Operating Systems Comps
Spring 2001
There are five questions on this exam. Please answer all five questions. You have 90 minutes to
complete this exam. Good luck!
1. Traditional Unix address spaces have three user-level virtual memory regions: one for static
text and data, one for the heap, and one for the stack. These regions have well-defined
locations and are internally contiguous. Modern Unix implementations have much more
flexible virtual memory support in which memory regions have been generalized to memory
objects . Memory objects can be dynamically created, and can have arbitrary locations and
sizes in a process' virtual address space. Each traditional memory regions is implemented as
one or more memory object.
Briefly describe two advantages and one disadvantage of the modern memory object approach
to virtual address spaces compared with the traditional approach.
2. The interfaces of many file systems are similar to that of Unix, where, to access a file, it must
first be "opened", after which the program can issue a series of reads and writes. When done,
it is "closed". The OPEN system call is given a file name (and possibly other parameters),
and it generally returns a file handle, which is then supplied as a parameter to subsequent
READ and WRITE system calls to operate on that file (and also to the CLOSE system call to
close it). Consider an alternative "simplified" design, where there are no OPEN and CLOSE
calls; rather, the READ and WRITE system calls are called with the file name as a parameter
(along with other parameters, including any that were originally supplied to the OPEN system
call, as appropriate).
Describe as many advantages or disadvantages to the alternative scheme. Why do most file
systems require files to be opened before being read and written? What is the purpose of the
CLOSE system call?
3. Alice and Bob had the following conversation over a game of foosball:
Alice: "I learned from the Lampson paper that protection mechanisms are just different
ways of expression a sparse access control matrix."
Bob: "Really? I just took an algorithms class where we studied and implemented sparse
matrices. You're implementing a free Linux-like operating system (Alix), so why don't
you use my code? It has a very compact representation of sparse matrices, with a default
value of 0.0, and while you won't need the other efficient operations such as matrix add,
multiply etc, This code has been proven correct!"
If Alice were to take Bob's code and plug it into Alix, would it probably satisfy all of the
needs of an access control system? Give reasons why or why not.
