Cairo VM

The Cairo Virtual Machine (Cairo VM) is a specialized, Turing-complete virtual machine designed to execute programs written in the Cairo programming language. It is the foundational runtime environment for Starknet, a Layer 2 validity rollup on Ethereum. Unlike the Ethereum Virtual Machine (EVM), the Cairo VM is built from the ground up to generate cryptographic proofs of computational integrity, specifically STARK proofs. This allows it to prove that a program was executed correctly without requiring every network node to re-execute the computation, enabling massive scalability.

At its core, the Cairo VM operates on a CPU architecture with a set of registers and a memory model that is fundamentally different from traditional VMs. It uses a concept called memory-constrained computation, where memory accesses are non-deterministically provided and must be proven consistent. This design, along with its native support for recursive proofs, makes it exceptionally efficient for generating zero-knowledge proofs (ZKPs). The VM's instruction set is optimized for the algebraic operations required by the STARK proof system, making proof generation faster and more cost-effective than general-purpose alternatives.

The primary use case for the Cairo VM is powering Starknet's decentralized prover network. When a user submits a transaction batch (a collection of many transactions) to Starknet, a prover node executes them on the Cairo VM and generates a STARK proof. This single, succinct proof is then posted to the Ethereum mainnet for verification. This architecture is what defines a validity rollup; the security of the Layer 2 state transitions is cryptographically guaranteed by Ethereum, while the heavy computation is offloaded to the Cairo VM. This enables Starknet to achieve high throughput and low transaction fees while maintaining Ethereum-level security.

Developers interact with the Cairo VM by writing smart contracts and applications in the Cairo programming language. The Cairo compiler (cairo-compile) translates high-level Cairo code into Sierra, an intermediate representation, and finally into Cairo Assembly (CASM), the bytecode executed by the VM. This toolchain ensures that any program proven by the Cairo VM is mathematically guaranteed to have been executed as written, eliminating the possibility of fraudulent state transitions. The VM's design is a critical innovation in scaling blockchain technology through cryptographic verification rather than mere replication of computation.

The Cairo Virtual Machine (Cairo VM) is a specialized, Turing-complete virtual machine designed to execute Cairo programs and generate STARK proofs of their computational integrity. Unlike general-purpose VMs like the Ethereum Virtual Machine (EVM), the Cairo VM is built from the ground up for verifiable computation, meaning its primary output is not just a state change but a cryptographic proof that the execution was performed correctly. This proof, known as a STARK proof, can be verified by anyone far more efficiently than re-running the computation, enabling massive scalability through validity rollups on Starknet.

At its core, the Cairo VM executes a low-level assembly-like language, Cairo assembly (CASM), which is compiled from higher-level Cairo code. Its architecture is centered around a concept called the Algebraic Execution Trace (AET). As the VM runs a program, it constructs this trace—a structured record of every computational step, register state, and memory access. This trace must satisfy a set of predefined polynomial constraints. The genius of the Cairo VM is that the correctness of the entire execution is equivalent to proving that these constraints hold, which is perfectly suited for efficient STARK proof generation.

A key innovation is the AIR (Algebraic Intermediate Representation). The constraints that define correct execution are expressed in AIR, which describes the relationships between rows in the execution trace using polynomials. The STARK prover (part of the VM's toolchain) uses this AIR to generate a proof that the trace is valid. The STARK verifier can then check this proof in milliseconds, regardless of the original computation's complexity. This decouples execution cost from verification cost, the foundation of Starknet's Layer 2 scaling.

The VM interacts with several critical components: the program counter tracks execution flow; memory is managed via a continuous, non-overlapping address space; and built-in functions (like poseidon hash or signature verification) are implemented as native, highly optimized circuits with their own dedicated constraints. These built-ins are essential for performance, as their execution is proven directly rather than being composed from many basic VM operations. This design makes operations common in zero-knowledge applications exceptionally efficient.

Finally, the Cairo VM's workflow integrates with the broader Starknet stack. A developer writes a Cairo contract, which is compiled to CASM and then executed by the Cairo VM. The VM produces the execution trace and a STARK proof. This proof is posted to Ethereum as part of a validity rollup batch, where the verifier contract on Ethereum attests to its validity, finalizing the state update. This entire process allows Starknet to inherit Ethereum's security while performing thousands of transactions off-chain, with the Cairo VM serving as the provable execution engine at its heart.

The Cairo Virtual Machine (Cairo VM) is the core execution engine of the StarkNet Layer 2 network. It is a specialized, Turing-complete virtual machine designed to run Cairo programs, which include all StarkNet smart contracts and system logic. Unlike the Ethereum Virtual Machine (EVM), the Cairo VM is built from the ground up to be STARK-friendly, meaning its execution trace is optimized to generate efficient cryptographic proofs. Every transaction and state update on StarkNet is computed within this isolated environment, producing an execution trace that serves as the input for the STARK prover.

Its primary role is to ensure deterministic execution, guaranteeing that given the same input, the Cairo VM will always produce the same output trace. This determinism is critical for validity proofs, as the prover and verifier must agree on the computational result without re-execution. The VM handles all standard operations—arithmetic, storage access, and control flow—but does so using Cairo's unique AIR (Algebraic Intermediate Representation) constraints. This design means the VM's operation is inherently verifiable, translating computational steps into a set of polynomial constraints that can be proven with STARKs.

Within the stack, the Cairo VM interacts directly with the StarkNet OS, which manages system calls, fee mechanics, and the interface with the sequencer. It executes the bytecode compiled from Cairo source code, updating a Merkle-Patricia trie that represents StarkNet's state. The output is not just a new state root, but a succinct proof attesting to the correctness of the transition. This architecture allows the L1 verifier contract on Ethereum to trustlessly verify the integrity of massive batches of L2 transactions by checking a single STARK proof, rather than re-running the computations.