Computers, such as microprocessors, are more and more often embedded in products that the user does not perceive as computers. This is the case for both consumer electronics (your CD player, mobile phone, etc.), home supplies (your micro-wave oven, washing machine, etc.), vehicles (the engine control in your car, airplane fly-by-wire control, etc.), building automation (elevator control, temperature control, etc.), as well as for industrial equipment. We want these things to be affordable, and we want to be able to trust their operation, in particular when our life depends on them. Additionally, in some cases, we want to be able to reconfigure, upgrade, or even program them, possibly by downloading software components from a trusted supplier via the Internet.
Most people think of a computer as the desktop type of computer (with keyboard and display), but the number of embedded computers is bigger than the number of all other types of computers, and the difference is increasing. Moreover, the variety of types of processors and operating systems is much larger in the embedded world, which includes the desktop types and many others. Reasons for this variety include optimization to reduce the product price and minimization of energy (from batteries) consumption.