1.describe the life circle of process   

A process is a dynamic entity, the entire life cycle of a process from creation to extinction.Processes typically go through three states.

• Ready state: the process has acquired all resources except the CPU and is waiting in the ready queue for the CPU to dispatch it


• Execution status: the CPU has been obtained and all required resources are running


• Blocking state (wait state) : the process abandons the processor because it is waiting for the required resource, or the process does not own the processor in the first place and other resources are not satisfied

State transition: ready state process to get the CPU scheduler will into execution status, the process of execution state if because dormancy or waiting for some resources will become a wait state, the process of execution state if time slice to will the end back into the ready state into the ready queue, waiting for the state of the process if other than the CPU resources would be into the ready state.
2.why each process must have one PCB.

The PCB is a special data structure set up by the system to manage processes.It is used by the system to record the external characteristics of the process and describe the change process of the process.At the same time, the system can use PCB to control and manage the process, so that the PCB is the only sign that the system is aware of the process.