Operating System: Case Study 6

Diagram 1.1: The Process State

In the Process State diagram, jobs move through the system. There are five stages Hold, Ready, Waiting, Running, and Finished. When a process is first created, it occupies the hold state. In this state, the process awaits admission to the "ready" state. This admission will be approved or delayed by process scheduler. Typically in most desktop computer system, this admission will be approved automatically; however for real time operating systems this admission may be delayed. In the Reading process has been loaded into main memory and is awaiting execution on a CPU (Central Processing Unit). There may be many "ready" processes at any one point of the system execution. For example, in a one processor system, only one process can be executing at any one time, and all other "concurrently executing" process will be waiting for execution. While in the Running process is a process which is currently executing on a CPU. From this state the process may exceed its
allocated time slice and be context switched out and back to "ready" by the operating system, it may indicate that it has finished and be terminated or it may block on some needed resource and be moved to a "waiting" state. Waiting process, a process is said to be blocked if it is waiting for some event to happen such that as an I/O completion before it can proceed. Note that a process is unable to run until some external event happens. And the last process is the terminated process which either from the "running" state by completing its execution or by explicitly being killed. In either of these cases, the process moves to the "terminated" state.

However, there are things that may arise in the process state. Why there is no transition from Ready to Waiting. The reason behind is that the typical process is the Waiting state is waiting for the external device response which must be received before the CPU can be used again. While in the ready state is making the advent for the usage of the CPU. So if a job is in the Ready state and cannot proceed for it because there is a failure of the required response, then the job must go back to hold state, not in the advent of waiting state. A good analogy is the instructor is holding a test questionnaire (Hold state), disseminate the test questionnaire to the students (Ready state), answering the questionnaire (Running state), unsuccessful answering the whole test questionnaire (Waiting state) because of the time (I/O request page fault) and eventually unfinished answering the questionnaire (Terminate state). They cannot finish answering the questionnaire
since there is only minimal time, so as what the process state occur when there is no transition from Ready to Waiting so the unfinished test questionnaire will be hold again by the teacher, not in the advent of successful answering. Another thing is there is no transition from Waiting to Running because the Process Scheduler (PS) selects processes from the Ready state for the CPU. Therefore, circumnavigating the ready queue would definitely make process management impossible. Like in the analogy aforementioned, if it is already time for answering the test questionnaire, the students can no longer answer the questionnaire so the students will hold it or like in the typical process state it will occur a context switching and back to the Ready state.