CSE 120: Homework #1

Out: Tuesday October 6
Due: Thursday October 15 at 9pm

Each question is worth 5 points. When a question asks you to describe or explain, your answers can be relatively brief. The [name] tags give scholarly attribution to authors of other OS textbooks who wrote the question (you do not need those textbooks to be able to answer the question, though). You may type or handwrite your answers.

You will submit your homework electronically via gradescope. Once you have logged in, follow these general instructions for uploading your homework (we will post to piazza when gradescope is ready for submissions).

1. [Anderson 2.1] For each of the three mechanisms that supports dual-mode operation — privileged instructions, memory protection, and timer interrupts — explain what might go wrong without that mechanism, assuming the system only had the other two. (In other words, if we just had memory protection and timer interrupts but not privileged instructions, what could go wrong; if we just had privileged instructions and timer interrupts, what could go wrong, etc.)

2. [Silberschatz] Some computer systems do not provide a privileged mode of operation in hardware. Is it possible to construct a secure operating system for these computers? Give arguments both that it is and that it is not possible. (By a secure operating system, we mean that a user program is not able to corrupt the kernel, prevent it from running, crash the system, violate memory protection, etc.)

3. [Silberschatz] Which of the following instructions should be privileged? Give a one-sentence explanation for why.

   a) Set value of timer
   b) Read the clock
   c) Clear memory
   d) Turn off interrupts
   e) Switch from user to monitor (kernel) mode
   

4. [Tanenbaum] For each of the following Unix system calls, give a condition that causes it to fail: open, read, fork, exec, unlink (delete a file). (Hint: We discussed some in lecture, and you can also explore the error semantics of these system calls using man on ieng6, e.g., man 2 fork.)

5. List two challenges an operating system faces when passing parameters between user and kernel mode (e.g., consider the differences between passing parameters via procedures in the same process versus between processes/applications). Describe how an operating system can overcome them.

6. [Crowley] Suppose the hardware interval timer only counts down to zero before signalling an interupt. How could an OS use the interval timer to keep track of the time of day?

7. [Anderson 2.13] Suppose you have to implement an operating system on hardware that supports interrupts and exceptions but does not have an explicit trap (syscall) instruction. Can you devise a satisfactory substitute for traps using interrupts and/or exceptions? If so, explain how. If not, explain why. (In this context, the trap instruction is the instruction used by a user-level process to invoke a system call in the operating system, i.e., the trap instruction is the system call instruction.)

8. Consider the following C program:

   #include <stdlib.h>
   
   int main (int argc, char *arg[])
   {
       if (fork ()) {
   	fork ();
       } else {
           fork ();
   	char *argv[2] = {"/bin/ls", NULL};
   	execv (argv[0], argv);
           fork ();
       }
   }
   

   a. How many total processes are created (including the first process running the program)? (Note that execv is just one of multiple ways of invoking exec, see man 3 exec for all possibilities.)

   b. How many times does the /bin/ls program execute?

   [Hint: You can always add debugging code, compile it, and run the program to experiment with what happens.]