ArbOS 20 “Atlas” Now Live: Bringing Ethereum Dencun Support to Arbitrum Chains

Offchain Labs
Offchain Labs
Published in
5 min readMar 14, 2024


Written by terence tsao and Lee Bousfield — March 14, 2024

We’re elated to announce the much-anticipated release of ArbOS 20, affectionately dubbed “Atlas.” This milestone marks a significant leap forward for the Arbitrum ecosystem, introducing Ethereum Dencun support (EIP-4844, EIP-1153, EIP-5656, and EIP-6780) to Arbitrum chains, paving the way for significantly improved costs and new use cases. ArbOS Atlas is the first named release in a series that will be named after the moons of planets, starting with the letter “A”, symbolizing our commitment to exploration, discovery, and the limitless potential of the Arbitrum ecosystem.

Understanding ArbOS: Powering Arbitrum Nitro

Arbitrum is powered by nodes that run Nitro. ArbOS is one part of Nitro that provides L2-specific logic, including producing and executing L2 blocks, managing the accounting for L1 costs, and supporting critical bridging functionalities.

With each iteration, ArbOS will evolve to bring new core technologies that will be common to all Arbitrum chains.

Bringing Ethereum Dencun to Arbitrum

ArbOS Atlas makes Arbitrum chains ready to support the Ethereum Dencun update that went live on Ethereum mainnet at 13:55 UTC, March 13, 2024. With ArbOS Atlas, developers and users on Arbitrum alike will benefit.

The biggest impact for most will be a big change in Ethereum L2 transaction costs. ArbOS Atlas provides Arbitrum chains with the ability to post user transactions to Ethereum using a new type of Ethereum transaction in the form of a Blob, reduce costs of those transactions (EIP-4844), new storage opcodes for cheaper transient data (EIP-1153), and cheaper memory copying (EIP-5656).

ArbOS Atlas also adds support for EIP-6780, aligning Arbitrum with EVM’s security posture and laying the groundwork for future EVM improvements.

Applications on Arbitrum will not have to be modified or take any explicit action to get the cost benefits of EIP-4844 (i.e. the whole chain opts-in with ArbOS 20 “Atlas”).

ArbOS Atlas has been audited, voted on, and approved by the Arbitrum DAO for all DAO-governed chains in a fully decentralized, on-chain voting process.

In addition to applying to DAO-governed chains, ArbOS Atlas can be deployed by Orbit chains independently of the Arbitrum DAO governance process; Orbit L2 roll-up chains that deploy Atlas will receive the cost benefits of EIP-4844, while all Orbit chains can upgrade to benefit from the other features in ArbOS Atlas. The ability of self-governed Orbit L2 chains to deploy ArbOS Atlas whenever they’re technically ready is an example of the freedom granted by the Arbitrum Expansion Program.

Because Arbitrum chains are highly customizable and can be deployed in a number of different configurations, Decun will have different impacts depending upon your implementation:

  • All L3 Rollup chains on top of Arbitrum One will see lower fees. No additional action is required.
  • L3 chains on top of Arbitrum One that use alternative data availability (DA), such as Arbitrum AnyTrust, Avail, Celestia, EigenDA, or NEAR DA, will see fees unchanged.
  • Self-governed Orbit L2 rollup chains must deploy ArbOS Atlas and also switch on blob posting to see lower fees. L2 rollup chains that don’t do both won’t see any fee changes.
  • Orbit L2 chains that use alternative DA will also see fees unchanged.

Additionally, as part of several changes rolled out in ArbOS Atlas, a security fix was rolled out to the Sequencer Inbox contract. This change addressed an arithmetic overflow vulnerability that would allow a Sequencer to disable the force inclusion mechanism. More information can be found here.

Additional Arbitrum gas cost reductions

On Layer 2 and Layer 3 chains, fees are a combination of two parts: execution gas, which charges for the use of resources on the L2/L3 chain itself, and data posting gas, which reflects the cost of posting data.

As EIP-4844 rolls out on Ethereum, L1 data posting costs are expected to be dramatically reduced. However, predicting the exact size of this reduction is challenging at this moment, as detailed by our research team.

In addition to reducing data posting fees via EIP-4844, ArbOS Atlas is also introducing major reductions in other fees for Arbitrum One, expected to be activated by March 18, 2024:

  • L1 surplus fee. Reduce the surplus fee per compressed byte from 32 gwei to 0.
  • L2 base fee. Reduce the minimum from 0.1 gwei to 0.01 gwei.

Note that this is additive to the reduction in L1 data posting fees that come with EIP-4844.

Net result: the combination of EIP-4844 and the forthcoming reduction in Arbitrum execution fees are expected to dramatically reduce transaction costs, benefitting all users.

Will stack with Arbitrum Stylus for even more savings

Pending DAO approval, Stylus is expected to become Arbitrum mainnet-ready in 2024. The cheaper transaction costs enabled via EIP-4844 and other ArbOS Atlas fee changes are expected to combine with Stylus’ potential 10x computational efficiencies and 100x memory cost improvements, for even greater savings than a typical Ethereum L2.

In combination with Stylus and EVM+, applications and use cases previously considered impossible to ship on Arbitrum are now within reach — especially those that produce high transaction volumes like Gaming, SocialFi, and exchanges.

How Offchain Labs contributed to Ethereum Dencun

The ArbOS Atlas upgrade is able to take advantage of Dencun’s new lower-cost fee market thanks to collaboration across L1 and L2, with Offchain Labs teams working to upgrade both Prysm—Offchain Lab’s Ethereum consensus layer software—and Arbitrum Nitro—the execution layer software that powers Arbitrum One. The implementations built by consensus layer client teams play a vital role in the specification process, testing the feasibility of designs created by Ethereum researchers, or the client teams themselves. In the case of Dencun, Prysm was the first team to release a prototype of a draft version of EIP-4844, and this prototype played a vital role in answering questions about how to add blob data to the existing gossip networks, enabling experiments to be conducted with real data on real networks. Prysm participated in all devnet and testnet forks and gave feedback on spec choices that resulted in a more secure final design.

To get a deeper dive into the changes that Dencun will bring to Ethereum mainnet and its impact on Arbitrum with ArbOS Atlas, check out this Offchain Labs AMA:

Uncovering Dencun • Feb 22, 2024

ArbOS Atlas Roll-Up As a Service (RaaS) providers committed to Ethereum Dencun

If you’re a builder, know that Arbitrum RaaS providers Altlayer, Caldera, Conduit, and Gelato are committed to upgrading your existing Orbit chains to ArbOS Atlas and providing Ethereum Dencun support.

To learn more about ArbOS Atlas and Ethereum Dencun support for Arbitrum

Tune into our upcoming live stream on Friday, March 15 at 9:30am (ET) / 1:30pm (UTC):

ArbOS 20 Atlas & Dencun Livestream • Mar 15, 2024



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