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tags: 大三
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# 作業系統期中筆記 I - 國立中興大學大資訊工程系 Autumn 2020 CH1-3

 

[作業系統期中筆記整理 - 國立中興大學大資訊工程系 Autumn 2020 (Part II)](https://hackmd.io/6i05ZGzzSBCpF_7FE50VIQ)

## CH1 Introduction

Computer is consisted of:
1. hardware
> OS have **device driver(software)** for each **device controller(hardware)** for management
2. operating system
> Between a user/app and the computer hardware
>
> Goal:
> Manage the computer hardware and provides an environment for apps to run
3. system and application programs
4. users

### Event-Driven OS

#### Interrupt
> **Device controller raises an interrupt** by asserting a signal on the interrupt-request line
>
> CPU **catches** the interrupt
>
> CPU **dispatches** it to the interrupt handler
>
> Interrupt handlers **serve** the device

#### Trap (aka. exception)
> A software-generated interrupt caused by
> 1. Error (e.g. 除以零)
> 2. Request for operating system service (e.g. system call)

*Q: Please list the three situations that the CPU would enter into the kernel mode.*
*A: 1. system call 2. interrupt 3. trap*

### OS structure
1. single-programming system
> One job at time -- very inefficient
2. multi-programming system
> **只有 I/O 時才會切換 process**
> run multiple programs keep either the CPU or I/O devices busy at all times
3. time-sharing system
> A logical extension of multi-programming
> 硬體設定計時器時間到了切換 processes

### Dual-mode and Multi-mode of Operations

To ensure proper execution of the system, most distinguish between the execution of OS code and user-defined code. Thus, hardware supports dual-mode or multi-mode operations.

+ Allow OS to protect itself and other system components.
+ Privileged instruction(特權指令) can only execute in kernel mode.

有些操作只有 OS 有權限做到，比如 CLI (一種拒絕 cpu interrupt 的指令)，只有 OS 有權使用該指令

Modern CPUs have at least two execution modes -- user modes and monitor modes(aka. supervisor mode, kernel mode). e.g. x86 has 4 kinds of modes, ARM has 7 kinds of modes.

## CH2 OS Structure

OS provides services to programs and users. Two sets of services:
1. Provides functions helpful to the user
+ user interface
> CLI(command-line Interface), GUI (Graphical User Interface)
+ program execution
+ I/O operations
+ File-system manipulation
+ Communications
+ Error detection
2. Ensure the efficient operation of the system via resource sharing
+ Resource allocation
> When multiple users running concurrently, resources must be allocated to each of them
+ Logging (記錄)
> Keep track of which users use how much and what kinds of resources
+ Protection and security


### System calls
+ An interface for app program to use OS services
+ e.g., open(), read(), write(), close(), ...etc
+ Even simple program may make heavy use of system calls

However, system calls are often accessed by programs via a high-level **Application Program Interface(API)** rather than directly call the system call. Three most common APIs
+ Win32 API for windows
+ POSIX API for POSIX-bases systems
+ Java API for the Java virtual machine(JVM)

There are managed by run-time support library

Standard C Library: C programming uses functions defined in libc, which provides a wrapper for a portion of system calls. (比如 printf 就是把 system call write() 做包裝，讓碼農直接 call printf 而不需要用 write())


Why use APIs rather than system call?
1. portability (可以跨平台)
2. Easier to use (printf 比 write 好用)

### Why app are OS specific
Fundamentally, app compiled on one OS usually not executable on other OSes, due to
1. At the system level
+ OSes have different binary format for executable
+ ELF for unix-based os
+ PF for windows
+ CPUs have different instruction sets
+ OSes define their own system call
2. At the application level
+ An applications call one set of APIs will ot work on an OS without the same API

### OS design and implementation

User goals and System goals
+ User goals – OS should be convenient to use, easy to learn, reliable, safe, and fast
+ System goals – OS should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient

早期的作業系統如 MS-DOS 是用 intel-8088 asm 寫的，用組語寫的好處是跑的比 C 快，但其實並沒有，因為現代強大的編譯器優化過後 C 跑的跟組語差不多快了。主要還是差在演算法和資結。

### OS structure
1. Monolithic structure
> 早期的作業系統採用此法
> **pros**: very little overhead
> **cons**: difficult to implement and extend
2. Layered approach
> OS is divided into a number of layers
> **pros**: simplicity of construction and debugging. information hiding (do not have to know the details of other layers, 類似網際網路 5 層架構，大家各司其職)
> **cons**: hard to determine the number of layers and hard to define functionality of each layer. 另外這個方法慢到靠北 (less efficient)
3. Microkernels
> motivation: due to UNIX became large and difficult to manage
> **pros**: easier to extend OS, easier to port the OS to new architectures, more reliable and more secure
> **cons**: performance overhead by message passing between services via <u>communication facility</u>
> e.g., Windows NT (microkernel), Windows XP (monolithic kernel)
4. Modules
> An OS consists of a core kernel and a set of loadable kernel modules (LKMs) that can dynamically loadable at run time
> **pros**:
> + more flexible than layered system because a module can call any other module
> + more efficient than microkernel (kernel module communications is **function call, not by message passing(message passing = communication facility 這個詞有可能會考哦**)
>
> **cons**:
> 講議沒寫
5. Hybrid systems
> 大雜燴
> Linux: monolithic + modular
> Windows: monolithic + micro kernel + modular

### System boot

Two-step booting process in PCs
> Bootstrap loader in ROM → boot program on disk → kernel

bootstrap program or bootstrap loader are
+ small piece of code stored in ROM or EEPROM
+ modern system replace BIOS with Unified Extensible Firmware Interface (UEFI)
+ locate the kernel, load it into memory, and start its execution.

## CH3 Process


+ stack: variables in function
+ heap: dynamic variables
+ data
+ uninitialized data (.bss)
+ initialized data (.data)
+ text (.code)

### Process control block (PCB)

*Q: What is purpose of process control block (PCB)?*
*A: Keep information associated with each process*

+ Process state
+ Program counter (PC)
+ CPU register
+ CPU scheduling information
+ Memory-management information
+ Accounting information
+ I/O state information

### Context switch

*Q: What is context?*
*A: Context is saved and represented in the PCB of a process. e.g., process id, cup reg, memory areas, various tables, etc.*

*Q: What is context switch?*
*A: CPU switch from one process to another process.*

*Q: The context is stored in a kernel data structure, what is this structure?*
*A: Program counter block (abbr. PCB)*

*Q: Provide a detailed description of all activates of a context switch. (= During context switch, what operations the operating system must perform?)*

*A: The system must save the state (context) of the old process and load the state (context) of new process from it’s PCB.*


### Process Scheduling


+ ready: waiting to be assigned to a processor
+ waiting: **waiting for some events to occur**
+ running: only one process can be running on any core


剩下的去看第五章

### Operation on processes

#### Process creation

A process may create several new processes
+ The creating process is called parent
+ The new processes are called children of that process

Execution
+ Parent and children execute concurrently, or
+ Parent waits until children terminate

Address space
+ `fork()` Child duplicate of parent, or has the same program and data as the parent
+ `fork() + exec()`Child has a new program loaded into it

#### Process termination

+ When a process terminates and before parent calls wait(), the terminated process becomes a **zombie process**.

+ If parent did not invoke wait() and is terminated, the child process becomes as **orphans**

### Interprocess communication (IPC)

> node.js 的 IPC 套件 https://www.npmjs.com/package/node-ipc

1. Message passing (message queue)
> 1. Useful for exchanging smaller amounts of data (via memory)
> 2. Easier to implement
> 3. Slow (due to system calls required)
> 4. No conflict need to be avoided
>
> ```c
> #include<mqueue.h>
> mq_open()
> mq_send()
> mq_reveive()
> mq_close()
> ```

2. Shared memory
> 1. Faster (due to no system calls required)
> 2. Need to protection and synchronization
> ```c
> #include<sys/mman.h>
> shm_open()
> mmap()
> shm_unlink()
> ```


**↑ 左為 message passing；右為 shared memory**

### Socket & Remote procedure calls (RPC)

RPC and RMI (Remote Method Invocation) are high level form of communication. Socket is low level


↑ **socket (TCP)**

#### RPC problems

> The main challenge of RPC is to look as much
like local procedure call, i.e., transparency

什麼是 transparency? Client & server may be different types of machine different data representation, e.g., big-endian vs. little-endian, ASCII, EBCDIC, 1's complement vs. 2's complement

Solution. 透過 External data representation (XDR). machine independent data representation. 實作上透過一個叫 stub 的 library (A library in the client and server)

+ Client stub (like sender)
+ Locates the server
+ Marshal params into a msgs
+ Calls OS (i.e., calling socket APIs) to send msg to the server
+ Waits for result returned from the server
+ **Machine-dependent data => XDR**
+ Server stub (like receiver)
+ Be blocked waiting for incoming msg by calling receive()
+ Unmarshal the params
+ Clls the corresponding procedure in the conventioanl way
+ Returns result to the caller.
+ **XDR => Machine-dependent data**

*Q: Explain the purpose of external data representation (XDR).*
*A: 為了解決client和server用不同平台而導致的data representation的問題。*

<style>
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<div id="author">
<p>111級中興資工</p>
<p>作者：<a href="https://github.com/liao2000">廖柏丞</a>
</div>