zkVM Technical Specification
Like many other execution environments, the zkVM has unique characteristics that developers may find helpful when programming. The following description of zkVM characteristics assume familiarity with systems programming concepts. Those who are not familiar may want to consult additional resources such as OsDevWiki to understand the terminology.
Introduction
The zkVM is a software emulator that implements a 32-bit RISC-V instruction set. One thing to note about the zkVM is that it does not use features that one typically finds in a traditional processor. This was done to enable the zkVM to efficiently generate proofs on commodity hardware. While the emulator is more simple than hardware processors, this is sufficient to run many useful computations such as Ethereum block production and cryptographic signature verification. The following section describes key characteristics of the zkVM implementation.
The zkVM Execution Model
The zkVM implements the RV32IM instruction set. This is the RV32I base
instruction along with the multiplication (M) feature. We do not implement
CRS instructions. This means that a program does not have what is typically
described as privileged and user modes. We do not implement loads or stores to
unaligned addresses. So all addresses must be multiples of 0x4 and all code
must be mapped to addresses divisible by 0x4.
We extend the RV32IM ISA by using the ECALL instruction. We do this in order
to add instructions that are specific to zero knowledge computing.
The zkVM does not support interrupts and there is no built-in notion of a scheduler. The zkVM runs programs using a single-thread environment without preemption, so there is also no support for atomic instructions. If the execution raises an exception such as an unaligned access, the execution terminates without executing exception handlers.
zkVM Memory Layout
The following table summarizes the layout of the zkVM memory
| Address Range | Size | Name | Description |
|---|---|---|---|
0x00000000 - 0x000003ff | 1 KB | Invalid page | This page of memory is invalid, so that dereferencing a NULL address will result in a failure |
0x00000400 - 0x0BFFFFFF | 192 MB | User Memory | Contains code, heap, and stack of the guest program |
0x0C000000 - 0x0CFFFFFF | 16 MB | System Memory | Contains RISC-V registers and non-leaf Merkle tree nodes for the page table |
0x0D000000 - 0x0DFFFFFF | 16 MB | Page Table | Contains the Page Table |
zkVM Memory Model
The zkVM executes instructions in order; in other words, instructions are never reordered and the zkVM's memory model is sequentially consistent. Unlike many processors, the zkVM has no notion of traditional memory caches and cache-coherency protocols implemented in the zkVM.