sdp1(os_process)

1. what's shell?

   A shell is a user interface for access to an operating system's services.In general,operating system shells use either a command-line interface(CLI) or graphical user interface(GUI),depending on a computer's role and particular operation.

   It acts as a command interpreter; it takes each command and passes it to the operating system. It then displays the results of this operation on your screen.
   

2.what's a process

 

  Algorithm: a logical procedure that in a finite number of steps solves a problem

  Program: formal expression of an algorithm by means of a programming language(needs runtime support) 

  Process: a sequence of operations performed by a  program in execution on a given set of input data.

 Shell and Process

  Whenever enter a command at the shell prompt,it invokes a program.While this program is running, it becomes into dynamic objec from static object,it's called a process. Your login shell is also a process,created upon your logging in and existing until you logout.

3.Process State Diagram

 After creating the process(submit),swapper(mid-term schedular) moves process between memory and swap area on the disk(hold),when the process gets enough resources,the process enters into Ready state(Ready means the process could immediately run but may have many "Ready",so use policy(Preemption),process which is ready assigned with cpu goes to running state,when time slice is consumed over,it goes to the end of the queue with wait state(Round Robin)).It runs in this cycle,till finishes(complete).

Preemption:Preemption as used with respect to operating systems means the ability of the operating system to preempt (that is, stop or pause) a currently scheduled task in favour of a higher priority task. The resource being scheduled may be the processor or I/O, among others.

Round Robin:Round-robin (RR) is one of the algorithms employed by process and network schedulers in computing.^[1] As the term is generally used, time slices are assigned to each process in equal portions and in circular order, handling all processes without priority (also known as cyclic executive).