ArbOS State Machine

ArbOS is the operating system layer that runs on top of the EVM. It manages L1/L2 pricing, retryable tickets, cross-chain messaging, and protocol-level state - all stored in the state trie under a dedicated system address.

State Layout

All ArbOS state lives at ARBOS_STATE_ADDRESS (0xa4b05...), organized into subspaces:

Each subspace uses a key derivation scheme: hash(parent_key || offset[0..31]) || offset[31], which produces unique non-colliding storage slots.

Versioning

ArbOS evolves through version upgrades stored in the block header's mix_hash field. Each version maps to an EVM spec:

The mix_hash header field encodes three values:

• Bytes 0-7: send_count (cumulative L2-to-L1 messages)
• Bytes 8-15: l1_block_number (L1 block at time of production)
• Bytes 16-23: arbos_format_version

Gas Pricing

Arbitrum uses a two-layer gas model: L1 data posting costs and L2 execution costs.

L1 Pricing

L1 costs cover data availability - the cost of posting transaction data to Ethereum L1.

• Transaction calldata is measured using Brotli compression to approximate actual batch size
• Units are priced at l1_price_per_unit (wei per compressed byte)
• An equilibration algorithm adjusts the L1 price to balance collected fees against actual batch posting costs
• Poster fees are deducted from the transaction's gas budget before EVM execution

L2 Pricing

L2 costs cover computation on the Arbitrum chain.

• Base fee uses an exponential backlog model: fees increase when demand exceeds the per-block gas limit
• The exponential function uses a degree-4 Horner's method polynomial approximation
• Three pricing models exist across versions:
  • Legacy (pre-v50): Simple gas accounting
  • SingleGas (v50): Unified gas constraint
  • MultiGas (v60): Eight resource dimensions tracked independently

Fee Distribution

After each transaction, collected fees are split:

Block Processing

Each block follows this sequence:

1. StartBlock internal transaction - updates L1 pricing, reaps expired retryable tickets, caches L1 block hashes, computes per-block gas limit
2. Batch posting report (if present) - records L1 batch costs for the pricing model
3. User transactions - each goes through start_tx (gas charging, poster fee) → EVM execution → end_tx (fee distribution, refunds)
4. Header derivation - mix_hash, extra_data (send root), and gas fields are computed from final state

Custom EVM Behavior

ArbReth modifies several EVM opcodes from their standard Ethereum behavior: