A bit, called mode bit, is added to the hardware of the computer to indicate current mode.
0: kernel mode
When a task is executed on behalf of the operating system
1: user mode
When a task is executed on behalf of the user
Switch
Whenever a trap or interrupt occurs, the hardware switches from user mode to kernel mode
i.e., change mode bit to 0
When a user application requests a service from the operating system (via a system call)
It must transition from user to kernel mode to fulfill the request
Privileged Instructions
The hardware allow privileged instructions to be executed only in kernel mode.
The instruction to switch to kernel mode is an example of a privileged instruction. Some other example include I/O control, timer management, and interrupt management.
