The following figure shows the clock rate and power for Intel x86 microprocessors over nine generations and 36 years. Both clock rate and power increased rapidly for decades, and then flattened off recently. The reason they grew together is that they are correlated.
| Also, the reason for their recent slowing is that we have run into the practical power limit for cooling commodity microprocessors. |
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The dominant technology for integrated circuits is called CMOS (complementary metal oxide semiconductor). For CMOS, the primary source of energy consumption is so-called dynamic energy—that is, energy that is consumed when transistors switch states from 0 to 1 and vice versa.
- Static energy occurs because of leakage current that flows even when a transistor is off.
- Dynamic energy
depends on the capacitive loading of each transistor and the voltage applied:
Energy ∝ Capacitive load × Voltage2
The energy of a single transition is half of the energy of a pulse during the logic transition of 0→1→0 or 1→0→1:∴ Energy ∝ ½ × Capacitive load × Voltage2
The power required per transistor is just the product of energy of a transition and the frequency of transitions, where the frequency switched is a function of the clock rate:∴ Power ∝ ½ × Capacitive load × Voltage2 × Frequency switched