IMS DB - PCB Mask

PCB stands for Program Communication Block. PCB Mask is the second parameter used in the DL/I call. It is declared in the pnkage 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 −

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 apppcation programs.

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.

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.

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 −

PCB Name is the name of the area which refers to the entire structure of the PCB fields.

PCB Name is used in program statements.

PCB Name is not a field in the PCB.

DBD Name

Points to note −

DBD name contains the character data. It is eight bytes long.

The first field in the PCB is the name of the database being processed and it provides the DBD name from the pbrary of database descriptions associated with a particular database.

Segment Level

Points to note −

Segment level is known as Segment Hierarchy Level Indicator. It contains character data and is two bytes long.

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.

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 −

Status code field contains two bytes of character data.

Status code contains the DL/I status code.

Spaces are moved to the status code field when DL/I completes the processing of calls successfully.

Non-space values indicate that the call was not successful.

Status code GB indicates end-of-file and status code GE indicates that the requested segment is not found.

Proc Options

Points to note −

Proc options are known as processing options which contain four-character data fields.

A Processing Option field indicates what kind of processing the program is authorized to do on the database.

Reserved DL/I

Points to note −

Reserved DL/I is known as the reserved area of the IMS. It stores four bytes binary data.

IMS uses this area for its own internal pnkage related to an apppcation program.

Segment Name

Points to note −

SEG Name is known as segment name feedback area. It contains 8 bytes of character data.

The name of the segment is stored in this field after each DL/I call.

Length FB Key

Points to note −

Length FB key is known as the length of the key feedback area. It stores four bytes of binary data.

This field is used to report the length of the concatenated key of the lowest level segment processed during the previous call.

It is used with the key feedback area.

Number of Sensitivity Segments

Points to note −

Number of sensitivity segments store four bytes binary data.

It defines to which level an apppcation program is sensitive. It represents a count of number of segments in the logical data structure.

Key Feedback Area

Points to note −

Key feedback area varies in length from one PCB to another.

It contains the longest possible concatenated key that can be used with the program’s view of the database.

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.