• Course: CS110
• Name: SooYoung Jung
• Homework Assignment #3
• Due Date: 10/1/00
• Date: 10/1/00
• Formatter: HTML

 

1. Point 3, S&G, 2.5, pg46
   Which of the following instructions should be privileged?
     
1. Set value of timer.
2. Read the clock.
3. Clear memory.
4. Turn off interrupts.
5. Switch from user to monitor mode.   
    
• d. Turn off interrupts.  Since privileged instructions to be executed in only monitor mode, turning off interrupts can be only done by operating system.  Other instructions (a, b, c, e) could be seen they are in user modes.

2. Point 3, S&G, 2.7, pg46
   Some early computers protected the operating system by placing it in a memory partition that could not be modified by either the user job or the operating system itself. Describe two difficulties that you think could arise with such a scheme.
   
   
• Since either of one is not accessible the memory partition, operating system should keep change user and monitor mode. This occasional alternation of instruction mode will not be easy to keep protection information. Also, attempt by a program executing in user mode to access monitor memory or other usersĄŻ memory will treat as a fatal error so this will loose some necessary execution or will be difficult to store information and execute them later.

3. 2-1 from Paul with the following changes:
   write only a version with all appropriate nops

4. 2-3 from Paul with the following changes:
   write only a version with all appropriate nops