---
eip: 7843
title: SLOTNUM opcode
description: Opcode to get the current slot number
author: Marc Harvey-Hill (@Marchhill)
discussions-to: https://ethereum-magicians.org/t/eip-7843-slotnum-opcode/22234
status: Draft
type: Standards Track
category: Core
created: 2024-12-06
---

## Abstract

This EIP proposes to add a new opcode `SLOTNUM` (`0x4b`), that returns the corresponding slot number for the current block.

## Motivation

There are currently two ways to get the current slot number onchain:

1) Calculate from the block timestamp. This requires hardcoding the chain slot length into a smart contract.

2) Provide the slot number as calldata and prove it against the beacon block root (using [EIP-4788](./eip-4788.md)).

Both of these approaches have significant drawbacks. (1) would break contracts in the event of a change to the slot length. (2) is expensive in terms of gas, encouraging approach (1) to be used instead.

A better approach is for the slot length to be abstracted away from applications, and instead the slot number can be calculated in the consensus layer client and exposed by an opcode. This paves the way for future changes to the slot length.

## Specification

A new opcode `SLOTNUM` is introduced at `0x4b`. It shall return one stack element.

### Output

One element `slotNumber` is added to the stack; it is equal to the corresponding slot number for this block. `slotNumber` is a `uint64` in big endian encoding.

### Gas Cost

The gas cost for `SLOTNUM` is a fixed fee of `2`.

### RPC changes

The slot number is calculated in the consensus layer and passed to the execution layer through the engine API.

#### Header extension

The header encoding shall be extended to include a `slotNumber` field of type `uint64`.

#### Engine API changes

New objects `ExecutionPayloadV4` and `PayloadAttributesV4` are introduced, adding a `slotNumber` field of type `uint64`. The methods `engine_newPayloadV4`, `engine_getPayloadV6`, and `engine_forkChoiceUpdateV4` are added, to use the new objects.

## Rationale

### Gas Price

The opcode is priced to match similar opcodes in the `W_base` set.

### Calculation in consensus layer

The slot number could alternatively be calculated in the execution layer using the timestamp, but it is more appropriate to calculate values pertaining to the beacon chain in the consensus layer. Additionally this avoids code duplication, as the slot number is already calculated in the consensus layer.

### ZK-VM proving

The `SLOTNUM` opcode should not increase the complexity of proving EVM execution, as it is similar to existing opcodes such as `TIMESTAMP`. The slot number is included in the block header rather than as a `engine_newPayload` parameter, ensuring that the block is self-contained for proving; no extra inputs to the circuit are required.

## Backwards Compatibility

No backward compatibility issues found.

## Test Cases

N/A

## Security Considerations

None.

## Copyright

Copyright and related rights waived via [CC0](../LICENSE.md).