Chapter 2 | Operating-System Structures¶

约 2691 个字 13 张图片预计阅读时间 13 分钟

Operating System Services¶

One set of operating-system services provides functions that are helpful to the user :

User interface - Almost all operating systems have a user interface (UI) 提供用户界面（UI），包括命令行界面（CLI）和图形用户界面（GUI），方便用户与计算机交互。
Program execution - The system must be able to load a program into memory and to run that program, end execution, either normally or abnormally (indicating error) 把程序装载到内存中并运行，程序运行结束后能正常或异常（出错）终止。
I/O operations - A running program may require I/O, which may involve a file or an I/O device.
File-system manipulation - The file system is of particular interest. Obviously, programs need to read and write files and directories, create and delete them, search them, list file Information, permission management.
Communications – Processes may exchange information, on the same computer or between computers over a network. 进程之间需要交换信息。
Error detection – OS needs to be constantly aware of possible errors

Another set of OS functions exists for ensuring the efficient operation of the system itself via resource sharing :

Resource allocation - When multiple users or multiple jobs running concurrently, resources must be allocated to each of them 合理分配CPU、内存、I/O设备等资源。
Accounting - To keep track of which users use how much and what kinds of computer resources 为资源管理、计费或优化提供依据。
Protection and security - The owners of information stored in a multiuser or networked computer system may want to control use of that information, concurrent processes should not interfere with each other 信息的所有者希望控制信息的使用权限。操作系统要防止进程间互相干扰，保护数据和资源的安全。

User Operating System Interface¶

CLl¶

CLI allows direct command entry

Sometimes implemented in kernel, sometimes by systems program
Sometimes multiple flavors implemented – shells
Primarily fetches a command from user and executes it

GUI¶

User-friendly desktop metaphor interface

Usually mouse, keyboard, and monitor
Icons represent files, programs, actions, etc
Various mouse buttons over objects in the interface cause various actions (provide information, options, execute function, open directory (known as a folder))

Many systems now include both CLI and GUI interfaces

System Calls¶

Programming interface to the services provided by the OS

系统调用是操作系统为程序提供服务的编程接口。

Typically written in a high-level language (C or C++)

Mostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call use

大多数情况下，程序通过高级 API 间接访问系统调用。

Three most common APIs are Win32 API for Windows, POSIX API for POSIX- based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM)

System call sequence to copy the contents of one file to another file

System Call Implementation¶

Typically, a number associated with each system call

System-call interface maintains a table indexed according to these numbers

每个系统调用都有一个编号，系统调用接口维护一个表，通过编号查找对应的系统调用。

The system call interface invokes intended system call in OS kernel and returns status of the system call and any return values

系统调用接口负责在内核中调用目标系统调用，并返回状态和结果。

The caller needs to know nothing about how the system call is implemented

调用者（程序员）无需关心系统调用的具体实现，只需遵循API规范，理解调用结果即可。

Just needs to obey API and understand what OS will do as a result call
Most details of OS interface hidden from programmer by API

System Call Parameter Passing¶

Three general methods used to pass parameters to the OS

Simplest: pass the parameters in registers
- In some cases, may be more parameters than registers
- 最简单的方法，直接把参数放在CPU寄存器中。如果参数太多，寄存器不够用时不适用。
Parameters stored in a block, or table, in memory, and address of block passed as a parameter in a register
- 把所有参数存放在内存中的一个块（或表）里，然后把这个块的地址作为参数传递。适合参数数量或长度不定的情况。
- This approach taken by Linux and Solaris
Parameters placed, or pushed, onto the stack by the program and popped off the stack by the operating system
- 程序把参数压入栈中，操作系统从栈中弹出参数。

Block and stack methods do not limit the number or length of parameters being passed

Parameter Passing via Table

Types of System Calls¶

Process control、File management、Device management、Information maintenance (e.g. time, date)、Communications、Protection

System Programs¶

System programs provide a convenient environment for program development and execution. They can be divided into:

File manipulation
Status information
File modification
Programming language support
Program loading and execution
Communications
Application programs

Provide a convenient environment for program development and execution

File management - Create, delete, copy, rename, print, dump, list, and generally manipulate files and directories

Status information

Some ask the system for info - date, time, amount of available memory, disk space, number of users
Others provide detailed performance, logging, and debugging information
Typically, these programs format and print the output to the terminal or other output devices
Some systems implement a registry - used to store and retrieve configuration information

File modification

Text editors to create and modify files
Special commands to search contents of files or perform transformations of the text

Programming-language support - Compilers, assemblers, debuggers and interpreters sometimes provided

Program loading and execution - Absolute loaders, relocatable loaders, linkage editors, and overlay-loaders, debugging systems for higher-level and machine language

Communications - Provide the mechanism for creating virtual connections among processes, users, and computer systems

Operating System Design and Implementation¶

Start by defining goals and specifications

Affected by choice of hardware, type of system

User goals and System goals

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

Policy: What will be done? 策略（确定具体做什么事）
Mechanism: How to do it? 机制（定义做事方式）

Tip

课上的举例

policy like c file

mechanism like PC

🌰

机制：实现了“分配和回收内存块”的基本操作。

策略：可以选择首次适应、最佳适应、最差适应等不同分配算法。

好处：更换分配策略时，内存管理机制无需大改。

The separation of policy from mechanism is a very important principle, it allows maximum flexibility if policy decisions are to be changed later

Simple Structure¶

以 MS-DOS 为例，说明早期操作系统的结构特点：

MS-DOS – written to provide the most functionality in the least space

Not divided into modules
Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

MS-DOS Layer Structure¶

虽然图中有层次，但实际上各层之间的界限并不严格。比如，应用程序可以直接调用MS-DOS设备驱动，甚至直接访问ROM BIOS驱动，而不一定非要通过上层的resident system program。

各部分之间耦合度高，接口不统一，导致系统难以维护和扩展。

应用程序可以直接操作底层硬件，容易导致系统崩溃或安全隐患。

Layered Approach¶

The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

With modularity, layers are selected such that each uses functions (operations) and services of only lower-level layers

Layered Operating System¶

操作系统被划分为若干层，每一层只依赖于它下方的那一层。

UNIX¶

UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts

System programs
The kernel
- Consists of everything below the system-call interface and above the physical hardware
- Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level

UNIX结构分为系统程序和内核两大部分。

UNIX System Structure¶

用户空间（the users）: 最上层是用户，包括所有使用计算机的人。
系统程序（shells and commands, compilers and interpreters, system libraries）
系统调用接口（system-call interface to the kernel） : 这是应用程序和内核之间的桥梁。用户程序通过系统调用接口请求内核服务（如文件操作、进程管理等）。
内核（Kernel）: 内核是UNIX操作系统的核心，负责所有底层资源管理和硬件控制。信号与终端处理、字符I/O系统、终端驱动、文件系统、交换、块I/O系统、磁盘和磁带驱动、CPU调度、页面置换、请求分页、虚拟内存。
内核与硬件接口（kernel interface to the hardware）: 这是内核与底层硬件之间的接口，内核通过它来控制和管理硬件设备。
硬件控制器和设备（terminal controllers, device controllers, memory controllers）

Microkernel System Structure¶

Moves as much from the kernel into “user” space

微内核只保留最基本、最核心的功能在内核态（kernel mode），如进程间通信（IPC）、内存管理、CPU调度等。

Communication takes place between user modules using message passing

Benefits:

Easier to extend a microkernel. 增加或修改系统服务时，只需更改用户态模块，不影响内核。
Easier to port the operating system to new architectures. 内核代码少，移植到新硬件平台更容易。
More reliable (less code is running in kernel mode). 大部分操作系统代码在用户态运行，内核态代码少，出错影响范围小。
More secure. 内核态代码少，攻击面小，安全性高。

Detriments:

Performance overhead of user space to kernel space communication. 用户态和内核态之间频繁消息传递，导致性能下降。

Architecture of A Typical Microkernel¶

Mac OS X Structure¶

更详细地展示了Darwin内核环境的内部结构。

application environments and common services（应用环境与通用服务）提供给应用程序的运行环境和常用服务。
BSD层。BSD是Berkeley Software Distribution的缩写，这一层提供了类Unix的系统接口，包括进程管理、文件系统、网络协议栈等，是macOS兼容Unix标准的关键部分。
Mach内核。Mach是一个微内核，负责最底层的任务，如进程/线程管理、内存管理、进程间通信（IPC）等。BSD层运行在Mach之上，利用Mach提供的底层机制实现更高层的操作系统功能。

Mac OS Structure¶

每一层都为上层提供服务和接口，应用程序可以直接调用上层或部分底层框架的功能。

applications（应用程序）用户直接使用的各种应用软件
user experience（用户体验层）提供图形界面、窗口管理、动画、系统交互等用户可见的体验功能。
application frameworks（应用框架）为应用开发者提供的各种API和框架。
core frameworks（核心框架）提供更底层的系统服务。
kernel environment (Darwin)（内核环境）最底层，是macOS的内核。它负责硬件管理、进程调度、内存管理、驱动支持等所有底层功能。

Modules¶

Most modern operating systems implement kernel modules

它将操作系统的各个核心功能划分为独立的模块（module），每个模块负责不同的功能。这些模块可以根据需要被动态加载或卸载到内核中。

Uses object-oriented approach. 类似于面向对象中的“对象”，职责单一，接口明确。
Each core component is separate. 核心组件分离。
Each talks to the others over known interfaces. Modules call each other instead of message passing. 已知接口通信。
Each is loadable as needed within the kernel. 按需加载。

Overall, similar to layers but more flexible

模块化结构类似于==分层结构==，但更加灵活。分层结构层次固定，模块化结构可以动态组合和扩展。

==微内核==将大部分服务移到用户态，通过消息传递通信，安全性高但性能有损耗。模块化内核则大部分模块仍在内核态，直接调用接口，性能更高。

Solaris Modular Approach¶

Other Structures¶

Unikernel: statically linked with the OS code needed.

Good for cloud service, APP boots in tens of milliseconds

Unikernel/Library OS结构的核心思想：应用、库和内核紧密集成，按需裁剪，极致精简，适合云原生和高安全性场景。