Computer Science 322
Operating Systems

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• Lecture assignment recap
• 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

Due at the start of class, Wednesday, April 7.

You need not submit answers to these questions, but you will have a chance to ask questions about them at the start of class.

1. SG&G Practice Exercise 8.4, p. 351
2. SG&G Exercise 8.24, p. 353

Please submit answers to these questions either as a hard copy (typeset or handwritten are OK) or by email to jteresco AT mtholyoke.edu by the start of class. We will discuss these questions at the start of class, so no late submissions are accepted.

1. SG&G Exercise 8.17, p. 352
2. SG&G Exercise 8.18, p. 353
3. SG&G Exercise 8.20, p. 353