1. Welcome to the Embedded World
1.1. First Steps in the Embedded World
Once upon a time, programming embedded systems was easy: all a developer needed when he wanted to start a new product was a good toolchain, consisting of
maybe an assembler
probably an EPROM burning device
and things could start. After some more or less short time, every register of the CPU was known, a variety of library routines had been developed and our brave developer was able to do his project with the more and more well-known system. The controllers had legacy interfaces like RS232, i2c or SPI which connected them to the outside world and the main difference between the controllers available on the market was the number of GPIO pins, UARTs and memory resources.
Things have changed. Hardware manufacturers have weakened the border between deeply embedded microcontrollers – headless devices with just a few pins and very limited computing power – and full-blown microprocessors. System structures became much more complicated: where our good old controllers have had just some interrupts with some small interrupt service routines, we today need complicated generic interrupt infrastructures, suitable for generic software frameworks. Where we’ve had some linearly mapped flash ROM and some data RAM we today have multi-stage-pipeline architectures, memory management units, virtual address spaces, on-chip-memory, caches and other complicated units, which is not exactly what the embedded system developer wants to program every other day.
Entering embedded operating systems. Although there are still some processors out there (like the popular ARM7TDMI based SoCs) which can be programmed the good old non-operating-system way with reasonable effort, it in fact is becoming more and more difficult. On the other hand, legacy I/O interfaces like RS232 are increasingly often replaced by modern plug-and-play aware communication channels: USB, FireWire (IEEE1394), Ethernet & friends are more and more directly integrated into today’s microcontroller hardware. Whereas some of these interfaces can “somehow” be handled the old controller-style way of writing software, the developer following this way will not be able to address the security and performance issues which come up with the modern network accessible devices.
During the last few years, more and more of the small-scale companies which developed little embedded operating systems have been pushed out of the market. Nearly no small company is able to support all different interfaces, communication stacks, development tools and security issues out there. New interfaces and -variants (like USB On-the-Go) are developed faster than operating system developers can supply the software for them. The result is a consolidation of the market: today we see that, besides niche products, probably only the largest commercial embedded operating system suppliers will survive this development.
Only the largest commercial…? There is one exception: when the same situation came up in the “mainstream” computer market at the beginning of the 1990es, people started to develop an alternative to the large commercial operating systems: Linux. Linux did never start with a ready-to-use solution: people had a problem, searched for a solution but didn’t find one. Then they started to develop one themselves, often several people did this in parallel, and in a huge community based evolution mechanism the best solutions found their way into the Linux kernel, which over the time formed one of the most reliable and performant kernels available today. This “develop-and-evolute” mechanism has shown its effectiveness over and over again in the server and desktop market of today.
1.2. From Server to Embedded
The fact that for most technical problems that might occur it may be possible to find somebody on the internet who has already worked on the same or another very similar problem, was one of the major forces behind the success story of Embedded Linux.
Studies have shown that more than 70% of the embedded developers are not satisfied with a black-box operating system: they want to adapt it to their needs, to their special hardware situation (which most times is Just Different than anything available). Embedded projects are even more variegated than desktop- or server projects, due to the fact that so many different embedded processors with lots of peripherals exist out there.
Linux has evolved from an i386-only operating system to a kernel running on nearly every modern 32 bit processor available today: x86, PowerPC, ARM, MIPS, m68k, cris, Super-H etc. The kernel supplies a hardware abstraction layer which lets our brave embedded developer once again concentrate on his very special problem, not on handling negligibilities like memory management.
But Linux is only half of the story. Besides the kernel, a Linux based embedded system consists of a “userland”: a filesystem, containing all the small tools which form a small Unix system. Only the combination of the kernel and a Userland lets the developer run “normal” processes on his x86 development machine as well as on his embedded target.
1.3. Linux = Embedded Linux
Whereas the mainstream developers were always able to use normal Linux distributions like SuSE, RedHat, Mandrake or Debian as a base for their applications, things are different for embedded systems.
Due to the restricted resources of these systems, distributions have to be small and should only contain those things that are needed for the application. Today’s mainstream distributions cannot be installed in less than 100 MiB without major loss of functionality. Even Debian, probably today’s most customizable mainstream distribution, cannot be shrunk below this mark without for example losing the packet management, which is an essential feature of using a distribution at all.
Additionally, source code for industrial systems has to be
Embedded developers usually want to know what’s in their systems – be it that they have to support their software for a long time span (something like 10-15 years are usual product lifetimes in automation applications) or that they have such a special scenario that they have to maintain their integrated source of their Userland themselves.