Simple operating systems (such as MS-DOS) always execute in a single CPU mode, but Unix-like operating systems use dual modes to effectively implement timesharing and resource allocation and protection. At any time in Linux, the CPU is either operating in a trusted kernel mode (where we can do everything we wish) or in a restricted user mode (where some operations are not allowed). All user processes execute in user mode, whereas the core kernel itself and most device drivers (except ones implemented in user space) run in kernel mode so that they have unrestricted access to the entire processor instruction set and to the full memory and I/O space.
When a user mode process needs to get access to peripherals, it cannot do it by itself, but it has to channel requests through device drivers or other kernel mode code via system calls, which play a major role...