构建嵌入式Linux系统：实践与详解（第二版）

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《嵌入式操作系统：实践方法（第二版）》是一本深入浅出的教科书和参考书籍，专为读者设计，通过实践构建一个完整的嵌入式操作系统，从源代码出发，以便于理解每个组件及其协同工作原理。本书详细介绍了如何创建引导加载程序、内核、文件系统、共享库、启动脚本、配置文件以及系统工具，目标是生成一个基于GNU/Linux的操作系统。第二版在原有内容基础上新增了虚拟机技术，如VirtualBox、Vagrant和Linux容器系统Docker的相关知识。本书主要涵盖以下主题： 1. GNU/Linux系统概述：首先介绍GNU/Linux系统的组成部分，包括进程管理、输入/输出和环境处理的基础概念。 2. 容器与底层技术：深入探讨容器的原理，以及它们依赖的内核技术。 3. 文件系统详解：对GNU/Linux文件系统进行详尽剖析，帮助读者理解其内部结构和功能。 4. 虚拟化嵌入式系统：指导读者如何在虚拟机环境中构建嵌入式系统，并开发能够在实际处理器上运行的嵌入式系统。 5. 编译器工具链：介绍编译器工具链的概念，以及针对BeagleBone和Raspberry Pi等平台的应用。 6. Openwrt：讲解如何为运行Openwrt操作系统的设备构建固件映像。本书强调实践性和结构清晰，非常适合对计算机科学有实践经验的本科生和研究生，也适用于在这个领域工作的专业人士。作者Alan Holt和Chi-Yu Huang以其专业背景，结合国际顾问团队，确保了内容的高质量和时效性。通过阅读本书，读者不仅能掌握理论知识，还能提升实际操作能力，从而更好地理解和应用嵌入式操作系统。

List of Tables

Table 1.1 GNU/Linux distributions .............................. 7

Table 2.1 Unix System V runlevels used by the Linux stand ard base

.... 15

Table 2.2 Summary of input/output streams

........................ 22

Table 2.3 Redirection operators

................................. 25

Table 4.1 File permissions

..................................... 72

Table 4.2 Master boot record sector structure

....................... 90

Table 4.3 Primary partition table

................................ 91

Table 4.4 Partition table entry

.................................. 91

Table 4.5 Partition entry

....................................... 94

Table 4.6 Partition entry

....................................... 99

Table 6.1 Null modem

........................................ 147

Table 7.1 GCC optimisation options

............................. 155

Table 8.1 Raspberry Pi models

................................. 173

Table 8.2 XBee Serial Numbers

................................. 180

Table 8.3 BeagleBoards technical speciﬁ cation

..................... 186

Table 9.1 Branches of Openwrt (at the time of writing this book)

....... 196

Table 9.2 Open-mesh models

................................... 200

Table 9.3 M32 speciﬁcation

.................................... 207

xix

2 1 Introduction

1.1 An O verview of Operating Systems

Operating systems are software components dedicated to the management of the

computer system’s hardware. Most computer systems (embedded or otherwise) oper-

ate under the control of an operating system. Unix-like operating systems, such as

GNU/Linux, comprise a kernel, software libraries and a number of utility programs.

The diagram in Fig. 1.1 shows the operating system architecture. A piece of software

called a bootloader is also required to start the kernel when the computer system is

powered. We discuss the bootloader in more detail in Chap. 2.

The kernel is a program that manages the resources of a computer system. It

allocates these resources between separate processes in a controlled way. Processes

are computer programs under execution and may be initiated by different users.

Resources are, therefore, not just allocated amongst processes, but also amongst

users. What distinguishes the kernel from the rest of the operating system software,

is the processor privilege level at which the respective code is executed. Most mod-

ern processors support multiple privilege levels. Intel 386 processors, for example,

support four privilege levels. GNU/Linux only uses two, level 0 (the most privileged)

for the executing kernel code and level 3 (least privileged) for running user code.

The reason for these different privilege levels is to prevent user code from directly

accessing certain hardware resources. For example, I/O port instructions can only be

accessed by kernel code. Clearly, user processes require access to peripheral devices

(I/O port instructions) on the computer system. A process can context switch into

kernel mode by issuing a software trap. In kernel mode, a process can access hardware

via the kernel’s own subsystems. Access to hardware is, therefore, controlled by the

kernel. Functions of the kernel can be characterised below:

Process management: Computer systems store a number of programs in their

memory. These programs may be part of the operating system’s utilities or soft-

ware written by a user as an application. A program under execution is called a

process. The system may execute multiple processes concurrently. The process

management system gives the illusion of processes running simultaneously, typ-

ically, by giving each process a time slice of the CPU (or CPUs). A scheduler

makes the decision as to which process is allocated a time slice. A process may

have its execution suspended and resumed several times before it terminates.

Fig. 1.1 Operating system overview

1.1 An Overview of Operating Systems 3

Memory management: Memory management is a complex function. In order to

manage memory efﬁciently and effectively, the kernel implements virtual mem-

ory. Virtual memory gives the appearance that a process has more memory than it

actually has (or even, more than the entire system has). The virtual and physical

memory are divided into pages. With paged memory a virtual address is divided

into two parts, namely, a virtual page frame number plus an offset into the page.

When a virtual memory location is referenced, the virtual page frame is translated

into a physical page frame number. The offset is then used to identify the speciﬁc

memory location within the page.

Filesystem: Persistent data is stored in ﬁles which in turn are organised in directo-

ries. The ﬁlesystem provides the functions and data structures required to manage

ﬁles and directories (as well as other ﬁles objects) within a disk partition.

Device management: In addition to the CPU and main memory, a computer system

will comprise a variety of peripheral devices. For example, disk drives, network

interfaces, input/output devices etc. Device drivers are pieces of software within

the kernel that control the devices.

1.2 Overview of Embedded Systems

Embedded systems are hard to deﬁne. The problem lies in the fact there are exceptions

to any rules one may conceive. While it is not easy to ﬁnd a characteristic that is

exclusive to embedded systems we shall examine a few below:

A Subsystem of a device or machine: Such systems are embedded within a more

sophisticated device or machine, an engine management system of a car or micro-

processor within an appliance, for example. Given this deﬁnition, we can view

a general processing computer as being made up of a number of embedded sys-

tems. In addition to the CPU and main memory, general processing computers

also include bus controllers, disk controllers, network interfaces and video con-

trollers. All of which could be considered embedded systems in their own right.

Furthermore, there are numerous non-subsystem examples of embedded systems,

WiFi access-points, network routers, set-top boxes, for example.

Dedicated application: Embedded systems are largely designed to perform a spe-

ciﬁc task such as monitoring a temperature sensor or controlling a valve. In the

case of consumer electronics devices may perform multiple tasks. Modern mobile

phones are rarely limited to just making telephone calls. Many are described as

smart phones and can support a plethora of applications. Given the increase in the

power and sophistication of embedded devices it is difﬁcult to distinguish them

from general processing systems.

Small footprint: A small footprint is a typical characteristic of embedded systems.

This is usually driven by the task it has to perform.

4 1 Introduction

Low power consumption: Many embedded systems are battery powered and need

to conserve energy consumption. Such systems may be static or mobile, either

way they operate at a distance from any mains power. There are obvious exemp-

tions to this rule. High speed routers and switches, for example, have powerful

processors for forwarding high volumes of network packets. Consequently energy

consumption will be high and mains supplied.

Low processing power and memory: This is somewhat a corollary of the charac-

teristic above. Whereas, in the past, embedded systems were almost exclusively

based on specially designed hardware, it is possible nowadays to develop embed-

ded systems on commercial off-the-shelf (COTS) hardware.

Real-Time systems: Real-time systems are systems that must respond to events

within strict time constraints. The term “real-time system” is sometimes used

synonymously with embedded systems. Real-time systems are almost always

dedicated to a single application and can be categorised as embedded systems.

However, not all embedded systems support real-time applications.

Requires specially designed operating system: This characteristic may have been

true in the past when embedded system had limited processing and memory

resources. It is still true to some extent but the increase in processor speed and

memory size means that operating systems that run on general purpose computing

platforms can be used on embedded systems. The Linux kernel for an embed-

ded system will almost certainly need to be customised to work on the target

platform (but this is true of any computer system). For devices that lack mem-

ory management units (MMUs) there is a separate Linux kernel branch called

uClinux. Systems that need to support real-time applications need a real-time

kernel. The vanilla Linux kernel does not support real-time. There is a real-time

version of Linux, namely, RTLinux which has an RTOS (real-time operating sys-

tem)microkernel.

Interact with the physical world Physical computing: is a branch of computing

concerned with sensing and responding to the analogue world. This application

is considered the domain of embedded systems.

Single board computer: Entire computer systems can be built on a single printed

circuit board (PCB). The PCB includes the CPU memory and peripheral devices.

Such systems are called single board computers (SBCs) and are well suited to

embedded system platforms. It is possible to go beyond single board computers

with system on a chip (SoC). This format too, lends itself to embedded system

design.

The distinction between an embedded system and a general processing system is

somewhat blurred. The Raspberry Pi for example, ﬁts many of the criteria above,

yet is primarily designed to function as a general processing computer. Given that it

shares many characteristics of an embedded system, it is being used extensively for

embedded applications.

We do not intend to get overly concerned with strict deﬁnitions of embedded

systems in this book. Our focus is on the core components of an embedded operating

system. For this reason, we omit graphical user interfaces and sophisticated user input

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