The SETL macro lets you retrieve records starting at the beginning of an indexed sequential data set or at any point in the data set. Processing that is to start at a point other than the beginning can be requested in the form of a record key, a key class (key prefix), or an actual address of a prime data record.

The key class is useful because you do not have to know the entire key of the first record to be processed. A key class consists of all the keys that begin with identical characters. The key class is defined by specifying the desired characters of the key class at the address specified in the lower-limit address of the SETL macro and setting the remaining characters to the right of the key class to binary zeros.

To use actual addresses, you must keep a record of where the records were written when the data set was allocated. The device address of the block containing the record just processed by a PUT-move macro is available in the 8-byte data control block field DCBLPDA. For blocked records, the address is the same for each record in the block.