# HW Protection

Here, "protection" does not mean protection on security-related issues(e.g. malwares, cyber attack, etc.).

---
## Dual-Mode Operation
### What to protect?
- Sharing system resources requires OS to ensure that **any incorrect program cannot cause other programs to execute incorrectly**

- Provide **hardware support** to differentiate between:
- User Mode: execution done on behalf of user
- Kernel Mode: execution done on behalf of OS

### How to differentiate?
- <span style="color:orange">**Mode bit added to hardware to indicate the current mode: kernel(0) or user(1)**</span>
- When an interrupt/trap or fault occurs, hardware switches to monitor mode
- <span style="color:orange">**Privileged instructions**</span>: (特權指令，一個寫死的set)
- <span style="color:orange">**Executed only in monitor mode**</span>
- <span style="color:orange">Requested by users (must come through **system calls**)</span>

使用者一定要透過system call才能執行特權指令，如果直接在自己寫的program執行，OS會去檢查mode bit，如果不是kernel mode就會終止本次執行

---
## I/O Protection
- <span style="color:orange">**All I/O instructions are privileged instructions**</span>
- Any I/O device is shared between users

- Must ensure a user program could never gain control of the computer (i.e. a user program stores a new address in the interrupt vector)
- 也就是說只保護I/O是不夠的
- 駭客有可能繞過I/O直接修改memory中interrupt vector裡面的地址，讓電腦去做其他指令，所以會需要memory protection

---
## Memory Protection
- Proctect
- interrupt vector and service routines
- **data access and over-write from other programs**

- HW support: <span style="color:orange">**two register for legal address determination**</span>
沒有透過OS就去access超出範圍的空間時，就會發生segmentation fault
- **Base register**: 連續空間的起始點(smallest legal physical memory address)
- **Limit register**: 這個空間size多少

---
## Hardware Address Protection

- **Load or change the values of base/limit register is privileged instructions**
- Example: malloc() -> will invoke system call to change base/limit register

---
## CPU Protection
- Prevent user program from not returning control
- 避免程式霸佔CPU(包括OS)
- HW support: <span style="color:orange">**Timer**</span> -- interrupts computer after specific period
- Timer is decremented every clock tick. When timer reaches 0, an interrupt occurrs
- Timer is commonly used to implement time-sharing
- **Load(over-write)-timer** is a privileged instruction

Timer到時間後就會丟出一個interrup來打斷目前執行的程式，然後OS就會接管並執行scheduling，決定下一支要執行的程式或是處理interrupt

---

#### 如何判斷這個指令是否為特權指令？
如果user擅自去call這個指令，會危害到系統時，這些指令就是特權指令

#### References
https://secinitiat.medium.com/%E7%89%B9%E6%AC%8A%E6%8C%87%E4%BB%A4-privileged-instruction-%E7%B3%BB%E7%B5%B1%E5%91%BC%E5%8F%AB-system-call-bead9402516