The developments represent the flowering of an embedded-Linux movement that has taken root during the past few years, thanks in part to the swirl of activity that has led to the porting of the kernel to various microprocessors.

Linux is gaining ground in the embedded area because it sports a small kernel, it's free and it reaps the benefits of open-source software — a large pool of engineers continually working to improve the code. In a paper he will present at Linux World, software expert Greg Ungerer of Moreton Bay Ventures Pty. Ltd. (Sumner Park, Australia) noted, "Development within the Linux community is fast and furious. And those developments are shared with the community at large with lightning speed."

Or, as one engineer put it at last year's company launch for TiVo Inc. (Sunnyvale), whose digital TV recorder runs on Linux, "It's free, it works. Why should we pay for some brand-name operating system?"

Clearly, Linux' most powerful draw in the embedded world is its open-source nature. Developers hoping to tailor the kernel to a particular application are free to mess with it — take it apart, compress it, mold it — provided they add their changes to the open-source pool.

One of the issues Linux must face in the embedded world is the stark difference in environment between a desktop PC and even the most elaborate embedded computer. In embedded applications, a kernel can't assume the presence of a monitor, keyboard or even a file system — restrictions that would appear to be prohibitive for a derivative of the legendarily file-happy Unix.

But at least one vendor, MontaVista, is finding solutions to those problems. The company has adapted Linux to run not only on embedded CPUs, but also on headless CompactPCI I/O processor boards. The company's founder, RTOS pioneer James Ready, said the experience has made him confident that Linux can be booted in even smaller, more restrictive environments, such as palmtop computers and deeply embedded processors.

Motorola's Computer Group, meanwhile, will announce at Linux World its alliance with Lineo Inc., the embedded-software spin-off of Linux distributor Caldera Inc., for embedded-Linux projects. The first offerings from the pair will be an ATX-form-factor server and a "network appliance," both Pentium-based. They are slated to ship in September.

In addition, a project dubbed uClinux, spearheaded by a Toronto company called Rt-Control Inc., is busy porting Linux to a variety of Motorola processors, including the MC68328, or Dragonball, which lies at the heart of the PalmPilot, and the Coldfire microcontroller.

For MontaVista, much of the challenge was in getting Linux to fit on an I/O board.

The initial target for the company's Linux port was a PC-compatible, Pentium-II-based chassis from Force Computers. Getting Linux to boot and run on the system's CPU board was not too tough, Ready said, since the CPU is essentially a PC motherboard complete with monitor and keyboard connections, disk and boot ROM.

But customers wanted to be able to run Linux not only on the CPU, but also on the system's equally powerful, and similarly configured, slave I/O boards. This presented more of a hurdle. The I/O boards included a Pentium II and 128 Mbytes of DRAM, but lacked keyboard, monitor or disk access.

Initial analysis, according to Ready, suggested that the job could be done. The full-blown Red Hat Linux configuration requires about 600 kbytes of memory and 150 Mbytes of disk space. Somehow this had to be trimmed to squeeze into the on-chip DRAM and flash.

The MontaVista team went through code, tossing out pieces that weren't strictly relevant to the embedded world but preserving all the things fundamental to Linux. The result was an 11.8-Mbyte compressed file that fit nicely into the I/O board's 16-Mbyte flash. When expanded at boot time, the file blossoms into a 28-Mbyte region of RAMdisk, a 15-Mbyte operating system memory area and about 8 Mbytes of required user memory, leaving plenty of room for applications.

Asymmetric cluster

Using the TCP/IP stack to communicate — via CompactPCI bus — to the Linux version running on the CPU, the kernel becomes part of an asymmetric cluster. The same application programming interfaces, tool chains, communications protocols and utilities work for the CPU board and the I/O processors.

Ready said it would be possible to continue the shrinking process further, trimming more code to fit into even less space. "We stopped when the operating system fit into this environment," he said. "There are more things we could easily have taken out. The big issue is providing enough space for the file system."

Similar efforts are under way worldwide. At Linux World, Moreton Bay's Ungerer will describe his efforts to port Linux to the Motorola Coldfire microcontroller, the processor chosen for Moreton's NETtel box.

The Coldfire port is part of the larger uClinux project, which began in 1998. Typically for a Linux endeavor, outside developers with contributions to add are being brought into the uClinux fold. Moreton's ColdFire port is one; another is a port of Linux to the Intel i960. Both ports, along with libraries and a complete development tool chain, are being included in the next uClinux CD-ROM, due out in a few weeks, uClinux cofounder D. Jeff Dionne said.

The uClinux effort "started from a project which was real-time monitoring of devices in substations," said Dionne, cofounder of Rt-Control. "We needed the ability to do TCP/IP, and the existing solutions like Vx were far too expensive for that kind of project."

Faced with the option of building TCP/IP support for their own kernel, Dionne and colleague Michael Durrant decided instead to use Linux's TCP/IP support by porting that kernel to the 68000.

UClinux is only a subset of Linux, not the entire OS. Durrant and Dionne found that the biggest adjustment to using a microcontroller for Linux was the lack of a management system, as Linux assumes virtual memory is available for the taking. They got around that by forcing the kernel to work only with physical memory addresses.

Dionne and Durrant now distribute their work through Rt-Control, which sells single-in-line memory modules and board kits as a jump-start toward building Internet appliances. The idea was to provide the Linux port to EEs who lacked time or experience to do their own kernel hacking, Durrant said.

Linux's main competition in embedded spaces is real-time operating systems. Linux itself is not an RTOS, although companies including Motorola are working on real-time extensions for the kernel.

But hard real-time processing often isn't required, and in many cases it's rendered irrelevant by processor speeds. "With the speed of the chips, the actual need for real-time is diminishing," said Brian Sparks, chief executive of Lineo. Lineo's plan is to build development tools that can indicate where real-time processing is needed, allowing developers to add real-time extensions only where necessary.

For his part, Ready of MontaVista sees no urgent need to give Linux hard real-time capabilities. "The majority of applications we are seeing today have requirements that are on the softer side of real-time," Ready said. "There are certainly parts of systems that have hard real-time requirements. But what we are seeing today is that these parts of the systems are getting shoved down into hardware, where the response is entirely deterministic. What's left after this process is a much softer level of requirements, and we are finding that — unlike desktop operating systems like Windows or NT — Linux is often demonstrably good enough."

Linux is making inroads into complex embedded applications as well. Telecom is a big target market for Motorola, due to the need for reliability, as are imaging and industrial automation, said Noel Lesniak, business manager for telecom platforms. But the company has no intention of scrapping Windows just yet. "Motorola Computer Group will continue in its other software initiatives," Lesniak said.

Realizing the limitations of Windows NT, Microsoft has begun querying communications carriers about their needs for the OS.

"We're working with Microsoft, Intel, Hewlett-Packard and Motorola — a number of major players — to go back and make the changes to Windows NT to provide the [up-time] availability we need to provide to our customers," said Andy Thomas, chief technologist for the enterprise voice solutions group of Nortel Networks in Richardson, Texas. In early July Nortel met with Microsoft to discuss such improvements. In Thomas' view, Windows 2000 "goes a long way" in improving reliability.

Lapses in reliability aren't completely NT's fault, Thomas said. "It's not typically because of the OS itself. It's what you put on it — you build a new driver, or you get an application that goes somewhere it shouldn't, and you get the blue screen of death."

Nortel does quite a bit of development work on Linux but is holding back on releasing Linux-run hardware for now, Thomas said.

Linux is also breaching consumer electronics, having found its way into the TiVo receiver, a digital video recorder based on IBM's 403GCX PowerPC. The operating system is there to manage all the traffic and housekeeping tasks — the flow of video and data moving simultaneously for capture and storage — inside the box.

The box uses real-time MPEG-2 encoding and decoding ICs to compress TV programs and a Quantum Corp. hard-disk drive to store up to 20 hours of TV programs.

— Ron Wilson and Junko Yoshida contributed to this story