There are five states are defined at a process's life cycle:

(1) New. The process is being created.

(2) Running. Instructions are being executed.

(3) Waiting. The process is waiting for some event to occur(like I/O).

(4) Ready. The process is waiting to be assigned to a processor.

(5) Terminated. The process has finished execution.

The following reasons describe the necessity of PCB:

(1)The process control block is the only indication that the process exists.

(2)PCB is the most important data structure for process management and control (process identifier information, processor state information, process scheduling information, process control information) ,for example: PCB contains CPU field protection. Record the CPU state at the time of interruption(important to recover a process).It also contains process priority, the urgency of the process, which make sure that the process can be executed by CPU orderly.