Mainframe Interview Questions

Customer Information Control System (CICS) is an online transaction processing monitor from IBM that acts as an interface between the operating system and application programs to provide rapid high-volume online transaction processing.

They communicate via service MVS request calls.

IDENTIFICATION DIVISION and PROGRAM-ID paragraph are mandatory for a compilation error free COBOL program.

- Use a COBOL II CALL statement to invoke a subprogram. This method is transparent to CICS, which sees only the one load module. 

- EXEC LINK is similar to a call; it invokes a separate CICS program and ends with a RETURN to the invoking program

- EXEC XCTL which transfers control to another CICS program and does not get control 

back. 

COBOL II allows for static calls which are more efficient than the LINK instruction which establishes a new run-unit. 

Autostatus is a protocol mode which causes the expansion of each DML statement to include a 'perform IDMS-Status' statement.

PCT (Program Control Table) - Defines each transaction containing a list of valid transaction identifiers (transid) where each transaction is paired with its matching program.

PPT (Program Processing Table) - Contains a list of valid program names and maps and whether a current version is in the CICS region or needs to be brought in as a new copy.

FCT (File Control Table) - Contains a list of files known to CICS, the dataset name and status (closed/open, enable/disabled)

TCT (Terminal Control Table)- A list of the terminals known to CICS.

The simplest DB2 index is the B-tree and the B-tree's top page is called the root page. The root page entries represent the upper range limits of the index and are referenced first in a search. 

DB2 use the multiple indexes to satisfy multiple predicates in a SELECT statement that are joined by an AND or OR.