The fixed portion of the program is defined as that portion that is logically treated as if it were always resident in memory. This portion of the program is composed of two types of computer storage segments: permanent segments and fixed overlayable segments.
A permanent segment is a segment in the fixed portion that cannot be overlaid by another part of the program.
A fixed overlayable segment is a segment in the fixed portion which can overlay, or be overlaid by, either another fixed overlayable segment or an independent segment. From the execution point of view, however, it is considered to be resident in internal storage. When a fixed overlayable segment is called, it is always made available in the state in which it was last used; GO TO statement branch addresses modified by ALTER are not returned to their initial states.
The number of the permanent segments in the fixed portion can be varied through the use of the SEGMENT-LIMIT clause.