Specification of an overlay program

Once you have designed an overlay structure, the program must be placed into that structure. You indicate to the binder the relative positions of the segments, the regions, and the control sections in each segment. Positioning is accomplished as follows:

Segments: Are positioned by OVERLAY statements. In addition, the overlay statement provides a means to equate each load point with a unique symbolic name. Each OVERLAY statement begins a new segment.
Regions: Are also positioned by OVERLAY statements. You specify the origin of the first segment of the region, followed by the word REGION in parentheses.
Control sections: Are positioned in the segment specified by the OVERLAY statement with which they are associated in the input sequence. However, the sequence of the control sections within a segment is not necessarily the order in which the control sections are specified.

The input sequence of control statements and control sections should reflect the sequence of the segments in the overlay structure from top to bottom, left to right, and region by region. This sequence is illustrated in later examples.

In addition, several special options are used with overlay programs. These options are specified on the EXEC statement for the binder job step and are described at the end of this section.

Note: If a program module in overlay structure is reprocessed by the binder, the OVERLAY statements and special options (such as OVLY) must be specified. If the statements and options are not provided, the output program module will not be in overlay structure.

The symbolic origin of every segment, other than the root segment, must be specified with an OVERLAY statement. The first time a symbolic origin is specified, a load point is created at the end of the previous segment. That load point is logically assigned a relative address at the doubleword boundary that follows the last byte in the preceding segment. Subsequent use of the same symbolic origin indicates that the next segment is to have its origin at the same load point.

In the sample single-region program, the symbolic origin names ONE and TWO are assigned to the two necessary load points, as shown in Figure 2. Segments 2 and 5 are at load point ONE; segments 3 and 4 are at load point TWO.

The following sequence of OVERLAY statements results in the structure in Figure 1. (The control sections in each segment are indicated by name.)

Control section CSA
Control section CSB
  OVERLAY ONE
Control section CSC
  OVERLAY TWO
Control section CSD
Control section CSE
  OVERLAY TWO
Control section CSF
  OVERLAY ONE
Control section CSG

Note: The sequence of OVERLAY statements reflects the order of segments in the structure from top to bottom and left to right.

Figure 1. Symbolic segment origin in single-region program ieab1v10