CMSC 412 Midterm #2 (Spring 2002)

1.) Virtual Memory (20 points)

a) Why would it be a bad idea to use a 2-level page table (page directory and page table) for a processor that supports 64-bit memory addresses (using a 4KB page size)?

b) Explain what is Belady’s anomaly and how it occurs.

2.) (20 Points) Given an implementation of counting semaphores (semaphores whose values range from 0 to some positive integer), implement binary semaphores (semaphores whose value is either 0 or 1).

P_binary:

V_binary:

3.) (25 points) Filesystems

a) Why can a hybrid index allocation strategy support random access to a file easier than a linked allocation scheme?

b) Explain how an Access Control List of users is both similar and different than a group.

c) Given a filesystem that supports only one directory (i.e. it’s not possible to create sub-directories) and 8 character filenames, explain how you could write a user level library that simulates hierarchical directories.

4.) (20 points) Disk and I/O Systems

a) Explain what the term variable geometry disk means.

b) If a disk spins at 7200 RPM, what is the average rotational latency in milliseconds?

5.) (15 points) Project

a) Explain why it is possible to use a single LDT for all user processes in the project once the paging code for project #3 is done.

b) How is the semaphore code from project #2 able to have multiple processes call createSemaphore for the same semaphore without causing a problem?