OS 1 st Exam Name Solution St # (Q1) (19 points) True/False. Circle the appropriate choice (there are no trick questions).

Transcription

1 OS 1 st Exam Name Solution St # (Q1) (19 points) True/False. Circle the appropriate choice (there are no trick questions). (a) (b) (c) (d) (e) (f) (g) (h) (i) T_ The two primary purposes of an operating system are to manage the resources of the computer and to provide a convenient interface to the hardware for programmers. _F When the CPU is in kernel mode, the CPU is prevented from accessing areas of memory that are not owned by the current process. F_ System calls do not change to privilege mode of the processor _F Each thread of a process has its own virtual address space. T Threads are cheaper to create than processes F Change from thread state BLOCKED to thread state RUNNING _F Change from process state READY to thread state BLOCKED T Given two processes in the READY state, one that is CPU-bound and one that is I/O bound, which process should be given a higher priority for running next (all other things being equal)? Answer: I/O bound process. What does the CPU hardware do when a trap or interrupt occurs? T_ the CPU saves the program counter and other registers (e.g. the stack pointer, status register, etc.) in memory, T_ switches to kernel mode, and T_ jumps to the address contained in the interrupt table entry corresponding to the interrupt that occurred. (Q2) (3 points) What is the difference between a trap, interrupt and Exception? A trap is generated by User Process An interrupt is generated by External Hardware An exception is generated by Program Error Example of exception: Divide By Zero (Q3) (3 points) Name three ways in which the processor can transition form user mode to kernel mode? Can the user execute arbitrary code after transition? _ Trap Instruction _ External Hardware Interrupt _ Exception

2 (Q4) (3 points) What needs to be saved and restored on a context switch between two threads in the same process? PC, SP and Registers (Q5) (4 points) Suppose a thread is running in a critical section of code, meaning that it has acquired all the locks through proper arbitration. Can it get context switched? Why or why not? Yes. Locks are independent of scheduler. Only threads running in kernel mode have their interrupt disabled. (Q6) (6 points) (a) Name the four conditions required for deadlock and give a brief (one sentence) description of each. Mutual Exclusion Hold and Wait No Preemption Circular Waite (b) Does a cyclic dependency (graph cycle) always lead to deadlock? No Multiple instants of a resource will not lead to a deadlock even in the presence of a cycle. (Q7) (4 points) When a process executes a fork() system call, a duplicate process (i.e. the child process) is created. How does the code in the processes since it is identical in both the parent and child processes know which process is the parent and which is the child? Return results of calling Fork() Zero for child PID of child for parent

3 Q8. (4 points) Recall that privileged instructions can only be executed in the kernel mode (i.e., can only be executed by the OS but not by user-level processes). For each instruction below, is it a protected instruction? (No explanations needed). (A) modify the PC register (program counter) Not Protected or Not Privileged (B) modify the SP register (stack pointer) Not Protected or Not Privileged (C) modify the register that controls kernel/user mode Protected or Privileged (D) direct access to an I/O device Protected or Privileged Q9 (8 points) You wrote the following two programs a.out and b.out, Program a.out program b.out main(){ main(){ Exec( b.out ); mpid = fork(); pid = fork(); if (mpid > 0) { if (pid == 0) { printf( 3: %d, mpid); printf( 1: %d, pid); printf( 4: %d\n, getppid()); } } else { printf( 5: %d, mpid); printf( 2: %d, pid); } wait(); } } After you have executed program a.out, what is the output of each print statement in the above two programs? Note: If the printf statement is not executed then answer with NULL. Also assume that a.out was assigned PID = 12 and the parent of program a.out has PID = 5. Assume each new child created will have PID equals (parent PID)+1. 1: NULL 2: NULL 3: 13 4: 5 5: 0 0r 13

4 (Q10) (16 points)) CPU Scheduling. Here is a table of processes and their associated arrival and running times. Process Arrival Time Expected CPU Running Time P1 0 5 P2 1 5 P3 5 3 P4 6 2 (a) Show the scheduling order for these processes under First-In-First-Out (FIFO), Shortest- Job First (SJF), and Round-Robin (RR) with a quantum = 1 time unit. Assume that the context switch overhead is 0 and new processes are added to the head of the queue except for FIFO. Time FIFO SJF RR (b) For each process in each schedule above, indicate the queue wait time and turnaround time (TRT). Scheduler Process 1 Process 2 Process 3 Process 4 FIFO queue wait FIFO TRT SJF queue wait SJF TRT RR queue wait RR TRT

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