Wasi function parameters

[itamarsch]

Hello, I am currently creating a wasi runtime as a side project, I have already Implemented most of the instructions and started implementing part of the wasi interface after reading online in multiple places I found two defenitions for the fd_write call,
1:

2:
https://wasix.org/docs/api-reference/wasi/fd_write

The first one implies I return a tagged union of the number of bytes written or the error, the second one (the one implemented by wasmtime and probably the correct one) Expects to receive a "pointer" to the amount of bytes written and returns and error if it failed, I only found the "correct" version in the wasix docs which isn't directly related to this project from what I saw.

So my question is, Where can I find the actual docs for the calling conventions of wasm syscalls that I need to implement as a runtime?