Processor modes

Contemporary processors incorporate a mode bit to define the execution capability of a program in the processor. This bit can be set to kernel mode or user mode. Kernel mode is also commonly referred to as supervisor mode, monitor mode or ring 0. In kernel mode, the processor can execute every instruction in its hardware repertoire, whereas in user mode, it can only execute a subset of the instructions. Instructions that can be executed only in kernel mode are called kernel, privileged or protected instructions to distinguish them from the user mode instructions. For example, I/O instructions are privileged. So, if an application program executes in user mode, it cannot perform its own I/O. Instead, it must request the OS to perform I/O on its behalf. The system may logically extend the mode bit to define areas of memory to be used when the processor is in kernel mode versus user mode. If the mode bit is set to kernel mode, the process executing in the processor can access either the kernel or user partition of the memory. However, if user mode is set, the process can reference only the user memory space. We frequently refer to two classes of memory user space and system space (or kernel, supervisor or protected space). In general, the mode bit extends the operating system's protection rights. The mode bit is set by the user mode trap instruction, also called a supervisor call instruction. This instruction sets the mode bit, and branches to a fixed location in the system space. Since only system code is loaded in the system space, only system code can be invoked via a trap. When the OS has completed the supervisor call, it resets the mode bit to user mode prior to the return.