Library /sys$common/syshlp/dbg$uihelp.hlb DEBUGUI, break_exception  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  An exception breakpoint suspends execution when an exception is
  signaled and before any exception handler declared by your program
  executes.  This allows you to step into the exception handler (if one
  is available) to check the flow of control.

  To set an exception breakpoint, choose On Exception from the Break
  menu.  The exception breakpoint triggers whenever any exception is
  signaled.