User mode vs kernel mode
• Modes of operation can be separated to user mode and kernel mode (also
called supervisor mode, system mode, or privileged mode)
• A bit, called the mode bit, is added to the hardware to indicate the current
mode: kernel (0) or user (1)
• We accomplish system this protection by designating some of the machine
instructions that may cause harm as privileged instructions
• The hardware allows privileged instructions to be executed only in kernel mode