Transition from User Mode to Kernel Mode

Operating System (OS) Operations

Operating System OS has two modes of Operation:-

1. User mode
2. Kernel mode (Supervisor mode, system mode or privileged mode)

A bit called mode bit is added to the hardware of the computer to indicate the current mode: kernel (0) or user (1). With the mode bit, we are able to distinguish between a task that is executed on behalf of the OS and one that is executed on behalf of the user.

Transition from User to Kernel Mode

When the computer system is executing on behalf of a user application, the system is in user mode. However, when a user application requests a service from the OS (via system call), it must transition from user to kernel mode to fulfill the request

At boot time, the hardware starts in kernel mode. The OS is then loaded and starts user
application in user mode.