Skip to main content

​
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:
OpcodeFrontierTangerine WhistleBerlin
BALANCE204002600 (cold) / 100 (warm)
EXTCODESIZE207002600 (cold) / 100 (warm)
EXTCODECOPY207002600 (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: 
// 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: 
// 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: 
// 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: 
// INSUFFICIENT: Can be bypassed during construction
modifier onlyEOA() {
    uint256 size;
    assembly { size := extcodesize(caller()) }
    require(size == 0);
    _;
}

​
Common Patterns

​
Checking Contract vs EOA

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

​
Reading Call Data Efficiently

// 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

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