contract at address a with input mem[in..(in+insize)) providing g gas and ether value v wei and output area mem[out..(out+outsize)) returning 0 on error (eg. out of gas) and 1 on success

This repo defines a simple helloPrecompiled precompiled contract which prints out a hello world message on the console of the ethereum node.

Refer to go-ethereum's original README on instructions to build the geth client.

Start a private instance of the geth. You can follow this very useful tutorial reference.

The demo folder contains a simple truffle project with a contract to call the helloPrecompiled precompiled contact. migrate this project to deploy the contract and call the callPrecompiled function.