Create operating_system.md

operating_system.md

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+유저 모드: 하드웨어 접근이 안됨, 접근 시도 할 시, 익셉션이 발동함. 운영체제의 익셉션 핸들러 --> 커널모드로 변환됨.
+
+cooperative approach: wait until a process makes a system call
+non-cooperative approach: os takes control
+- problem: 해킹에 취약함, 프로그램에 버그가 있어도 컴퓨터가 먹통이 될 수 있음.
+Earlier versions of OSs used to take the cooperative approach.
+
+interrupt: an unexpected event from outside the process
+exception: an unexpected even from within the process
+
+timer interrupt: (intel) interrupt number 0.
+
+
+context switch: written in assembly
+
+scheduling:
+- preemptive: can take cpu time from a running job
+- non-preemptive: cannot take cpu time from a running job
+  - sjf: smallest average turnaround time for a non-preemptive
+    - problem: 간발의 차로 큰 job가 들어오면, 결국 다 늦어짐.
+    -