I/O Protection
A user process might disrupt normal operation of the system by issuing the illegal I/O instructions, by accessing memory locations and addresses within the operating system itself, or by refusing to surrender CPU. We can use of several mechanisms to ensure that such disruptions should not take place in the system.
To prevent the users from performing the illegal I/O, we define all the I/O instructions to be as privileged instructions. Therefore users cannot issue the I/O instructions directly; they should do it by making use of the operating system. For I/O protection and security to be complete, we should be sure that the user program can never gain control of the computer in the monitor mode. If it could, then the I/O protection could be compromised.
Consider a computer executing in the user mode. It will switch to the monitor mode whenever an interrupt occurs, jumping to the address decided from the interrupt vector. If the user program, as part of its execution, saves a new address in the interrupt vector, this new address can overwrite the previous address with the address in the user program. Then, when a corresponding trap occurred, the hardware would switch to the monitor mode and transfer the control through the modified interrupt vector table to the user program, resulting it to gain control of the computer in monitor mode. Hence we require all I/O instructions and instructions for changing contents of the system space in the memory which is to be protected. A user process could request the privileged operation by executing a system call like read (for reading the file).
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