Program communication block (PCB) lists

Use the correct format of program communication block (PCB) lists and generated program specification block (GPSB) PCB lists in your application program.

PCB list format

This is the format of a PCB:

[IOPCB]
[Alternate PCB ... Alternate PCB]
[DB PCB ... DB PCB]
[GSAM PCB ... GSAM PCB]

Each PSB must contain at least one PCB. An I/O PCB or alternate PCB is required for transaction management calls, and an I/O PCB is required for most system service calls. DB PCBs for DL/I databases are used only with the IMS Database Manager, but can be present even though your program is running under DCCTL or TM Batch. (A DB PCB can be a full-function PCB, a DEDB PCB, or an MSDB PCB.) GSAM PCBs can be used with DCCTL or TM batch.

Format of a GPSB PCB list

A generated program specification block (GPSB) has the following format:

[IOPCB]
[Alternate PCB]

A GPSB contains only an I/O PCB and one modifiable alternate PCB. It can be used by all transaction management application programs, and permits access to the PCBs specified without the need for PSBGEN.

The PCBs in a GPSB have predefined PCB names. The name of the I/O PCB is IOPCBbb. The name of the alternate PCB is TPPCB1bb.

PCB summary

I/O PCBs and alternate PCBs can be used in various types of application programs.

TM Batch Programs: Alternate PCBs are always included in the list of PCBs supplied to the program by IMS TM. The I/O PCB is always present in the PCB list regardless of the CMPAT options specified in PSBGEN.
BMPs, MPPs, and IFPs: The I/O PCB is always present in the PCB list and is always the first address in the list, regardless of the CMPAT options specified in the PSB. The PCB list always contains the address of the I/O PCB followed by the addresses of any alternate PCBs, followed by the addresses of the DB PCBs.