3) Which of the following instructions should be privileged? 1) Set value of timer. 2) Read the clock. 3) Clear memory. 4) Issue a trap instruction. 5) Turn off interrupts. 6) Modify entries in device-status table. 7) Switch from user to kernel mode. 8) Access I/O device. 4) What is the purpose of system calls? 5) What is the main advantage of the layered approach to system design? What are the disadvantages of the layered approach