Q. How does the distinction among kernel mode and user mode function as a rudimentary form of protection (security) system?

Answer: The distinction among kernel mode and user mode provides a rudimentary form of protection in the following manner. Certain instructions could be executed merely when the CPU is in kernel mode. Likewise hardware devices could be accessed only when the program is executing in kernel mode. Control over when disrupt could be enabled or disabled is also possible only when the CPU is in kernel mode. Therefore the CPU has very limited capability when executing in user mode thereby enforcing protection of critical resources.