Deep Dive into WebAssembly Runtime Architecture: Parsing, Execution, Memory Management, and WASI

This article explores the core architecture and underlying mechanisms of the WebAssembly (Wasm) runtime, moving beyond its narrow definition as a portable binary format to examine its broader ecosystem and foundational components.

The Wasm runtime primarily consists of a module loader and parser, an execution engine, and host interaction interfaces like WASI. The parsing phase involves loading and decoding the binary format using LEB128 variable-length encoding, validating module correctness through a single-pass type-checking algorithm, and instantiating modules by resolving symbols and performing dynamic linking for imported functions.

The execution engine operates on a stack-based virtual machine model, utilizing zero-address instructions for high code density and simplified validation. Function execution relies on call stacks composed of frames containing local variable areas and operand stacks. Linear memory management provides a contiguous, sandboxed address space divided into data, BSS, stack, and heap regions, with layout strategies configurable via linker options.

void foo() {
   int a = 1;
   int b = 2;
   int c = (a + b) * 5;
   return c;
}

Initially designed for manual memory management to maximize performance, Wasm is evolving to support automatic garbage collection (GC) through the Phase 3 GC proposal. This advancement introduces structured types like struct and array, reference types, and typed function references, enabling seamless interoperability with host environments and reducing the need for language-specific GC implementations within Wasm modules.

Finally, the WebAssembly System Interface (WASI) provides a modular, capability-based set of APIs that allow Wasm modules to securely interact with host operating systems outside the browser. By standardizing POSIX-like functionalities such as file I/O, networking, and clocks, WASI enables Wasm to run as a portable, sandboxed executable across diverse platforms.

The article concludes by emphasizing that while Wasm MVP and 2.0 have established a strong foundation, ongoing proposals in GC, interface types, and system interfaces will continue to expand its capabilities, making it a versatile runtime for modern, cross-language applications.