Dual mode of operation is the distinction between execution of user mode and kernel mode (supervisor mode, system mode, and privileged mode). A mode bit is added to the hardware to indicate the current mode: Kernel(0) or user(1). Dual mode of operation provides us with more protection to the operating system. For example, if a kernel instruction is executed under user mode, the hardware does not execute it, although identifies it as an illegal execution and traps it to the operating system.
