Dual-Mode Operation

• Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.
• Provide hardware support to differentiate between at least two modes of operation.
• User mode- execution done on behalf of a user.
• Monitor mode- execution done on behalf of operating system.
• Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1).
• When an interrupt or fault occurs hardware switches to monitor mode.
• Privileged instructions can be issued only in monitor mode.