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Overview

Opcodes: 0xa0 (LOG0) to 0xa4 (LOG4)` Introduced: Frontier (EVM genesis) The LOG family of instructions emits event logs that external systems (off-chain indexers, monitoring services) can capture and process. Each instruction encodes a fixed number of topics (indexed parameters) and flexible data, enabling efficient event filtering without on-chain computation.

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Instruction Set

OpcodeNameTopicsStack Items
0xa0LOG002 (offset, length)
0xa1LOG113 (offset, length, topic0)
0xa2LOG224 (offset, length, topic0, topic1)
0xa3LOG335 (offset, length, topic0, topic1, topic2)
0xa4LOG446 (offset, length, topic0, topic1, topic2, topic3)

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Gas Cost

All LOG instructions cost: 
375 gas (base)
+ 375 gas per topic
+ 8 gas per byte of data
Examples:
  • LOG0 with empty data: 375 gas
  • LOG1 with 32 bytes: 375 + 375 + 256 = 1006 gas
  • LOG4 with 64 bytes: 375 + (4 × 375) + 512 = 2387 gas
Memory expansion costs apply when reading data beyond current allocation.

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Key Constraints

EIP-214 (Static Call Protection): LOG instructions cannot execute in static call context. Attempting to log during a STATICCALL reverts with StaticCallViolation. 
// This will revert
function badLog() external view {
    // emit event - reverts in view functions
}
Data Limit: Data size is limited by available gas and memory. No hard cap exists, but practical limits depend on transaction gas budget.

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Common Usage

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Event Indexing

event Transfer(address indexed from, address indexed to, uint256 value);

function transfer(address to, uint256 amount) public {
    balances[msg.sender] -= amount;
    balances[to] += amount;
    emit Transfer(msg.sender, to, amount);  // Compiler generates LOG2
}

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Event Filtering

Off-chain services use topics for fast filtering without parsing all event data: 
// Listen for Transfer events from specific address
const logs = await getLogs({
  address: tokenAddress,
  topics: [
    keccak256("Transfer(address,indexed address,indexed uint256)"),
    null,                                    // Match any 'from'
    "0xaddressToFilterFor"                   // Match specific 'to'
  ]
});

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Multiple Events

A transaction can emit multiple logs, which are returned in order: 
event Approval(address indexed owner, address indexed spender, uint256 value);

function approve(address spender, uint256 amount) public {
    allowances[msg.sender][spender] = amount;
    emit Approval(msg.sender, spender, amount);  // LOG2
    return true;
}

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Implementation Notes

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Topic Encoding

Topics are 256-bit values. For dynamic types (strings, arrays), the keccak256 hash is used: 
event StringLog(string indexed data);
// Topic is keccak256(data), not the string itself

event DynamicLog(uint256[] indexed arr);
// Topic is keccak256(abi.encode(arr)), not individual values

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Data vs Topics

  • Topics (0-4): Indexed parameters, optimized for efficient filtering
  • Data: Non-indexed parameters, stored but not indexed
event Transfer(
  address indexed from,    // Topic 1
  address indexed to,      // Topic 2
  uint256 value           // Data (non-indexed)
);

// Generates: LOG2 with topics=[from, to] and data=abi.encode(value)

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References