Dual Mode Operation
To ensure the proper operation, we should protect and guard the operating system and all other programs and their data from any malfunctioning of the program. Protection is required for any shared resources. Instruction set of a modern CPU has majorly two kinds of instructions which are privileged instructions and non-privileged instructions. Privileged instructions can be made in use to perform hardware operations which a normal user process should not be capable of performing, such as communicating with the I/O devices. If the user process tries to execute the privileged instruction, a trap should be obtained and process should be terminated or stopped prematurely. At the same time, a piece of the operating system code must be permitted to execute the privileged instructions. In order for the CPU to be able to differentiate between the user process and an operating system code, we require two separate modes of the operation: user mode and monitor mode (also known as supervisor mode, system mode, or the privileged mode). A bit, known as the mode bit, is added to hardware of the computer to indicate the present mode: monitor mode (0) or user mode (1). With the mode bit we are efficient to distinguish between the task which is executed on the behalf of the operating system and one which is executed on the behalf of user.
The concept of the privileged instructions also gives us the means for the user to interact with the operating system by telling it to perform some designated tasks which only the operating system should perform. A user process can make a request to the operating system to perform such tasks for it by executing the system call. Whenever a system call is made or trap, an interrupt, or signal is generated, CPU mode is switched to the system mode before the relevant kernel code begins to execute. The CPU mode is switched back to the user mode before control is transferred back to user process. This is illustrated by the diagram drawn below
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