Finding unexpected storage overwrite errors in PL/I

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

2 FIELD1(2) CHAR(8);
2 FIELD2 CHAR(8);
 CTR = 3;         /* an invalid index value is set */
 FIELD1(CTR) = 'TOO MUCH';

Find the address of FIELD2 with the command:

DESCRIBE ATTRIBUTES FIELD2

Suppose the result is X'00521D42'. 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('00521D42'px,8)

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