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While not a multitasking operating system, the Prizm's OS appears to have support for a fixed amount of "processes", which only run when in foreground. A process usually comes into foreground on user request.
The amount of processes supported appears to be two. One is the main process, and the other is the child process. All built-in OS apps and add-ins run as the child process, except eActivity, which runs as the main process. When a strip inside a eActivity file is open, it runs as the child process, and alternating between the strip and the eActivity document equates to selecting what process is in foreground.
When another app or add-in is selected on the Main Menu, or when another strip is launched (or the running app inside the strip is changed), the old app in the child process terminates, and the new one takes place. GetKey appears to play an important role in process switching, as it is one of the few syscalls that handle proper Main Menu access and switching between a strip and the eActivity document.
It is not clear in which process things like the Main Menu or screenshot capture screen run - it is actually possible that there's a third process set up for the Main Menu.
Each process has a separate stack and possibly a separate heap.