# OS Chapter2
###### tags: `作業系統`

## *User Interface(UI)*

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**CLI (command line interface)**
- command interpreter allows direct command entry

- implemented in kernel or by systems program

- 使用者透過**shell**下指令
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**GUI**
- 圖形介面
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## *System Calls*
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- Programming interface to the services provided by the OS

- System calls are mostly accessed by programs via a high-level API rather than direct system call

- Comment API
- Win32 API for windows
- POSIX API for Linux
- Java API for JVM
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- API-System Call
※open() -> 呼叫trap
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- System Call parameter passing
- pass in regeister
- sotred in a block , or table , in memory
- pushed onto the stack by the programm , popped off the stack by os
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## *System Services*
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Most users' view of the operation system is defined by system programs , not system calls
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## *Linkers and Loaders*
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- **Linker** : conbines object file into single binary executable file

- **Loader** :
- kernel的載入器
- bring binary executable file into memory to be executed

- Relocation : assigns final addresses to program and adjusts code and data in program to match addresses

- Dynamically Linked Libraries(DLLs) : are loaded when neede.

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## *Application Binary Interface(ABI)*
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It defines how different components of binary code can interface for a given operating system on a given architecture.
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## *OS design and implementation*
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Different design consideration
- User goal : OS should be convenient to use , easy to learn.
- System goal : OS should be easy to design , implement and maintain.
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Important principle
Policy : What will be done?
Mechanism : How to do it?
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## *OS structure*
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- Monolithic structure - Unix
- a large number of functions for one level
- 缺點 : 功能越多，東西也越來越多

- Linux system structure
- modular design 模組化

- Layered approach
- OS is divided into a number of layers
- 缺點 : 但對於OS來說無法分得太清楚

- Microkernels
- 只留下基本、必要的
- communication takes place between user modules using message passing
- 缺點 : 變簡單了但效率變不好
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- Modules : many OS implement loadable kernel modules(LKMs) , 動態需要時才載入

- Hybird System : 混合型model
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System Boot
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