collapseos/apps/README.md

# User applications

This folder contains code designed to be "userspace" application. Unlike the
kernel, which always stay in memory. Those apps here will more likely be loaded
in RAM from storage, ran, then discarded so that another userspace program can
be run.

That doesn't mean that you can't include that code in your kernel though, but
you will typically not want to do that.

## Userspace convention

We execute a userspace application by calling the address it's loaded into.

This means that userspace applications must be assembled with a proper `.org`,
otherwise labels in its code will be wrong.

The `.org`, it is not specified by glue code of the apps themselves. It is
expected to be set either in the `user.h` file to through `zasm` 3rd argument.

That a userspace is called also means that an application, when finished
running, is expected to return with a regular `ret` and a clean stack.

Whatever calls the userspace app (usually, it will be the shell), should set
HL to a pointer to unparsed arguments in string form, null terminated.

The userspace application is expected to set A on return. 0 means success,
non-zero means error.

A userspace application can expect the `SP` pointer to be properly set. If it
moves it, it should take care of returning it where it was before returning
because otherwise, it will break the kernel.

Apps in Collapse OS are design to be ROM-compatible, that is, they don't write
to addresses that are part of the code's address space.

By default, apps set their RAM to begin at the end of the binary because in
most cases, these apps will be ran from RAM. If they're ran from ROM, make sure
to set `USER_RAMSTART` properly in your `user.h` to ensure that the RAM is
placed properly.