Operating mechanism:
1. Interrupt (external) ===== kind of advice behavior (such as inserting a keyboard)
2. system call (active reaction) === request one kind of behavior
3. Abnormal (internal) ===== wrong handling behavior
Relationship between calls and program interface system, the interface system call encapsulated in
Interrupt :( Example) a keyboard key is pressed, a pulse is generated by the key signal to the control CPU, CPU receives an interrupt signal, the behavior of the signal read by detecting a data bus,
