Fig. 14. A protected subsystem to implement the grade-keeping system described in the text. P₁, which can be invoked by all students in the subject, is programmed to return the caller's grade for a particular assignment or the distribution of all grades for an assignment. P₂ which can be invoked by the teaching assistants for the subject, is programmed to allow the addition of new grades to the record but to prevent changing a grade once it is entered. P₃, which can be invoked only by the instructor, is programmed to read or write on request any data in the grade record.