CSCI 350 Fall 2001 Quiz 5

Statistics:

mean     46.154 (600.000/13)
stddev   3.060
median   47.000
midrange 44.000-48.750

#   avg
1	7.31 / 8
2	7.38 / 9
3	8.46 / 9
4	7.85 / 8
5	7.38 / 8
6	7.77 / 8

1. [8 pts] Name a primary purpose of an operating system.

   An operating system manages hardware resources (providing easy access to user programs, allocating resources between programs, and protecting resources from damage by programs).

2. [9 pts] Name three specific pieces of data found in a typical page table entry.
   • page frame number (telling where page is in physical memory)
   • invalid bit (tracking whether page is currently in physical memory)
   • dirty bit (telling whether the page in memory has been modified)
   • referenced bit (telling whether the page in memory has been accessed)

3. [9 pts] What distinguishes a thread from a process?

   Each process gets its own set of resources (memory, file descriptors, etc), while threads share the same set of resources within a process.

4. [8 pts] We discussed the relative merits of implementing threads as a kernel-independent user library (user-level threads) and of implementing threads via system calls to the kernel (kernel-level threads). Explain an advantage of one of the approaches over the other. Be sure to mention which approach your point supports.

   Some of the better answers:

   • User-level threads gain efficiency, since they do not incur the significant cost of switching frequently between user mode and kernel mode in the CPU.

   • When a thread incurs a page fault, other kernel-level threads can continue working, whereas user-level threads must stall.

   • Programs using user-level threads rely less heavily on operating system support and hence are more portable.

5. [8 pts] We discussed three capabilities that TCP provides on top of IP. Name two of them.
   • TCP provides reliable delivery of messages.

   • TCP provides program-to-program delivery (whereas IP only delivers between computers.

   • TCP provides a continuous connection (whereas IP packets are independent of each other).

   • [8 pts] What is the purpose of the sliding window concept in TCP?

     It allows the computer to send large messages more quickly by keeping several segments of the message in transit at once.