HARDWARE PROTECTION

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 operations.

1. Users mode - execution done on behalf of a user.
2. 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 instruction can be issued only in monitor mode.

I/O Protection

• All I/O instruction are priviliged instruction.
• Must ensure that a user program could never gain control of the computer in monitor mode (i.e., a user program that, as part of its execution, stores a new address in the interrupt vector).

Memory Protection

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

– base register – holds the smallest legal physical memoryaddress.

– limit register – contains the size of the range.

• Memory outside the defined range is protected.

CPU Protection

• to prevent a user programs gets stuck in infinite loop and never returning back to the os