---
eip: 2327
title: BEGINDATA opcode
author: Martin Lundfall (@MrChico)
discussions-to: https://ethereum-magicians.org/t/new-opcode-begindata/3727
status: Draft
type: Standards Track
category: Core
created: 2019-10-28
---

## Simple Summary
<!--"If you can't explain it simply, you don't understand it well enough." Provide a simplified and layman-accessible explanation of the EIP.-->
Introduces a new opcode `BEGINDATA`, which indicates that the remaining bytes of the contract should be regarded as data rather than contract code.

## Abstract
<!--A short (~200 word) description of the technical issue being addressed.-->
It is common for smart contracts to efficiently store data directly in the contract bytecode. Examples include constant variables, compiler metadata and the contract runtime during the init phase. Currently, such data is not distinguished from normal bytecode and is still being analysed for `JUMPDEST`s by EVM interpreters. This EIP introduces a new opcode `BEGINDATA` at byte `0xb6`, which marks the remainding bytecode as data, indicating to EVM interpreters, static analysis tools and chain explorers that the remaining bytes do not represent opcodes.

## Motivation
<!--The motivation is critical for EIPs that want to change the Ethereum protocol. It should clearly explain why the existing protocol specification is inadequate to address the problem that the EIP solves. EIP submissions without sufficient motivation may be rejected outright.-->
The `BEGINDATA` opcode has been suggested before as part of the EIP `Subroutines and Static Jumps for the EVM` [EIP-615](https://eips.ethereum.org/EIPS/eip-615) as a way to determine the position of jumptables in contract bytecode. It is here introduced in its own right in order to exclude data from the `JUMPDEST` analysis of contracts, making it impossible to jump to data. This makes it easier for static analysis tools to analyse contracts, allows disassemblers, chain explorers and debuggers to not display data as a mess of INVALID opcodes and may even provide a marginal improvement in performance. Additionally, it paves the way for suggestions such as [EIP 1712](https://github.com/ethereum/EIPs/pull/1712) to disallow unused opcodes, jumptables [EIP-615](https://eips.ethereum.org/EIPS/eip-615) and speculative proposals to disallow for deployment of contracts with stack usage violations.

## Specification
<!--The technical specification should describe the syntax and semantics of any new feature. The specification should be detailed enough to allow competing, interoperable implementations for any of the current Ethereum platforms (go-ethereum, parity, cpp-ethereum, ethereumj, ethereumjs, and [others](https://github.com/ethereum/wiki/wiki/Clients)).-->
While computing the valid `JUMPDEST`s of a contract, halt analysis if `BEGINDATA` is encountered. A `JUMP` to a value higher than the `PC` value of `BEGINDATA` should throw with a `BAD_JUMP_DESTINATION` error. Bytes past `BEGINDATA` remain accessible via `CODECOPY` and `EXTCODECOPY`.
If `BEGINDATA` is encountered during contract execution, it has the same semantics as `STOP`. It uses 0 gas.

## Rationale
<!--The rationale fleshes out the specification by describing what motivated the design and why particular design decisions were made. It should describe alternate designs that were considered and related work, e.g. how the feature is supported in other languages. The rationale may also provide evidence of consensus within the community, and should discuss important objections or concerns raised during discussion.-->
The byte `0xb6` was chosen to align with [EIP-615](https://eips.ethereum.org/EIPS/eip-615).
The choice to `STOP` if `BEGINDATA` is encountered is somewhat arbitrary. An alternative would be to be to abort the execution with an out-of-gas error.

## Backwards Compatibility
<!--All EIPs that introduce backwards incompatibilities must include a section describing these incompatibilities and their severity. The EIP must explain how the author proposes to deal with these incompatibilities. EIP submissions without a sufficient backwards compatibility treatise may be rejected outright.-->
The proposal will not change any existing contracts unless their current behaviour relies upon the usage of unused opcodes.

## Test Cases
<!--Test cases for an implementation are mandatory for EIPs that are affecting consensus changes. Other EIPs can choose to include links to test cases if applicable.-->
Test cases should include:
1) A contract which jumps to a destination `X`, where `X` has a pc value higher than the `BEGINDATA` opcode, and the byte at `X` is `0x5b`. This should fail with a `BAD_JUMP_DESTINATION` error.
2) A contract which encounters the `BEGINDATA` opcode (should stop executing the current call frame)

## Implementation
<!--The implementations must be completed before any EIP is given status "Final", but it need not be completed before the EIP is accepted. While there is merit to the approach of reaching consensus on the specification and rationale before writing code, the principle of "rough consensus and running code" is still useful when it comes to resolving many discussions of API details.-->
Not yet.

## Copyright
Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).