【正文】 ime critical 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 approach depends mostly on the CPU interrupt structure and contextswitch hardware support. This approach is sufficient for a large range of realtime requirements. Even without the realtime extensions, Linux does 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 characterlevel serial ports at a full 56KBPS without losing any data. 6 Some realtime hardware and software Linux APIs to consider are RTLinux, RTAI, EL, and LinuxSRT. RTLinux is a hard realtime Linux API originally developed at the New Mexico Institute of Technology. RTAI (DIAPM) is a spinoff of the RTLinux realtime API that was developed by programmers at the Department of Aerospace Engineering, Polytechnic Politeico di Milano (DIAPM). EL/IX is a proposed POSIXbased hard realtime Linux API being promoted by Red Hat. And LinuxSRT is a soft realtime alternative to realtime APIs, which provides performanceenhancing capabilities to any Linux program without requiring that the program be modified or repiled. 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 response latencies Some realtime embedded systems need to respond quickly to external events in order to acplish 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. Six. Configuration procedures Now let39。 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 where NETtel is used as a remote control device. Matrix Orbital: This an optional, but not remended, 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 20x4 VFDs, plus a 240x64 graphic LC (128x128 on the way). Communication with the displays is acplished via either RS232 or I2C, both of which are standard on all of their modules. A prehensive mand set is included in the modules39。s onboard 4MB flash memory. Calibri: CalibriTM133 is a readytouse, pact, multipurpose work appliance that uses Embedded Linux as its operating system. It offers a highly efficient and lowcost solution to firewall, VPN, and routing demands. EmbeddedPla: EmbeddedPla has created a PostPCera puter that es loaded with MontaVista39。s ipEngine1 is a creditcard sized singleboard puter with Embedded Linux support. It utilizes a PowerPCbased CPU and provides an array of onboard peripherals, including Ether, LCD/Video Controller, USB, Serial I/O, and a 16K gate userconfigurable FPGA. BSE39。s safe to say that some form of Linux can run on just about any puter 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 wellknown small footprint Embedded Linux versions: ETLinux a plete Linux distribution designed to run on small industrial puters, especially PC/104 modules. LEM a small (8 MB) multiuser, worked Linux version that runs on 386s. LOAF Linux On A Floppy distribution that runs on 386s. uClinux Linux for systems without MMUs. 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, X10 controllers, MP3 players, and other such embedded applications. Software and hardware requirements Several userinterface tools and programs enhance the versatility of the Linux basic kernel. It39。s still a more economical choice. Three. Different types of Embedded Linux systems There are already many examples of Embedded Linux systems。s a lightweight operating system to bring to the embedded market. Another benefit of using an open source operating system like Embedded Linux over a traditional realtime operating system (RTOS), is that the Linux development munity tends to support new IP and other protocols faster than RTOS vendors do. For example, more device drivers, such as work interface card (NIC) drivers and parallel and serial port drivers, are available for Linux than for mercial operating systems. The core Linux operating system itself has a fairly simple microkernel architecture. Networking and file systems are layered on top of the microkernel in modular fashion. Drivers and other features can be either piled in or added to the kernel at runtime as loadable modules. This provides a highly modular buildingblock approach to constructing a custom embeddable system, which typically uses a bination of custom drivers and application programs to provide the added functionality. An embedded system also often requires generic capabilities, which, in order to avoid reinventing the wheel, are built with offtheshelf programs and drivers, many of which are available for mon peripherals and applications. Linux