November 1998, Issue 100

Embedded RT-Linux (Part 1):
General Introduction

RUNNING SUPPORT

Although the recommended minimum size for running Linux with X-Windows is usually 8–16 MB, it’s overkill for running Linux in an embedded system. What is the minimum configuration we need?

To run Linux on a PC architecture, we need at least an i386-class machine because Linux has to run in 32-bit protected mode. Several embedded ’386 boards out there should run Linux just fine.

In fact, I was even able to boot and run Linux on my 16-MHz ’386SX laptop with 5 MB of memory. It runs fine, although you probably don’t want to compile a program or run an Xserver on it. But then, that’s not the point of a minimum system.

The minimum memory configuration to load a standard Linux kernel and have it be able to do something is 4 MB. It should be possible to configure and build a minimum size kernel that should work in about 2 MB.

Now we have an i386 and 4 MB of memory, what else do we need? Well, to boot Linux, we need a boot device.

Linux can’t use flash memory as disks, but it can boot from it. In fact, Linux can load and initialize the kernel using any kind of boot device the BIOS can handle. So, it’s possible to boot Linux from a flash memory after all, but it can’t access the boot flash-memory disk once it’s loaded.

You may ask, what good is it to boot from flash memory if you can’t access the disk after booting? Well, there are at least two techniques you can use to initialize an initial RAM disk while booting, and this RAM-disk image can contain the essentials of what’s needed to run Linux.

What do we need to run Linux once the kernel has booted? By incorporating our system into one Linux program, we can get away with a small set of files. Listing 1 has the details.

/lib
/lib/libc.so.5
/lib/ld-linux.so.1
/bin
/bin/sh
/bin/insmod
/etc
/etc/ld.so.cache
/dev
/dev/console
/dev/null
/dev/ram
/dev/systty
/dev/tty1
/dev/tty2
/dev/tty3
/dev/tty4
/linuxrc

The most critical components are the console device entry /dev/console and whatever device entries that are needed. Once booted, the kernel opens and runs whatever program or command script is contained in /linuxrc. That’s about all you need for a minimal configuration.

Even though Linux wasn’t designed as an embedded OS, it ended up having features like small initial size and capability for RAM disks and modular device drivers. These features are there because the developers wanted Linux to be easy to install.

A single floppy can hold a complete mini-Linux system, complete with enough utilities to format and initialize the file system on a hard disk and do a bootstrap install of Linux from another medium. We can use this to our advantage by including our embedded applications, instead of the installation utilities to build an embedded Linux system.

Now let’s look at a system I’ve been using to play around with embedded Linux. It’s based on Motorola’s embedded Pentium SBC, the NLX 55, shown in Photo 1.

The motherboard contains everything needed to build a system and run an embedded application. It has a 233-MHz Pentium MMX CPU, slots for two DIMM modules, 10-/100-Mbps Ethernet controller, dual-IDE controller, floppy control, and SVGA controller. It also contains a flash-card socket on the motherboard.

SanDisk’s CompactFlash is a flash-based media that contains an IDE controller on the card itself. To the system, it looks just like another IDE drive. Motorola also sells a starter system that includes a case and power supply as well as a standard IDE drive for development.

With this system, you can develop embedded applications while running off the hard disk and build a flash-card module of our embedded system. Then, just reboot, change the boot sequence in the BIOS setup, and have the system boot from the flash card to test your application. Next month, I’ll use it to build a flash card-based Linux installation.