All processes waiting for I/O requests to be completed, they wait in I/O queue. But with вЂњcopy on writeвЂќ this is much less expensive.ДЄll processes in the ready state are held in the ready queue. Copying data from the parent process is very expensive. I/O states such as file handlers also need to be copied, this is medium expensive.

The new page table must be set up for its address space, this step is more expensive. When a process is created, the new PCB must be constructed. Upon completion of I/O or event, the process will be put into the ready state again. A running process incurring I/O or event wait will be put into wait. A running process calling exit will result this process to be terminated. A running state gets interrupted and put back to ready - that's preemption. The new process is that admitted into the ready state, a scheduler dispatches a process into running state. This is quite common with embedded applications and a few early operating systems.ДЄs a process executes, it has 5 possible states. Where each thread can access the data of every other thread and threats can share instructions. This multi-programming model can be implemented in an unprotected way. And the switch is triggered by preemptive timer, voluntary yield, I/O, and other things. Switching from one virtual CPU to another simply means saving a state block and loading a new state block. Each virtual CPU uses a structure to hold program counters stack pointers registers etc. On a single processor the illusion of multiple processors comes from multiplexing in time. A short take-home message - kernel mode is a mode forД¬PU operation, it is not a mode for process. Why and how kernel model protects address spaces requires an understanding Transitions from user mode to kernel mode can be achieved by system calls, Some instructions are prohibited in user mode, User mode in which normal programs execute, Hardware to provide at least 2 modes: kernel mode or protected mode MMU maintains a consistent view of how different process address Performed in hardware by memory management unit, a.k.a. Mapping of virtual addresses to physical addresses is often, note: not always, The addresses known and used by CPU are called virtual addresses. Other processes or that of the operating system. The simple policy is processes are not allowed to read and write memory of Process address space means a group of memory addresses used by a process. How does address translation help with 'protection'? The key mechanisms for protecting processes from each other are: The first purpose of operating system is 'protection' among multi-processes

Dual mode processing operating system