Describe the life circle of process and  why each process must have one PCB.

The process has three basic states:Ready,Running,Blocked.

And the process will undergo state transition:When the ready state process gets cpu scheduling, it will become the execution state.If you sleep or wait for a certain resource, it will become a wait state. If the time is up, the process will be heavy.The new ready state is placed at the end of the ready queue, and the process waiting for the state gets resources other than cpu will become ready.

PCB is the only sign of the existence of the process, the operating system can only perceive the existence of the process according to the PCB, according to the data of the PCBto manage and control the process.