Numbering sequence in a hierarchy: movement and position

The terms movement and position are used when talking about how segments are accessed when an application program issues a call. They are used to help describe the numbering sequence in a hierarchy.

When talking about movement through the hierarchy, it always means moving in the sequence implied by the numbering scheme. Movement can be forward or backward. When talking about position in the hierarchy, it means being located (positioned) at a specific segment.

A segment is the smallest piece of data IMS can store. If an application program issues a Get-Unique (GU) call for the student segment BAKER (see Figure 2), the current position is immediately after the BAKER segment occurrence. If an application program then issues an unqualified GN call, IMS moves forward in the database and returns the PASS segment occurrence.