Hardware Protection

hardware protection

Hardware Protection

  • Dual-Mode Operation
  • I/O Protection
  • Memory Protection
  • CPU Protection

Dual-Mode Operation

  • 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).

 

Memory Protection

  • 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.

Hardware Protection

  • 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.

CPU Protection

  • 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.

  • Timer commonly used to implement time-sharing.
  • Time also used to compute the current time.
  • Load-timer is a privileged instruction.

 

 

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