Overlay process

The overlay process is initiated when a control section in virtual storage references a control section not in storage. The control program determines the segment that the referenced control section is in and, if necessary, loads the segment. When a segment is loaded, it overlays any segment in storage with the same relative origin. Any segments in storage that are lower in the path of the overlaid segment can also be overlaid. An exclusive reference can also cause segments higher in the path to be overlaid. No overlay occurs if a control section in storage references a control section in another segment already in storage.

The portion of the control program that determines when overlay is to occur is the overlay supervisor that uses special tables to determine when overlay is necessary. These tables are generated by the binder and are part of the output program module. The special tables are the segment table and the entry table(s). Figure 1 shows the location of the segment and entry tables in the sample program.

Figure 1. Location of segment and entry tables in an overlay module

Because the tables are present in every overlay module, their size must be considered when planning the use of virtual storage. The storage requirements for the tables are given in Special considerations. A detailed discussion of the segment and entry tables follows.