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.
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.
Specification
Stack Input:
Stack Output:
Gas Cost: 3 (GasFastestStep)
Operation:
temp = stack[top]
stack[top] = stack[top - 2]
stack[top - 2] = temp
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
Examples
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)
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]
}
}
Assembly Usage
assembly {
push1 0xa
push1 0xb
// Stack: ['a', 'b']
swap1
// Stack: ['b', 'a'] - 'a' and 'b' swapped
}
Gas Cost
Cost: 3 gas (GasFastestStep)
All SWAP1-16 operations cost the same despite different stack depths accessed.
Comparison:
| Operation | Gas | Note |
|---|
| SWAP1 | 3 | Swap with 2nd item |
| DUP1-16 | 3 | Same cost tier |
| POP | 2 | Cheaper |
Common Usage
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)
}
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
}
Stack Depth Requirements
Minimum Depth
// SWAP1 requires 2 items
assembly {
push1 0x01
// Only 1 items - SWAP1 will fail!
swap1 // StackUnderflow
}
Safe Usage
assembly {
push1 0x01
push1 0x02
// Exactly 2 items - safe
swap1 // Success
}
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;
}
Edge Cases
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" }
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" }
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
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)
References
