Process control: achieve a variety of state of the process of conversion 
ponder: When a process from one queue to another queue, but there is no corresponding PCB and a new state record, this error will cause the system
to prevent the above problems, so we use primitive
primitive (atomic): not allowed to be interrupted only at one go
nature of primitive: to enable or disable interrupt instruction (interrupter)
when I interrupter closed, even if you interrupt the signal, is useless (with I did a bit like a class-based approach)
