Linux在嵌入式的应用外文翻译

Linux 应用于嵌入式系统 UNIX 引起了全然不同的情绪 因其能力和灵活性而受到一些人的青睐 但因 其复杂且神秘的命令而受到另一些人的藐视 在计算世界里 UNIX 已建立起一 个褒义不一的声誉 易用性 UNIX 因其过于神秘 不易记忆和含义模糊的大量键盘命令而名声不佳 这 些命令每个都有很多命令行开关 这可能造成昏药而不易记忆 它的SVR4 版 本拥有 2000 多个命令 其中很多功能可通过管道和重定向进行组合 这反映了 UNIX 的基本设计思想之一 生成数量很大的专用和模块化命令 把它们结合起来 就能完成各种复杂的任务 虽然 UNIX 过去基本上限于大学和应用开发公司中的软件专业人员使用 其 复杂的命令行语法和由此而得到的灵活性被看作一个优点而不是问题 但是在 面向商业的市场中使用 UNIX 却产生了一个重大的缺陷 系统越灵活 它就 变得越难学会和操作 UNIX 的原本 或脚本 语言提供某种形式的帮助 利用原本语言 系统管 理员能很快地把系统裁剪成满足一组用户的需求 克服 UNIX 神秘命令语言之困难的另一种方法是用图形用户接口GUI 如 Motif SunView 或 OpenLook 然而 图形用户接口在已经很复杂的系统上增添了 另一层不兼容性问题 Motif 已经移植到差别非常大的不同体系结构上 由于它仿效 Presentation Manager 的风格 而且在外观与感觉上也许是最接近于诸如 Microsoft Windows 一类的 PC 接口 SunView 由于有很大的装机数和很多应用 程序员已熟悉按它的规范写软件 所以也是一种主流图形用户接口 二进制兼容性 UNIX 的销售者以某种羡慕的心情关注着DOS 世界中可能的大量简装应用程 序 并承诺不同系统的二进制兼容的应用程序即将面世 这些承诺远未让人满足 虽然二进制兼容性尚未获得 但现在不同机器上共享数据和应用程序正在变得更 加容易 可移植性 与多数操作系统源码相比 UNIX 程序容易移植 它用 C 语言编写 而不用 汇编语言 使用 UNIX 能移植到不同的体系结构上 但是把UNIX 移植到一个新 系统上也不是一件一蹴而就的事情 常常是要几个人年的工作 还可能造成故障 和失灵 从而在性能上产生难以捉摸的不一致性 这些故障往往是难以识别与纠 正的 拥有你自己的计算机操作系统的源码 既有益 也有害 如果操作系统缺少 某些所需功能 公司自己拥有源码能大大增强公司进行必要修改的能力 另一方面 具有新的或修改特性的操作系统定制版本 在日后可能出现与更 新的版本或购买的应用程序不兼容的问题 为什么用 LINUX 用于接口 监控 通信和控制应用程序的职能专用系统和设备越来越要求 高级的现代操作系统的这些服务 许多这样的系统需要如下的高级性能 高分辨 率和用户友好的图形用户界面 TCP IP 链接 用可靠的闪存固态盘代替常规的 磁盘机 支持 32 位的超高速 CPU 使用大存储器阵列 以及似乎是无限容量 的存储设备 包括 CD ROM 和硬盘 这不是以前的独立代码 自己写的核 或简单的老是DOS 那些日子已永 远过去 另外也考虑到硬件和芯片迅速加速的革新步伐 伴随着老设备相当快地 淘汰 结合这两种情况 就能知道为什么对商用实时操作系统供应商而言 跟上 硬件设备的不断出新已变成巨大的挑战 及时地支持最新设备 甚至不去理会不 愿退让的 大力推销的 逐渐过时的芯片组 你需要大量和不断的资材投入 如果 商用实时操作系统供应商必须奋力紧跟硬件发展的话 那么编写独立的代码或写 自己的核 这种单枪匹马的做法一定是毫无意义的 因为选择范围很小 嵌入式系统的开发商面临这样的一种困境 一方面 今天高度复杂的 且授权的智能嵌入式系统 基于最新的芯片和硬 件性能 所需要的正式流行的高档操作系统 如Windows 提供的那种能力 精致性 以及通用性 另一方面 嵌入式系统要求非常高的可靠性 不停机 无人照管的操作 加上有能力把操作系统改编成符合应用系统的独特的要求 其窘境是 通用桌面操作系统 如Windows 不能很好地适应于类似设备的 嵌入式系统的独特需求 然而 商用实时操作系统 虽然设计成满足嵌入式应用 的可靠性和配置灵活性的要求 但由于他们缺乏标准化以及没有能力跟上技术 的速度发展步伐 它们日益不合需要 开发人员做什么 幸运的是 一种新的 令人兴奋的选择系统已经出现 开放源码 Linux Linux 提供功能强大的和高级系统管理设施 丰富的设备支持 在可靠 性和鲁棒性 以及广泛详尽的文档方面有极好的声誉 最好的 对系统开发人员 而言 是 Linux 不要钱 有完全免费的源代码 Linux 是不是像 Windows 那样太大以及需要系统资源太多 以致不能满足嵌 入式系统的约束要求呢 与 Windows 不同 Linux 本来就是模块化的 并且能 够很容易缩减成紧缩配置 这种配置几乎与DOS 差不多大 甚至能放到一张软 盘上 此外 因为 Linux 源码是免费可用的 所以可以按照独特的嵌入式系统要 求改编该操作系统 这样 并不令人惊奇 开放源码 Linux 已建成了一个新的操作系统开发和 支持范例 在那里数以千计的开发人员继续贡献于不断发展的Linux 代码库 此外 几十家面向 Linux 的软件公司已经出现 他们热切支持那些为建立从 工厂自动化到智能设备范围很广的应用系统的开发人员的要求 小 Linux 对许多嵌入式系统 为了适应诸如RAM 固态盘 处理机速度 以及功耗的 约束 嵌入的 Linux 的主要任务是 使系统所需的资源最小 嵌入式操作系统可 能需要从一个芯片盘或紧凑闪存固态盘上自举 或者自举和运行在没有显示器和 键盘 无头 操作 的环境 或经有以太网连接 从远程设备装入应用程序 现成的小 Linux 有许多来源 其中有日益增多的面向应用的Linux 配置和 分发版 这些都被修改成适应于特定的应用 例如路由器 防火墙 互联网 网络设备 网络服务器 网关等 你也可能选择建立你自己喜欢的嵌入式Linux 从一个标准分发版开始 略 去不要的模块 虽然如此 还应该考虑从别人的工作配置基础上开始你的工作 因为他们的版本的源仅是完全合法的 而且也是被鼓励 实时 Linux 许多嵌入式系统需要对现实世界的一些事件可预 测 并且受限响应 这样 的实时系统包括工厂自动化 数据采集和控制系统 音频 视频应用 以及许多 其他计算机化的产品和设备 什么是 实时系统 通常接受的 实时 性能 的定义是 现实世界时间必须在确定的 可预测的 以及在相对短的时间间隔内 得到响应 虽然 Linux 不是一个实时操作系统 Linux 内核不提供所需要的事件优先 级和抢占功能 但当前有几个扩充选项可用 这些选项把实时能力带给基于 Linux 的系统 最通常的方法是双内核方法 用这个方法 通用 非实时 OS 运行作为实时内核的一个任务 通用操作系统提供诸如磁盘读 写 LAN 通信 串行 并行 I O 系统初始化 内存管理等功能 而实时内核处理时限世界事件 你可以把这个看作两者兼得 因为它能够保持流行的通用操作系统好处 而增加 了实时 OS 的能力 就 Linux 来说 你能保持与标准 Linux 兼容 而以非干扰的 方式增加了实时功能 当然 也可以专研并修改 Linux 把改变成实时操作系统 因为它的源码是 公开可用的 但如果这样做 你会面临这样严重缺点 即不论特性方面 还是驱 动程序方面都不能与主流 Linux 同步前进 简言之 你的制定 Linux 将不能从 Linux 的不断进展中获得好处 而这种进展是世界范围内数以千计的开发人员共 同协力的结果 Linux 是一个操作系统 它担当计算机系统硬件与软件间的通信服务 Linux 内核包含了你在任何操作系统所期望的所有特性 原文 Using Linux in Embedded Systems UNIX evokes a wide range of emotions loved by some for its power and flexibility despised by others for its eomplex and arcane commands UNIX has established a checkered reputation in the world of computing EASE OF USE UNIX is infamous for its glut of arcane non mnemonic and cryptic keyboard commands each with many command line switches which can be incredibly confusing to remember Its SVR 4 implementation contains more than 2 000 comands Many of these functions can be combined using pipes and redirection This illustrates one of UNIX s design fundamentals the creation of a large assortment of very specialized and modular commands that can be combined to accomplish complex tasks While UNIX was essentially limited to use by software professionals at universities and in applications development houses its complex command line syntax and resulting flexibility were considered an advantage rather than a problem But this same flexibility also creates a major drawback for using UNIX in a business oriented market the more flexible a system is the more difficult it becomes to learn and operate UNIX s script languages provide one form of help Using scripts a system administrator can tailor the system to a set of users needs quickly Another factor mitigating the difficulties of UNIX s arcane command language are the Graphic User Interfaces GUIs such as Motif SunView or OpenLook GUIs however place another level of incompatibility problems on what is already a complex system Motif has been ported to the most different architectures and because it follows the Presentation Manager style is perhaps closest in look and feel to a PC interface such as Microsoft Windows Sun View is also dominant because of its large installed base and the numbers of applications programmers who have become familiar with writing software to its specifications BINARY COMPATIBILITY UNIX marketers have looked with some envy at the huge base of shrink wrapped applications available in the DOS world and have promised that binary compatible applications for systems is just around the corner These promises have yet to be met in any significant way While binary compatibility is not yet available it is getting easier to share data and applications across different machines PORTABILITY Compared with most operating system sources UNIX code is quite move UNIX to different architectures But a UNIX port to a new system is not a trivial matter offen taking several man years of work and can result in glitches and malfunctions which may create very subtle inconsistencies in performance These bugs are often difficult to identify and correct Having the source code available for your computer s operating system is beneficial and detrimental if the OS lacks certain desirable featuers having the sources in house greatly enhances a company s ability to make necessary changes On the other hand the customized version of the operating system with its new or modified features may later present compatibility problems with newer releases or purchased application Why Linux Intelligent dedicated systems and applicances used in interface monitoring communications and control applications increasingly demand the services of a sophisticated state of the art operating systems Many such systems require advanced capabilities like high resolution and user friendly graphical user interfaces GUIs TCP IP connectivity substitution of reliable and low power flash memory solid state disk for conventional disk drives support for 32 bit ultra high speed CPUs the use of large memory arrays and seemingly infinite capacity storage devices including CD ROMs and hard disks This is not the stuff of yesteryear s standalone code roll your own kernels or plain old DOS No those days are gone forever Then too consider the rapidly accelerating pace of hardware and chipset innovation accompanied by extremely rapid obsolescence of the older devices Combine these two and you can see why it s become an enormous challenge for commercial RTOS vendors to keep up with the constant churning of hardware devices Supporting the newest devices in a timely manner even just to stay clear of the unrelenting steamroller of chipset obsolescence takes a large and constant resource commitment If it s a struggle for the commercial RTOS vendors to keep up going it alone by writing standalone code or a roll your own kernel certainly makes no sense With the options narrowing embedded system developers find themselves faced with a dilemma On the one hand today s highly sophisticated and empowered intelligent embedded systems based on the newest chips and hardware capabilities demand noting less than the power sophistication and currency of support provided by a popular high end operating system like windows On the other hand embedded systems demand extremely high reliability for non stop unattended operation plus the ability to customize the OS to match an application s unique requirements The dilemma is common desktop operating system such as Windows are not well adapted to the similar equipment to the unique needs of embedded system However commercial real time operating system although designed to meet the embedded application reliability and configuration flexibility requirements but due to their lack of standardization and inability to keep up with the speed of technology development which is undesirable Developers do Fortunately a new exciting alternative systems have emerged the open source Linux Linux provides powerful and advanced system management facilities A wealth of equipment support the reliability and robustness and extensive documentation has an excellent reputation The best for developers is Linux not money are completely free source code Linux is it right Like Windows is too large and the need for system resources too much so can not meet the requirements of embedded system Unlike Windows Linux is a modular and can easily be reduced into compact configuration this configuration is almost almost DOS even on a single floppy disk In addition because the Linux source code is freely available so it can be in accordance with the unique requirements of embedded systems to adapt the operating system So not surprisingly the open source Linux has built a new operating system development and support the paradigm where thousands of developers continue to contribute to the development of the Linux code library In addition dozens of Linux oriented software companies have emerged they eager to support those built from factory automation to intelligent devices are a wide range of application systems development personnel requirements Small Linux For many embedded systems such as RAM in order to adapt to the solid state disk processor speed and power constraint embedded Linux is a major task the system resources required minimum Embedded operating system may need from a chip disk or compact flash solid state disk bootstrap or bootstrap and operation in the absence of the display and the keyboard headless operation environment or by the Ethernet connection from a remote device in application Small ready made Linux has many sources including the increasing application oriented Linux configuration and distribution these are modified into adapted to specific application For example router firewall Internet network equipment network server gateway You may also choose to build your own like embedded Linux from a standard distribution version omitting don t module Even so also should be taken from the work of others on the basis of the allocation to begin your work because their version of the source is perfectly legal but also be encouraged Real time Linux Many embedded systems need to real world events can be predicted and limited response Such a system include factory automation data acquisition and control system audio video applications as well as many other computer products and equipment What is real time system The generally accepted real time performance is defined the real world time must be identified predictable and within a relatively short time interval in response Although Linux is not a real time operating system Linux cores do not provide the required priority and preemption features but currently a few expansion opti