【正文】 s are actually more about deterministic behavior than raw speed.The bare Linux kernel is not real time. No guarantees are provided for deterministic interrupt handling times and the numbers can vary greatly depending on what the system load is when the interrupt occurs. RTLinux was designed to add hard real time characteristics to the Linux kernel.First released in February 1997 by Michael Barabanov and Victor Yodaiken of the New Mexico Institute of Technology, RTLinux is not a separate, modified version of the Linux kernel. After considering this possibility, it was deemed to be a maintenance and patibility nightmare2. Instead RTLinux was designed as a small, independent realtime kernel and a set of replacements for the Linux kernel routines that enable and disable interrupts. RTLinux also supplies an API that allows the user to create tasks in the realtime kernel and municate with processes in the Linux kernel via FIFOs or shared memory. Figure graphically depicts how the system works. When RTLinux is included and the Linux kernel calls the CLI or STI to disable enable interrupts, respectively, the request gets routed to an RTLinux routine that remembers the current Linux interrupt state. When interrupts occur they are always handled by the RTLinux kernel and are then passed on to the Linux kernel if they were not handled and interrupts are enabled. This allows RTLinux to always react to every interrupt without the Linux kernel getting in the way. This also gets around the problem of the Linux kernel not being able to handle nested interrupts since RTLinux always sees the interrupt first and can ensure that the kernel is never reentered.RTLinux is not the only player in this arena. Real Time Application Interface (RTAI) takes a similar approach in extending the existing Linux kernel with real time features. They have a few features, such as periodic timers, that are not present in RTLinux but do not have the wide popularity of RTLinux. If you believe the rumors, these two products will merge in some future version.There are some problems with the features present in both RTLinux and RTAI. First of all it is clear that the features were bolted on as an afterthought instead of being an intrinsic part of the design. Although RTLinux does an admirable job of acplishing it’s goals it tends to be useful in certain classes of applications such as data acquisition rather then being more generally applicable. Another issue is that RTLinux does nothing to address the problem of priority inversion, which can be the cause of serious bugs that are hard to reproduce and track down. RealTime FIFOsInterrupt Control HardwareRealTime KernelRealTime TasksLinuxLinux Processes Figure 1: Block Level design of RTLinuxGraphical User InterfacesAs the desktop puter has e to rely on GUIs rather than text based interfaces, many embedded systems have also been adopting them. A GUI in an embedded system may e in many different flavors including small resolution LCDs, an optional VGA output port, or a fully integrated screen with either vertical or horizontal orientation. Although similar in concept to desktop GUIs, embedded systems have some requirements, such as the kernel footprint, that are uniquely their own.Linux has always suffered from a fragmented market in the area of GUIs. Although the XWindow the closest thing to a standard, there are many popular petitors including emulators for the Win32 API and the look and feel of Windows 95. Embedded systems are certainly free to use any of these systems that are targeted for the desktop, but there are several products that specifically target the unique needs of embedded systems.Lineo Systems is one provider of a version of embedded Linux with enhancements and mercial support. One of the options that they provide is a custom toolkit for building an embedded GUI. Microwindows and NanoGUI are two public, open source projects that were merged to provide a free library for emulating the Win32 API for use on Linux systems. There are certainly other options – these are just the tip of the iceberg.There is another, separate route that many embedded devices have chosen to take when providing a GUI. Since network and Internet connectivity are so mon these days, many systems choose to include a web server and offer up their user interface via HTML. This allows the user to access the device remotely from any location with any platform that has a web browser. It also places fewer requirements on the size and shape of the device since it doesn’t have to include a display. Development Tool SupportEmbedded Linux developers get the best of both worlds when it es to development tools. Not only do they get to utilize the same GNU tool chain a。 Footprint and runtime memory restrictions Economic considerations Configurability雖然嵌入式Linux似乎在適應(yīng)設(shè)備的數(shù)量方面打得贏了Windows CE，但這兩者之間是有很大的重疊的。此外，其受歡迎程度似乎是迅速上升，并且眾多開發(fā)商都在迅速積累經(jīng)驗(yàn)。這僅僅是嵌入式Linux設(shè)備的冰山一角。研究人員又增加了新的硬件驅(qū)動(dòng)程序和一個(gè)專門的閃存文件系統(tǒng)。只有一張信用卡大小，大約有一節(jié)9伏電池那么厚， Itsy使用一枚30200MHz的StrongArm SA – 1100處理器并且擁有32MB的內(nèi)存和32 MB的Flash ROM 。ColdFire處理器是沒有MMU的。這個(gè)系統(tǒng)是嵌入式Linux在高端應(yīng)用的一個(gè)很好的例子，它能夠支持多種硬件和通信方式。 全球市場快覽 當(dāng)然，這一切都是為了創(chuàng)造一些有趣的和有用的東西。 當(dāng)然，這一切都不是免費(fèi)的。隨著Linux在臺(tái)式機(jī)和嵌入式系統(tǒng)的日益普及，開發(fā)商們所能得到的合同和收益都會(huì)隨之而增長。當(dāng)您要跟蹤一個(gè)討厭的錯(cuò)誤的時(shí)候一個(gè)不可取代的步驟是通過源代碼。 近來,另一個(gè)令人關(guān)注的問題已經(jīng)在Linux社區(qū)引發(fā)了很多討論，實(shí)際上RTLinux是有專利的。Lineo’s Embedix的版權(quán)費(fèi)是隨著不同的操作系統(tǒng)功能和額外的軟件包包括GUI工具包和Web瀏覽器而變化的。 經(jīng)濟(jì)因素 一般而言Linux最吸引人的地方就在于它是免費(fèi)的。此外，已經(jīng)有許多人找到了GNU內(nèi)核調(diào)試器的漏洞，并且它很難使用，特別令人沮喪的是在開發(fā)嵌入式系統(tǒng)設(shè)備驅(qū)動(dòng)程序上這一點(diǎn)是非常普遍的。 GNU包含了編譯器，調(diào)試器，匯編程序和鏈接程序，能夠支持各種各樣的微處理器，是軟件開源精神一個(gè)最好的證明。還有，不管設(shè)備的大小和形狀都只有很少的要求，因?yàn)樗鼪]有顯示器。當(dāng)然，還有其他的選擇，這些只是冰山的一角。嵌入式系統(tǒng)當(dāng)然是免費(fèi)使用這些針對(duì)臺(tái)式機(jī)的系統(tǒng)，但也有一些產(chǎn)品專門針對(duì)特殊需求的嵌入式系統(tǒng)。一個(gè)擁有GUI的嵌入式系統(tǒng)可能會(huì)有多種不同的選擇，包括低分辨率液晶顯示器，可選的VGA輸出端口，或一個(gè)完全集成的屏幕，不管是縱向或橫向的。首先，很顯然這些功能特性都是后來加上去的，而不是作為固有的組成部分而設(shè)計(jì)的。實(shí)時(shí)應(yīng)用程序接口（ RTAI ）采取了類似的做法擴(kuò)展現(xiàn)有的Linux內(nèi)核的實(shí)時(shí)功能。當(dāng)中斷發(fā)生時(shí)，他們總是由RTLinux的內(nèi)核接手，然后傳遞給Linux內(nèi)核，如果Linux內(nèi)核不處理則中斷啟用。RTLinux被