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PCB stands for Program Communication Block. PCB Mask is the second parameter used in the DL/I call. It is declared in the linkage section. Given below is the syntax of a PCB Mask −
01 PCB-NAME. 05 DBD-NAME PIC X(8). 05 SEG-LEVEL PIC XX. 05 STATUS-CODE PIC XX. 05 PROC-OPTIONS PIC X(4). 05 RESERVED-DLI PIC S9(5). 05 SEG-NAME PIC X(8). 05 LENGTH-FB-KEY PIC S9(5). 05 NUMB-SENS-SEGS PIC S9(5). 05 KEY-FB-AREA PIC X(n).
Here are the key points to note −
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For each database, the DL/I maintains an area of storage that is known as the program communication block. It stores the information about the database that are accessed inside the application programs.
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The ENTRY statement creates a connection between the PCB masks in the Linkage Section and the PCBs within the program’s PSB. The PCB masks used in a DL/I call tells which database to use for operation.
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You can assume this is similar to specifying a file name in a COBOL READ statement or a record name in a COBOL write statement. No SELECT, ASSIGN, OPEN, or CLOSE statements are required.
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After each DL/I call, the DL/I stores a status code in the PCB and the program can use that code to determine whether the call succeeded or failed.
PCB Name
Points to note −
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PCB Name is the name of the area which refers to the entire structure of the PCB fields.
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PCB Name is used in program statements.
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PCB Name is not a field in the PCB.
DBD Name
Points to note −
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DBD name contains the character data. It is eight bytes long.
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The first field in the PCB is the name of the database being processed and it provides the DBD name from the library of database descriptions associated with a particular database.
Segment Level
Points to note −
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Segment level is known as Segment Hierarchy Level Indicator. It contains character data and is two bytes long.
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A segment level field stores the level of the segment that was processed. When a segment is retrieved successfully, the level number of the retrieved segment is stored here.
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A segment level field never has a value greater than 15 because that is the maximum number of levels permitted in a DL/I database.
Status Code
Points to note −
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Status code field contains two bytes of character data.
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Status code contains the DL/I status code.
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Spaces are moved to the status code field when DL/I completes the processing of calls successfully.
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Non-space values indicate that the call was not successful.
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Status code GB indicates end-of-file and status code GE indicates that the requested segment is not found.
Proc Options
Points to note −
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Proc options are known as processing options which contain four-character data fields.
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A Processing Option field indicates what kind of processing the program is authorized to do on the database.
Reserved DL/I
Points to note −
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Reserved DL/I is known as the reserved area of the IMS. It stores four bytes binary data.
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IMS uses this area for its own internal linkage related to an application program.
Segment Name
Points to note −
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SEG Name is known as segment name feedback area. It contains 8 bytes of character data.
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The name of the segment is stored in this field after each DL/I call.
Length FB Key
Points to note −
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Length FB key is known as the length of the key feedback area. It stores four bytes of binary data.
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This field is used to report the length of the concatenated key of the lowest level segment processed during the previous call.
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It is used with the key feedback area.
Number of Sensitivity Segments
Points to note −
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Number of sensitivity segments store four bytes binary data.
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It defines to which level an application program is sensitive. It represents a count of number of segments in the logical data structure.
Key Feedback Area
Points to note −
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Key feedback area varies in length from one PCB to another.
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It contains the longest possible concatenated key that can be used with the program’s view of the database.
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After a database operation, DL/I returns the concatenated key of the lowest level segment processed in this field, and it returns the length of the key in the key length feedback area.
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