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Power Reduction Techniques For Microprocessor Systems. Presented by William Lane. Material and graphics presented here are from: Power Reduction Techniques For Microprocessor Systems Vasanth Venkatachalam and Michael Franz ACM Computing Surveys, Vol. 37, No3, September 2005, pp. 195 - 237.

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Power reduction techniques for microprocessor systems l.jpg

Power Reduction Techniques For Microprocessor Systems

Presented by William Lane

Material and graphics presented here are from:

Power Reduction Techniques For Microprocessor Systems

Vasanth Venkatachalam and Michael Franz

ACM Computing Surveys, Vol. 37, No3, September 2005, pp. 195 - 237


Slide2 l.jpg

“Power consumption is a major factor that limits the performance of computer systems.”

Most of the power a computer consumes ends up as heat.

The above diagram shows the power density of a few microprocessors.

In some cases heat generation and power consumption may not be an issue.

For example: Home computer used for gaming vs. Cell phone


Forms of power consumption l.jpg
Forms of power consumption: performance of computer systems.”

  • Dynamic

    • Switched capacitance

    • Short circuit current

  • Static

    • Leakage

Static power loss seems to be where most of the total power is ending up.


Dynamic power loss l.jpg
Dynamic Power Loss performance of computer systems.”

  • Switched capacitance:

    • 85-90% of dynamic power loss

    • Energy used in charging or dissipating capacitors at outputs of circuits

  • Short Circuit Current:

    • 10-15% of dynamic power loss

    • Energy used for pairs of transistors that switch current with opposite polarity cause a momentary short circuit


Equation for switched capacitance power loss l.jpg
Equation for Switched Capacitance power loss performance of computer systems.”

P = aCV2f

P = power loss a = activity factor

V = supply voltage f = clock frequency

C = capacitance


Methods of reducing switched capacitance power loss l.jpg
Methods of reducing Switched Capacitance power loss performance of computer systems.”

  • Decreasing the capacitance

    • This can worsen the performance.

  • Decreasing activity level

    • This can be difficult because generally we try to fit more on to computer chips. Ex. Clock Gating

  • Reduce clock frequency

    • Reduces the performance and may not end up reducing the energy used.

  • Decrease supply voltage

    • Lower supply voltage can increase time delay of the circuit which would then need a lower clock frequency.


Dynamic voltage scaling l.jpg
Dynamic Voltage Scaling performance of computer systems.”

  • A combination of adjusting the supply voltage and clock frequency are used to increase or decrease the power of the processor. Adjusting these two together gives the best results.

  • Many modern processors use such a method to control the speed of the processor.

  • DVS can be used for consuming less power while also keeping the temperature of the chip cooler.


Static power loss l.jpg
Static Power Loss performance of computer systems.”

  • Reduce supply voltage

    • Uses less power

  • Reduce the size of the circuit

    • Clock gating

    • Technology Mapping (tool for circuit design)

  • Cool computer

    • Recent study showed that cooling the processor by 50 degrees reduced leakage by up to a factor of 5

  • Change threshold voltage of transistors

    • Increasing the threshold voltage lowers the amount of leakage through the transistor, but it then takes more voltage to switch the transistor. Ex: Adaptive Body Biasing


Further studies in power consumption l.jpg
Further studies in power consumption performance of computer systems.”

  • Buses

    • Reduce the amount of switching in a bus. Invert signal to decrease number of bits that change.

      11111 to 00001 4 bits change

      11111 to 11110 1 bit change

    • Low Swing Bus

      • Use lower voltage for the bus

    • Bus segmentation

      • Shut down parts of bus not used


Further studies in power consumption10 l.jpg
Further studies in power consumption performance of computer systems.”

  • Memory

    • Each memory access costs energy

      • Decrease number of memory access

      • Decrease amount of energy needed to access memory

    • Scratch pad memory

      • Used mostly in embedded application; programmer defines what data to store in the cache.

    • Instruction level cache

      • Stores the next few instructions only. Any memory references are removed. Located between L1 cache and processor

    • Adaptive Cache

      • Turns off part off cache to save energy. Can be used with drowsy cache to save data in cache when cache is turned off.


A few open questions l.jpg
A few open questions performance of computer systems.”

  • Is it more energy efficient to run a program using a more powerful processor which runs for less time, or is it better to run a processor that is less powerful but finishes in more time? Which processor uses more energy?

    • The speedup of a program is not linear to the speedup of the processor.

  • Is it better to use multiple slow processors or one fast processor? Which uses more energy?

  • For a laptop computer would you pick a processor that would burn through your battery quickly but gives you more computing power?


In conclusion l.jpg
In conclusion performance of computer systems.”

  • The energy efficiency of a processor needs to be weighed against the application of the processor.

  • Most often it is a choice between a more powerful processor and a more energy efficient one.


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