Comprehensive Examination
Operating Systems
Fall 2001
This exam has 4 questions, all of equal value. Answer all the questions to the best of your ability.
For each question, keep you answer on the same page (you may use the other side of the page).
WRITE YOUR ID ON EACH INDIVIDUAL PAGE. Good luck!
1. In the layered approach to operating system design, the system is composed of horizontal
layers where a higher layer can call on the resources of a lower layer, but not vice-versa.
However, in some situations, it may be necessary for a lower layer to initiate the passing of
information up to a higher layer in an unscheduled way, so that it is not simply a question of
constructing a return to a method or procedure call. In order to preserve the integrity of the
layered approach, this is often done using "callbacks". Explain how this works and what the
basic data structures and methods/classes are that are required to support it.
2a. Briefly describe each of the following page replacement algorithms in terms of (1) the page it
chooses to evict, and (2) what information the operating system must keep track of, if any, to
make that choice.
a) Random (RAND)
b) First In First Out (FIFO)
c) Least Recently Used (LRU)
d) Most Recently Used (MRU)
2b. State which page replacement algorithm the operating system should use to best handle the
following applications, and briefly justify your answer.
a) Personal workstation (multiprocessing: mail, text editing, web browsing, applications
with good locality)
b) Web search engine (single application: gigantic hash tables larger than physical
memory, very little locality)
c) Web server (single application: nearly every object on the file system is eventually
accessed, but accesses have excellent locality)
d) Scientific computing (single application: gigantic arrays larger than physical
memory)
e) TiVo, ReplayTV (single application: storing and replaying compressed video files
much larger than physical memory)
f) Palmtop, cell phone (multiprocessing: applications fit entirely in physical memory)
