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Overview

Opcode: 0x85 Introduced: Frontier (EVM genesis) DUP6 duplicates the 6th stack item and pushes it to the top of the stack. The original 6th item remains in place.

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Specification

Stack Input: 
[..., value, item5, ..., item1]
Stack Output: 
[..., value, item5, ..., item1, value]
Gas Cost: 3 (GasFastestStep) Operation: 
value = stack[depth - 6]
stack.push(value)

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Behavior

DUP6 copies the 6th-from-top stack item without removing it. Requires stack depth ≥ 6. Key characteristics:
  • Requires stack depth ≥ 6
  • Original value unchanged
  • New copy pushed to top
  • StackUnderflow if depth < 6
  • Stack depth increases by 1

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Examples

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Basic Usage

import { handler_0x85_DUP6 } from '@tevm/voltaire/evm/stack/handlers';
import { createFrame } from '@tevm/voltaire/evm/Frame';

// Duplicate 6th item
const frame = createFrame({
  stack: [600n, 500n, 400n, 300n, 200n, 100n],
  gasRemaining: 1000n
});

const err = handler_0x85_DUP6(frame);

console.log(frame.stack); // [600n, 500n, 400n, 300n, 200n, 100n, 100n] - 6th item duplicated
console.log(frame.gasRemaining); // 997n (3 gas consumed)

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Solidity Compilation

contract Example {
    function deepAccess() public pure {
        // Access deep stack value
        assembly {
            // Stack has 6 items
            dup6  // Duplicate 6th item to top
        }
    }
}

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Assembly Usage

assembly {
    push1 0x01
    push1 0x02
    push1 0x03
    push1 0x04
    push1 0x05
    push1 0x06
    // Stack: [0x01, 0x02, 0x03, 0x04, 0x05, 0x06]

    dup6
    // Stack: [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 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
DUP63Duplicate 6th item
PUSH1-323Same cost tier
POP2Cheaper

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Common Usage

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Deep Stack Access

function complex() public pure {
    assembly {
        // Build deep stack
        let v1 := 1
        let v2 := 2
        let v3 := 3
        let v4 := 4
        let v5 := 5
        let v6 := 6

        // Access v1 from depth 6
        dup6
    }
}```

### 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

// DUP6 requires 6 items on stack
assembly {
    push1 0x01
    push1 0x02
    push1 0x03
    push1 0x04
    push1 0x05
    // Only 5 items - DUP6 will fail!
    dup6  // StackUnderflow
}

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Safe Usage

assembly {
    push1 0x01
    push1 0x02
    push1 0x03
    push1 0x04
    push1 0x05
    push1 0x06
    // Exactly 6 items - safe
    dup6  // Success
}

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Implementation

/**
 * DUP6 opcode (0x85) - Duplicate 6th stack item
 *
 * Stack: [..., value, ...] => [..., value, ..., value]
 * Gas: 3 (GasFastestStep)
 */
export function handler_0x85_DUP6(frame: FrameType): EvmError | null {
  const gasErr = consumeGas(frame, FastestStep);
  if (gasErr) return gasErr;

  if (frame.stack.length < 6) {
    return { type: "StackUnderflow" };
  }

  const value = frame.stack[frame.stack.length - 6];
  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, 100n, 100n, 100n, 100n]  // Only 5 items
});

const err = handler_0x85_DUP6(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_0x85_DUP6(frame);
console.log(err); // { type: "StackOverflow" }

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Out of Gas

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

const err = handler_0x85_DUP6(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, 0n, 0n, 0n, 0n, MAX]
});

handler_0x85_DUP6(frame);
console.log(frame.stack[frame.stack.length - 1]); // MAX (duplicated)

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References