Linux is making steady progress in the embedded arena. Because Linux is covered under the GPL (see Resources later in this article), anyone interested in customizing Linux to his PDA, palmtop, or wearable device can download the kernel and applications freely from the Internet and begin porting or developing. Many Linux flavors cater to the embedded/realtime market. These include RTLinux (Real-Time Linux), uclinux (Linux for MMUless devices), Montavista Linux (Linux distributions for ARM, MIPS, PPC), ARM-Linux (Linux on ARM), and others (see Resources for links to these and other terms and products mentioned in this article).
Embedded Linux development broadly involves three tiers: the bootloader, the Linux kernel, and the graphical user interface (or GUI). In this article, we will focus on some basic concepts involving these three tiers; we will provide some insights into how the bootloader, kernel, and filesystem interact; and we will investigate some of the numerous options available for the filesystem, GUI, and bootloaders.
Bootloaders
The bootloader is usually the first piece of code that will be executed on any hardware. In conventional systems like desktops, the bootloader is normally loaded into the MBR (Master Boot Record), or the first sector of the disk where Linux resides. Normally, BIOS will transfer control to the bootloader in the case of desktops or other systems. This poses an interesting question: who loads the bootloader onto the embedded devices, which (in most cases) don't have BIOS?
Two general techniques are used to address this problem: specialized software and tiny bootcode.
Specialized software can directly interact with the flash device on the system remotely and install the bootloader at the given location in flash. Flash devices are special chips that act like storage devices, and they are persistent -- that is, the contents are not erased on reboot.
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