Embedded Linux Applications: An Overview
By Darrick Addison2005-04-30
Advantages/Disadvantages of Using Linux for Your Embedded System
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Although 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 and 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.
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 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 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 it'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 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 (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 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 functionality.
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 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 processor 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 addition 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 system's 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 than 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 currently 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, it's still a more economical choice.
Different types of Embedded Linux systems
There are already many examples of Embedded Linux systems; it's 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 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 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, X-10 controllers, MP3 players, and other such embedded applications.
Flash memory
Flash RAM is a special kind of memory that most Palm devices use to store the operating system. It has the advantage of allowing operating system upgrades, and it is also used in digital cellular phones, digital cameras, LAN switches, PC cards, digital set top boxes, embedded controllers, and other small devices. An embedded system, like Embedded Linux, does not require a disk drive, although a number of other memory organizations are possible. So if, let's say, Linux runs out of flash memory, it may use part of the flash memory as a read-only file system to store extra programs and static data.
Tutorial Pages:
» From Wrist Watches to Cluster-Based Supercomputers
» Emergence of Embedded Systems
» Advantages/Disadvantages of Using Linux for Your Embedded System
» Software and Hardware Requirements
» Hardware Platform Options
» Real-Time Embedded Linux Applications
» Configuration Procedures
» Creating a Bootdisk
» Installing TinyLogin and Login Dependencies
» Summary
» Resources
First published by IBM DeveloperWorks
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