> ## 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.

# Context Instructions

> EVM opcodes for accessing execution environment information

<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

Context instructions provide access to information about the current execution environment, including addresses, call data, gas prices, and external account state. These opcodes (0x30-0x3f) form the foundation for contracts to understand their execution context and interact with the broader blockchain state.

## Instruction Categories

### Environment Context (0x30-0x34)

Basic execution environment information:

* **ADDRESS (0x30)** - Get currently executing account address
* **BALANCE (0x31)** - Get balance of an account
* **ORIGIN (0x32)** - Get transaction origination address
* **CALLER (0x33)** - Get immediate caller address
* **CALLVALUE (0x34)** - Get deposited value in current call

### Call Data Access (0x35-0x37)

Reading input data passed to the contract:

* **CALLDATALOAD (0x35)** - Load 32 bytes from calldata
* **CALLDATASIZE (0x36)** - Get size of calldata
* **CALLDATACOPY (0x37)** - Copy calldata to memory

### Code Introspection (0x38-0x39)

Accessing the currently executing code:

* **CODESIZE (0x38)** - Get size of executing code
* **CODECOPY (0x39)** - Copy executing code to memory

### Transaction Context (0x3a)

Transaction-level information:

* **GASPRICE (0x3a)** - Get transaction gas price

### External Account Access (0x3b-0x3f)

Querying other accounts on the blockchain:

* **EXTCODESIZE (0x3b)** - Get size of external account's code
* **EXTCODECOPY (0x3c)** - Copy external account's code to memory
* **RETURNDATASIZE (0x3d)** - Get size of return data from last call
* **RETURNDATACOPY (0x3e)** - Copy return data to memory
* **EXTCODEHASH (0x3f)** - Get keccak256 hash of external account's code

## Gas Costs

Context instructions have varying gas costs based on their complexity and hardfork:

**Simple Environment Access (2 gas):**

* ADDRESS, ORIGIN, CALLER, CALLVALUE
* CALLDATASIZE, CODESIZE, GASPRICE, RETURNDATASIZE

**Call Data Operations (3+ gas):**

* CALLDATALOAD: 3 gas
* CALLDATACOPY, CODECOPY, RETURNDATACOPY: 3 + memory expansion + copy cost

**External Account Access (700+ gas):**

* BALANCE: 700 gas (Istanbul+)
* EXTCODESIZE: 700 gas (Tangerine Whistle+)
* EXTCODECOPY: 700 + memory expansion + copy cost
* EXTCODEHASH: 700 gas (Constantinople+)

## Access Cost Evolution

External account access costs have changed significantly across hardforks to prevent DoS attacks:

| Opcode      | Frontier | Tangerine Whistle | Berlin                   |
| ----------- | -------- | ----------------- | ------------------------ |
| BALANCE     | 20       | 400               | 2600 (cold) / 100 (warm) |
| EXTCODESIZE | 20       | 700               | 2600 (cold) / 100 (warm) |
| EXTCODECOPY | 20       | 700               | 2600 (cold) / 100 (warm) |
| EXTCODEHASH | -        | -                 | 2600 (cold) / 100 (warm) |

## Security Considerations

### tx.origin vs msg.sender

**Critical distinction:**

* `ORIGIN (0x32)` - Original transaction sender (never changes)
* `CALLER (0x33)` - Immediate caller (changes with each call)

**Vulnerability:**

```solidity theme={null}
// VULNERABLE: Uses tx.origin for authorization
function withdraw() public {
    require(tx.origin == owner);  // Can be exploited!
    // ...
}
```

**Attack scenario:**

1. Attacker deploys malicious contract
2. Owner calls attacker's contract
3. Attacker's contract calls victim's `withdraw()`
4. `tx.origin == owner` passes, funds stolen

**Safe pattern:**

```solidity theme={null}
// SAFE: Use msg.sender for authorization
function withdraw() public {
    require(msg.sender == owner);  // Correct!
    // ...
}
```

### Return Data Bounds

RETURNDATACOPY can revert if copying beyond actual return data:

```solidity theme={null}
// Can revert if returndata size < offset + length
assembly {
    returndatacopy(dest, offset, length)
}
```

### External Code Checks

EXTCODESIZE returns 0 for:

* Externally owned accounts (EOAs)
* Contracts during construction (before constructor completes)

**Bypass example:**

```solidity theme={null}
// INSUFFICIENT: Can be bypassed during construction
modifier onlyEOA() {
    uint256 size;
    assembly { size := extcodesize(caller()) }
    require(size == 0);
    _;
}
```

## Common Patterns

### Checking Contract vs EOA

```solidity theme={null}
function isContract(address addr) view returns (bool) {
    uint256 size;
    assembly { size := extcodesize(addr) }
    return size > 0;
}
```

### Reading Call Data Efficiently

```solidity theme={null}
// Load single value
function getValue() pure returns (uint256 value) {
    assembly {
        value := calldataload(4)  // Skip function selector
    }
}

// Copy calldata to memory
function copyData(uint256 offset, uint256 length) pure returns (bytes memory) {
    bytes memory data = new bytes(length);
    assembly {
        calldatacopy(add(data, 0x20), offset, length)
    }
    return data;
}
```

### Verifying Code Hash

```solidity theme={null}
function verifyImplementation(address impl, bytes32 expected) view returns (bool) {
    bytes32 hash;
    assembly { hash := extcodehash(impl) }
    return hash == expected;
}
```

## Implementation Reference

Context instruction handlers are implemented in:

* TypeScript: `/src/evm/context/`
* Zig: `/src/evm/context/handlers_context.zig`

Each instruction follows the standard handler pattern:

1. Pop operands from stack
2. Consume gas (including memory expansion for copy operations)
3. Access context information or external state
4. Push result to stack
5. Increment program counter

## References

* [Yellow Paper](https://ethereum.github.io/yellowpaper/paper.pdf) - Appendix H (Virtual Machine Specification)
* [EVM Codes - Context Instructions](https://www.evm.codes/#30?fork=cancun)
* [EIP-150](https://eips.ethereum.org/EIPS/eip-150) - Gas cost changes (Tangerine Whistle)
* [EIP-1052](https://eips.ethereum.org/EIPS/eip-1052) - EXTCODEHASH opcode
* [EIP-2929](https://eips.ethereum.org/EIPS/eip-2929) - Gas cost increases (Berlin)
* [SWC-115: Authorization through tx.origin](https://swcregistry.io/docs/SWC-115)