About the kernel mode:

When the kernel is in kernel mode, all tasks
are stopped and all devices can be used by
the kernel regardless whether they have been
opened by any process or not.

The kernel is usually in kernel mode during
initialization and during shutdown. It may
also be in kernel mode when an error occurs
and the kernelpanic () routine is executed. 

=> there are two functions:

1. k_enter_kmode ()
2. k_leave_kmode ()

they disable (1.)/enable (2.) the following:

- the scheduler
- maskable interrupts
- device control by process (kernel controls all device in kmode)
- (what else ?)