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Overview

Opcode: 0x81 Introduced: Frontier (EVM genesis) DUP2 duplicates the 2nd stack item and pushes it to the top of the stack. The original 2th item remains in place.

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Specification

Stack Input: 
[..., value, item1, ..., item1]
Stack Output: 
[..., value, item1, ..., item1, value]
Gas Cost: 3 (GasFastestStep) Operation: 
value = stack[depth - 2]
stack.push(value)

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Behavior

DUP2 copies the 2th-from-top stack item without removing it. Requires stack depth ≥ 2. Key characteristics:
  • Requires stack depth ≥ 2
  • Original value unchanged
  • New copy pushed to top
  • StackUnderflow if depth < 2
  • Stack depth increases by 1

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Examples

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Basic Usage

import { handler_0x81_DUP2 } from '@tevm/voltaire/evm/stack/handlers';
import { createFrame } from '@tevm/voltaire/evm/Frame';

// Duplicate 2th item
const frame = createFrame({
  stack: [200n, 100n],
  gasRemaining: 1000n
});

const err = handler_0x81_DUP2(frame);

console.log(frame.stack); // [200n, 100n, 100n] - 2th item duplicated
console.log(frame.gasRemaining); // 997n (3 gas consumed)

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Solidity Compilation

contract Example {
    function swap() public pure {
        uint256 a = 10;
        uint256 b = 20;
        // Access a again
        // Stack: [b, a]
        // DUP2  // Stack: [b, a, a]
    }
}

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Assembly Usage

assembly {
    push1 0x01
    push1 0x02
    // Stack: [0x01, 0x02]

    dup2
    // Stack: [0x01, 0x02, 0x01] - first item duplicated
}

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Gas Cost

Cost: 3 gas (GasFastestStep) All DUP1-16 operations cost the same despite different stack depths accessed. Comparison:
OperationGasNote
DUP23Duplicate 2th item
PUSH1-323Same cost tier
POP2Cheaper

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Common Usage

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Access Previous Value

assembly {
    let a := 10
    let b := 20
    dup2  // Get 'a' again
    // Stack: [b, a, a]
}```

### Efficient Copies

```solidity
// Instead of multiple loads
assembly {
    let value := sload(slot)  // Expensive
    // Use value
    let value2 := sload(slot) // Wasteful!
}

// Use DUP to reuse
assembly {
    let value := sload(slot)  // Load once
    dup1                       // Copy
    // Use both copies
}

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Conditional Logic

assembly {
    let condition := calldataload(0)
    dup1  // Keep condition for later
    iszero
    jumpi(skip)
    // Use condition again
    skip:
}

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Stack Depth Requirements

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Minimum Depth

// DUP2 requires 2 items on stack
assembly {
    push1 0x01
    // Only 1 items - DUP2 will fail!
    dup2  // StackUnderflow
}

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Safe Usage

assembly {
    push1 0x01
    push1 0x02
    // Exactly 2 items - safe
    dup2  // Success
}

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Implementation

/**
 * DUP2 opcode (0x81) - Duplicate 2nd stack item
 *
 * Stack: [..., value, ...] => [..., value, ..., value]
 * Gas: 3 (GasFastestStep)
 */
export function handler_0x81_DUP2(frame: FrameType): EvmError | null {
  const gasErr = consumeGas(frame, FastestStep);
  if (gasErr) return gasErr;

  if (frame.stack.length < 2) {
    return { type: "StackUnderflow" };
  }

  const value = frame.stack[frame.stack.length - 2];
  const pushErr = pushStack(frame, value);
  if (pushErr) return pushErr;

  frame.pc += 1;
  return null;
}

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Edge Cases

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Stack Underflow

// Insufficient stack depth
const frame = createFrame({
  stack: [100n]  // Only 1 item
});

const err = handler_0x81_DUP2(frame);
console.log(err); // { type: "StackUnderflow" }

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Stack Overflow

// Stack at maximum, can't add more
const frame = createFrame({
  stack: new Array(1024).fill(0n)
});

const err = handler_0x81_DUP2(frame);
console.log(err); // { type: "StackOverflow" }

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Out of Gas

// Insufficient gas
const frame = createFrame({
  stack: [100n, 100n],
  gasRemaining: 2n  // Need 3
});

const err = handler_0x81_DUP2(frame);
console.log(err); // { type: "OutOfGas" }

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Maximum Value

// Duplicate max uint256
const MAX = (1n << 256n) - 1n;
const frame = createFrame({
  stack: [0n, MAX]
});

handler_0x81_DUP2(frame);
console.log(frame.stack[frame.stack.length - 1]); // MAX (duplicated)

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References