Question

This is a question for my undergraduate Operating Systems class. Thanks in advance!

6. Instructions related to accessing IO devices are typically privileged instructions, that is, they can be executed in kernel mode but not in user mode. Give a reason why these instructions are privileged.

Answer 1

   We already know that the OS(Operating System) has two modes of operation in it. In which one is user mode and the other is kernel mode. Privileged instruction means that it can harm any operating system or system when it is executed by some malicious programs or processes. Privileged instructions are executed in the Kernel mode only. I/O instruction comes to the side of the Kernel mode and supports truly to the Operating System. All these instructions are called only by using the System Calls. Whenever at the time the system call is executed then at first it sets the Kernel mode bit to "0" and then it will execute the system call. After the completion of the execution before exiting from the I/O system call then it sets the user mode bit to "1" and then it transfers the whole control back to the user. It does not mean that to give the whole powers and control to the user process.

   In my opinion, if the I/O instructions have become the duty to take care of them by the user, then the users may misuse them which includes deleting the data in the disk etc. Then the I/O instructions will be privileged so that then the Operating System will check whether the user's authentications is correct or not. It will verify the user's authentication to perform any I/O operation. If you are not performing the operation it will check whether you have performed the I/O operation before or not.