‘TTR’ in PDS and PDSE

Partitioned Data Set uses an addressing scheme called TTR (Track-Track-Record) which is based on the DASD geometry. TTR addresses are stored in hexadecimal format. So an address of X’002E26′ would mean track number X’002E’ and record X’26′. The name TTR comes from the fact that first two bytes of the address denote track number and third byte denotes record number. This dependence on the DASD geometry makes it very difficult to migrate PDS from one type of DASD to another one, e.g. from 3380 to 3390.

The PDSE addressing scheme is not dependent on the physical device geometry. It uses a ’simulated’ 3-byte TTR address to locate the members & the records which makes the migration easier. Incidentally, this simulation of addresses places some limitations on the number of members and the number of records per member in a PDSE. A TTR address of X’000001′ in a PDSE points to the directory. The addresses from X’000002′ to X’07FFFF’ point to the first record of each member, which is why there is a limit of 524,286 members. The addresses from X’100001′ to X’FFFFFF’ point to records within each member, which is why there is a limit of 15,728,639 records in each member.