先进的内存优化技术的低功耗嵌入式处理器

In a relatively short span of time, computers have evolved from huge mainframes to small
and elegant desktop computers, and now to low-power, ultra-portable handheld devices.
With each passing generation, computers consisting of processors, memories and peripherals
became smaller and faster. For example, the first commercial computer UNIVACI costed $1
million dollars, occupied 943 cubic feet space and could perform 1,905 operations per
second [94]. Now, a processor present in an electric shaver easily outperforms the early
mainframe computers.
The miniaturization is largely due to the efforts of engineers and scientists that made the
expeditious progress in the microelectronic technologies possible. According to Moore’s
Law [90], the advances in technology allow us to double the number of transistors on
a single silicon chip every 18 months. This has lead to an exponential increase in the
number of transistors on a chip, from 2,300 in an Intel 4004 to 42 millions in Intel Itanium
processor [55]. Moore’s Law has withstood for 40 years and is predicted to remain valid for
at least another decade [91].
Not only the miniaturization and dramatic performance improvement but also the significant
drop in the price of processors, has lead to situation where they are being integrated into
products, such as cars, televisions and phones which are not usually associated with computers.
This new trend has also been called the disappearing computer, where the computer
does not actually disappear but it is everywhere [85].
Digital devices containing processors now constitute a major part of our daily lives.
A small list of such devices includes microwave ovens, television sets, mobile phones, digital
cameras, MP3 players and cars. Whenever a system comprises of information processing
digital devices to control or to augment its functionality, such a system is termed an embedded
system. Therefore, all the above listed devices can be also classified as embedded systems.
In fact, it should be no surprise to us that the number of operational embedded systems has
already surpassed the human population on this planet [1].
Although the number and the diversity of embedded systems is huge, they share a set of
common and important characteristics which are enumerated below:
(a) Most of the embedded systems perform a fixed and dedicated set of functions. For
example, the microprocessor which controls the fuel injection system in a car will
perform the same functions for its entire life-time.