Add application-abi describing WASI application lifecycle

design/application-abi.md

@@ -0,0 +1,31 @@
+WASI Application ABI
+====================
+
+In addition to the APIs defined by the various WASI [modules](modules.md) there
+are also certain expectations that the WASI runtime places on an application
+that wishes to be portable across WASI implementations.
+
+This document descibes how a conforming WASI application is expected to behave
+in terms of lifecycle (startup, shutdown, etc) any exports it is expected to
+provide in order to for the embedder to load and run
+
+Lifecycle
+---------
+
+A WASI program may contain any number exports, but the embedded attributes
+specific meaning the following optional function exports:
+
+- `__wasi_init` - Called after WebAssemembly instancation but before any other
+  functions.
+- `__wasi_main` - Runs the programs main entry points.
+- `__wasi_term` - The logical inverse of `__wasi_init`.  Optionally called
+  before modules destruction.  No other functions within program will be called
+  after this one.
+
+Linear Memory
+-------------
+
+All WASI programs are expected to share a linear memory with the embedder.  The
+memory can either be imported from the embedder or exports to the embedder.  If
+exported the memory must be named `__wasi_memory`.  If imported the import must
+be named `memory` from a module names `wasi`.