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Overview

Opcode: 0x37 Introduced: Frontier (EVM genesis) CALLDATACOPY copies a specified range of call data bytes to memory. Out-of-bounds bytes are zero-padded.

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Specification

Stack Input: 
destOffset (memory offset)
offset (calldata offset)
length (bytes to copy)
Stack Output: 
[]
Gas Cost: 3 + memory expansion + (length / 32) * 3 (rounded up) Operation: 
destOffset = stack.pop()
offset = stack.pop()
length = stack.pop()
memory[destOffset:destOffset+length] = calldata[offset:offset+length]

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Behavior

Copies length bytes from calldata starting at offset to memory starting at destOffset. Zero-pads if calldata bounds exceeded.

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Examples

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Basic Copy

function copyCalldata() public pure {
    assembly {
        // Copy entire calldata to memory at 0
        calldatacopy(0, 0, calldatasize())
    }
}

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Proxy Pattern

fallback() external payable {
    assembly {
        calldatacopy(0, 0, calldatasize())
        let result := delegatecall(gas(), impl, 0, calldatasize(), 0, 0)
        returndatacopy(0, 0, returndatasize())
        switch result
        case 0 { revert(0, returndatasize()) }
        default { return(0, returndatasize()) }
    }
}

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Gas Cost

Base: 3 gas Memory expansion: Variable Copy cost: 3 gas per 32-byte word (rounded up)

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Common Usage

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Forwarding Calls

function forward(address target) public {
    assembly {
        let size := calldatasize()
        calldatacopy(0, 0, size)
        call(gas(), target, 0, 0, size, 0, 0)
    }
}

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Security

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Bounds Validation

Check offsets don’t overflow when adding length.

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Implementation

export function calldatacopy(frame: FrameType): EvmError | null {
  const destOffsetResult = popStack(frame);
  if (destOffsetResult.error) return destOffsetResult.error;

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

  const lengthResult = popStack(frame);
  if (lengthResult.error) return lengthResult.error;

  // Gas calculation + copying logic
  // See full implementation in codebase

  frame.pc += 1;
  return null;
}

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References