OS & Hardware

Which combination of information does a page-table entry usually contain?

  1. APhysical page number plus valid, protection, accessed and dirty bits
  2. BThe page's actual data contents and the time it was last written to
  3. CThe name of the process using the page and its total usage time
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Answer: A. Physical page number plus valid, protection, accessed and dirty bits

A page-table entry holds the physical page number plus a valid (present) bit showing whether the page is in memory, permissions for read, write, execute and user-mode access, and accessed (reference) and dirty bits set by hardware. The OS uses the reference bit for page-replacement decisions and the dirty bit to decide whether a page must be written back when evicted. Data contents and process names are not stored there.

  • A. Correct. It holds the physical page number and protection and status bits.
  • B. Data lives in the page itself, not in the table.
  • C. Process names and usage times are kept elsewhere.
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