The segments themselves are composed of pages, to simplfy core memory management. When a process is created, its DS is loaded with the supervisor's program and data segments. Multics' ring protection mechanism is used to cause a program to become more privileged when it executes supervisor code.
Some segments are directories.
The processor, when it tries to access a segment with this switch ON, will generate a missing segment fault , and will run the segment fault handler in the supervisor. The segment fault handler checks the active switch in the branch corresponding to the segment, which says whether there is a page table for the segment. If not, it activates the segment. In any case, it connects the DS entry. In order to activate a segment, its segment map and length are copied from its branch into a free ASTE, the active switch in its branch is turned ON, and pointers to the ASTE and PT are stored along with it.