8.) Hardware Protection

Dual-Mode Operation
Sharing system resources requires operating system to ensurethat an incorrect program cannot cause other programs toexecute incorrectly.

Provide hardware support to differentiate between at least twomodes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also supervisor mode or system mode) –execution done on behalf of operating system.

Mode bit added to computer hardware to indicate the currentmode: 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.

I/O Protection

All I/O instructions are privileged instructions. Must ensure that a user program could never gain control ofthe computer in monitor mode (i.e., a user program that, aspart of its execution, stores a new address in the interruptvector).

Memory Protection

Must provide memory protection at least for the interrupt vectorand the interrupt service routines. In order to have memory protection, add two register determine the range of legal addresses a program may access:

– base register – holds the smallest legal physicaladdress.

– limit register – contains the size of the range.

Memory outside the defined range is protected.

CPU Protection

Timer – interrupts computer after specified period to ensure operating system maintains control:

• Timer is decremented every clock tick
• When timer reaches the value 0, an interrupt occurs
  
  
  Timer commonly used to implement time sharing.

Time also used to compute the current time.

Load-timer is a privileged instruction.