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Chapter 9 Virtual Memory Silberschatz - Operating System Concepts 9th…

- - - - We can gain additional ordering information by recording the reference bits at regular intervals. We can keep an 8-bit byte for each page in a table in memory. At regular intervals (say, every 100 milliseconds), a timer interrupt transfers control to the operating system. The operating system shifts the reference bit for each page into the high-order bit of its 8-bit byte, shifting the other bits right by 1 bit and discarding the low-order bit.
    - - The basic algorithm of second-chance replacement is a FIFO replacement algorithm. When a page has been selected, however, we inspect its reference bit. If the value is 0, we proceed to replace this page; but if the reference bit is set to 1, we give the page a second chance and move on to select the next FIFO page.
    - - We can enhance the second-chance algorithm by considering the reference bit and the modify bit as an ordered pair. With these two bits, we have the following four possible classes:
        
        (0, 0) neither recently used nor modified—best page to replace
        
        (0, 1) not recently used but modified—not quite as good, because the page will need to be written out before replacement
        
        (1, 0) recently used but clean—probably will be used again soon
        
        (1, 1) recently used and modified—probably will be used again soon, and the page will be need to be written out to disk before it can be replaced
  - - - Least frequently used
      - Most frequently used
- - - - To split m frames among n processes.
    - - We allocate available memory to each process according to its size