title: Why should "accessing I/O device" be a privileged instruction?
body: <p>While studying <code>Operating System Concepts, 9th Edition,</code> </p>
  
  <p>I got stuck in Exercise #1.6. </p>
  
  <p>1.6 Which of the following instructions should be privileged?</p>
  
  <p>a. Set value of timer.</p>
  
  <p>b. Read the clock.</p>
  
  <p>c. Clear memory.</p>
  
  <p>d. Issue a trap instruction.</p>
  
  <p>e. Turn off interrupts.</p>
  
  <p>f. Modify entries in device-status table.</p>
  
  <p>g. Switch from user to kernel mode.</p>
  
  <p>h. Access I/O device.</p>
  
  <p><strong>Answer:</strong></p>
  
  <p>The following operations need to be privileged: Set value of timer, clear
  memory, turn off interrupts,modify entries in device-status table, access
  I/O device. The rest can be performed in user mode.</p>
  
  <p>Why should "accessing I/O device" be a privileged instruction?</p>