System Hardware Protection

Protection refers to a mechanism which controls the access of programs, processes, or users to the resources defined by a computer system. Need of Protection:

• To prevent the access of unauthorized users and
• To ensure that each active programs or processes in the system uses resources only as the stated policy,
• To improve reliability by detecting latent errors.

Hardware protection is divided into 3 categories: CPU protection, Memory Protection, and I/O protection. These are explained as following below.

CPU Protection

OS must prevent the user program from getting stuck in an infinite loop or not calling system services and never return control to CPU. For this purpose timer is used which interrupts the CPU after specified periods to ensure OS maintains control. The timer period may be variable or fixed. A fixed rate clock and a counter is used to implement a variable timer. The OS initialzes the counter with some positive number. The counter is decremented with every clock tick. When counter reaches 0, a timer interrupt is generated and control is transfer is to next process. Thus timer is used to prevent the program from running too long.

Memory Protection

The area of a memory where process resides is known as process address space. OS ensures that a process cannot access memory address outside its address space. That is, a process is not allowed to access other process address space. Therefore, there is a need to determine the range of legal addresses a process can access. Using two registers, OS provide this memory protection. These register are

• 1   Base Address: holds the smallest legal address
• 2.  Limit Register: specifies the size of range

WWhen a process is loaded into the memory, the base address is initialized with starting address of the process address space and limit register is initialized with its size. Memory outside the range is protected by having hardware  that compare every address generated in user mode with the register. Any attempt to access the memory outside range treated as a fatal error.