CSE 221: Homework 1

Fall 2005

Due: Section A (Marzullo): Wednesday, October 19, 2005 at the start of class (5pm)
Due: Section B (Voelker): Thursday, October 20, 2005 at the start of class (9:30am)

Answer the following questions. Support your answers with material from the referenced papers, or with your own critical arguments, as appropriate. I am interested in your justifications as much as the answer itself. There may not necessarily be a "right" answer, although some answers may be easier to justify. Finally, do not use shorthand -- write your answers using complete sentences.

  1. The Multics paper lists five principles for protection in an operating system. Critique Hydra according to those principles. To what extent does Hydra implement each principle?

  2. Critique Pilot according to the same principles. For each, if you think that Pilot does not implement the principle, explain whether it is important (or not) that it does not.

  3. Both Pilot and Swift used typesafe languages to implement the system, and both reported significant benefits in doing so. Why do you think the operating systems we use are not implemented in type-safe languages?

  4. Pilot made a strong argument for tailoring the design and implementation of operating systems to personal computers. We have also seen commercial operating systems like MSDOS and MacOS tailored towards personal computers as well. Why do you think we still run multi-user timesharing systems like Unix on our PCs? (Note: Consider, for example, the requirements we have of the systems that we use today.)

  5. Operating systems go to great lengths to provide isolation and protection among processes executing on the system. Process debugging, however, represents a necessary, interesting feature that is made more difficult by process isolation and protection, and requires support from the operating system to function correctly. To the extent possible and where appropriate, when answering the following questions support your answers with approaches for debugging support found in the papers you have read to this point (e.g., Tenex, Pilot, Lampson Protection).

    1. Why must a traditional operating system like Unix explicitly provide support for process debugging?
    2. List two operations that a debugger must perform that require support from the operating system.
    3. Because processes are protected and isolated from each other, operating systems must also provide support for communication and coordination among processes. Why can't debuggers just use the support that operating systems already provide for process communication and coordination?
    4. Do language runtime environments like Java and Perl require operating system support for debugging programs in those languages? Why or why not?
    5. When working on an operating system, developers also need to use a debugger on the operating system itself. Why is debugging the kernel of an operating system more challenging than debugging a user-level process? What is one option for where to run a kernel debugger?