This topic is intended to help you use ISAM programs with VSAM data sets. The system no longer supports use of indexed sequential (ISAM) data sets. The information in this topic is shown to facilitate conversion to VSAM.

Although the ISAM interface is an efficient way of processing your existing ISAM programs, all new programs that you write should be VSAM programs. Before you migrate to z/OS V1R7 or a later release, you should migrate indexed sequential data sets to VSAM key-sequenced data sets. Existing programs can use the ISAM interface to VSAM to access those data sets and need not be deleted. During data set conversion you can use the REPRO command with the ENVIRONMENT keyword to handle the ISAM "dummy" records. For information about identifying and migrating ISAM data sets and programs prior to installing z/OS V1R7, see z/OS Migration.

The z/OS system no longer supports the creation or opening of indexed sequential data sets.

VSAM, through its ISAM interface program, allows a debugged program that processes an indexed sequential data set to process a key-sequenced data set. The key-sequenced data set can have been converted from an indexed-sequential or a sequential data set (or another VSAM data set) or can be loaded by one of your own programs. The loading program can be coded with VSAM macros, ISAM macros, PL/I statements, or COBOL statements. That is, you can load records into a newly defined key-sequenced data set with a program that was coded to load records into an indexed sequential data set.

Figure 1 shows the relationship between ISAM programs processing VSAM data with the ISAM interface and VSAM programs processing the data.

Figure 1. Use of ISAM Processing Programs

There are some minor restrictions on the types of processing an ISAM program can do if it is to be able to process a key-sequenced data set. These restrictions are described in Restrictions on the Use of the ISAM Interface.

Significant performance improvement can be gained by modifying an ISAM program that issues multiple OPEN and CLOSE macros to switch between a QISAM and BISAM DCB. The ISAM program can be modified to open the QISAM and BISAM DCBs at the beginning of the program and to close them when all processing is complete. The performance improvement is proportional to the frequency of OPEN and CLOSE macros in the ISAM program.