User Mode and Kernel Mode

Mode Bit

• 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.