WebAssembly’s binary format is designed to be executable efficiently on a variety of operating systems and instruction set architectures, on the Web and off the Web.

Assumptions for Efficient Execution

Execution environments which, despite limited, local, nondeterminism, don’t offer the following characteristics may be able to execute WebAssembly modules nonetheless. In some cases they may have to emulate behavior that the host hardware or operating system don’t offer so that WebAssembly modules execute as-if the behavior were supported. This sometimes will lead to poor performance.

As WebAssembly’s standardization goes forward we expect to formalize these requirements, and how WebAssembly will adapt to new platforms that didn’t necessarily exist when WebAssembly was first designed.

WebAssembly portability assumes that execution environments offer the following characteristics:

  • 8-bit bytes.
  • Addressable at a byte memory granularity.
  • Support unaligned memory accesses or reliable trapping that allows software emulation thereof.
  • Two’s complement signed integers in 32 bits and optionally 64 bits.
  • IEEE 754-2008 32-bit and 64-bit floating point, except for a few exceptions.
  • Little-endian byte ordering.
  • Memory regions which can be efficiently addressed with 32-bit pointers or indices.
  • wasm64 additionally supports linear memory bigger than 4 GiB with 64-bit pointers or indices.
  • Enforce secure isolation between WebAssembly modules and other modules or processes executing on the same machine.
  • An execution environment which offers forward progress guarantees to all threads of execution (even when executing in a non-parallel manner).
  • Availability of lock-free atomic memory operators, when naturally aligned, for 8- 16- and 32-bit accesses. At a minimum this must include an atomic compare-and-exchange operator (or equivalent load-linked/store-conditional).
  • wasm64 additionally requires lock-free atomic memory operators, when naturally aligned, for 64-bit accesses.


WebAssembly does not specify any APIs or syscalls, only an import mechanism where the set of available imports is defined by the host environment. In a Web environment, functionality is accessed through the Web APIs defined by the Web Platform. Non-Web environments can choose to implement standard Web APIs, standard non-Web APIs (e.g. POSIX), or invent their own.


Portability at the C/C++ level can be achieved by programming to a standard API (e.g., POSIX) and relying on the compiler and/or libraries to map the standard interface to the host environment’s available imports either at compile-time (via #ifdef) or run-time (via feature detection and dynamic loading/linking).