Lecture 2 -

Hardware Modes

Who actually gets to control the hardware?

The application? It would be simple and quick, but isn't safe at all.

The OS? Can act on behalf of the application, which gives us protection.

Challenge: Protection with Restrictions

How do we execute code with restricted privileges?

Hardware Support: Dual Mode Operation

• Kernel mode - privileged. Execution with full access to the hardware
• User mode - restricted. Only able to execute instructions granted by OS (ie non privileged). Limits on memory accesses, only allowed to access own memory. There is a timer interrupt that regularly gives the kernel the ability to take control from a user process.

On x86, mode stored in EFLAGS register. On MIPS, mode stored in status register.