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Overview

Opcode: 0x35 Introduced: Frontier (EVM genesis) CALLDATALOAD reads 32 bytes from the call data (input data passed to the contract) starting at a specified offset. Bytes beyond call data bounds are zero-padded.

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Specification

Stack Input: 
offset (uint256)
Stack Output: 
data (bytes32 as uint256)
Gas Cost: 3 (GasFastestStep) Operation: 
offset = stack.pop()
data = calldata[offset:offset+32]  // zero-padded if out of bounds
stack.push(data)

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Behavior

CALLDATALOAD loads exactly 32 bytes from call data, reading from the specified byte offset. If the offset extends beyond call data, the remaining bytes are padded with zeros. Key characteristics:
  • Always returns 32 bytes (256 bits)
  • Zero-pads when offset + 32 > calldata.length
  • Big-endian byte order
  • Does not revert on out-of-bounds access

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Examples

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Basic Usage

import { calldataload } from '@tevm/voltaire/evm/context';
import { createFrame } from '@tevm/voltaire/evm/Frame';

// Load from calldata
const calldata = new Uint8Array([
  0x12, 0x34, 0x56, 0x78, // First 4 bytes
  ...new Array(28).fill(0xAB) // Next 28 bytes
]);

const frame = createFrame({
  calldata,
  stack: [0n] // offset = 0
});

const err = calldataload(frame);

// Result: 0x12345678AB...AB (32 bytes)

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Function Arguments

contract Example {
    function process(uint256 x, uint256 y) public pure returns (uint256) {
        // Calldata layout:
        // 0x00-0x03: function selector (4 bytes)
        // 0x04-0x23: first argument (x)
        // 0x24-0x43: second argument (y)

        uint256 firstArg;
        uint256 secondArg;

        assembly {
            firstArg := calldataload(4)   // Skip selector
            secondArg := calldataload(36) // 4 + 32
        }

        return firstArg + secondArg;
    }
}

​
Zero Padding

// Calldata shorter than 32 bytes
const shortCalldata = new Uint8Array([0xFF, 0xEE]);

const frame = createFrame({
  calldata: shortCalldata,
  stack: [0n]
});

calldataload(frame);

// Result: 0xFFEE000000000000000000000000000000000000000000000000000000000000

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Gas Cost

Cost: 3 gas (GasFastestStep) CALLDATALOAD shares the same cost tier with:
  • ADD, SUB (0x01, 0x03): 3 gas
  • NOT, ISZERO (0x19, 0x15): 3 gas
  • Comparison operations: 3 gas
Comparison:
  • CALLDATALOAD (read 32 bytes): 3 gas
  • CALLDATACOPY (copy N bytes): 3 + memory + copy cost
  • MLOAD (read from memory): 3 gas

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Common Usage

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Function Selector Extraction

function getSelector() public pure returns (bytes4) {
    bytes4 selector;
    assembly {
        // First 4 bytes contain function selector
        let data := calldataload(0)
        selector := shr(224, data) // Shift right 224 bits (32-4)*8
    }
    return selector;
}

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Manual ABI Decoding

function decodeUint256(uint256 offset) public pure returns (uint256 value) {
    assembly {
        value := calldataload(offset)
    }
}

function decodeAddress(uint256 offset) public pure returns (address addr) {
    assembly {
        let data := calldataload(offset)
        addr := and(data, 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF)
    }
}

​
Dynamic Array Length

function getArrayLength(uint256 arrayOffset) public pure returns (uint256 length) {
    assembly {
        // Array length is stored at arrayOffset
        length := calldataload(arrayOffset)
    }
}

​
Calldata Validation

function validateCalldata() public pure returns (bool) {
    assembly {
        // Check if calldata is at least 36 bytes (selector + 1 uint256)
        if lt(calldatasize(), 36) {
            revert(0, 0)
        }

        // Load and validate first parameter
        let param := calldataload(4)
        if gt(param, 1000) {
            revert(0, 0)
        }
    }
    return true;
}

​
Security

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Out-of-Bounds Reading

// Safe: automatically zero-padded
function readAtOffset(uint256 offset) public pure returns (uint256) {
    uint256 value;
    assembly {
        value := calldataload(offset)
        // If offset >= calldatasize(), returns 0
    }
    return value;
}

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Function Selector Validation

function onlyCorrectSelector() public pure {
    assembly {
        let selector := shr(224, calldataload(0))
        // Verify expected selector
        if iszero(eq(selector, 0x12345678)) {
            revert(0, 0)
        }
    }
}

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Calldata Bounds Check

function safeRead(uint256 offset) public pure returns (uint256) {
    require(offset + 32 <= msg.data.length, "Out of bounds");

    uint256 value;
    assembly {
        value := calldataload(offset)
    }
    return value;
}

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Implementation

import { consumeGas } from "../Frame/consumeGas.js";
import { popStack } from "../Frame/popStack.js";
import { pushStack } from "../Frame/pushStack.js";

/**
 * CALLDATALOAD opcode (0x35) - Load 32 bytes from calldata
 *
 * Stack: [offset] => [data]
 * Gas: 3 (GasFastestStep)
 */
export function calldataload(frame: FrameType): EvmError | null {
  const gasErr = consumeGas(frame, 3n);
  if (gasErr) return gasErr;

  const offsetResult = popStack(frame);
  if (offsetResult.error) return offsetResult.error;
  const offset = offsetResult.value;

  if (offset > 0xffffffffn) {
    // Offset beyond reasonable range, return zero
    const pushErr = pushStack(frame, 0n);
    if (pushErr) return pushErr;
  } else {
    const off = Number(offset);
    let result = 0n;

    // Load 32 bytes (zero-padded if out of bounds)
    for (let i = 0; i < 32; i++) {
      const idx = off + i;
      const byte = idx < frame.calldata.length ? frame.calldata[idx] : 0;
      result = (result << 8n) | BigInt(byte);
    }

    const pushErr = pushStack(frame, result);
    if (pushErr) return pushErr;
  }

  frame.pc += 1;
  return null;
}

​
Edge Cases

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Offset Beyond Calldata

const frame = createFrame({
  calldata: new Uint8Array([0xFF]),
  stack: [100n] // Offset beyond calldata
});

calldataload(frame);
console.log(frame.stack[0]); // 0n (all zeros)

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Partial Overlap

// 4 bytes calldata, offset = 1
const frame = createFrame({
  calldata: new Uint8Array([0xAA, 0xBB, 0xCC, 0xDD]),
  stack: [1n]
});

calldataload(frame);
// Result: 0xBBCCDD00...00 (3 bytes data, 29 bytes padding)

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Zero Offset Empty Calldata

const frame = createFrame({
  calldata: Bytes(),
  stack: [0n]
});

calldataload(frame);
console.log(frame.stack[0]); // 0n

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Maximum Offset

const frame = createFrame({
  calldata: new Uint8Array(100),
  stack: [(1n << 256n) - 1n] // Max u256
});

calldataload(frame);
console.log(frame.stack[0]); // 0n (out of bounds)

​
References