What is the Ethereum Virtual Machine (EVM)?

One machine, run everywhere

The EVM is a virtual computer. It runs code the same way, safely, on every Ethereum node.¹ Nodes run it to carry out smart contracts.¹ It is the place where smart contract code actually runs.³

A machine that changes state

A blockchain like Bitcoin is often described as a shared ledger: a record of activity that must follow set rules.¹ Ethereum's state holds more. It has every account and balance, plus a machine state that changes from block to block by fixed rules.¹ The EVM defines those rules.¹

The EVM behaves like a maths function. Give it the same input, and it always gives the same output.¹ In short: take the old state, apply a batch of valid transactions, and you get the new state.¹ That is how every node can run the same code and reach the same result.¹

From Solidity to bytecode

The EVM cannot read languages like Solidity. Code is first compiled into bytecode: low-level machine instructions.⁴ Bytecode runs as small steps called opcodes.¹ Some do basic sums, such as ADD and SUB. Others are special to blockchains, such as BALANCE and BLOCKHASH.¹

The EVM is a stack machine. It works on a pile of values up to 1,024 items deep, and each item is 256 bits.^1,5

Sealed off from the outside

Code running in the EVM has no access to the network, to files, or to other programs on the computer.³ So a smart contract needs an oracle to learn outside facts. ethereum.org says this is by design: relying on outside data could put consensus at risk.⁶

Where data lives

Each contract has its own storage, which lasts between function calls and between transactions.³ The EVM also has memory, which is wiped after each transaction.¹ Storage is costly to read, and even more costly to change.³ A contract can only read and write its own storage.³

Gas: the meter on the machine

Gas measures how much computing work an operation needs.² Each transaction sets a gas limit: the most gas it may use. This stops mistakes or attacks that would loop forever.^2,7 The Yellow Paper calls the EVM "quasi-Turing-complete" for this reason: gas puts a ceiling on the total work.⁵

A standard ETH transfer uses 21,000 gas.² Set a limit of 50,000 for one, and 21,000 is used. The other 29,000 comes back to you.² If a contract runs out of gas halfway, the EVM undoes its changes, but all the gas is still used up.² You pay the fee whether a transaction succeeds or fails.²

Many versions, one rulebook

Ethereum nodes run software called execution clients, and each one includes its own EVM.^1,8 There are also stand-alone versions in Python, C++, JavaScript and Rust.¹ All of them must follow the rules set out in the Yellow Paper.¹