Finding unexpected storage overwrite errors in COBOL

During program run time, some storage might unexpectedly change its value and you want to find out when and where this happened. Consider this example where the program changes more than the caller expects it to change.

   05  FIELD-1     OCCURS 2 TIMES
                       PIC X(8).
   05  FIELD-2         PIC X(8).
 PROCEDURE DIVISION.
*                    ( An invalid index value is set )
     MOVE 3 TO PTR.
     MOVE "TOO MUCH" TO FIELD-1( PTR ).

Find the address of FIELD-2 with the command:

DESCRIBE ATTRIBUTES FIELD-2

Suppose the result is X'0000F559'. To set a breakpoint that watches for a change in storage values starting at that address for the next 8 bytes, issue the command:

AT CHANGE %STORAGE(H'0000F559',8)

When the program runs, z/OS® Debugger halts if the value in this storage changes.