WebAssembly MVP (Wasm 1.0) defined linear memory using 32-bit integer indexes (i32), restricting each Wasm module instance to a maximum of 65,536 pages (4 GiB). For modern heavy computational workloads running in the browser — such as AAA game engines (Unreal Engine 5), massive LLM inference runtimes (llama.cpp/WebLLM), scientific simulation kernels, 3D CAD modeling, and Gaussian Splatting databases — 4 GiB of virtual address space causes catastrophic out-of-memory crashes. The Memory64 proposal introduces 64-bit index types (i64), allowing WebAssembly modules to address up to 16 Exabytes of virtual linear memory directly.

Historically, a Wasm module could only possess a single linear memory shared by all functions, libraries, and third-party plugins. If a third-party C/Rust library had a buffer overflow vulnerability, it could corrupt the entire application state. The Multi-Memory specification allows a module to define, import, and export multiple isolated linear memories: for example, Memory 0 for the secure host application heap, Memory 1 for untrusted user scripts, and Memory 2 for a protected shadow call stack. Memory instructions explicitly specify which memory they operate on (e.g. i32.load (memory 1)), creating mathematical capability-based isolation with zero runtime performance cost.

In 32-bit Wasm, engines reserve a 4 GiB virtual address space surrounded by a 4 GiB guard region; out-of-bounds accesses automatically trigger a hardware MMU page fault caught by the OS signal handler, avoiding explicit bounds-checking instructions. For Memory64, reserving an exabyte guard region is impossible. Engines utilize a hybrid strategy: they allocate a large dynamic virtual reservation (e.g. 32 to 64 GiB) backed by hardware guard pages, and emit combined register-width clamp instructions (e.g. CMP + JAE trap) only for offsets exceeding the reservation, preserving native C++ execution speeds.

Under the Multi-Memory specification, bulk memory instructions take two memory index immediates: "memory.copy ". For instance, "memory.copy 0 1" copies bytes directly from an untrusted plugin's isolated memory (Memory 1) into the host's primary heap (Memory 0) in optimized SIMD vectorized chunks, without needing to bounce through intermediate JavaScript typed arrays.