Protected mode:
Protection against execution of inadmissible functions is hardware implemented inside processors (CPU), and it is active while the CPU operates in protected mode. Besides the protection itself, protected mode gives several important advantages, and this is why it has become the main operating mode for modern CPUs. For application programs and for the user the protected mode resembles a virtual shell, disabling all actions, which may inflict any harm to vital functions of operating system. This is the main factor of high reliability, inherent to modern operating systems
Real Mode:
Real mode is a "defenseless" mode.This mode is required by BIOS: it must have free access to computer hardware for performing the Post test.Just because of this mission all processors are automatically set into real mode each time thepower supply is switched on. Operating systems too can't obtain control over computer's hardware unless protection is disabled, and therefore must start while CPU runs in a "defenseless" real mode.
Having got total control over computer, operating systems of the Windows clone prepare protection data structure so that after switching to protected mode the highest privilege level is granted to Windows OS itself, whereas both the user and applications are given the third (the lowest) privilege level. Since then and for ever the user wouldn't be allowed to change this allocation of rights. Because of the same reason all opportunities beyond the limits of Windows' API become inaccessible for the user and for his application programs in protected mode
