Abstract
There are two kind of software that runs in our system. They are system software and application software. Operating System is system software that is responsible for handling all other processes that runs on our system. At a time there are various processes that demands for CPU and resources, since job executed only when the job is allotted to CPU. The order of assigning job to CPU is called CPU scheduling. Assigning of CPU to many jobs depends on various factors like their service time, waiting time, priority, response time etc. Once the job has assigned CPU, it do not leave CPU till it's completion in Non-Preemptive scheduling. In Preemptive there is relaxation to leave CPU after assigning it to any job. Generally CPU leaves when it demands for some resources or wants some others task to be done before it, than it resides in main memory or in secondary memory and go in to waiting state or suspend state. There are five states of any job i.e. new, ready, running, waiting and terminating state.