sacred

sacred adj.  Reserved for the exclusive use of something (an
   extension of the standard meaning).  Often means that anyone may
   look at the sacred object, but clobbering it will screw whatever it
   is sacred to.  The comment "Register 7 is sacred to the interrupt
   handler" appearing in a program would be interpreted by a hacker
   to mean that if any *other* part of the program changes the
   contents of register 7, dire consequences are likely to ensue.