By now you should have an understanding of the IMS online environment, as well as how to design terminal screens using the IMS Message Format Service. You will now learn what it takes, as a systems engineer, to code online IMS programs that can perform the input/output processing of data transmitted via physical devices (such as 3278 terminals).
Let's review the data base (DB) portion of IMS. Every IMS application program uses a Program Specification Block (PSB) to control its access to IMS resources. When the resources are databases, the PSB contains a Program Communication Block (PCB) for each database that the application program needs to access. A data base-type PCB not only specifies the database or Data Base Descriptor (DBD), you need to access, but also those segments within the database you need to access.
The segment specification provides IMS with your logical view of the database. Additionally, your access rights or processing options could be specified at the PCB and/or segment level. Processing options include the ability to insert, delete, retrieve, or replace segments within the database.
For IMS to logically tie databases to your application program through the PSB, you had to specify a pointer (address) for each PCB in the PSB. The order of the pointers in your program determined which database IMS would access, and therefore had to coincide with the order of the PCBs in the PSB.
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