LOG Instructions (0xa0-0xa4)

Event logging with 0-4 indexed topics for contract communication

This page is a placeholder. All examples on this page are currently AI-generated and are not correct. This documentation will be completed in the future with accurate, tested examples.

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.

Instruction Set

Opcode	Name	Topics	Stack Items
0xa0	LOG0	0	2 (offset, length)
0xa1	LOG1	1	3 (offset, length, topic0)
0xa2	LOG2	2	4 (offset, length, topic0, topic1)
0xa3	LOG3	3	5 (offset, length, topic0, topic1, topic2)
0xa4	LOG4	4	6 (offset, length, topic0, topic1, topic2, topic3)

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.

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.

Common Usage

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
}

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'
  ]
});

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;
}

Implementation Notes

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

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)

References

• LOG Instruction Reference (evm.codes)
• EIP-214 (New opcode: STATICCALL)
• Solidity Events Documentation
• Yellow Paper - Section 9 (Execution Model)