There's stories and then there's stories. The ones with any worth change your life forever, perhaps only in a small way, but once you've heard them, they are forever a part of you. You nurture them and pass them on, and the giving only makes you feel better. The others are just words on a page.
- Charles de Lint
An extremely important function of an operating system is to protect the user from being hurt, either maliciously or accidentally, by other users; that is, protect him when other users are executing or changing their programs, files, or databases. The operating system must insure inviolability. As well as protecting users from each other, the operating system must also protect itself from users who, whether maliciously or accidentally, might "crash" the system.
Students are great challengers of protection mechanisms. When the systems programming course is given at M.I.T., we find that due to the large number of students participating it is very difficult to personally grade every program run on the machine problems. So for the very simple problems - certainly the first problem which may be to count the number of A's in a register and leave the answer in another register - we have written a grading program that is included as part of the operating system. The grading program calls the student's program and transfers control to it. In this simple problem the student's program processes the contents of the register, leaves his answer in another register, and returns to the grading program. The latter checks to find out if the correct number has been left in the answer register. Afterwards, the grading program prints out a listing of all the students in the class and their grades. For example:
VITA KOHN -- CORRECT
RACHEL BUXBAUM -- CORRECT
JOE LEVIN -- INCORRECT
LOFTI ZADEH -- CORRECT
On last year's run, the computer listing began as follows:
JAMES ARCHER -- CORRECT
ED MCCARTHY -- CORRECT
ELLEN NANGLE -- INCORRECT
JOHN SCHWARTZ -- MAYBE
(We are not sure how John Schwartz did this; we gave him an A in the course.)
- John J. Donovan, Systems Programming
