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The University of Texas at Dallas Computer Science Program
Operating Systems Principles—Exam II
Notes:
1. Answer all questions in the space provided.
2. The proctoring TAS will not answer any questions. If there is
ambiguity, write your assumption clearly and proceed.
3. For all questions, show how your steps in arriving at the answer.
1. Explain how Unix’s memory management (demand paging) implements the working
set principle.
2. Explain why preemption is not needed at times when no new jobs arrive in shortest
remaining time first scheduling algorithm.
3. Consider a set of 8 processes with following CPU time needs: 2, 4, 5, 8, 9, 10, 15, 19.
Find the schedule for a 2-CPU system to execute all of the 8 processes in the least
amount of time if preemption is allowed.
4. You are to redesign the Unix V6 file system so that “small” files can be larger than
4Kbytes. Disk block addresses are 4 Bytes long (instead of 2 Bytes), disk block size is
4Kbytes (instead of 512 Bytes). Components nlinks, uid and gid are 2 Bytes long (instead
of 1 Byte) and size is stored in 6 Bytes (instead of 3 Bytes). Components actime and
modtime remain unchanged. Each I-node is to be stored entirely within one disk block.
Thus, an I-node cannot span (be split across) two disk blocks. For this situation, find
(a) Find the smallest size of I-nodes.
(b) Find the maximum number of elements of addr array.
(c) Compute max size of “small” file.
5. The read/write (r/w) head is at track # 50 and the following sequence of requests (only
track numbers are shown) have arrived: 90, 25, 80, 15, 45, 60. Show the schedule and the
# of tracks moved for the (i) Scan algorithm (for this assume that the r/w head in moving
in the direction of decreasing track numbers) and (ii) Shortest Seek Time First algorithm.
6. NTSC TV-quality video stored in a compressed format needs 3 Mega bits per second
(=3x2^20/8 Bytes per second). The video (say a movie) is stored contiguously with 8
blocks (sectors) oer tracks and 2 Kbytes per block. Compute the minimum RPM
(revolutions per minute) of the disk drive. (Assume that the time to move the r/w head
from a track to an adjacent track is negligible.)