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An application program which includes DL/I calls cannot execute directly. Instead, a JCL is required to trigger the IMS DL/I batch module. The batch initialization module in IMS is DFSRRC00. The application program and the DL/I module execute together. The following diagram shows the structure of an application program which includes DL/I calls to access a database.

The application program interfaces with IMS DL/I modules via the following program elements −

• An ENTRY statement specifies that the PCBs are utilized by the program.
• A PCB-mask co-relates with the information preserved in the pre-constructed PCB which receives return information from the IMS.
• An Input-Output Area is used for passing data segments to and from the IMS database.
• Calls to DL/I specify the processing functions such as fetch, insert, delete, replace, etc.
• Check Status Codes is used to check the SQL return code of the processing option specified to inform whether the operation was successful or not.
• A Terminate statement is used to end the processing of the application program which includes the DL/I.

Segments Layout

As of now, we learnt that the IMS consists of segments which are used in high-level programming languages to access data. Consider the following IMS database structure of a Library which we have seen earlier and here we see the layout of its segments in COBOL −

01 LIBRARY-SEGMENT.
   05 BOOK-ID        PIC X(5).
   05 ISSUE-DATE     PIC X(10).
   05 RETURN-DATE    PIC X(10).
   05 STUDENT-ID     PIC A(25).
	
01 BOOK-SEGMENT.
   05 BOOK-ID        PIC X(5).
   05 BOOK-NAME      PIC A(30).
   05 AUTHOR         PIC A(25).
	
01 STUDENT-SEGMENT.
   05 STUDENT-ID     PIC X(5).
   05 STUDENT-NAME   PIC A(25).
   05 DIVISION       PIC X(10).

Application Program Overview

The structure of an IMS application program is different from that of a Non-IMS application program. An IMS program cannot be executed directly; rather it is always called as a subroutine. An IMS application program consists of Program Specification Blocks to provide a view of the IMS database.

The application program and the PSBs linked to that program are loaded when we execute an application program which includes IMS DL/I modules. Then the CALL requests triggered by the application programs are executed by the IMS module.

IMS Services

The following IMS services are used by the application program −

• Accessing database records
• Issuing IMS commands
• Issuing IMS service calls
• Checkpoint calls
• Sync calls
• Sending or receiving messages from online user terminals