operating systems:

Question 1: What is multiprogramming? What is timesharing?

Question 2: Which of the following instructions should be allowed only in kernel mode? (a) Set the time-of-day clock. (b) Disable all interrupts. (c) Read the time-of-day clock. (d) Change the memory map.

Question 3: Consider a computer system that has cache memory, main memory (RAM) and disk, and the operating system uses virtual memory. It takes 2 nsec to access a word from the cache, 10 nsec to access a word from the RAM, and 20 ms to access a word from the disk. If the cache hit rate is 90% and main memory hit rate (after a cache miss) is 98%, what is the average time to access a word?

Question 5: What is the biggest advantage of implementing thread in user space? What is the biggest disadvantage?

Question 6: Five batch jobs A through E, arrive at a computer center at almost the same time. They have estimated running times of 7, 6, 3, 5, and 9 minutes. Their (externally determined) priorities are 3, 2, 1, 5, and 4, respectively, with 5 being the highest priority. For each of the following scheduling algorithms, determine the mean process turn- around time. Ignore process switching overhead. (a) Round robin (with time slice of 2 minute). (b) Priority scheduling. (c) First-come, first served (run in order A, B, C, D, E). (d) Shortest job first.

Question 7: A RAID can fail if two or more of its drives crash within a short time interval. Suppose that the probability of one drive crashing in a given hour is X. What is the probability of a k-drive RAID failing in a given hour? Question 8: Disk requests come in to the disk driver for cylinders 25, 40, 7, 32, 22, 5 and 15, in that order. A seek takes 10 msec per cylinder moved. How much seek time is needed for (a) First-come, first served. (b) Shortest Seek First (SSF). (c) Elevator algorithm (initially moving upward). In all cases, the arm is initially at cylinder 17.