CMPS 111 Spring 2003 Midterm Exam May 8, Name: ID:

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1 CMPS 111 Spring 2003 Midterm Exam May 8, 2003 Name: ID: This is a closed note, closed book exam. There are 20 multiple choice questions and 5 short answer questions. Plan your time accordingly. Part I: Multiple Choice [2.5 points each]. Write the letter of your answer to the left of each question. If none of the answers are correct, write None. If more than one is correct, write the letter of all correct answers. 1. Multiprogramming allows: a. Multiple processes to execute at exactly the same time b. The OS to use the hardware more efficiently c. Processes to run faster 2. System calls: a. Protect important kernel data structures from user code b. Increase the performance of the operating system c. Provide a rich and flexible API for software developers to use 3. Which are all system calls: a. waitpid(), undo(), mount(), open() b. fork(), exit(), lseek(), kill() c. chmod(), chdir(), read(), main() 4. In the process state transition diagram, the transition from the Running state to the Waiting state indicates that the running process a. Has blocked on an I/O operation b. Is done waiting for an I/O operation c. Has been preempted by another process 5. In a user-level thread package, threads are scheduled by: a. The user b. The process c. The kernel 6. Which are all goals of various CPU scheduling algorithms: a. Turnaround time, No Starvation, and First-Come-First-Serve b. Responsiveness, Utilization, and Timeliness c. Fairness, Throughput, and I/O Bandwidth 1

2 7. Starvation is when: a. A set of processes is waiting for an event that only another process in the set can cause b. A processes could run but doesn't get to because other processes keep getting in ahead of it c. A set of processes are prevented from simultaneously accessing a shared data structure 8. The main difference between Semaphores and Monitors and Condition Variables is: a. That Semaphores can support event notification b. That Semaphores can sometimes be used in situations where Monitors and Condition Variables cannot c. That Semaphores are more powerful, doing things that Monitors and Condition variables cannot 9. Which is true: a. With appropriate parameter settings, round-robin can emulate any other algorithm b. Shortest job first has nice properties, but is hard to implement c. First-come-first-serve maximizes average turnaround time 10. Preemption is when: a. A process terminates because of an error b. A process blocks on a resource c. A process blocks so another process can run 11. The four conditions that must hold in order for deadlock to occur are: a. Mutual exclusion, hold-and-wait, circular wait, and no preemption b. No preemption, no starvation, circular wait, and mutual exclusion c. Hold-and-wait, circular wait, no starvation and mutual exclusion 12. One difference between swapping and paging is: a. Who is doing the memory management, the application or the OS b. The degree of multiprogramming each can support c. The type of fragmentation that can occur 13. Which is false: a. The memory usage of bitmapped free memory management depends on the amount of memory in the system b. Linked list free memory management requires extra hardware c. Linked list free memory management incurs more overhead when memory is freed 14. The main advantage of multilevel page tables is that they: a. Use less memory b. Speed up page table references c. Reduce the complexity of the paging system 2

3 15. TLBs reduce the amount of time it takes to: a. read data from disk b. perform an address translation c. load a page table into memory 16. Optimal page replacement: a. Is usually the best choice in practice b. Is a good idea, but you can never really know what is optimal c. Is useful as a basis for comparing other algorithms 17. Which suffers most from internal fragmentation a. Large pages b. Medium page c. Small pages 18. Suppose you have a memory access stream that references pages 0, 1, and 2 over and over again, in that order, and you have only 2 pages of physical memory for the process, which page replacement algorithm will generate the most page faults a. Least Recently Used (LRU) b. Most Recently Used (MRU) c. Random 19. A shell: a. Is a useful program b. Is an important part of the kernel c. Is a useful system call 20. Lottery scheduling: a. Is the default for DLXOS b. Could starve an unlucky process for a while c. Behaves exactly like round robin, but executes the processes in a random order 3

4 Part II: Short answers [10 points each] 1. Explain system calls in a way that would be understandable to someone who has not yet taken CMPS 111. What are they? Why do we need them? What do they do? How do they work? Etc. System calls provide an abstract interface to hardware and the OS functionality. The protect important kernel data structures and prevent processes from interfering with each others operation, code, or data. System calls appear to be normal function calls in a program, but actually call a special instruction called trap, that switches the processor from user mode to kernel mode. User mode disables certain operations that user processes are not allowed to execute, while kernel mode enables all operations. In kernel mode, the system figures out which request has been made by the user process that called the system call, decides whether or not the operation will be allowed, and either returns an error code or performs the operation and returns the result. Examples of system calls include read(), write(), fork(), exec(), and many others. 4

5 2. Virtual memory a) [7 points] Many modern systems use virtual memory. Why? Virtual memory has several advantages. It allows processes to be written as though they use all of the memory in the system. It also allows them to be written without knowledge about the actual amount of memory in the system. Finally, it allows processes to use less actual memory by only storing in memory the data that they are currently using. Also, the hardware to implement virtual memory is fairly simple and well-understood, so the overhead of implementing it is not too great. b) [3 points] In what situations might a system designer decide not to use virtual memory? There are many reasons why a system designer might decide not to use virtual memory. First, it simply might not be needed in a very simple system with a lot of memory. Also, the overhead of iimplementing it (in hardware and software) might be considered too great. And in some real-time systems, the relatively unbounded delays that virtual memory misses cause may be considered too high. Other reasons also exist. 5

6 3. Given four page frames and the memory reference string and given three physical pages, show which references will cause page faults and which pages will be in each page frame after each reference. a) LRU * * * * * * * * * * * P P P P b) FIFO * * * * * * * * * * * * * P P P P c) Second Chance * * * * * * * * * * * P x x x 0x P x 5x 5x 5x 5 5 2x 2x x x P x 4x 4x x 2x 2x 2x 2x P x x 5x 5x 5x 2 2x 2x

7 4. We have discussed several situations in which hardware features have been developed to directly support operating systems needs. Give two examples of this and describe how they are used. Examples: - MMU for virtual memory support - Trap instruction to allow for protection of kernel operations and data - CPU processing modes: user and kernel to protect kernel and processes - DMA: direct memory access to allow for parall device and CPU operation - Interrupts to allow for asynchronous I/O

8 5. Given the following processes: Process Arrival Time Burst Time Priority (lower number=higher priority) a) Show the schedule that would occur with non-preemptive Shortest Job First scheduling (Process, Start, End): (0, 0, 10), (3, 10, 13), (1, 13, 17), (2, 17, 22) b) Show the schedule that would occur with Round Robin (time quantum = 2) (0, 0, 2), (0, 2, 4), (1, 4, 6), (2, 6, 8), (0, 8, 10), (1, 10, 12), (3, 12, 14), (2, 14, 16), (0, 16, 18), (3, 18, 19), (2, 19, 21), (0, 21, 22) c) Show the schedule that would occur with Preemptive Priority (0, 0, 3), (1, 1, 4), (2, 4, 7), (3, 7, 10), (2, 10, 12), (1, 12, 15), (0, 15, 22) 8