UNIVERSITY OF HOUSTON Department of Computer Science COSC 6360-Operating Systems FINAL PART I December 10, 1990 Be concise but complete. The examination is closed text but you can have four pages of notes. 1. What conditions must an encryption function fulfill to be usable for digital signatures and public-key cryptosystems? (10 points) THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE READING LIST. IT WILL BE NOT BE ASKED AGAIN. 2. A time-sharing system has 20 terminals. The average think time is 8 seconds. The average disk access time is 25 ms. Each user job requires on the average 750 ms of CPU time and 24 disk accesses. The CPU utilization are respectively 75% and 60%. (10 points) (a) What is the disk utilization? (b) What is the system's throughput? obs/s (c) What is the system's response time? (d) What is the absolute maximum for the system's throughput? obs/s (e) What is the absolute minimum for the system's response time? THIS QUESTION WAS RELATED TO A PAPER THAT IS NOT ANYMORE IN THE READING LIST. IT WILL BE NOT BE ASKED AGAIN. 3. Sketch the solution proposed by Tanenbaum et al. to build object capabilities that can be easily downgraded without going back to the server. (10 points) 4. What is the difference between optimistic and pessimistic replication strategies? (10 points) 5. What are the advantages and disadvantages of Sprite's delayed-write policy? 6. What are the advantages and disadvantages of Andrew's call- back policy? Department of Computer Science COSC 6360-Operating Systems FINAL PART II OS QUALIFYING EXAMINATION PART A December 10, 1990 Be concise but complete. The examination is closed text but you can have four pages of notes. 1. Are local disks back to stay? While the workstations of the previous generations were gen- erally diskless, today's workstations often come packaged with a local hard disk. (a) What technical reasons can we give for that change? ( 8 points) (b) What technical advances are needed before the balance swings again in favor of diskless workstations? (6 points) (c) What is the likelyhood of having these advances achieved and implemented in the near future? (6 points) 2. File System Security: Sketch a protection mechanism for a network file system that pro- vides the same amount of protection as the current UNIX file sys- tem but would operate correctly in environments where client sites cannot be trusted and messages can be easily intercepted. (You don't have the right to assume the existence of any special hardware, including Amoeba's F-boxes.) (20 points)