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fa11final-sample-solutions

fa11final-sample-solutions - Final Exam Sample Questions...

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Final Exam Sample Questions Solutions Page 1/7 SAMPLE PROBLEMS 1. Consider a FAT-based (File Allocation Table) file system. Entries in the table are 16 bits wide. A user wants to install a disk with 131072 512-byte sectors. a. What is a potential problem? Each 16 byte entry in the table is an address of a sector on the disk. The OS can address 65,536 sectors, and the disk has more than that. b. Describe a solution to this problem and explain the trade-offs involved. Make each FAT entry access a logical sector that is 2 physical sectors. This trades increased internal fragmentation against maintaining the size of the FAT, and backward compatibility. Alternatively, you can increase the size of the FAT to be 131,072 17-bit entries, which increases the table size to 278,528 bytes (at least) and increases the complexity of the decoding process. It also ruins any backward compatibility. 2. Generally we’ve talked about each operating system component in isolation. This question asks you to think about ways in which they interoperate. For each pair of systems below, give a specific way that they interact (or that they could interact). Writing that the file system and I/O system interact because they both use the disk is not worth more than a point, and may be worth none. Writing that the file system and I/O system interact when they determine the mapping from logical blocks physical blocks which impacts the size of file system structures, and the efficiency of the disk usage because larger logical blocks imply more internal fragmentation on the disk is a more complete answer. a. How does a demand paged, lazy loaded virtual memory system interact with the process scheduling and creation system? A demand paging system will be at its worst at process creation, with an empty address space. The two systems have to work together to pre-fetch a reasonable working set without overloading the paging system by bringing in the entire address space. If the VM system supports shared text (or copy on write), the two need to work together to take advantage of it. b. Name another way (not the example above) that the file system and the hard disk drivers in the I/O system interact. Feature coordination. If the disk implements interleaving, the file system implementation should not do its own interleaving. The result can be to make both systems worse that useless - actually detrimental. Similar negotiations have to be done for features like disk caching or bad block renaming.
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Final Exam Sample Questions Solutions Page 2/7 3. On some computer, the clock interrupt handler needs 2 msec (including context switch overhead) per clock tick to execute, and the clock runs at 75 Hz. What fraction of the CPU time is devoted to the clock? The clock interrupt handler is run 75 times every second, which takes 75*2 = 150ms. This is 15% of the CPU time.
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