Skip to main content

​
Overview

Opcode: 0x3c Introduced: Frontier (EVM genesis) EXTCODECOPY copies bytecode from an external account into memory.

​
Specification

Stack Input: 
address (uint160 as uint256)
destOffset (memory offset)
offset (code offset)
length (bytes to copy)
Stack Output: 
[]
Gas Cost: 700 (access) + 3 + memory expansion + (length / 32) * 3

​
Behavior

Copies length bytes from external account’s code at offset to memory at destOffset. Zero-pads if code bounds exceeded.

​
Examples

​
Copy Contract Code

function getExternalCode(address account) public view returns (bytes memory code) {
    assembly {
        let size := extcodesize(account)
        code := mload(0x40)
        mstore(code, size)
        extcodecopy(account, add(code, 0x20), 0, size)
        mstore(0x40, add(add(code, 0x20), size))
    }
}

​
Verify Implementation

function verifyCode(address account, bytes32 expectedHash) public view returns (bool) {
    bytes memory code;
    assembly {
        let size := extcodesize(account)
        code := mload(0x40)
        extcodecopy(account, add(code, 0x20), 0, size)
    }
    return keccak256(code) == expectedHash;
}

​
Gas Cost

Base: 700 gas (Tangerine Whistle+) Memory expansion: Variable Copy cost: 3 gas per word Berlin+: 2600 (cold) / 100 (warm) + memory + copy

​
Common Usage

​
Clone Factory

function clone(address implementation) internal returns (address instance) {
    bytes memory code;
    assembly {
        let size := extcodesize(implementation)
        code := mload(0x40)
        extcodecopy(implementation, add(code, 0x20), 0, size)
    }
    // Deploy cloned code
}

​
Implementation

export function extcodecopy(
  frame: FrameType,
  host: BrandedHost
): EvmError | null {
  // Pop address, destOffset, offset, size
  // Calculate gas costs
  // Copy external code to memory
  // See full implementation in codebase

  frame.pc += 1;
  return null;
}

​
References