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
- Kauri original title: Ethereum 101 - Part 7 - The EVM
- Kauri original link: https://kauri.io/ethereum-101-part-7-the-evm/a7ac47d26eab4ce899a865619122d42e/a
- Kauri original author: Wil Barnes (@wil)
- Kauri original Publication date: 2019-02-13
- Kauri original tags: bytecode, ethereum, 101, virtual-machine, opcode, ethereum-virtual-machine, evm
- Kauri original hash: QmWSA6oxhDQD6A96cCmqRPU3iXYugKoqRzZVRHtopbMsTq
- Kauri original checkpoint: QmZSRFGq9bnBLosiVwSTANrDR9YdXbWkwG71aw35jAjyLo