CS 3733 Operating Systems, Spring 2006 Exam 1 Comments

Grade distribution:

• 90-95: 4
• 80-89: 4
• 70-79: 8
• 60-69: 9
• 50-59: 4
• below 50: none
• Average: 72

1. 1a) It is true that a hard link is a link, but saying this gives no information. Say what it is, not what it does.
2. 1 a) and b) Do not say that it points to something unless it contains a pointer. In a) it contains a number and in b) it contains a name.
3. 1c) asks about a directory entry, not a directory.
4. 1d) You need to give at least 4 different thing to get full credit. File times counts as one thing, not 3.
5. 2a) Don't just say it has a count of things without saying what the things are.
6. 2b i) The count is not incremented because the entry does not exist before the open.
7. 2b ii) You must also say what happens when the count becomes 0.
8. 2b v)
   • The answer does not depend on whether open was called before or after the fork.
   • The answer depends on the state of the process when the fork is executed.
   • The process may not call open at all.
   • FDT entries may exist because they were copied from the parent process (which may have gotten them from its parent), for example as in the usual case with standard input, output and error.
9. 2b v and vi) You must say which entries are affected since (unlike with open and close) there is not a direct correspondence between the operation and an entry.
10. 4a)
    • It is not sufficient to just give an example of a critical section, such as a section of code that modifies a shared variable.
    • It is not correct that you are not allowed to lose the CPU during execution of a critical section.
    • What is important is that if you do, another process (or thread) does not execute its critical section.
    • It is not sufficient to say that no two processes execute their critical sections at the same time since this will never occur on a single CPU system.
    • It is important to understand the difference between the terms simultaneously and concurrently.
11. 4b)
    • Bounded waiting does not put a bound on time.
    • Time depends on the speed of the CPU and the number of processes competing for the CPU.
    • Bounded waiting refers to the number of times an event may occur.
    • This is hardware and load independent.
12. 4c) In order to get some credit for this part you had to discuss the case in which one process is trying to enter its CS while the other process tries to enter its CS twice.