Ethereum 101 - Part 7 - The EVM

Quick Overview

The EVM is the part of Ethereum that handles smart contract deployment and execution. Simple value transfer transactions from one EOA to another don’t need to involve it, practically speaking, but everything else will involve a state update computed by the EVM. At a high level, the EVM running on the Ethereum blockchain can be thought of as a global decentralized computer containing millions of executable objects, each with its own permanent data store.

Citation: "Mastering Ethereum, Section 13 - The Ethereum Virtual Machine" authors Andreas Antonopoulos and Gavin Wood Ph.D.

The EVM's been mentioned throughout this documentation, but we haven't gone into much detail. The EVM is the Ethereum Virtual Machine, it is the Turing complete virtual machine that handles all of the transaction processing on the Ethereum network. It is a complete 256 bit virtual machine that serves to execute arbitrary EVM bytecode.

EVM bytecode

Bytecode is the machine code that the high-level smart contract languages are compiled into. It looks like this: 

608060405234801561001057600080fd5b506040516020806108b28339810180604052602081101561003057600080fd5b8101908080519060200190929190505050336000806101000a81548173ffffffffffffffffffffffffffffffffffffffff021916908373ffffffffffffffffffffffffffffffffffffffff16021790555060018060008060009054906101000a900473ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff168152602001908152602001600020600001819055508060ff166002816100fa9190610101565b5050610154565b81548183558181111561012857818360005260206000209182019101610127919061012d565b5b505050565b61015191905b8082111561014d5760008082016000905550600101610133565b5090565b90565b61074f806101636000396000f3fe60806040526004361061005c576000357c0100000000000000000000000000000000000000000000000000000000900480635c19a95c14610061578063609ff1bd146100b25780639e7b8d61146100e3578063b3f98adc14610134575b600080fd5b34801561006d57600080fd5b506100b06004803603602081101561008457600080fd5b81019080803573ffffffffffffffffffffffffffffffffffffffff169060200190929190505050610172565b005b3480156100be57600080fd5b506100c76104c7565b604051808260ff1660ff16815260200191505060405180910390f35b3480156100ef57600080fd5b506101326004803603602081101561010657600080fd5b81019080803573ffffffffffffffffffffffffffffffffffffffff169060200190929190505050610543565b005b34801561014057600080fd5b506101706004803603602081101561015757600080fd5b81019080803560ff169060200190929190505050610640565b005b6000600160003373ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff16815260200190815260200160002090508060010160009054906101000a900460ff16156101d257506104c4565b5b600073ffffffffffffffffffffffffffffffffffffffff16600160008473ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff16815260200190815260200160002060010160029054906101000a900473ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff161415801561030057503373ffffffffffffffffffffffffffffffffffffffff16600160008473ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff16815260200190815260200160002060010160029054906101000a900473ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff1614155b1561036f57600160008373ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff16815260200190815260200160002060010160029054906101000a900473ffffffffffffffffffffffffffffffffffffffff1691506101d3565b3373ffffffffffffffffffffffffffffffffffffffff168273ffffffffffffffffffffffffffffffffffffffff1614156103a957506104c4565b60018160010160006101000a81548160ff021916908315150217905550818160010160026101000a81548173ffffffffffffffffffffffffffffffffffffffff021916908373ffffffffffffffffffffffffffffffffffffffff1602179055506000600160008473ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff16815260200190815260200160002090508060010160009054906101000a900460ff16156104aa57816000015460028260010160019054906101000a900460ff1660ff1681548110151561048b57fe5b90600052602060002001600001600082825401925050819055506104c1565b816000015481600001600082825401925050819055505b50505b50565b6000806000905060008090505b6002805490508160ff16101561053e578160028260ff168154811015156104f757fe5b906000526020600020016000015411156105315760028160ff1681548110151561051d57fe5b906000526020600020016000015491508092505b80806001019150506104d4565b505090565b6000809054906101000a900473ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff163373ffffffffffffffffffffffffffffffffffffffff161415806105eb5750600160008273ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff16815260200190815260200160002060010160009054906101000a900460ff165b156105f55761063d565b60018060008373ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff168152602001908152602001600020600001819055505b50565b6000600160003373ffffffffffffffffffffffffffffffffffffffff1673ffffffffffffffffffffffffffffffffffffffff16815260200190815260200160002090508060010160009054906101000a900460ff16806106a857506002805490508260ff1610155b156106b35750610720565b60018160010160006101000a81548160ff021916908315150217905550818160010160016101000a81548160ff021916908360ff160217905550806000015460028360ff1681548110151561070457fe5b9060005260206000200160000160008282540192505081905550505b5056fea165627a7a723058209061ffc04667804683fe01748db07db99f66b416464677c76a87e047d3ff2a430029

This is not human readable code. If you had some free time, it could be reverse engineered, but that's not always a value-adding task. Further, you shouldn't be interacting with contracts on the blockchain unless you also have their high-level source code and application binary interface (ABI).

Deployment vs runtime bytecode

The above is the deployment bytecode of the HelloWorld.sol Solidity smart contract we deployed earlier in the Smart Contract section. Deployment bytecode is the runtime bytecode wrapped in auxiliary code to foster successful deployment of the contract. After successful contract deployment, the runtime bytecode resides alone at its new contract address.

EVM Assembly

The solidity compiler can print out the EVM assembly of our HelloWorld.sol contract in human readable format:

    /* "HelloWorld.sol":109:871  contract HelloWorld // defining the contract... */
  mstore(0x40, 0x80)
    /* "HelloWorld.sol":231:395  constructor() // constructor function, optional, executed once upon deployment and cannot be called again... */
  callvalue
    /* "--CODEGEN--":8:17   */
  dup1
    /* "--CODEGEN--":5:7   */
  iszero
  tag_1
  jumpi
    /* "--CODEGEN--":30:31   */
  0x00
    /* "--CODEGEN--":27:28   */
  dup1
    /* "--CODEGEN--":20:32   */
  revert
    /* "--CODEGEN--":5:7   */
tag_1:
    /* "HelloWorld.sol":231:395  constructor() // constructor function, optional, executed once upon deployment and cannot be called again... */
  pop
    /* "HelloWorld.sol":362:388  greeting = "Hello, World." */
  0x40
  dup1
  mload
  swap1
  dup2
  add
  0x40
  mstore
  dup1
  0x0d
  dup2
  mstore
  0x20
  add
  0x48656c6c6f2c20576f726c642e00000000000000000000000000000000000000
  dup2
  mstore
  pop
    /* "HelloWorld.sol":362:370  greeting */
  0x00
    /* "HelloWorld.sol":362:388  greeting = "Hello, World." */
  swap1
  dup1
  mload
  swap1
  0x20
  add
  swap1
  tag_4
  swap3
  swap2
  swap1
  tag_5
  jump  // in
tag_4:
  pop
    /* "HelloWorld.sol":109:871  contract HelloWorld // defining the contract... */
  jump(tag_6)
tag_5:
  dup3
  dup1
  sload
  0x01
  dup2
  0x01
  and
  iszero
  0x0100
  mul
  sub
  and
  0x02
  swap1
  div
  swap1
  0x00
  mstore
  keccak256(0x00, 0x20)
  swap1
  0x1f
  add
  0x20
  swap1
  div
  dup2
  add
  swap3
  dup3
  0x1f
  lt
  tag_8
  jumpi
  dup1
  mload
  not(0xff)
  and
  dup4
  dup1
  add
  or
  dup6
  sstore
  jump(tag_7)


[ ---- cut ---- ]



    tag_29:
      tag_30
      swap2
      swap1
    tag_31:
      dup1
      dup3
      gt
      iszero
      tag_32
      jumpi
      0x00
      dup2
      0x00
      swap1
      sstore
      pop
      0x01
      add
      jump(tag_31)
    tag_32:
      pop
      swap1
      jump
    tag_30:
      swap1
      jump  // out

    auxdata: 0xa165627a7a723058205a6ad79adf0bb2db43f8594df4cf90d9ddac2dcc7fdec3406884535056226e4c0029
}

This EVM assembly has been truncated. It's actually quite long. This is a little easier to interpret than the raw bytecode.

Quick primer on the EVM instruction set

The instruction set consists of many operations called opcodes. Each opcode is a computational step with an explicit gas cost. Some examples:

Opcode Name Description Gas Notes
0x01 ADD Addition operation 3 Simple computational steps
0x02 MUL Multiplication operation 5 N/A
0x31 BALANCE Retrieve balance of given account 400 See higher costs
0x54 SLOAD Load word from storage 200 N/A
0x55 SSTORE Save word to storage 20000 High cost to store a 256-bit word
0xf4 DELEGATECALL Perform a message call to another account in the context of the calling account Varies Need to be cautious when using this opcode

Some opcodes cost 0 gas. For example, opcodes that halt execution are gas-less opcodes. For example, the 0x00 STOP opcode halting execution costs 0 gas. Opcodes that terminate a transaction generally use no gas. Some other exceptions that will force the EVM to terminate a transaction are invalid opcodes, invalid jump destinations (the EVM is able to jump to arbitrary positions only if lands on a valid jump-destination), and stack underflows.

Notes on EVM performance

On mainnet, the EVM generally executes bytecode slower than one would expect of other virtual machines. The salient reason for this is that each operation must be executed by every full node in the network in order to achieve a trust-less environment. This is by design. The EVM was designed to achieve decentralized consensus across the whole network, and as a result, computation speeds are slower and costs are higher than those of a centralized network. The upside is that Ethereum network experiences near immutability, significantly improved fault tolerance, and zero downtime.

Additionally, the EVM's gas metering mechanism ensures that miners receive compensation for including the transaction in a block. This also prevents programs from looping eternally. Eventually the transaction will exceed its gas limit, the transaction will immediately halt and rollback all sandboxed state changes. The only state changes resulting from the transaction is the sender's nonce incremented by one and the gas costs up until transaction failure are paid to the miner for their computational effort.

Learning more

The EVM is a subject that should be discussed at length. We won't go into any more detail, as the purpose of this section is to simply introduce users to the internal mechanisms of the EVM. If this topic is interesting to you, then we recommend the Ethereum Virtual Machine (EVM) Awesome List as a good starting point: - https://github.com/ethereum/wiki/wiki/Ethereum-Virtual-Machine-(EVM)-Awesome-List