• Real or Native mode: the original 8086 mode restricted to 1 MB of memory and only one program at a time (except for special memory-resident programs which run in the background, called TSR [terminate and stay resident] programs)

• Protected mode: introduced with 80286 processor; supports multitasking (running multiple programs) each program's assigned memory is protected from other programs (hence protected mode); supports extended memory; 80386 and later processors can switch on the fly between real and protected modes

• Virtual real mode: introduced with 80386 processors, it runs real mode from within protected mode to allow DOS programs to run under MS Windows - i.e. open a DOS window