計(jì)算機(jī)外文翻譯--嵌入式_linux_應(yīng)用概述

1、<p>　　畢業(yè)設(shè)計(jì)外文資料翻譯</p><p><b>　　（譯文）</b></p><p>　　譯文題目： Embedded Linux applications: An overview </p><p>　　原文題目： 嵌入式 Linux 應(yīng)用：概述 </p><p>

2、　　原文出處： http://www.ibm.com/developerworks/linux/library/l-embl/index.html </p><p>　　Embedded Linux applications: An overview</p><p>　　Linux now spans the spectrum of computing applications, incl

3、uding IBM's tiny Linux wrist watch, hand-held devices (PDAs and cell phones), Internet appliances, thin clients, firewalls, industrial robotics, telephony infrastructure equipment, and even cluster-based supercompute

4、rs. Let's take a look at what Linux has to offer as an embedded system, and why it's the most attractive option currently available.</p><p>　　One. Emergence of embedded systems</p><p>　

5、　The computers used to control equipment, otherwise known as embedded systems, have been around for about as long as computers themselves. They were first used back in the late 1960s in communications to control electrom

6、echanical telephone switches. As the computer industry has moved toward ever smaller systems over the past decade or so, embedded systems have moved along with it, providing more capabilities for these tiny machines. Inc

7、reasingly, these embedded systems need to be connected to som</p><p>　　Off-the-shelf operating systems for embedded systems began to appear in the late 1970s, and today several dozen viable options are avail

8、able. Out of these, a few major players have emerged, such as VxWorks, pSOS, Neculeus, and Windows CE. </p><p>　　Two. Advantages/disadvantages of using Linux for your embedded system</p><p>　　A

9、lthough most Linux systems run on PC platforms, Linux can also be a reliable workhorse for embedded systems. The popular "back-to-basics" approach of Linux, which makes it easier and more flexible to install an

10、d administer than UNIX, is an added advantage for UNIX gurus who already appreciate the operating system because it has many of the same commands and programming interfaces as traditional UNIX.</p><p>　　The

11、typical shrink-wrapped Linux system has been packaged to run on a PC, with a hard disk and tons of memory, much of which is not needed on an embedded system. A fully featured Linux kernel requires about 1 MB of memory. H

12、owever, the Linux micro-kernel actually consumes very little of this memory, only 100 K on a Pentium CPU, including virtual memory and all core operating system functions. With the networking stack and basic utilities, a

13、 complete Linux system runs quite nicely in 500 K of mem</p><p>　　Another benefit of using an open source operating system like Embedded Linux over a traditional real-time operating system (RTOS), is that th

14、e Linux development community tends to support new IP and other protocols faster than RTOS vendors do. For example, more device drivers, such as network interface card (NIC) drivers and parallel and serial port drivers,

15、are available for Linux than for commercial operating systems. </p><p>　　The core Linux operating system itself has a fairly simple micro-kernel architecture. Networking and file systems are layered on top o

16、f the micro-kernel in modular fashion. Drivers and other features can be either compiled in or added to the kernel at run-time as loadable modules. This provides a highly modular building-block approach to constructing a

17、 custom embeddable system, which typically uses a combination of custom drivers and application programs to provide the added functionality. </p><p>　　An embedded system also often requires generic capabilit

18、ies, which, in order to avoid re-inventing the wheel, are built with off-the-shelf programs and drivers, many of which are available for common peripherals and applications. Linux can run on most microprocessors with a w

19、ide range of peripherals and has a ready inventory of off-the-shelf applications. </p><p>　　Linux is also well-suited for embedded Internet devices, because of its support of multiprocessor systems, which le

20、nds it scalability. This capability gives a designer the option of running a real-time application on a dual processor system, increasing total processing power. So you can run a Linux system on one processor while runni

21、ng a GUI, for example, simultaneously on another processor. </p><p>　　The one disadvantage to running Linux on an embedded system is that the Linux architecture provides real-time performance through the add

22、ition of real-time software modules that run in the kernel space, the portion of the operating system that implements the scheduling policy, hardware-interrupts exceptions and program execution. Since these real-time sof

23、tware modules run in the kernel space, a code error can impact the entire system's reliability by crashing the operating system, which can be a</p><p>　　An off-the-shelf RTOS, on the other hand, is desig

24、ned from the ground up for real-time performance, and provides reliability through allocating certain processes a higher priority than others when launched by a user as opposed to by system-level processes. Processes are

25、 identified by the operating system as programs that execute in memory or on the hard drive. They are assigned a process ID or a numerical identifier so that the operating system may keep track of the programs currently

26、executing </p><p>　　Three. Different types of Embedded Linux systems</p><p>　　There are already many examples of Embedded Linux systems; it's safe to say that some form of Linux can run on

27、just about any computer that executes code. The ELKS (Embeddable Linux Kernel Subset) project, for example, plans to put Linux onto a Palm Pilot. Here are a couple of the more well-known small footprint Embedded Linux ve

28、rsions:</p><p>　　ETLinux -- a complete Linux distribution designed to run on small industrial computers, especially PC/104 modules. </p><p>　　LEM -- a small (<8 MB) multi-user, networked Linu

29、x version that runs on 386s. </p><p>　　LOAF -- "Linux On A Floppy" distribution that runs on 386s.</p><p>　　uClinux -- Linux for systems without MMUs. Currently supports Motorola 68K,

30、MCF5206, and MCF5207 ColdFire microprocessors.</p><p>　　uLinux -- tiny Linux distribution that runs on 386s.</p><p>　　ThinLinux -- a minimized Linux distribution for dedicated camera servers, X-

31、10 controllers, MP3 players, and other such embedded applications.</p><p>　　Software and hardware requirements</p><p>　　Several user-interface tools and programs enhance the versatility of the L

32、inux basic kernel. It's helpful to look at Linux as a continuum in this context, ranging from a stripped-down micro-kernel with memory management, task switching and timer services to a full-blown server supporting a

33、 complete range of file system and network services.</p><p>　　A minimal Embedded Linux system needs just three essential elements: </p><p>　　A boot utility </p><p>　　The Linux micro

34、-kernel, composed of memory management, process management and timing services </p><p>　　An initialization process</p><p>　　To doing anything useful while remaining minimal, you also need to add

35、: </p><p>　　Drivers for hardware </p><p>　　One or more application processes to provide the needed functionality</p><p>　　As additional requirements become necessary, you might also

36、 want:</p><p>　　A file system (perhaps in ROM or RAM) </p><p>　　TCP/IP network stack </p><p>　　A disk for storing semi-transient data and swap capability </p><p>　　A 32

37、-bit internal CPU (required by all complete Linux systems)</p><p>　　Four. Hardware platform options</p><p>　　Choosing the best hardware can be complex because of internal company politics, prej

38、udices, legacies of other projects, a lack of complete or accurate information, and cost, which should take into account the total product costs and not just the CPU itself. Sometimes a fast, inexpensive CPU can become e

39、xpensive once bus logic and the delays necessary to make it work with other peripherals are considered. To calculate the necessary speed of a CPU for any given project, start with a realistic view</p><p>　　B

40、right Star Engineering: Bright Star Engineering's ipEngine-1 is a credit-card sized single-board computer with Embedded Linux support. It utilizes a PowerPC-based CPU and provides an array of on-board peripherals, in

41、cluding Ethernet, LCD/Video Controller, USB, Serial I/O, and a 16K gate user-configurable FPGA. BSE's Embedded Linux configuration allows Linux to be booted from the ipEngine's on-board 4MB flash memory.</p>

42、;<p>　　Calibri: CalibriTM-133 is a ready-to-use, compact, multipurpose network appliance that uses Embedded Linux as its operating system. It offers a highly efficient and low-cost solution to firewall, VPN, and r

43、outing demands.</p><p>　　EmbeddedPlanet: EmbeddedPlanet has created a PostPC-era computer that comes loaded with MontaVista's HardHat Linux. Powered by a PowerPC-based computing engine and matching I/O c

44、ard, Linux Planet comes in a colorful translucent case with a touchscreen and access to digital and analog I/O. </p><p>　　Eurotech: Eurotech provides embedded PC SBC and sponsors ET-Linux, a complete Linux s

45、ystem designed to run on small industrial computers and based on glibc 2.1.2. </p><p>　　Microprocess Ingenierie: Microprocess develops, produces, and sells standard and customized products for the industrial

46、 and embedded market. Microprocess has a global activity in real-time software and great expertise in systems integration. Its products, like the 740 PowerPC compactPCI board, can be ordered with a standard distribution

47、of Linux or an Embedded Linux version.</p><p>　　Moreton Bay: Moreton Bay is releasing their NETtel 2520 and NETtel 2500 range of Linux-based Internet routers. These small, easy-to-connect intelligent router

48、solutions are engineered to offer a simple, secure, and affordable extranet-friendly Virtual Private Network (VPN) for flat networks. The NETtel router family runs an Embedded Linux kernel. A development kit is available

49、; it enables customized code to be stored in flash memory and executed inside the NETtel. The code may contain special en</p><p>　　Matrix Orbital: This an optional, but not recommended, addition. Matrix Orbi

50、tal manufactures a line of serial LCDs and VFDs, which many Linux users are including in their embedded systems. The product line ranges from 8x2 to 40x4 character LCDs, 20x2 and 20x4 VFDs, plus a 240x64 graphic LC (128x

51、128 on the way). Communication with the displays is accomplished via either RS232 or I2C, both of which are standard on all of their modules. A comprehensive command set is included in the modules' BIOS. </p>

52、<p>　　Five. Real-time Embedded Linux applications</p><p>　　One of the most important issues with embedded systems is the need for a real-time operating system. The definition of real-time here varies q

53、uite a bit. To some people, real-time means responding to an event in the one-microsecond range, to others it is 50 milliseconds. The hardness of real-time also varies quite a bit. Some systems need hard real-time respon

54、se, with short deterministic response latencies to events. However, on many systems, when analyzed closely, we see a response time require</p><p>　　RT-Linux (Linux with real-time extensions) contains time cr

55、itical functions to provide precise control over interrupt handling, through the use of an interrupt manager, and does a good job of making sure that critical interrupts get executed when needed. The hardness of this app

56、roach depends mostly on the CPU interrupt structure and context-switch hardware support. This approach is sufficient for a large range of real-time requirements. Even without the real-time extensions, Linux does pretty w

57、e</p><p>　　Some real-time hardware and software Linux APIs to consider are RTLinux, RTAI, EL, and Linux-SRT. RTLinux is a hard real-time Linux API originally developed at the New Mexico Institute of Technolo

58、gy. RTAI (DIAPM) is a spin-off of the RTLinux real-time API that was developed by programmers at the Department of Aerospace Engineering, Polytechnic Politecnico di Milano (DIAPM). EL/IX is a proposed POSIX-based hard re

59、al-time Linux API being promoted by Red Hat. And Linux-SRT is a soft real-time alter</p><p>　　See the Resources section later in this article for information on the above and for some Web sites offering diff

60、erent flavors of software extensions, development tools, support, and training courses for the standard Linux operating system. </p><p>　　Short deterministic response latencies</p><p>　　Some rea

61、l-time embedded systems need to respond quickly to external events in order to accomplish a specific task. A custom microcontroller embedded inside a missile, for example, needs to respond quickly to external events such

62、 as moving targets, weather, humans, etc., before instructing the missile to target a specific object in its surrounding environment. Short deterministic response latencies mean that the embedded system can determine the

63、 time it will take to respond to an external event. </p><p>　　Six. Configuration procedures</p><p>　　Now let's take a look at how to make LEM, a small, embeddable Linux distribution, which

64、provides both network and X server. You can download this distribution, although it is not essential. You will need a full Linux distribution to build your own Embedded Linux operating system, which will contain everythi

65、ng you need (utilities, sources, compiler, debugger, and documentation). Here is a list of the software that can be used to make LEM:</p><p>　　TinyLogin: TinyLogin is a suite of tiny UNIX utilities for handl

66、ing logging into, being authenticated by, changing one's password for, and otherwise maintaining users and groups on an embedded system. It also provides shadow password support to enhance system security. TinyLogin

67、is, as the name implies, very small, and makes an excellent complement to BusyBox on an embedded System.</p><p>　　BusyBox: BusyBox is a multicall binary used to provide a minimal subset of POSIX-style comman

68、ds and specialized functions. It is geared toward the very small, such as boot floppies, embedded systems, etc. Specifically it is used in the Debian Rescue/Install system (which inspired development on the original Busy

69、Box ), the Linux Routeur Project, LEM, lineo, and others. Busybox is being maintained by Erik Andersen.</p><p>　　Ash: Ash is a very small Bourne shell.</p><p>　　Sysvinit: Sysvinit is the most us

70、ed init package for Linux. We will use init and the C version of the start-stop-daemon. </p><p>　　See the Resources section for more information on these items. </p><p>　　Seven. Creating a boot

71、disk</p><p>　　A bootdisk is basically a miniature, self-contained Linux system on a floppy diskette. It can perform many of the same functions that a complete full-size Linux system performs. The following m

72、aterial is based on the Bootdisk-HOWTO (see Resources).</p><p>　　Step 1. Bios All PC systems start the boot process by executing code in ROM (specifically, the BIOS) to load the sector from sector 0, cylind

73、er 0 of the boot drive. The boot drive is usually the first floppy drive (designated A: in DOS and /dev/fd0 in Linux). The BIOS then tries to execute this sector. On most bootable disks, sector 0, cylinder 0 contains eit

74、her: </p><p>　　Code from a boot loader such as LILO, which locates the kernel, loads it, and executes it to start the boot proper </p><p>　　The start of an operating system kernel, such as Linux

75、</p><p>　　If a Linux kernel has been raw copied to a diskette, a hard drive, or another media, the first sector of the disk will be the first sector of the Linux kernel itself. This first sector will continu

76、e the boot process by loading the rest of the kernel from the boot device. </p><p>　　Step 2. The boot loader You will use a boot loader like LILO to operate the boot process. It allows the development and p

77、roduction platforms to co-exist on the same hardware and permits switching from one to the other just by rebooting. The LILO boot loader is loaded by the bios. It then loads kernels or the boot sectors of other operating

78、 systems. It also provides a simple command line interface to interactively select the item to boot with its options. See Resources for more information on LI</p><p>　　Step 3. The kernel The kernel checks t

79、he hardware and mounts the root device and then looks for the init program on the root filesystem and executes it.</p><p>　　Step 4. InitInit is the parent of all other processes that will run on your Linux

80、OS. It will watch its child processes and start, stop, re-launch them if needed. Init takes all information from /etc/inittab. </p><p>　　Step 5. InittabThe file /etc/inittab/ refers to scripts named /etc/rc

81、... to do the system setup. It also has entries for the getty tool to handle the login process. </p><p>　　Step 6. The login process There is one getty available in the inittab file for each console allowed

82、for the users. Getty will launch /bin/login to verify the user password. </p><p>　　Step 7. Creating a new partition From the LFS-HOWTO (see Resources): Before we can build our new Linux system, we need to h

83、ave an empty Linux partition on which we can build our new system. If you already have a Linux Native partition available, you can skip this step and the following one. Start the fdisk program (or cfdisk if you prefer th

84、at program) with the appropriate hard disk as the option (like /dev/hda if you want to create a new partition on the primary master IDE disk). Create a Linu</p><p>　　Step 8. Creating an ext2 file system on t

85、he new partition From the LFS-HOWTO (see Resources): To create a new ext2 file system we use the mke2fs command. Give $LFS as the only option, and the file system will be created. From now on I'll refer to this newl

86、y created partition as $EMBPART. $EMBPART should be substituted with the partition you have created. </p><p>　　Step 9. Mounting the partition To access the newly created filesystem, you have to mount it. To

87、 do this, create an /mnt/hda? directory and type the following at the shell prompt: </p><p>　　mkdir /mnt/hda?</p><p>　　mount $EMBPART /mnt/hda?</p><p>　　If you created your partitio

88、n on /dev/hda4 and you mounted it on /mnt/hda4, then you'll need to return to the step where you copied a file to $dollar;EMBPART/usr/sbin, and copy that file to /mnt/hda4/usr/bin. Do this after the last command in S

89、tep 14 (Copy the file in $EMBPART/usr/sbin).</p><p>　　Step 10. Populating the filesystem The root filesystem must contain everything needed to support a full Linux system. We will build a directory structur

90、e not that far from the File Hierarchy Standard (see Resources). </p><p>　　Step 11. DirectoriesThe mkdir function in the new mounted filesystem creates the following directories:</p><p>　　/proc

91、 Directory stub required by the proc filesystem /etc System configuration file /sbin Critical System binaries /bin Basic binaries considered part of the system/lib Shared Libraries to provide run-time support /

92、mnt Mount point for maintenance /usrAdditional utilities and applications </p><p>　　cd /mnt/hda? </p><p>　　mkdir bin dev home proc sbin usr boot etc liv mnt root tmp var </p><p>

93、　　mkdir -p usr/bin usr/sbin usr/share usr/lib </p><p>　　mkdir -p etc/config etc/default etc/init.d etc/rc.boot </p><p>　　mkdir -p etc/rc0.d etc/rc1.d etc/rc2.d etc/rc3.d etc/rc4.d etc/rc5.d etc/

94、rc6.d etc/rcS.d </p><p>　　/devThe dev directory is the stub required to perform devices input / output. Each file in this directory may be created using the mknod function. You may save time by directly cop

95、ying the required dev entries from your desktop Linux, using the following instruction:cp -dpR /dev /mnt</p><p>　　Eight. Installing TinyLogin and login dependencies</p><p>　　TinyLogin (see the

96、 Resources section to install it) will give us the following tools in less than 35Kb: /bin/addgroup, /bin/adduser, /bin/delgroup, /bin/deluser, /bin/login, /bin/su, /sbin/getty, /sbin/sulogin, /usr/bin/passwd. </p>

97、;<p>　　Please refer to your main distribution doc or man pages for a full description of those commands. </p><p>　　Step 12. Configuring TinyLogin From the TinyLogin README: TinyLogin is modularized t

98、o help you build only the components you need, thereby reducing binary size. To turn off unwanted TinyLogin components, simply edit the file tinylogin.def.h and comment out the parts you do not want using C++ style (//)

99、comments. </p><p>　　Step 13. Installing TinyLoginAfter the build is complete, a tinylogin.links file is generated, which is then used by make install to create symlinks to the tinylogin binary for all compi