> ## Documentation Index
> Fetch the complete documentation index at: https://voltaire.tevm.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# DUP2 (0x81)

> Duplicate 2nd stack item

<Warning>
  **This page is a placeholder.** All examples on this page are currently AI-generated and are not correct. This documentation will be completed in the future with accurate, tested examples.
</Warning>

## 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.

## Specification

**Stack Input:**

```
[..., value, item1, ..., item1]
```

**Stack Output:**

```
[..., value, item1, ..., item1, value]
```

**Gas Cost:** 3 (GasFastestStep)

**Operation:**

```
value = stack[depth - 2]
stack.push(value)
```

## 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

## Examples

### Basic Usage

```typescript theme={null}
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)
```

### Solidity Compilation

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

### Assembly Usage

```solidity theme={null}
assembly {
    push1 0x01
    push1 0x02
    // Stack: [0x01, 0x02]

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

## Gas Cost

**Cost:** 3 gas (GasFastestStep)

All DUP1-16 operations cost the same despite different stack depths accessed.

**Comparison:**

| Operation | Gas | Note               |
| --------- | --- | ------------------ |
| DUP2      | 3   | Duplicate 2th item |
| PUSH1-32  | 3   | Same cost tier     |
| POP       | 2   | Cheaper            |

## Common Usage

### Access Previous Value

````solidity theme={null}
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
}
````

### Conditional Logic

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

## Stack Depth Requirements

### Minimum Depth

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

### Safe Usage

```solidity theme={null}
assembly {
    push1 0x01
    push1 0x02
    // Exactly 2 items - safe
    dup2  // Success
}
```

## Implementation

<Tabs>
  <Tab title="TypeScript">
    ```typescript theme={null}
    /**
     * 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;
    }
    ```
  </Tab>
</Tabs>

## Edge Cases

### Stack Underflow

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

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

### Stack Overflow

```typescript theme={null}
// 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" }
```

### Out of Gas

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

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

### Maximum Value

```typescript theme={null}
// 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)
```

## References

* [Yellow Paper](https://ethereum.github.io/yellowpaper/paper.pdf) - Section 9.1 (Stack Operations)
* [EVM Codes - DUP2](https://www.evm.codes/#81?fork=cancun)
* [Solidity Assembly - dup2](https://docs.soliditylang.org/en/latest/yul.html#evm-opcodes)