As you add records to an indexed sequential data set, the system inserts each record in its proper sequence according to the record key. The remaining records on the track are then moved up one position each. If the last record does not fit on the track, it is written in the first available location in the overflow area. A 10-byte link field is added to the record put in the overflow area to connect it logically to the correct track. The proper adjustments are made to the track index entries. This procedure is illustrated in Figure 1.

Subsequent additions are written either on the prime track or as part of the overflow chain from that track. If the addition belongs after the last prime record on a track but before a previous overflow record from that track, it is written in the first available location in the overflow area. Its link field contains the address of the next record in the chain.

For BISAM, if you add a record that has the same key as a record in the data set, a "duplicate record" condition is shown in the exception code. However, if you specified the delete option and the record in the data set is marked for deletion, the condition is not reported and the new record replaces the existing record. For more information about exception codes, see z/OS DFSMS Macro Instructions for Data Sets.