Context Instructions

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Overview

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Instruction Categories

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Environment Context (0x30-0x34)

• 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

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Call Data Access (0x35-0x37)

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

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Code Introspection (0x38-0x39)

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

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Transaction Context (0x3a)

• GASPRICE (0x3a) - Get transaction gas price

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External Account Access (0x3b-0x3f)

• 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

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Gas Costs

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

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

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

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Access Cost Evolution

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Security Considerations

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tx.origin vs msg.sender

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

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

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

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Return Data Bounds

// Can revert if returndata size < offset + length
assembly {
    returndatacopy(dest, offset, length)
}

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External Code Checks

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

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

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Common Patterns

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Checking Contract vs EOA

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

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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;
}

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Verifying Code Hash

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

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Implementation Reference

• TypeScript: /src/evm/context/
• Zig: /src/evm/context/handlers_context.zig

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

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References

• Yellow Paper - Appendix H (Virtual Machine Specification)
• EVM Codes - Context Instructions
• EIP-150 - Gas cost changes (Tangerine Whistle)
• EIP-1052 - EXTCODEHASH opcode
• EIP-2929 - Gas cost increases (Berlin)
• SWC-115: Authorization through tx.origin