The fixed portion is defined as that part of the object program which is logically treated as if it were always in memory. This portion of the program is composed of fixed permanent segments, and fixed overlayable segments.
A fixed permanent segment is a segment in the fixed portion which cannot be overlaid by any other part of the program.
A fixed overlayable segment is a segment in the fixed portion which, although logically treated as if it were always in memory, can be overlaid by another segment to optimize memory utilization.