Records are written in the prime area when the data set is allocated or updated. The last track of prime data is reserved for an end-of-file mark. The portion of Figure 1 labeled cylinder 1 illustrates the initial structure of the prime area. Although the prime area can extend across several noncontiguous areas of the volume, all the records are written in key sequence. Each record must contain a key; the system automatically writes the key of the highest record before each block.

When the ABSTR option of the SPACE parameter of the DD statement is used to generate a multivolume prime area, the VTOC of the second volume, and of all succeeding volumes, must be contained within cylinder 0 of the volume.