The internal variable for debugging support. Different bits mean the following (subject to change):
Debug subroutine enter/exit.
Switch off optimizations.
Preserve more data for future interactive inspections.
Keep info about source lines on which a subroutine is defined.
Start with single-step on.
Note that some bits may be relevent at compile-time only, some at run-time only. This is a new mechanism and the details may change.