# Overview

This page specifies the system-contract registry and the protocol-level rules that apply to system contracts in MegaETH. It defines the system-contract set.

## Specification

### Distinction from Precompiles

Both [precompiles](/spec/megaevm/precompiles.md) and system contracts reside at fixed addresses and provide protocol-level functionality, but they differ in a fundamental way:

* **Precompiles** are stateless and pure. They perform a deterministic computation on their input and return output without reading or writing on-chain state. The EVM handles precompile calls entirely outside normal bytecode execution.
* **System contracts** are stateful. They have deployed bytecode, maintain persistent storage, and participate in normal EVM state access (including `SLOAD`, `SSTORE`, and account balance). Some of their functions are subject to [call interception](/spec/system-contracts/interception.md) for protocol-level side effects, but the contracts themselves exist as ordinary accounts with code and storage.

### Stable Registry

A node MUST recognize the following contracts as system contracts:

| Contract                                                                       | Address                            | Since                                        | Purpose                                    |
| ------------------------------------------------------------------------------ | ---------------------------------- | -------------------------------------------- | ------------------------------------------ |
| [Oracle](/spec/system-contracts/oracle.md)                                     | `ORACLE_CONTRACT_ADDRESS`          | [MiniRex](/spec/network-upgrades/minirex.md) | Off-chain data key-value storage           |
| [High-Precision Timestamp](/spec/system-contracts/high-precision-timestamp.md) | `HIGH_PRECISION_TIMESTAMP_ADDRESS` | [MiniRex](/spec/network-upgrades/minirex.md) | Sub-second timestamp oracle service        |
| [KeylessDeploy](/spec/system-contracts/keyless-deploy.md)                      | `KEYLESS_DEPLOY_ADDRESS`           | [Rex2](/spec/network-upgrades/rex2.md)       | Deterministic cross-chain deployment       |
| [MegaAccessControl](/spec/system-contracts/mega-access-control.md)             | `MEGA_ACCESS_CONTROL_ADDRESS`      | [Rex4](/spec/network-upgrades/rex4.md)       | Volatile-data access control               |
| [MegaLimitControl](/spec/system-contracts/mega-limit-control.md)               | `MEGA_LIMIT_CONTROL_ADDRESS`       | [Rex4](/spec/network-upgrades/rex4.md)       | Query remaining compute-gas budget         |
| [SequencerRegistry](/spec/system-contracts/sequencer-registry.md)              | `SEQUENCER_REGISTRY_ADDRESS`       | [Rex5](/spec/network-upgrades/rex5.md)       | System address and sequencer role registry |

> **Unstable** — Rex5 entries (`SequencerRegistry`, Oracle v2.0.0) are under active development and may change before network activation.

### Deployment Semantics

System contracts MUST be available at their specified addresses when the corresponding spec is active. Their availability is gated by [spec](/spec/hardfork-spec.md) activation.

### Call Interception

Some system-contract functions are handled at the EVM level rather than exclusively by contract bytecode. The generic interception mechanism — when it fires, how selectors are matched, how calls that do not match fall through to bytecode, and the gas and call-scheme rules — is specified on the [Call Interception](/spec/system-contracts/interception.md) page.

Each system-contract page defines which of its functions are intercepted and what each interception does:

* [Oracle — `sendHint`](/spec/system-contracts/oracle.md#hint-forwarding): performs a side effect (hint forwarding), then falls through to bytecode.
* [KeylessDeploy — `keylessDeploy`](/spec/system-contracts/keyless-deploy.md#interception-scope): intercepted at depth 0; executes deployment in a sandbox instead of on-chain bytecode.
* [MegaAccessControl](/spec/system-contracts/mega-access-control.md#interception-scope): all three functions are fully intercepted.
* [MegaLimitControl](/spec/system-contracts/mega-limit-control.md#interception-scope): `remainingComputeGas` is fully intercepted.

### Backward Compatibility Rule

Any change to system-contract semantics, bytecode-visible interface behavior, or interception rules MUST be introduced by a new spec. Stable behavior for an already-activated spec MUST remain unchanged.

## Constants

| Constant                           | Value                                        | Description                                     |
| ---------------------------------- | -------------------------------------------- | ----------------------------------------------- |
| `ORACLE_CONTRACT_ADDRESS`          | `0x6342000000000000000000000000000000000001` | Stable Oracle system-contract address           |
| `HIGH_PRECISION_TIMESTAMP_ADDRESS` | `0x6342000000000000000000000000000000000002` | Stable high-precision timestamp wrapper address |
| `KEYLESS_DEPLOY_ADDRESS`           | `0x6342000000000000000000000000000000000003` | Stable KeylessDeploy system-contract address    |
| `MEGA_ACCESS_CONTROL_ADDRESS`      | `0x6342000000000000000000000000000000000004` | MegaAccessControl address                       |
| `MEGA_LIMIT_CONTROL_ADDRESS`       | `0x6342000000000000000000000000000000000005` | MegaLimitControl address                        |
| `SEQUENCER_REGISTRY_ADDRESS`       | `0x6342000000000000000000000000000000000006` | SequencerRegistry address                       |

## Spec History

* [MiniRex](/spec/network-upgrades/minirex.md) introduced the Oracle and High-Precision Timestamp contracts.
* [Rex2](/spec/network-upgrades/rex2.md) introduced KeylessDeploy.
* [Rex4](/spec/network-upgrades/rex4.md) — added MegaAccessControl and MegaLimitControl contracts.
* [Rex5](/spec/network-upgrades/rex5.md) — added SequencerRegistry; upgraded Oracle to v2.0.0 (dynamic authority).


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