IMS control the structure of, and access to, its databases via DBDs (Database Definitions), PSBs (Program Specification Blocks) and ACBs (Access Control Blocks). DBDs, PSBs and ACBs are generally not the responsibility of the application programmer, but you need to know why they are needed and how they can affect your program. IMS databases are defined in a Database Definition (DBD). DBDs contain such information as the segment types, their place in the hierarchy (what the parent segment types is), and the segment keys (if any).

Program Specification Blocks (PSBs) Use the Program Specification Block (PSB) generation utility before you execute an application program under IMS, to describe the program and its use of logical terminals and logical data structures through a program specification block (PSB) generation.

The PSB generation statements supply the identification and characteristics of the IMS resources to be used. These program communication blocks (PCBs) represent message destinations and databases used by the application program. In addition, there must be a statement supplying characteristics of the application program itself. There must be one PSB for each message, batch, or Fast Path program. The name of the PSB and its associated application program must be the same in a telecommunications system.

If you require only an I/O PCB and a single, modifiable alternate PCB, you can use a generated PSB (GPSB) to describe the resources required for your application program. GPSBs can be used in any online environment, and are typically used in DCCTL application programs. You do not need to perform PSBGEN for GPSBs.

Access Control Blocks (ACBs) are generated by IMS as an expansion of information contained in the PSB in order to speed access to the applicable DBDs. It is the ACB, rather than the PSB that IMS uses when running your application program. In batch programs, ACBs can either be 'GEN'ed ahead of time, or at execution time, depending on the JCL used to invoke the program. The advantage of building the ACB at execution time is that you are assured of the latest changes to the PSB and DBD(s) being incorporated into the ACB. The disadvantage is the overhead of building the ACB every time you run the program. In CICS, you have no such choice; the ACBs must be built ahead of time, and it wasteful to build an ACB every time you execute an on-line transaction. .

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