Hardware Protection

Sharing system resources require the 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.
User mode – execution is done on behalf of a user.
Monitor mode (also kernel mode or system mode) – execution is done on behalf of the 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 Privileged instructions can be issued only in monitor mode.
monitor user ,Interrupt/fault ,set user mode

I/O Protection

All I/O instructions are privileged instructions.
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).

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

✦ Base register – holds the smallest legal physical memory address.

✦ Limit register – contains the size of the range.

When executing in monitor mode, the operating system has unrestricted access to both monitor and user’s memory.
The load instructions for the base and limit registers are privileged instructions.

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

✦ A timer is decremented every clock tick.

✦ When the timer reaches the value 0, an interrupt occurs.