An exclusive reference is made when the external reference in the requesting segment is to a symbol defined in a segment not in the path of the requesting segment. Exclusive references are either valid or invalid.

An exclusive reference is valid only if there is also an inclusive reference to the requested control section in a segment common to both the segment to be loaded and the segment to be overlaid. The same symbol must be used in both the common segment and the exclusive reference. In Figure 1, a reference from segment B to segment A is valid because there is an inclusive reference from the common segment to segment A. (An entry table in the common segment contains the address of segment A. The overlay does not destroy this table.)

Figure 1. Inclusive and exclusive references

In this same figure, a reference from segment A to segment B is invalid because there is no reference from the common segment to segment B. A reference from segment A to segment B can be made valid by including, in the common segment, an external reference to the symbol used in the exclusive reference to segment B.

Another way to eliminate exclusive references is to arrange the program so that the references that cause overlay are made in a higher segment. For example, you could eliminate the exclusive reference shown in Figure 1 by writing a new module to be placed in the common segment. The new module's only function would be to reference segment B. The code in segment A could then be changed to reference the new module instead of segment B. Control then would pass from segment A to the common segment, where the overlay of segment A by segment B would be initiated.