Computer Science 330
Operating Systems

Fall 2020, Siena College

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Agenda

• Announcements
  • The next readings/activities from OS zyBook: Chapter 7, due now, but please complete them this week if you have not already
  • OS zyBook: Chapter 8, due next Monday
  • The next Term Project this week: discuss ideas and form groups through Wednesday, proposals due Friday
• Last question of the midterm (classical problems of synchronization) recap
• Programming Project 2: CPU Scheduler Simulation [HTML] [PDF] thoughts and a look at some sample solution code
• Lab 7: Unix System Programming [HTML] [PDF] discussion
• Programming Project 3: The Roger Bacon Shell [HTML] [PDF] discussion
  • make good use of Git and GitHub (Issues, maybe even branches)
• Memory Management
  • Paging
    • break up logical memory into pages, physical memory into frames
    • page size: the size of these pages/frames
    • can allow virtual memory
    • translate addresses by looking up a frame number using the page number, which is part of the logical address
    • store the translations in a page table in memory
    • a memory access now requires a page table access then the memory access - slow!
    • help: translation lookaside buffer - associative memory that stores a subset of table entries
    • TLBs are small, but locality helps it to have an excellent hit rate in practice
    • page table too large? page it - multilevel page tables
    • another option: inverted page tables
    • memory protection with paging: valid/invalid bits
    • shared pages
  • 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