> ## Documentation Index
> Fetch the complete documentation index at: https://docs.phylax.systems/llms.txt
> Use this file to discover all available pages before exploring further.

# PhEVM

> The Phylax EVM - a specialized execution environment for assertion validation

PhEVM (Phylax EVM) is a specialized EVM implementation that executes [assertion](/credible/glossary#assertion) bytecode during transaction validation. It extends the standard EVM with additional capabilities for state inspection and comparison.

## Purpose

PhEVM serves as the execution engine within the [Assertion Enforcer](/credible/assertion-enforcer). When a transaction needs validation, the Assertion Enforcer invokes PhEVM to run all relevant assertions against the transaction's state changes.

See the [system architecture diagram](/credible/architecture-overview#system-architecture) for how PhEVM fits into the overall Credible Layer architecture.

## Key Capabilities

### State Access

PhEVM can access blockchain state at multiple points during transaction execution:

* **Pre-transaction state**: The blockchain state before the transaction executes
* **Post-transaction state**: The blockchain state after the transaction executes
* **Pre-call state**: State before a specific internal call within the transaction
* **Post-call state**: State after a specific internal call within the transaction

This enables assertions to compare values across different execution points, something impossible with standard Solidity.

### Cheatcodes

PhEVM implements special precompiles called [cheatcodes](/credible/glossary#cheatcodes) that provide capabilities beyond standard EVM operations:

* **Snapshot reads**: `loadStateAt` with explicit `ForkId` values for pre-transaction, post-transaction, pre-call, and post-call state
* **Call introspection**: Access to call inputs, call traces, and execution context
* **State change tracking**: Monitor storage slot modifications during execution
* **Log access**: Retrieve transaction logs for validation

These cheatcodes are exposed through the [`credible-std`](/credible/credible-std-overview) library and documented in the [Cheatcodes Reference](/credible/cheatcodes-reference).

### Off-Chain Execution

PhEVM runs entirely off-chain as part of the [Assertion Enforcer](/credible/assertion-enforcer) sidecar. This provides several advantages:

* **Higher gas limits**: Assertions can perform complex computations that would be prohibitively expensive on-chain
* **No on-chain overhead**: Validation doesn't consume block gas or affect transaction costs
* **Parallel execution**: Multiple assertions can be validated simultaneously

## How It Works

When the Assertion Enforcer receives a transaction for validation:

1. **State Preparation**: PhEVM receives the pre-transaction and post-transaction state snapshots
2. **Assertion Loading**: Relevant assertion bytecode is loaded from cache (fetched from [Assertion DA](/credible/assertion-da) during deployment)
3. **Execution**: PhEVM executes each assertion function against the prepared states
4. **Result**: If any assertion reverts, the transaction is flagged as invalid

For the complete validation flow, see [Assertion Enforcer](/credible/assertion-enforcer).