Module Structure

General

module starts with preamble, which indicates version of Wasm
modules can contain [0, n] sections (all sections are optional, module can be empty)
- named sections
  - must appear at specific place
  - is validated
- custom sections
  - can appear anywhere
  - may share section name
  - example: name section with symbols names for debugging
there is WebAssembly Text Format (WAT)
- human readable representation using s-expressions

Types - list of unique function signatures that will be used in module (includes imports)
Import - all imports used in module (Functions, Table, Memory, Global imports)
Functions - list of all functions in module. Position in the list represent the index of function body in Code. Value represent corresponding type from Types.
Table - typed list of references, that can't be stored in linear memory (every access is bound check)
Memory - linear memory of module (ArrayBuffer)
- each page is 64KB
- multiple instances of Wasm module can share the same Memory
Global - global variables definitions
Export - list of objects exported to host, once the module is instantiated
Start - index of entry point function
- can be used for initialization
- this function is called before any of exports are callable by JS host
- if there is main() then Start will be set to it
Element - data loaded into Table during instantiaton
Code - body of functions
Data - data loaded into Memory during instantiaton