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

  Information about the characteristics and the location of source files
  is embedded in the debug symbol table of your program.  If a source
  file has been moved to a different directory since compile time, the
  debugger might not find the file.  To direct the debugger to your
  source files, use the SET SOURCE command at the command-entry prompt.