As mentioned several times in this chapter cost and power are often at least as

As mentioned several times in this chapter cost and

This preview shows page 59 - 61 out of 86 pages.

As mentioned several times in this chapter, cost and power are often at least as important as performance in the embedded market. In addition to the cost of the processor module (which includes any required interface chips), memory is often the next most costly part of an embedded system. Recall that, unlike a desktop or server system, most embedded systems do not have secondary storage; instead, the entire application most reside in either FLASH or DRAM (as described in Chapter 5). Because many embedded systems, such as PDAs and cell phones, are constrained by both cost and physical size, the amount of memory needed for the application is critical. Likewise, power is often a determining factor in choosing a processor, especially for battery-powered systems. As we saw in Figure 1.24 on page 56, the power for the five embedded proces- sors we examined varies by more than a factor of 10. Clearly, the high perfor- mance AMD K6, with a typical power consumption of 9.3 W, cannot be used in environments where power or heat dissipation are critical. Figure 1.27 shows the relative performance per watt of typical operating power. Compare this figure to Figure 1.25 on page 57, which plots raw performance, and notice how different the results are. The NEC VR4122 has a clear advantage in performance per watt, but is the second lowest performing processor! From the viewpoint of power con- sumption the NEC VR4122, which was designed for battery-based systems, is the big winner. The IBM PowerPC displays efficient use of power to achieve its high performance, although at 6 watts typical, it is probably not be suitable for most battery-based devices. 1.8 Another View: Power Consumption and Efficiency as the Metric
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1.9 Fallacies and Pitfalls 59 The purpose of this section, which will be found in every chapter, is to explain some commonly held misbeliefs or misconceptions that you should avoid. We call such misbeliefs fallacies. When discussing a fallacy, we try to give a counter- example. We also discuss pitfalls —easily made mistakes. Often pitfalls are gen- eralizations of principles that are true in a limited context. The purpose of these sections is to help you avoid making these errors in machines that you design. Fallacy: The relative performance of two processors with the same ISA can be judged by clock rate or by the performance of a single benchmark suite. As processors have become faster and more sophisticated, processor performance in one application area can diverge from that in another area. Sometimes the in- struction set architecture is responsible for this, but increasingly the pipeline structure and memory system are responsible. This also means that clock rate is FIGURE 1.27 Relative performance per watt for the five embedded processors. The power is measured as typical operating power for the processor, and does not include any interface chips.
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