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Portability.md

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Portability

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.

Execution environments which, despite limited, local, non-determinism, 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 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.
  • Heaps bigger than 4GiB with 64-bit addressing may be added later, though it will be done under a feature test so it won't be required for all WebAssembly implementations.
  • 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).

Developer-exposed APIs (such as POSIX) are expected to be portable at a source-code level through WebAssembly libraries which will use feature detection. These libraries aren't necessarily standardized: WebAssembly will follow the extensible web manifesto's lead and expose low-level capabilities that expose the possibilities of the underlying platform as closely as possible. WebAssembly therefore standardizes a lower-level abstraction layer, and expects libraries to offer portable APIs.