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Overview

Opcode: 0x90 Introduced: Frontier (EVM genesis) SWAP1 exchanges the top stack item with the 2nd item from the top. Only these two positions change - all other items remain in place.

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Specification

Stack Input: 
[..., valueN, top]
Stack Output: 
[..., top, valueN]
Gas Cost: 3 (GasFastestStep) Operation: 
temp = stack[top]
stack[top] = stack[top - 2]
stack[top - 2] = temp

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Behavior

SWAP1 exchanges positions of the top item and the item at position 2 from top. Requires stack depth ≥ 2. Key characteristics:
  • Requires stack depth ≥ 2
  • Only two items change positions
  • Middle items unchanged
  • StackUnderflow if depth < 2
  • Stack depth unchanged

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Examples

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Basic Usage

import { handler_0x90_SWAP1 } from '@tevm/voltaire/evm/stack/handlers';
import { createFrame } from '@tevm/voltaire/evm/Frame';

// Swap top with 2nd item
const frame = createFrame({
  stack: [200n, 100n],
  gasRemaining: 1000n
});

const err = handler_0x90_SWAP1(frame);

console.log(frame.stack); // [100n, 200n] - positions 0 and 1 swapped
console.log(frame.gasRemaining); // 997n (3 gas consumed)

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Solidity Compilation

contract Example {
    function simpleSwap(uint256 a, uint256 b) public pure returns (uint256, uint256) {
        // Return in reverse order
        return (b, a);  // Compiler uses SWAP1
        // Stack: [a, b] => [b, a]
    }
}

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Assembly Usage

assembly {
    push1 0xa
    push1 0xb
    // Stack: ['a', 'b']

    swap1
    // Stack: ['b', 'a'] - 'a' and 'b' swapped
}

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Gas Cost

Cost: 3 gas (GasFastestStep) All SWAP1-16 operations cost the same despite different stack depths accessed. Comparison:
OperationGasNote
SWAP13Swap with 2nd item
DUP1-163Same cost tier
POP2Cheaper

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Common Usage

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Argument Reordering

// Function expects (b, a) but has (a, b)
assembly {
    // Stack: [a, b]
    swap1
    // Stack: [b, a]
    call(...)
}```

### Efficient Reordering

```solidity
// Reorder for function call
assembly {
    // Have: [value, to, token]
    // Need: [token, to, value]
    swap2  // [token, to, value]

    // Call transfer(token, to, value)
    call(gas(), target, 0, 0, 100, 0, 0)
}

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Storage Optimization

assembly {
    let slot := 0
    let value := 42
    // Stack: [slot, value]

    // SSTORE needs (slot, value) but we have them reversed
    // No swap needed in this case, but if we did:
    swap1
    // Stack: [value, slot]
    sstore
}

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Stack Depth Requirements

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Minimum Depth

// SWAP1 requires 2 items
assembly {
    push1 0x01
    // Only 1 items - SWAP1 will fail!
    swap1  // StackUnderflow
}

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Safe Usage

assembly {
    push1 0x01
    push1 0x02
    // Exactly 2 items - safe
    swap1  // Success
}

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Implementation

/**
 * SWAP1 opcode (0x90) - Swap top with 2nd item
 *
 * Stack: [..., valueN, top] => [..., top, valueN]
 * Gas: 3 (GasFastestStep)
 */
export function handler_0x90_SWAP1(frame: FrameType): EvmError | null {
  const gasErr = consumeGas(frame, FastestStep);
  if (gasErr) return gasErr;

  if (frame.stack.length <= 1) {
    return { type: "StackUnderflow" };
  }

  const topIdx = frame.stack.length - 1;
  const swapIdx = frame.stack.length - 2;
  const temp = frame.stack[topIdx];
  frame.stack[topIdx] = frame.stack[swapIdx];
  frame.stack[swapIdx] = temp;

  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 items, need 2
});

const err = handler_0x90_SWAP1(frame);
console.log(err); // { type: "StackUnderflow" }

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Out of Gas

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

const err = handler_0x90_SWAP1(frame);
console.log(err); // { type: "OutOfGas" }

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Identity Swap

// Swap same values
const frame = createFrame({
  stack: new Array(2).fill(42n)
});

handler_0x90_SWAP1(frame);
console.log(frame.stack); // All still 42n

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Maximum Values

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

handler_0x90_SWAP1(frame);
console.log(frame.stack[0]); // 1n (was at top)
console.log(frame.stack[1]); // MAX (was at bottom)

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References