Computer Science 330
Operating Systems

Fall 2025, Siena University

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Agenda

• Announcements
  • The next readings/activities from OS zyBook
    • Chapter 16, Sections 4-5, due Monday before class
  • Programming Project 4: Basic Shell [HTML] [PDF] due Monday night
    • FreeBSD changes to readline
  • Lab 7: Interprocess Communication [HTML] [PDF] due Wednesday night
  • Programming Project 5: Roger Bacon Shell with Pipes and Redirection [HTML] [PDF] parsing functionality due in 2 weeks
• Exam 2 information
  • In class, Monday, November 17
  • Same ground rules as Exam 1
  • Accounts for 10% of course grade
  • Topics are from Chapters 9, 12, 14 and 16 of OS zyBook, Labs 3, 4, 5, 6, Programming Projects 3 and 4
  • Practice problems out, we can spend some time on these in class next Friday, please look at them by then
• Demand paging
  • only bring in pages that are actually requested
  • when a page is referenced that is not in memory - generate a page fault
  • page fault traps to OS, bring in the page
  • allows a mechanism for virtual memory
  • if a page fault occurs and no free frame is available, we need to make one - send a frame (the victim) back to disk
  • performance will depend on the page fault rate
  • low page fault rate results in reasonably low effective access times, high page fault rates will make effective access times very high
  • locality of reference will make for a reasonably good effective access time in most circumstances
• Page replacement algorithms
  • Least Recently Used (LRU)
    • select the page that we have not used for the longest time
    • actually used in practice
    • direct implementation may involve timestamps or a stack of page references
    • may be too expensive to implement directly
  • LRU approximation algorithms
    • Second-chance or Clock Algorithm uses one reference bit and a clock pointer, treating the frames of memory as a circular array
    • Gold's Clock Algorithm/third-chance also considers a "dirty" bit to try to avoid selecing pages that need to be written back to disk because they are more expensive
  • Counting algorithms
    • Least-frequently used (LFU)
    • Most-frequently used (MFU)
    • not obvious why or when these are useful - something to think about
    • direct implementations may suffer from same overhead costs as a direct implementation of LRU
• Looking at the exercises for OS zyBook Chapter 16, Sections 1-3
  • 16.1.1, 16.1.2, 16.1.3, 16.1.4, 16.1.5
  • 16.2.1, 16.2.2, 16.2.3, 16.2.4
  • 16.3.1, 16.3.2
• Lab/Project time

Terminology

• virtual memory
• demand paging
• present bit
• page fault
• page replacement
• modified bit
• reference string
• optimal page replacement
• FIFO page replacement
• LRU page replacement
• referenced bit
• aging register
• aging page replacement
• second chance page replacement
• third chance/not recently used page replacement