Computer Science 330
Operating Systems

Fall 2025, Siena University

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Agenda

• Announcements
  • The next readings/activities from OS zyBook: Chapter 9, Sections 2-4, due Friday
  • Programming Project 1: CPU Scheduler Simulation [HTML] [PDF] wrap up this week
  • Finish up Lab 3: Introduction to pthreads [HTML] [PDF] this week
  • Exam next Monday!
• For fun: drawing the United States entirely from memory
• Concurrency
  • the producer-consumer problem
  • critical sections
    • requirements: mutual exclusion, prevent lockout, prevent starvation
    • 2-process algorithms
    • n-process algorithms
  • hardware support for process synchronization
    • atomic test-and-set
    • bounded wait using atomic test-and-set
    • atomic swap instruction
• In-class assignment, email your responses, for 10 lab points. Submit as plain text in Canvas.

  We looked at Peterson's Algorithm in class as a software solution to the critical section problem for two processes. Another solution is due to Dekker:

  • Shared data

    int turn=0;
    boolean flag[2];
    flag[0]=false;
    flag[1]=false;
    

  • Process P_i

    while (1) {
      flag[i]=true;
      while (flag[j]) {
        if (turn != i) {
          flag[i]=false;
          while (turn != i);
          flag[i]=true;
        }
      }
    
        /* critical section */
    
      turn=j;
      flag[i]=false;
    
        /* non-critical section */
    
    }
    

  Question 1: State the conditions that must be guaranteed for an approach such as this to be considered a solution to the critical section problem (3 points)
  Question 2: Explain, as precisely as you can, how each of the these conditions is satisfied by Dekker's Algorithm. (5 points)

• Looking at the exercises for OS zyBook Chapter 9, Section 1

Terminology

• concurrency
• critical section
• mutual exclusion
• lockout
• starvation
• deadlock

Examples

• https://github.com/SienaCSISOperatingSystems/prodcons-examples