Operating system

Kernel mission

• Virtualize the hardware.

  • Computational model.

• Enforce protection policies and provide protection mechanisms.

  • Against involuntary mistakes.

  • Against non-authorized activities.

Protection rings

• Different levels of privilege.

  • A set of concentric rings.

  • The CPU uses them to prevent non-privileged code from running privileged instructions.

    • e.g. IN/OUT, TLB manipulation.

• Nowadays processors have 4 rings.

  • But OS’s usually use only two of them.

  • 0 (supervisor/kernel mode) and 3 (user mode).

• Transfer of control between rings requires special gates.

  • The ones that are used by system calls (syscalls).