Thumb code density is better than that achieved by

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Unformatted text preview: There is no development visible at the time of writing which suggests a change on the same scale as RISC, but instruction sets continue to evolve to give better support for efficient implementations and for new applications such as multimedia. 1.7 Design for low power consumption Since the introduction of digital computers 50 years ago there has been sustained improvement in their cost-effectiveness at a rate unparalleled in any other technical endeavour. As a side-effect of the route taken to increased performance, the power consumption of the machines has reduced equally dramatically. Only very recently, however, has the drive for minimum power consumption become as important as, and in some application areas more important than, the drive for increased performance. This change has come about as a result of the growing market for battery-powered portable equipment, such as digital mobile telephones and lap-top computers, which incorporate high-performance computing components. Following the introduction of the integrated circuit the computer business has been driven by the win-win scenario whereby smaller transistors yield lower cost, higher performance and lower power consumption. Now, though, designers are beginning to design specifically for low power, even, in some cases, sacrificing performance to achieve it. The ARM processor is at the centre of this drive for power-efficient processing. It therefore seems appropriate to consider the issues around design for low power. Where does the power go? The starting point for low-power design is to understand where the power goes in existing circuits. CMOS is the dominant technology for modern high-performance digital electronics, and has itself some good properties for low-power design, so we start by looking at where the power goes in a CMOS circuit. A typical CMOS circuit is the static NAND gate, illustrated in Figure 1.2 on page 4. All signals swing between the voltages of the power and ground rails, Vdd and Vss, Until recently a 5 volt supply was standard, but many modern CMOS processes require a lower supply voltage of around 3 volts and the latest technologies operate with supplies of betwe...
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