The IBM i operating system contains a very powerful facility for locating logic errors in programs. It is generally referred to as "DEBUG" and when it is being used on a program the program is said to be in "DEBUG mode".
The most powerful feature of DEBUG is that it allows an executing program to be stopped at any point and the content of fields used by the program to be displayed or changed.
By using this facility, logic errors in programs can be quickly and easily located. In fact some people feel that it is this facility that enables programmers on the IBM i to achieve such high rates of productivity.