November 1998, Issue 100

Embedded RT-Linux (Part 1):
General Introduction

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WHY LINUX?

Naturally, this question is tugging at your brain. There are several reasons to consider Linux as an embedded operating system for many applications.

For one thing, it’s freely available and includes a TCP/IP stack. Also, it has multiarchitecture support for Intel, 68k, PowerPC, Alpha, Sparc, SMP, and others. So, you can obtain and begin using Linux with very little trouble.

One of the great benefits of using Linux is the large support network that exists for it, and let’s not forget the fact that Linux supports many devices and bus architectures right out of the box. Of course, Linux is somewhat modular and there are many tools and programming languages available. With features like this, Linux starts to make more sense, huh?

Let’s talk about these points in more detail. I already mentioned that Linux is freely available. Obviously, that’s nice because you won’t pay royalties for embedding Linux in your product. But, you need to make sure all of the modules and components you plan to use for your project are freely available. Most are.

You can download most Linux distributions from the Internet. But, this can be tedious over a slow link, so most of us opt to buy a CD-ROM. Yet, being able to download a distribution over the ’Net is extremely useful if you’re in the field and need to upgrade you embedded system or get a new version of a device driver.

Linux has a TCP/IP stack built in. The TCP/IP implementation is very robust, since many Internet-service providers use Linux for their high-throughput web servers. The programming interface for the TCP/IP stack is the standard Socket API.

Linux runs on a multitude of architectures. I’m only going to talk about the Intel port, but it’s good to know that you’re not stuck with Intel. Some of the details in each port are different, but for the most part, Linux is Linux, at least at the application level.

Linux is ported to 68k, which includes VME-bus modules, Alpha, and PowerPCs. And, there’s a port in progress for MIPS processors.

The US Robotics Palm Pilot may be the smallest system that Linux is ported for. It’s a PDA based on Motorola’s 68328 Dragonball processor. There’s also work in progress to port Linux to smaller Intel processors, like the 80188.

Since Linux has such a great following, there’s a large support network. Although many people provide Linux support by writing or porting applications and drivers in an effort to attain fame and glory, there are also several companies that are providing commercial support for Linux. Often, these companies reintegrate the fixes into the Linux kernel development effort.

There is yet another side effect to this large support group. Many of the fame-and-glory developers are students who have learned OS internals by observing the Linux kernel, and they’ll eventually enter the job market.

Consequently, the number of software engineers familiar with Linux is growing, and they’re even proficient at the kernel level. That’s good news, and perhaps one compelling reason why much of Linux will stay freely available with the GPL.

There are a great number of device drivers available for Linux. Because Linux has been ported to so many different architectures, it supports various bus architectures, like ISA bus, PCMCIA, PCI, VME bus, S-bus, and others.

Many vendors of PC-compatible cards have Linux drivers available. While the quality of device drivers for Linux varies, many popular cards and chip sets are well supported. Also, there’s hope of fixing or adapting the device driver for your purposes, since sources are available for most device drivers.

Drivers are available for just about all of the Ethernet chip sets, all standard AT peripherals (COM, IDE, floppy, etc.), and many VGA chip sets. Also, many CD-ROMs and SCSI devices are supported. Table 1 shows a list of devices that are supported in an older distribution of RedHat.

However, a flash file-system driver isn’t supported. That’s unfortunate. It would be nice for working with embedded systems. Hopefully, someone is working on it.

But, all is not lost—it’s still possible to boot Linux from a flash file system. Flash disks, like SanDisk’s CompactFlash devices, work under Linux because it acts just like a IDE drive. Next month, I’ll look at these issues in detail when I embed Linux.

Linux supports dynamic module loading, which means libraries and devices drivers can be loaded by the OS after boot time to extend the base functionality of the kernel. Dynamic modules are nothing new, but they do make configuration easy.

You build a simple kernel that can be configured to deal with different device configurations by changing a configuration file. This construction also improves memory use, since only the required device drivers are loaded when needed. Of course, you can still configure a Linux kernel to be static by compiling in all the device drivers it needs for a particular application.

Many applications and programming languages are available for Linux. This feature might not be important for building embedded systems, but it’s nice to have access to all the applications on your development system.

The normal compiler used for Linux is the GNU C compiler (a complete C, C++, and Objective C compiler). The companion debugger (gdb) is flexible and can even deal with remote debugging via Ethernet or serial port. This capability means that you can debug your embedded system remotely from a desktop or, in my case, a laptop. Of course, you can also run the debugger on the console of the embedded system.

In conjunction with the GNU C compiler and related tools, you’ll find commercial compiler and development environments. I listed one compiler vendor in my sources, and you can find others in the Linux Journal.