PCBs and PSB

A program specification block (PSB) used in a DBCTL environment can contain I/O PCBs, alternate PCBs, database PCBs (DB PCB), or GSAM PCBs.

I/O PCB

In a DBCTL environment, an I/O PCB is needed to issue DBCTL service requests. Unlike the other types of PCB, it is not defined with PSB generation, but if the application program is using an I/O PCB, this has to be indicated in the PSB scheduling request.

Alternate PCB

An alternate PCB defines a logical terminal and can be used instead of the I/O PCB when it is necessary to direct a response to a terminal. Alternate PCBs appear in PSBs used in a CICS®-DBCTL environment, but are used only in an IMS™ DC environment. CICS applications using DBCTL cannot successfully issue commands that specify an alternate PCB, an MSDB PCB, or a GSAM PCB. However, a PSB that contains PCBs of these types can be scheduled successfully in a CICS-DBCTL environment.

Alternate PCBs are included in the PCB address list returned to a call level application program. In an EXEC DLI application program, the existence of alternate PCBs in the PSB affects the PCB number used in the PCB keyword.

DB PCB

A DB PCB is the PCB that defines an application program's interface to a database. One DB PCB is needed for each database view used by the application program. It can be a full-function PCB, a DEDB PCB, or an MSDB PCB.

GSAM PCB

A GSAM PCB defines an application program's interface for GSAM operations.

When using DBCTL, a CICS program receives, by default, a DB PCB as the first PCB in the parameter list passed to it after scheduling. However, when your application program can handle an I/O PCB, you indicate this using the SYSSERVE keyword on the SCHD command. The I/O PCB is then the first PCB in the parameter address list passed back to your application program.

I/O PCBs and alternate PCBs in various types of application programs

DB batch programs

Alternate PCBs are always included in the list of PCBs supplied to the program by DL/I irrespective of whether you have specified CMPAT=Y. The I/O PCB is returned depending on the CMPAT option.

If you specify CMPAT=Y, the PCB list contains the address of the I/O PCB, followed by the addresses of any alternate PCBs, followed by the addresses of any DB PCBs.

If you do not specify CMPAT=Y, the PCB list contains the addresses of any alternate PCBs followed by the addresses of the DB PCBs.

BMP programs, MPPs, and IFPs

I/O PCBs and alternate PCBs are always passed to BMP programs. I/O PCBs and alternate PCBs are also always passed to MPPs and to IFP application programs.

The PCB list contains the address of the I/O PCB, followed by the addresses of any alternate PCBs, followed by the addresses of the DB PCBs.

CICS programs with DBCTL

The first PCB always refers to the first DB PCB whether you specify the SYSSERVE keyword.

The following table summarizes the I/O PCB and alternate PCB information. The first column lists different DB environments, the second and third column specify if the I/O PCB or alternate PCB, respectively, is valid in the specified environment.

Table 1. Summary of PCB information
Environment	EXEC DLI: I/O PCB count included in PCB(n)	EXEC DLI: Alternate PCB count included in PCB(n)
CICS DBCTL¹	No	No
CICS DBCTL²	No	No
BMP	Yes	Yes
Batch³	No	Yes
Batch⁴	Yes	Yes

Notes:

SCHD command issued without the SYSSERVE option.
SCHD command issued with the SYSSERVE option for a CICS DBCTL command or for a function-shipped command which is satisfied by a remote CICS system using DBCTL.
CMPAT=N specified on the PSBGEN statement.
CMPAT=Y specified on the PSBGEN statement.

Format of a PSB

The following is the format of a PSB.

[IOPCB]
[Alternate PCB ... Alternate PCB]
[DBPCB ... DBPCB]
[GSAMPCB ... GSAMPCB]

Each PSB must contain at least one PCB. The I/O PCB must be addressable in order to issue a system service command. An alternate PCB is used only for IMS online programs, which can run only with the Transaction Manager. Alternate PCBs can be present even though your program does not run under the Transaction Manager. A DB PCB can be a full-function PCB, a DEDB PCB, or an MSDB PCB.