Computer System Structures System Operation Protecting Processes from each other Instruction protection Address space protection Interrupt protection Textbook Silberschatz, Chapter 2

Process Protection Goal: Run multiple applications in such a way that they are protected from one another Keep User Programs from Crashing OS Keep User Programs from Crashing each other Keep Parts of OS from crashing other parts. Some of the required mechanisms: Dual Mode Operation - provides Instruction protection Address Translation - provides Address space protection Interrupt Protection - protects interrupts from other interrupts

Instruction protection Dual mode provides protection for instructions distinguishing instructions for User and Kernel modes.

Address Space, Address Translation A group of memory addresses used by something (process, module,…) Each program (process) and kernel has potentially different address spaces. Address Translation: Translate from Virtual Addresses (emitted by CPU) into Physical Addresses (of memory) Mapping often performed in Hardware by Memory Management Unit (MMU)

Address space protection Simple Policy: Programs are not allowed to read/write memory of other Programs or of Operating System  The mechanism of the change of the mapping also should be protected.

Interrupt protection Mechanisms: Interrupt deferring interrupts levels interrupt masking interrupt disabling When the interrupt happens the hardware Disables all other interrupts to protect this interrupt from the new coming ones. RTI reenables interrupts on hardware level (inside the instruction), returns to user mode if the interrupt is enabled in the handling routine then the routine could be interrupted.  The mechanism of the interrupt control also should be protected (special instructions in kernel mode).