In IMS DB (Information Management System Database), PCBs (Program Communication Blocks) are defined to establish communication between application programs and the IMS (Information Management System) control program. Defining PCBs involves specifying the necessary information and parameters required for application programs to access IMS databases.

Here's how PCBs are defined in IMS DB :

Specify PCB Fields : PCBs are defined using a PCB layout or PCB definition. The PCB layout specifies the fields or attributes that make up the PCB. These fields include information such as segment names, keys, data buffers, and status indicators.

Identify Program Name : Each PCB is associated with a specific application program or transaction code. The program name or transaction code is specified in the PCB definition to uniquely identify the program requesting database access.

Define Database Operations : PCBs define the database operations that the associated application program is authorized to perform. This includes specifying the type of database access (e.g., read, write, update, delete), the target database segments, and any additional parameters required for the operations. Allocate Data Buffers : PCBs allocate data buffers for storing database records retrieved or modified by application programs. The size and structure of the data buffers are defined in the PCB layout to accommodate the data returned from database operations.

Set Error Handling Mechanisms : PCBs define error handling mechanisms to handle exceptions and error conditions that may occur during database access operations. This includes specifying error codes, error handling routines, and status indicators to report the outcome of database operations to the application program.

Define Security and Access Control : PCBs define security and access control policies to enforce data integrity and protect sensitive information. Access privileges, authentication requirements, and authorization rules can be specified in the PCB definition to restrict access to database resources.

Compile and Link : Once the PCB definition is complete, it is compiled and linked with the application program code. This process generates executable code that incorporates the PCB information and establishes communication between the application program and IMS DB.