CS3MH3/6MH3 Final 2005 Page 1 COMP SCI 3MH3/6MH3. Day Class Duration of examination: two hours McMaster University Final Examination April 2005

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1 CS3MH3/6MH3 Final 2005 Page 1 Name Student Number Instructor: S. Qiao COMP SCI 3MH3/6MH3 Day Class Duration of examination: two hours McMaster University Final Examination April 2005 This examination paper includes 5 pages and 16 questions. You are responsible for ensuring that your copy of the paper is complete. Bring any discrepancy to the attention of your invigilator. SPECIAL INSTRUCTIONS: This paper must be returned with your answers. Use of McMaster standard (Casio-FX991) calculator only is allowed. 1. (2 marks) Which of the following is true? You may answer none if you think all the choices are A thread changes its state from running to ready when it calls (a) semaphore->p() (b) Sleep() (c) Yield() 2. (2 marks) Which of the following statements is true. You may answer none if you think all the choices are A stack is allocated when (a) the program is compiled (b) the program is linked (c) a process is created for the program 3. (2 marks) Which of the following statements is true. You may answer none if you think all the choices are The dirty bit in a page map table entry is set whenever the page (a) is referenced (b) is written (c) is corrupted Answer: (b)

2 CS3MH3/6MH3 Final 2005 Page 2 4. (2 marks) Which of the following is false? You may answer none if you think all the choices are true. The inverted page table is necessary for (a) virtual memory (b) replacing pages (c) mapping virtual page numbers into physical page numbers 5. A system uses paged segmentation. The virtual address is 32 bits divided into segment number (4 bits) page number (16 bits) offset (12 bits) The physical address is 28 bits. The segment table entry has the structure: page table address (28 bits) page table size (number of pages) protection (R, RW, 0) The page tables are in physical memory. The page table entry structure is: Suppose the segment table: At location 100: 010E, 0114, 0115,... At location 2000: 021A, 0203, 0221,... All numbers in hexadecimal. physical page number (16 bits) R RW (i) (2 marks) What is the page size in this system? Answer: 2 12 = 4K bytes (ii) (6 marks) Translate the following two virtual addresses into physical addresses. Indicate violation(s) if there is any write Answer: segment 0, read only violation read Answer: segment 2, virtual page 1, physical page 203, address

3 CS3MH3/6MH3 Final 2005 Page 3 6. (2 marks) The system call SpaceId Exec(char *name) establishes a process given an executable file name pointed to by name and returns SpaceId, which is (a) an index to the process table (b) the pointer to the process (thread) structure (c) the pointer to the address space structure You may answer none if you think all the choices are 7. (5 marks) The following is an implementation of the exception handler for the system call open(filename). Assuming the functions FindMyPid(), GetEntry(spaceId), and AddFile(ofp) are implemented correctly, fill in the missing line(s). void syscallopen() { OpenFileId fid = -1; OpenFile* fp; } // get virtual address of file name char *name = kernel->machine->readregister(4); // call file system to open fp = kernel->filesystem->open(name); if (fp!= NULL) { SpaceId myid = kernel->processtable->findmypid(); // find my space id ProcessEntry *myentry = kernel->processtable->getentry(myid); // get my PCB fid = myentry->addfile(fp); // add fp to my opne file table } kernel->machine->writeregister(2, fid); Before calling file system to open, call machine readmemory to translate virtual addresses into physical addresses and copy the file name from user space to kernel space, one byte at a time. 8. (2 marks) In a demand paging system, what is the maximum number of TLB misses that can be caused by one single MIPS instruction? Assume that the TLB is one-way associative? (a) one (b) two (c) infinite You may answer none if you think all the choices are

4 CS3MH3/6MH3 Final 2005 Page 4 9. (2 marks) Which of the following is false? You may answer none if you think all the choices are true. A process running a user program changes its mode from user to system when it calls (a) printf() (b) exit(0) (c) open(filename) Answer: none 10. (2 marks) Which of the following exception handlers does not increment the program counter? You may answer none if you think they all increment the program counter. (a) TLBmiss(badVirtualAddress) (b) open(filename) (c) join() 11. (2 marks) Hold-and-wait is one of four necessary conditions for deadlocks. Which of the following statements is false? (a) Hold-and-wait cannot happen in a uniprocessing system. (b) Hold-and-wait cannot happen if resources can be shared. (c) Hold-and-wait cannot happen in a system using dynamic resource allocation. If you think all the choices are true, you may answer none. 12. A CPU scheduling uses priority algorithm. Each process is given a base priority. A smaller number represents a higher priority. In t seconds, the scheduling priority is calculated by adding the adjustment adj = recent CPU usage r t to the base priority. The decaying rate r is obtained from 0.1 = r 5n, where n is the average number of ready processes in the last minute. (i) (2 marks) When a process changes from CPU bound to I/O bound, its scheduling priority gets (a) higher (b) lower (c) unchanged (ii) (2 marks) Which of the following statements is false? (a) This scheduling method favors CPU bound processes. (b) This scheduling method favors I/O bound processes. (c) This scheduling method is adaptive. next page...

5 CS3MH3/6MH3 Final 2005 Page (2 marks) Which of the following is true? You may answer none if you think all the choices are (a) NetworkInput schedules a networkinput interrupt when a packet is received (b) NetworkOutput periodically schedules networkoutput interrupts (c) There is always exactly one pending timer interrupt. 14. (2 marks) In UNIX, the logical record size of a file is (a) sector (b) byte (c) bit Answer: (b) 15. (4 marks) In a file system (Nachos), the file header fits in a disk sector (128 bytes). The file header structure is: class FileHeader { public: }; private: int numbytes; int numsectors; int datasectors[numdirect]; All data blocks are direct. What is the maximum file size? Show your calculations. Note that in C, sizeof(int) is 4. Answer: (( )/4) 128 = 3,840 bytes. 16. (2 marks) Which of the following is true? You may answer none if you think all the choices are Comparing with file structure with only direct data blocks, file structure with indirect data blocks (a) allows larger maximum file size (b) permits faster random disk access (c) saves disk space END!