How did Linux become a mainstream embedded operating system? Chris - PDF document

How did Linux become a mainstream embedded operating system? Chris Simmonds 2net Limited Winchester, UK chris@2net.co.uk Abstract —This paper is based on a timeline showing the tend to have slow processors, and small amounts of memory various stages of development that Linux went through as it and storage. Looking at it this way brings into the definition of evolved from a desktop operating system for PCs to a flexible embedded devices such as smart phones and tables, which are operating system which could be scaled down to embedded obviously computers, but which use many of the same devices. Since the early beginnings of embedded Linux in the late techniques as the first group. I have taken a rather broad definition, which includes both definitions. With that in mind, 1990's, many people have contributed time and know-how that has resulted in today's Linux-based embedded operating systems. let's see how all this came about. This paper highlights some of the key milestones, and the events II. T HE EARLY DAYS : 1995 TO 1999 and people behind them. Keywords—Linux I. I NTRODUCTION The birth of Linux can be pinpointed to 25th August 1991 when Linus Torvalds, then a student at the University of Helsinki, announced his project on the comp.os.minix newsgroup in a famous post that began: Hello everybody out there using minix - I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since april, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things). Illustration 1: Early days Linus' motivation was to run a Unix-like operating system on his desktop. However, the open source license and During the period 1995 to 1999 the foundations were laid availability of the code via the Internet, meant that many that would enable Linux to become an embedded operating developers and enthusiasts were able to get access to the code. system. Pretty soon Linux was running on all sorts of different systems In 1995, Bruce Perens began work on a utility cammed This paper is about a significant sector of the Linux Busybox that would enable him to create a simple Linux installed base, generally known as “embedded Linux”. It is system that would fit onto a 3.5 inch floppy disk. His aim was hard to pin down exactly what “embedded” means. One to make it easy to install the Debian Linux distribution by definition is “everything that contains a computer but doesn't allowing it to be booted from a floppy disk drive, which was look like one”, such as printers, wireless routers, smart TVs, fitted to almost all desktop PCs at that time. Both Busybox and smart thermostats and even smart light bubs. Another definition a Linux kernel would fit into at 1.44 MiB floppy disk. emphasizes the lack of system resources: embedded devices www.embedded-world.eu

Although it was not on his mind at the time, embedded III. 1999: T HINGS S TART TO H APPEN systems also tend to have a small amount of storage, and so Busybox was the ideal solution to creating a small, useful subset of a Linux distribution. Even today, a large proportion of embedded Linux systems use Busybox. An important stepping stone was the Linux Router Project (LRP) [1], created in 1997 by Dave Cinege. He took the idea of a bootable floppy disk and added some networking utilities which could turn any PC with two or more network cards, and the right drivers, into a router. While not strictly an embedded application, it demonstrated that a useful system could be put together in a small footprint. It also formed the basis that would lead to Linux being used on many network devices. A further step in that direction came a year later when David Täht and Greg Retkowski published a document titled the "Arlan Wireless Howto" [2], which described the process of making a wireless connection between two points using Arlan cards, and so created what was arguably the first embedded Illustration 2: Things start to happen Linux wireless router [3]. As well as having a small amount of storage, another 1999 is the year that embedded Linux started to make an characteristic of embedded devices is the variety of CPU impact as the first high-profile commercial products began to architectures employed, for reasons of cost, low power and the emerge. range of on-chip features. During this period, Linux was ported TiVo [7] shiped their first-generation DVR (Digital video to several architectures that are important for embedded Linux: recorder) in March of that year. TiVo was one of the first 1995: MIPS consumer products to run Linux, and established Linux in a product segment that it dominates today. Under the hood, the 1996: Motorola 68000 and the IBM/Motorola PowerPC first TiVo had a PowerPC 403 processor, 16 MiB RAM and a 1999: ARM 14 GiB hard drive. In 1998, support for the Motorola Dragonball processor Axis Communication [8] released the 2100 network camera. Axis created the jffs filesystem, as already noted. The was added, as part of the effort to run Linux on the 3Com Palm Pilot PDA. The Dragonball did not have a memory hardware spec for the 2100 was ETRAX-100 risc cpu, 8/16 management unit (MMU), and so this work created a fork of MiB RAM, 2 MiB NOR flash, uClinux 2.0.33 [9]. Linux that was known as micro controller Linux, or more The Kerbango 100E Internet radio [10] was announced. commonly, uClinux [4]. It also lead to the creation of a new, Although it never shipped as a product, it did generate a lot of small C library called uClibc [5], which is still in use today. press coverage, especially when 3com, which at that time was uClinux was merged into mainline Linux with the 2.6 release very large company, bought the product a year later. The in 2003. Kerbango was important in another way. Much of the software The final piece of the jigsaw was support for memory based was implemented by a new Silicon Valley company called storage technologies, especially flash memory. David Montavista, and it proved a good training ground for many developers. Woodhouse added the Memory Technology Devices (MTD) layer [6]. MTD creates a framework for accessing flash As well as Montavista, several other embedded Linux memory and is the basis of the various flash-based file systems software companies came into being in 1999. Lineo was that follow. Flash memory is a key component of most founded by Bryan Sparks. Lineo did not last as long as embedded devices. Shortly afterwards, Axis Communications Montavista, but also made important contributions to used the MTD later to create a robust file system for flash embedded Linux. In Germany, Software consultancy Denx was memory. It was called jffs (journalling flash filesystem) and formed. It began work on PPCBoot, merging code from later evolved into jffs2 which is used in a large number of fadsboot (Dan Malek) and 8xxrom-0.3.0 (Magnus Damm). devices through to today PPCBoot 0.41 was released July 2000, initially for MPC 8xx boards. PPCBoot evolved into U-Boot which is used to boot Linux on many millions of embedded boards today.