title: calculate the effective access time
body: <p>This is a paragraph from <a href="https://rads.stackoverflow.com/amzn/click/com/1118063333" rel="noreferrer" rel="nofollow noreferrer">Operating System Concepts, 9th edition</a> by Silberschatz et al:</p>
  
  <blockquote>
    <p>The percentage of times that the page number of interest is found in
    the TLB is called the hit ratio. An 80-percent hit ratio, for example,
    means that we find the desired page number in the TLB 80 percent of
    the time. If it takes 100 nanoseconds to access memory, then a
    mapped-memory access takes 100 nanoseconds when the page number is in
    the TLB. If we fail to find the page number in the TLB then we must
    first access memory for the page table and frame number (100
    nanoseconds) and then access the desired byte in memory (100
    nanoseconds), for a total of 200 nanoseconds. (We are assuming that a
    page-table lookup takes only one memory access, but it can take more,
    as we shall see.) To find the effective memory-access time, we weight
    the case by its probability: effective access time = 0.80 × 100 + 0.20
    × 200
    = 120 nanoseconds</p>
  </blockquote>
  
  <p>but in the 8th edition of the same book
  <img src="https://i.stack.imgur.com/b8t7f.png" alt="enter image description here"></p>
  
  <p>I'm confused with the </p>
  
  <blockquote>
    <p>effective access time</p>
  </blockquote>
  
  <p>Can someone explain it for me?</p>