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EVM

By · Reviewed & last updated · Part of Web3 & Solidity Tutorial

The EVM (Ethereum Virtual Machine) is a stack-based, 256-bit-word VM. Solidity compiles to EVM bytecode. Every Ethereum node runs every transaction deterministically — that’s how consensus works.

The three storage areas + gas mental model

EXAMPLE
// Storage (persistent, very expensive — ~20,000 gas per slot write)
contract Bank {
    mapping(address => uint256) private balances;   // lives in storage
}

// Memory (per-call, cheap — bytes32 / arrays you allocate)
function sum(uint256[] memory xs) external pure returns (uint256 t) {
    for (uint256 i; i < xs.length; ++i) t += xs[i];
}

// Calldata (input bytes, read-only, cheapest)
function echo(bytes calldata data) external pure returns (bytes memory) {
    return data;
}

// Cheap-to-expensive (rough)
//   calldata read         < memory ops < storage read < storage write
//   non-zero storage slot:  ~20,000 gas (first write)
//   subsequent updates:     ~5,000
//   zero-to-zero:            free (refund)

// Tips that save real money
//  - Pack storage slots: two uint128 fit in one slot
//  - Use 'unchecked { ++i; }' in for loops where overflow is impossible
//  - Emit events instead of storing log-like data
//  - Pass arrays as 'calldata' when they're read-only inputs

Why it matters

Every public on-chain function is an auction: users bid gas to get included. Every storage slot you avoid is real money saved by every caller, every call, forever.

Tip: Tweak the snippet with Try it Yourself », then sit the quiz at the bottom of the page.

Example

Example
// EVM = Ethereum Virtual Machine. Stack-based, 256-bit words.
// Solidity compiles to EVM bytecode.
// Storage is expensive; memory is cheap and per-call.
Try it Yourself »

Discussion

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