collapseos/apps

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.

Memory management

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.

Applications that are ran as a shell (the "shell" app, of course, but also, possibly, "basic" and others to come) need a manual override to their main RAMSTART constant: You don't want them to run in the same RAM region as your other userspace apps because if you do, as soon as you launch an app with your shell, its memory is going to be overwritten!

What you'll do then is that you'll reserve some space in your memory layout for the shell and add a special constant in your user.h, which will override the basic one (remember, in zasm, the first .equ for a given constant takes precedence).

For example, if you want a "basic" shell and that you reserve space right after your kernel RAM for it, then your user.h would contain .equ BAS_RAMSTART KERNEL_RAMEND.

You can also include your shell's code directly in the kernel by copying relevant parts of the app's glue unit in your kernel's glue unit. This is often simpler and more efficient. However, if your shell is a big program, it might run into zasm's limits. In that case, you'd have to assemble your shell separately.