CS439: Principles of Computer Systems

Discussion Section 8 Problem Set

Due in Section on April 12, 2024

The problem set must be completed before section and brought to section. To ensure that your problem set is turned in correctly and that you receive credit for discussion section, you must follow these guidelines exactly.

Include the following header at the top of your file:

Discussion Section Problem Set #:
Student Name:
EID:
If I had to talk like an animal, I would pick...


Partner Name:
EID:
If I had to talk like an animal, I would pick...

You may either type your answers or write them by hand, but you must bring a hard copy to discussion section.

In discussion section, your TA will check that you worked with your partner, you made a good effort on the problem set, you are participating in the discussions in section, and you are ready and able to answer when called upon. If all of those are true, you will receive credit for discussion section that day. Note that a "good effort" on the problem set includes that all answers are in words unique to you and your partner and not copied from anywhere else, including the slide deck.

What are "privileged instructions"? Name three situations in which privileged instructions are required.
Given the following pseudocode:
```
int i = 0;

int main(void)
{ 
   fork();

   if(/*CHILD*/)
   {
      i = i - 1;
      return SUCCESS;
   }
   if(/*PARENT*/)
   {
      i = i + 1;
      wait for child to terminate;

      printf("i = %d\n", i);
   }
}
```
- What is the output? Describe in detail why it is that way.
- Are there other possible outputs? Why or why not? If so, give another example of what the printed values of ``i'' might be and explain how to force the program to execute the same way each time.

Now consider this pseudocode involving kernel threads:
```
int i = 0;

void thread1(void *)
{
   i = i - 1;
}

void thread2(void *)
{
   i = i + 1;
}

int main(void)
{
   createThread(thread1);
   createThread(thread2);

   /* wait for thread 1 and thread 2 to end */

   printf("i = %d\n", i);
}
```
- What is the output? Describe in detail why it is that way.
- Are there other possible outputs? Why or why not? If so, give another example of what the printed values of ``i'' might be and explain how to force the program to execute the same way each time.
Considering all the pieces of the operating system we have discussed, describe the steps the OS takes when performing a context switch. Assume that processes are waiting in the ready queue.

Which is the best (fastest) network on which to implement a remote-memory read that sends a 100 byte packet from machine A to machine B and then sends a 8000 byte packet from machine B to machine A?
1. A network with 200 microsecond processing overhead, 10 Mbyte/s bandwidth, 20 microsecond latency
2. A network with 20 microsecond processing overhead, 10 Mbyte/s bandwidth, 200 microsecond latency
3. A network with 20 microsecond processing overhead, 1 Mbyte/s bandwidth, 2 microsecond latency
4. A network with 2 microsecond processing overhead, 1 Mbyte/s bandwidth, 20 microsecond latency

Using the traceroute command, trace the hops a packet takes from one of the UTCS machines to a website in Europe. Inspect the trace. What do you notice? Do you see any machines you recognize? ISPs? Cities? Please be sure to tell us what website you used.
Perform the trace again. How does the second time affect the output?

Describe the steps necessary to prepare a server to accept connections. How does it accept a connection?