The index entry portion of an index record takes up all of the record that is left over after the header, the free control interval entries, if any, and the unused space.

Figure 1 shows the format of the index entry portion of an index record. To improve search speed, index entries are grouped into sections, of which there are approximately as many as the square root of the number of entries. For example, if there are 100 index entries in an index record, they are grouped into 10 sections of 10 entries each. (The number of sections does not change, even though the number of index entries increases as free control intervals get used.)

Figure 1. Format of the Index Entry Portion of an Index Record

The sections, and the entries within a section, are arranged from right to left. IXHLEO in the header gives the displacement from the beginning of the index record to the control information in the leftmost index entry. IXHSEO gives the displacement to the control information in the leftmost index entry in the rightmost section. You calculate the displacement of the control information of the rightmost index entry in the index record (the entry with the lowest key) by subtracting IXHFLPLN from IXHLL in the header (the length of the control information in an index entry from the length of the record).

Each section is preceded by a 2-byte field that gives the displacement from the control information in the leftmost index entry in the section to the control information in the leftmost index entry in the next section (to the left). The last (leftmost) section's 2-byte field contains 0s.

Figure 2 gives the format of an index entry.

Figure 2. Format of an Index Record