Embedded Linux Primer: A Practical, Real-World Approach

• apache Implements the popular Apache web server.

• telnet-server Contains files necessary to implement telnet server functionality, which allows you to establish Telnet sessions to your embedded target.

• glibc Standard C library

• busybox Compact versions of dozens of popular command line utilities commonly found on UNIX/Linux systems.[15]

This is the purpose of a Linux distribution as the term has come to be used. A typical Linux distribution comes with several CD-ROMs full of useful programs, libraries, tools, utilities, and documentation. Installation of a distribution typically leaves the user with a fully functional system based on a reasonable set of default configuration options, which can be tailored to suit a particular set of requirements. You may be familiar with one of the popular desktop Linux distributions, such as RedHat or Suse.

A Linux distribution for embedded targets differs in several significant ways. First, the executable target binaries from an embedded distribution will not run on your PC, but are targeted to the architecture and processor of your embedded system. (Of course, if your embedded Linux distribution targets the x86 architecture, this statement does not apply.) A desktop Linux distribution tends to have many GUI tools aimed at the typical desktop user, such as fancy graphical clocks, calculators, personal time-management tools, email clients and more. An embedded Linux distribution typically omits these components in favor of specialized tools aimed at developers, such as memory analysis tools, remote debug facilities, and many more.

Another significant difference between desktop and embedded Linux distributions is that an embedded distribution typically contains cross-tools, as opposed to native tools. For example, the gcc toolchain that ships with an embedded Linux distribution runs on your x86 desktop PC, but produces binary code that runs on your target system. Many of the other tools in the toolchain are similarly configured: They run on the development host (usually an x86 PC) but operate on foreign architectures such as ARM or PowerPC.

2.4.1. Commercial Linux Distributions

There are several vendors of commercial embedded Linux distributions. The leading embedded Linux vendors have been shipping embedded Linux distributions for some years. Linuxdevices.com, a popular embedded Linux news and information portal, has compiled a comprehensive list of commercially available embedded Linux distributions. It is somewhat dated but is still a very useful starting point. You can find their compilation at www.linuxdevices.com/articles/AT995240555 8.html.

2.4.2. Do-It-Yourself Linux Distributions

You can choose to assemble all the components you need for your embedded project on your own. You will have to decide whether the risks are worth the effort. If you find yourself involved with embedded Linux purely for the pleasure of it, such as for a hobby or college project, this approach might be a good one. However, plan to spend a significant amount of time assembling all the tools and utilities your project needs, and making sure they all interoperate together.

For starters, you will need a toolchain. Gcc and binutils are available from www.fsf.org and other mirrors around the world. Both are required to compile the kernel and user-space applications for your project. These are distributed primarily in source code form, and you must compile the tools to suit your particular cross-development environment. Patches are often required to the most recent 'stable' source trees of these utilities, especially when they will be used beyond the x86/IA32 architecture. The patches can usually be found at the same location as the base packages. The challenge is to discover which patch you need for your particular problem and/or architecture.

2.5. Chapter Summary

This chapter covered many subjects in a broad-brush fashion. Now you have a proper perspective for the material to follow in subsequent chapters. In later chapters, this perspective will be expanded to develop the skills and knowledge required to be successful in your next embedded project.

• Embedded systems share some common attributes. Often resources are limited, and user interfaces are simple or nonexistent, and are often designed for a specific purpose.

• The bootloader is a critical component of a typical embedded system. If your embedded system is based on a custom-designed board, you must provide a bootloader as part of your design. Often this is just a porting effort of an existing bootloader.

• Several software components are required to boot a custom board, including the bootloader and the kernel and file system image.

• Flash memory is widely used as a storage medium in embedded Linux systems. We introduced the concept of Flash memory and expand on this coverage in Chapters 9 and 10.

• An application program, also called a process, lives in its own virtual memory space assigned by the kernel. Application programs are said to run in user space.

• A properly equipped and configured cross-development environment is crucial to the embedded developer. We devote an entire chapter to this important subject in Chapter 12.

• You need an embedded Linux distribution to begin development of your embedded target. Embedded distributions contain many components, compiled and optimized for your chosen architecture.

2.5.1. Suggestions for Additional Reading

Linux Kernel Development, 2nd Edition

Robert Love

Novell Press, 2005

Understanding the Linux Kernel

Daniel P. Bovet & Marco Cesati

O'Reilly & Associates, Inc., 2002

Understanding the Linux Virtual Memory Manager

Bruce Perens

Prentice Hall, 2004