A short (8k) loader program. This is responsible for copying the rest to a RAM card (if available), then invoking the main app. The second half is used to copy Selector entries to RAMCard on first boot.
The file is present in the original distribution as DESKTOP1 but is
renamed DESKTOP.SYSTEM in many disk images to be launched at boot.
The main app (DESKTOP, or optionally SELECTOR) is invoked by
loading only the first segment, which in turn loads the rest of the
segments of the file.
desktop.system.s contains all of the code.