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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: a userspace application is expected to return.
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.