CSCI 4210 Operating Systems CSCI 6140 Computer Operating Systems Sample Final Exam Questions (document version 1.0) WITH SELECTED SOLUTIONS

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1 CSCI 4210 Operating Systems CSCI 6140 Computer Operating Systems Sample Final Exam Questions (document version 1.0) WITH SELECTED SOLUTIONS Overview Our final exam will be on Thursday, May 10, 2018 from 11:30AM-2:30PM in West Hall Auditorium (please arrive early and sit toward the front with empty seats next to you on both sides). The final exam will be 180 minutes; therefore, if you have 50% additional time, you will have 270 minutes; if you have 100% additional time, you will have 360 minutes. The final exam will count as 20% of your final course grade. The final exam will be comprehensive. Therefore, please refer to the sample exam documents for Exam 1 and Exam 2. You may bring two double-sided (or four single-sided) 8.5 x11 crib sheets containing anything you would like; crib sheets will not be collected. Final exam and final course grades will be made available via Rainbow Grades on Submitty. Final exams will not be handed back or available per review; however, exams will be graded multiple times to ensure correctness and consistency of grading.

2 Sample Final Exam Questions 1. Contiguous Memory Allocation: Consider a contiguous memory allocation scheme with dynamic partitioning for a 64MB physical memory with five pre-allocated processes (i.e., A, B, C, D, and E) that just happen to have memory requirements that are evenly divisible by 1MB. Given new processes F, G, and H) that arrive (almost) simultaneously in the order shown below, show how memory allocation occurs for each of the given placement algorithms. Arrival Order ProcessID Memory Requirements ======================================================= 1 F 2,987,642 bytes 2 G 4,002,016 bytes 3 H 6,202,454 bytes ======================================================= Note that if a process cannot be placed, be sure to state that, then move on to the next process. Do not perform defragmentation. For the memory allocation shown below, apply the First-Fit algorithm: AAAAAAAAA...B BBBBBB...CCCCC CCCC...DDDDDDDDD DDDD...EEE (each character here represents 1MB) For the memory allocation shown below, apply the Best-Fit algorithm: AAAABBBBBBBBBBBB BBBBBBBB...CCCC CCCCC...DDDDDDDD DDDDDD...EEE For the memory allocation shown below, apply the Next-Fit algorithm, with process D being the last-placed process: AAAAAAAAAA...B BBBBBBBB...CCC CCCCCC...DDDDDDD DDDD...EEEE 2. For each of the above algorithms, how much space is unused after the processes are allocated to memory? 3. For each of the above, what kind of fragmentation occurs (i.e., internal or external)? 2

3 SOLUTIONS TO 1, 2, AND 3: First, please note that 1MB is 2 20 or 1,048,576 bytes. After the First-Fit algorithm is applied: AAAAAAAAAFFF...B (F uses 2,987,642 bytes of 6MB free) BBBBBBGGGG.CCCCC (G uses 4,002,016 bytes of 5MB free) CCCC...DDDDDDDDD DDDDHHHHHH...EEE (H uses 6,202,454 bytes of 9MB free) There is therefore external fragmentation, calculated as follows: ( 6MB - 2,987,642 ) + ( 5MB - 4,002,016 ) + 3MB + ( 9MB - 6,202,454 ) After the Best-Fit algorithm is applied: AAAABBBBBBBBBBBB BBBBBBBBGGGGCCCC (G uses 4,002,016 bytes of 4MB free) CCCCCFFFDDDDDDDD (F uses 2,987,642 bytes of 3MB free) DDDDDDHHHHHH.EEE (H uses 6,202,454 bytes of 7MB free) There is therefore external fragmentation, calculated as follows: ( 4MB - 4,002,016 ) + ( 3MB - 2,987,642 ) + ( 7MB - 6,202,454 ) After the Next-Fit algorithm is applied: AAAAAAAAAA...B BBBBBBBB...CCC CCCCCC...DDDDDDD DDDDFFFGGGG.EEEE (F uses 2,987,642 bytes and G uses 4,002,016 bytes of 8MB free) (H cannot be placed) There is therefore external fragmentation, calculated as follows: ( 8MB - 2,987,642-4,002,016 ) + 5MB + 5MB + 3MB 3

4 4. Non-contiguous Memory Allocation: Consider a non-contiguous memory allocation scheme in which a logical memory address is represented using 32 bits. Of these bits, the high-order 12 bits represent the page number; the remaining bits represent the page offset. What is the total logical memory space (i.e., how many bytes are addressed)? SOLUTION: 2 32 How many pages are there? SOLUTION: 2 12 What is the page size? SOLUTION: 2 20 What is the frame size? SOLUTION: 2 20 (same as page size) How does logical memory address 23,942,519 (binary ) map to physical memory (i.e., what is the logical page number and page offset)? SOLUTION: page number is (i.e., ); page offset is If a process requires 78,901,234 bytes of memory, how many pages will it require? SOLUTION: ans = ceil( 78,901,234 / 2 20 ) How many bytes are unused due to external fragmentation? SOLUTION: 0 (there is no external fragmentation, because the entirety of physical memory is partitioned into equally sized frames) How many bytes are unused due to internal fragmentation? SOLUTION: ( 2 20 ans ) - 78,901,234 Given that the page table is stored entirely in memory and a memory reference takes 100 nanoseconds, how long does a paged memory reference take? SOLUTION: 200 ns Adding a translation look-aside buffer (TLB) with a TLB access time of 15 nanoseconds, how long does a paged memory reference take if a TLB hit occurs? SOLUTION: 115 ns Given a TLB hit ratio of 84%, what is the effective memory access time (EMAT)? SOLUTION: ns ns 4

5 5. Virtual Given a page reference string and a 3-frame memory, how many page faults occur for the following page replacement algorithms: FIFO; OPT; LRU; LFU. Repeat the above for a 4-frame memory. Page Reference Stream: Given a page reference string and a working set delta, identify the working set at the point indicated below. Page Reference Stream: ^ Use a delta of 3, 5, 8, then 10. SOLUTION: working set with delta of 3 is {1, 3, 4, 7}; working set with delta of 5 is {1, 3, 4, 7, 8}; and working set with delta of 8 is also {1, 3, 4, 7, 8} working set with delta of 10 is also {1, 3, 4, 7, 8, 9} 5

6 7. Filesystems: In a Linux filesystem with a block size of Q = 4096 bytes and inodes with 15 direct block pointers, what is the maximum file size? Assume that an indirect inode block has a maximum of 1024 pointers. Further, how many inodes are required for a file of size 33,333 bytes? How about for a file of size 307,200,000 bytes? And for a file of size 307,200,000,000 bytes? 6

7 8. Input/Output: Why is buffering important in an operating system? 9. What is a disk context switch and how does it differ from a CPU context switch? 10. Given the disk access reference string below and assuming the disk arm is initially at track 20, how many disk arm movements are required to reach all of the tracks using the FCFS algorithm? Disk Access Reference String: 44, 20, 95, 4, 50, 52, 47, 61, 87, Repeat question 10 using the SSTF, SCAN, C-SCAN, LOOK, and C-LOOK algorithms. 7

CSCI 4210 Operating Systems CSCI 6140 Computer Operating Systems Sample Final Exam Questions (document version 1.1) WITH SELECTED SOLUTIONS

CSCI 4210 Operating Systems CSCI 6140 Computer Operating Systems Sample Final Exam Questions (document version 1.1) WITH SELECTED SOLUTIONS Overview The final exam will be on Tuesday, May 17, 2016 from