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

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

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

4. 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)

5. 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.

6. 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).