Mainframe Interview Questions and Answers

Q1. Explain Paging Concept in memory.  
Whenever a particular data item is requested, the CPU hits primary memory or RAM first to see if the record corresponding to the data item is present in any page in primary memory. If not, it does paging and brings in the correspong page from secondary/auxiliary memory or hard disk to primary memory and then returns the data item to the user.

Q2. Explain why we can not define occurs clause at 01 level.
01 is of the record level. We repeat the fields within a record not the record itself. So occurs clause can not be used at 01 level.

Q3. What is Defragmentation and What is its advantage?
The process of combining the scattered allocated spaces for a file in a single or few contiguous areas rather than multiple areas is called defragmentation. Defragmentation reduces the number the seeks to retrieve the data.

Q4. Explain IS NUMERIC Clause.
IS NUMERIC Clause is used to check if any item is numeric or not. It returns TRUE when the item against which it is used contains only numbers(0 to 9). The item can be +ve or -ve. IS NUMERIC can be used against Numeric, Alpha Numeric, Packed Decimal fields whether signed or not. 

Q5. What is downtime and what might be the reason?
Downtime is the very brief period that the system is not available. It is generally done for maintenance activities for the mainframe. In the rare cases, it is due to system outage.

Q6. Explain internal Sort and External Sort.
Internal Sort:
SORT file-1 ON ASCENDING/DESCENDING KEY key[,key1,….] USING file-2 GIVING file-3.
USING can be substituted by INPUT PROCEDURE IS para-1 THRU para-2
GIVING can be substituted by OUTPUT PROCEDURE IS para-1 THRU para-2.

file-1 is the sort (work) file and must be defined as SD entry in FILE SECTION.
file-2 is the input file for the SORT and must be defined as FD entry in FILE SECTION
file-3 is the out file from the SORT and must be defined as FD entry in FILE SECTION.
file-1, file-2 & file-3 should not be opened explicitly.

INPUT PROCEDURE is executed before the sort and records must be RELEASED to the sort work file from the input procedure.
OUTPUT PROCEDURE is executed after all records have been sorted. Records from the sort work file must be RETURNED one at a time to the output procedure.

External sort is the one which is coded in JCL. External sort is more efficient.

Q7. What is SLA?
SLA is Service Level Agreement between the customer and the vendor. It is significant in support systems wherein the terms could be like response should be given within two hours that an abend has occurred or for online systems, the system should not be down during user daytime.

Q8. Explain difference between CONTINUE and NEXT SENTENCE.
Continue transfers the control to next statement after the scope terminator. Next sentence transfers the control to statement after the first period encountered.

Q9. What is a Configuration Management system, give examples?
A configuration management system is typically used in development environments to maintain versioning of the developmental code to avoid any overlay of code by different developers working on the same code. It helps maintain the integrity and consistency of the code across environments.
CHANGEMAN and ENDEVOR are examples of configuration management in mainframe.

Q10. Can I redefine X(10) with X(20). What happens if I do so?
Yes.
Example:
01 WS-1 PIC X(10).
01 WS-2 REDEFINES WS-1 PIC X(20).

MOVE ‘MY NAME IS MAINFRAME’ TO WS-2.
DISPLAY WS-1
The above statement displays ‘MY NAME IS’
DISPLAY WS-2
The above statement displays ‘MY NAME IS MAINFRAME’