嵌入式 Linux 应用：概述---毕业设计外文翻译.doc

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1、毕业设计外文资料翻译（译文）题目名称：Embedded Linux applications: An overview学 院： 计算机科学技术 专业年级： 计算机科学与技术07级 学生姓名： xx 班级学号： 1班 16号 指导教师： xx 二一一年 四 月 十 日译文题目： Embedded Linux applications: An overview 原文题目： 嵌入式 Linux 应用：概述 原文出处：Internet source : -works/cn/linux/embed/embl/overview/index.htmlEmbedded Linux applications:

2、An overviewLinux now spans the spectrum of computing applications, including IBMs 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 supercomputers. Lets take

3、a look at what Linux has to offer as an embedded system, and why its the most attractive option currently available.One. Emergence of embedded systemsThe computers used to control equipment, otherwise known as embedded systems, have been around for about as long as computers themselves. They were fi

4、rst used back in the late 1960s in communications to control electromechanical 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. Increasingly,

5、these embedded systems need to be connected to some sort of network, and thus require a networking stack, which increases the complexity level and requires more memory and interfaces, as well as, you guessed it, the services of an operating system. Off-the-shelf operating systems for embedded system

6、s began to appear in the late 1970s, and today several dozen viable options are available. Out of these, a few major players have emerged, such as VxWorks, pSOS, Neculeus, and Windows CE. Two. Advantages/disadvantages of using Linux for your embedded systemAlthough most Linux systems run on PC platf

7、orms, 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 and administer than UNIX, is an added advantage for UNIX gurus who already appreciate the operating system because it has many of the sa

8、me commands and programming interfaces as traditional UNIX.The 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. However, the Linux

9、 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 complete Linux system runs quite nicely in 500 K of memory on an Intel 386 microprocessor, with

10、an 8-bit bus (SX). Because the memory required is often dictated by the applications needed, such as a Web server or SNMP agent, a Linux system can actually be adapted to work with as little as 256 KB ROM and 512 KB RAM. So its a lightweight operating system to bring to the embedded market. Another

11、benefit of using an open source operating system like Embedded Linux over a traditional real-time operating system (RTOS), is that the 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

12、 (NIC) drivers and parallel and serial port drivers, are available for Linux than for commercial operating systems. The core Linux operating system itself has a fairly simple micro-kernel architecture. Networking and file systems are layered on top of the micro-kernel in modular fashion. Drivers and

13、 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 custom embeddable system, which typically uses a combination of custom drivers and application programs to provide the added func

14、tionality. An embedded system also often requires generic capabilities, 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 wide range of

15、peripherals and has a ready inventory of off-the-shelf applications. Linux is also well-suited for embedded Internet devices, because of its support of multiprocessor systems, which lends it scalability. This capability gives a designer the option of running a real-time application on a dual process

16、or system, increasing total processing power. So you can run a Linux system on one processor while running a GUI, for example, simultaneously on another processor. The one disadvantage to running Linux on an embedded system is that the Linux architecture provides real-time performance through the ad

17、dition 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 software modules run in the kernel space, a code error can impact the entire syste

18、ms reliability by crashing the operating system, which can be a very serious vulnerability for real-time applications. An off-the-shelf RTOS, on the other hand, is designed from the ground up for real-time performance, and provides reliability through allocating certain processes a higher priority t

19、han others when launched by a user as opposed to by system-level processes. Processes are 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 c

20、urrently executing and of their associated priority levels. Such an approach ensures a higher reliability (predictability) with the RTOS time than Linux is capable of providing. But all-in-all, its still a more economical choice. Three. Different types of Embedded Linux systemsThere are already many

21、 examples of Embedded Linux systems; its safe to say that some form of Linux can run on 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

22、Linux versions:ETLinux - a complete Linux distribution designed to run on small industrial computers, especially PC/104 modules. LEM - a small (8 MB) multi-user, networked Linux version that runs on 386s. LOAF - Linux On A Floppy distribution that runs on 386s.uClinux - Linux for systems without MMU

23、s. Currently supports Motorola 68K, MCF5206, and MCF5207 ColdFire microprocessors.uLinux - tiny Linux distribution that runs on 386s.ThinLinux - a minimized Linux distribution for dedicated camera servers, X-10 controllers, MP3 players, and other such embedded applications.Software and hardware requ

24、irementsSeveral user-interface tools and programs enhance the versatility of the Linux basic kernel. Its 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 co

25、mplete range of file system and network services.A minimal Embedded Linux system needs just three essential elements: A boot utility The Linux micro-kernel, composed of memory management, process management and timing services An initialization processTo doing anything useful while remaining minimal

26、, you also need to add: Drivers for hardware One or more application processes to provide the needed functionalityAs additional requirements become necessary, you might also want: A file system (perhaps in ROM or RAM) TCP/IP network stack A disk for storing semi-transient data and swap capability A

27、32-bit internal CPU (required by all complete Linux systems)Four. Hardware platform optionsChoosing the best hardware can be complex because of internal company politics, prejudices, legacies of other projects, a lack of complete or accurate information, and cost, which should take into account the

28、total product costs and not just the CPU itself. Sometimes a fast, inexpensive CPU can become expensive 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 of how f

29、ast the CPU needs to run to accomplish a given task and triple it. Also, determine how fast the bus needs to run. If there are secondary buses, such as a PCI bus, consider them also. A slow bus (that is, one that is saturated with DMA traffic) can significantly slow down a fast CPU. Here are some of

30、 the best hardware solutions for Embedded Linux applications.Bright Star Engineering: Bright Star Engineerings 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, including Ethernet, LCD/V

31、ideo Controller, USB, Serial I/O, and a 16K gate user-configurable FPGA. BSEs Embedded Linux configuration allows Linux to be booted from the ipEngines on-board 4MB flash memory.Calibri: CalibriTM-133 is a ready-to-use, compact, multipurpose network appliance that uses Embedded Linux as its operatin

32、g system. It offers a highly efficient and low-cost solution to firewall, VPN, and routing demands.EmbeddedPlanet: EmbeddedPlanet has created a PostPC-era computer that comes loaded with MontaVistas HardHat Linux. Powered by a PowerPC-based computing engine and matching I/O card, Linux Planet comes

33、in a colorful translucent case with a touchscreen and access to digital and analog I/O. Eurotech: Eurotech provides embedded PC SBC and sponsors ET-Linux, a complete Linux system designed to run on small industrial computers and based on glibc 2.1.2. Microprocess Ingenierie: Microprocess develops, p

34、roduces, and sells standard and customized products for the industrial 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 of Linux o

35、r an Embedded Linux version.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 solutions are engineered to offer a simple, secure, and affordable extranet-friendly Virtual Private Network (VPN

36、) for flat networks. The NETtel router family runs an Embedded Linux kernel. A development kit is available; it enables customized code to be stored in flash memory and executed inside the NETtel. The code may contain special encryption or authentication protocols, or some local monitoring script wh

37、ere NETtel is used as a remote control device.Matrix Orbital: This an optional, but not recommended, addition. Matrix Orbital 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

38、 20x4 VFDs, plus a 240x64 graphic LC (128x128 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. Five. Real-time Embedded Linux applicationsOne of the m

39、ost important issues with embedded systems is the need for a real-time operating system. The definition of real-time here varies quite 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 qu

40、ite a bit. Some systems need hard real-time response, with short deterministic response latencies to events. However, on many systems, when analyzed closely, we see a response time requirement that is actually near real-time. Often the real-time requirement is a tradeoff of time and buffer space. Wi

41、th memory getting cheaper, and CPUs getting faster, near real-time is now more typical than hard real-time and many commercial operating systems that claim to be real-time are far from being hard real-time. Usually, when you get into the detailed design of these systems, there are warnings that the

42、drivers interrupts and applications must be very carefully designed in order to meet real-time requirements. RT-Linux (Linux with real-time extensions) contains time critical functions to provide precise control over interrupt handling, through the use of an interrupt manager, and does a good job of

43、 making sure that critical interrupts get executed when needed. The hardness of this approach 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

44、pretty well at keeping up with multiple streams of events. For example, a Linux PC system on a low end Pentium is able to keep multiple 10BaseT interfaces executing effectively, while simultaneously running character-level serial ports at a full 56KBPS without losing any data.Some real-time hardware

45、 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 Technology. RTAI (DIAPM) is a spin-off of the RTLinux real-time API that was developed by programmers at the Department of Aerospace En

46、gineering, Polytechnic Politecnico di Milano (DIAPM). EL/IX is a proposed POSIX-based hard real-time Linux API being promoted by Red Hat. And Linux-SRT is a soft real-time alternative to real-time APIs, which provides performance-enhancing capabilities to any Linux program without requiring that the

47、 program be modified or recompiled. See the Resources section later in this article for information on the above and for some Web sites offering different flavors of software extensions, development tools, support, and training courses for the standard Linux operating system. Short deterministic res

48、ponse latenciesSome real-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 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 time it will take to respond to an external event.