Modes of execution
Two modes : user mode and a privileged mode called the kernel mode.
Why? It is necessary to protect the OS and key OS tables such as PCBs from interference by user programs.
In the kernel mode, the software has complete control of the processor and all its hardware.
When a user makes a system call or when an interrupt transfers control to a system routine, an instruction to change mode is executed. This mode change will result in an error unless permitted by OS.