> ## Documentation Index
> Fetch the complete documentation index at: https://voltaire.tevm.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# LOG Instructions (0xa0-0xa4)

> Event logging with 0-4 indexed topics for contract communication

<Warning>
  **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.
</Warning>

## 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`.

```solidity theme={null}
// 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

```solidity theme={null}
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:

```typescript theme={null}
// 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:

```solidity theme={null}
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:

```solidity theme={null}
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

```solidity theme={null}
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)](https://www.evm.codes/#a0)
* [EIP-214 (New opcode: STATICCALL)](https://eips.ethereum.org/EIPS/eip-214)
* [Solidity Events Documentation](https://docs.soliditylang.org/en/latest/contracts.html#events)
* [Yellow Paper](https://ethereum.github.io/yellowpaper/paper.pdf) - Section 9 (Execution Model)