Kernel Abstraction

Role of Kernel

Central role of OS: Protection

Central role of Kernel: Implementing Protection

• Reliability
  • prevent crashing from other app, OS
  • 따라서 OS 자체는 굉장히 튼튼하고 “Bulletproof” 되어야 함.
• Security
  • some user, apps are less trustworthy
  • OS need to limit the Scope
  • 유저 코드가 os 를 망가트리는 것을 방지!
• Privacy
  • 여러 user, 여러 계정 등
• Efficienty
  • without protection, one app might occupy all resources

Kernel Abstraction

kernel is lowest level of software that is running on the system,

with FULL access to anything with HW.

• Kernel is “trusted” to do anything
• other app is “untrusted” so that it runs in restricted env.

User mode vs Kernel mode

Dual-mode Operation

• 프로그램은 로드된 이후 OS가 실행시키면 레지스터 값 바꾸기, 메모리에 읽고 쓰기, PC 증감하기 등을 한다.

• 각 프로그램이 서로 영향을 미치지 않고 잘 실행되려면?

  • is it referencing part of its memory?
  • is it trying to branch into someone’s code?
  • is it directly accessing disk?

• Dual mode Operation

  • user mode: 각 inst를 실행해도 되는지 확인한다.
  • kernel mode: protection을 끄고 실행한다.

  오른쪽 그림이 간략하게 나타내고 있다.

Three HW support for Dual-mode

1. Privileged Instructions
   • kernel mode에서만 실행 가능
   • ex. change privilege level, disable inturrupts, …
   • 그런거 실행하면 exception 발생시킴.