Embedding Linux in a Commercial Product
Most Linux systems run on PC platforms; however, Linux can also be a reliable workhorse for embedded systems. This article gives an overview of embedded systems and demonstrates what is involved in using Linux in a commercial embedded system.
The computers used to control equipment, or embedded systems, have been around for almost as long as computers themselves.
In communications, they were used back in the late 1960s to control electro-mechanical telephone switches and were called “Stored Program Control” systems. The word “computer” was not as ubiquitous back then, and the stored program referred to the memory that held the program and routing information. Storing this logic, instead of hard-wiring it into the hardware, was a real breakthrough concept. Today, we take it for granted that this is the way things work.
These computers were custom-designed for each application. By today's standards, they look like a collection of mutant deviants, with strange special-purpose instructions and I/O devices that were integrated with the main computing engine.
The microprocessor changed that by providing a small, low-cost, CPU engine that could be used as a building block in a larger system. It imposed a rigid hardware architecture based on peripherals connected by a bus and provided a general purpose programming model, which simplified programming.
Software also advanced along with the hardware. Initially, only simple program development tools were available for creating and testing software. The runtime software for each project was usually written entirely from scratch. This was almost always written in assembly language or macro languages, because compilers were often buggy and lacked decent debuggers. The idea of software building blocks and standardized libraries did not come into vogue until the mid 1970s.
Off-the-shelf operating systems for embedded systems began to appear in the late 1970s. Many of these were written in assembly language, and could be used only on the microprocessor for which they were written. When microprocessor became obsolete, so did its operating system, unless it was rewritten to run on a newer microprocessor. Today, many of these early systems are just a faint memory; does anyone remember MTOS? When the C language came along, operating systems could be written in an efficient, stable and portable manner. This had instant appeal to management, because it held the hope of preserving the software investment when the current microprocessor became obsolete. This sounded like a good story in a marketing pitch. Operating systems written in C became the norm and remain so today. In general, reusability of software has taken hold and is doing rather nicely.
My favorite OS in the early 1980s was the Wendon operating system. For about $150, you received a library of C source code. It was a kit, and you built your own operating system by choosing components—kind of like ordering dinner from a Chinese menu. For example, you could pick a task scheduler algorithm and a memory management scheme from a list of possibilities in the library.
A number of commercial operating systems for embedded systems sprang to life in the 1980s. This primordial stew has evolved to the present-day stew of commercial operating systems. Today, there are a few dozen viable commercial operating systems from which to choose. A few big players have emerged, such as VxWorks, pSOS, Neculeus and Windows CE.
Many embedded systems do not have any operating system at all, just a control loop. This may be sufficient for very simple ones; however, as systems grow in complexity, an operating system becomes essential or the software grows unreasonably complex. Sadly, there are some horribly complex embedded systems that are complex only because the designers insisted they did not need an operating system.
Increasingly, more embedded systems need to be connected to some sort of network, and hence, require a networking stack. Even the doorknob in many hotels has an embedded microprocessor connected to a network.
For simple embedded systems that are just coded in a loop, adding a network stack may raise the complexity level to the point that an operating system is desirable.
In addition to a variety of commercial operating systems, there is an amazing number of proprietary operating systems. Many of these are created from scratch, such as Cisco's IOS; others are derived from some other operating system. For example, many network products are derived from the same version of the Berkeley UNIX system, because it has complete networking capability. Others are based on public domain operating systems such as KA9Q from Phil Karn.
Linux as an embedded OS is a new candidate with some attractive advantages. It is portable to many CPUs and hardware platforms, stable, scalable over a wide range of capabilities and easy to use for development.